Handbook of Colorants Chemistry, Volume 2: In Painting, Art and Inks 3110777002, 9783110777000

Citation preview

Ingo Klöckl Handbook of Colorants Chemistry

Also of Interest Handbook of Colorants Chemistry Volume 1: Dyes and Pigments Fundamentals Klöckl, 2023 ISBN 978-3-11-077699-7, e-ISBN 978-3-11-077711-6

Chemistry for Archaeology Heritage Sciences Reiche, Alfeld, Radtke, Hodgkinson, 2020 ISBN 978-3-11-044214-4, e-ISBN 978-3-11-044216-8 Chemical Analysis in Cultural Heritage Sabbatini, van der Werf (Eds.), 2020 ISBN 978-3-11-045641-7, e-ISBN 978-3-11-045753-7

Encyclopedia of Color, Dyes, Pigments Pfaff, 2022 Volume 1. Antraquinonoid Pigments – Color Fundamentals ISBN 978-3-11-058588-9, e-ISBN 978-3-11-058807-1 Volume 2. Color Measurement – Metal Effect Pigments ISBN 978-3-11-058684-8, e-ISBN 978-3-11-058710-4 Volume 3. Mixed metal Oxide Pigments – Zinc Sulfide Pigments ISBN 978-3-11-058686-2, e-ISBN 978-3-11-058712-8

Ingo Klöckl

Handbook of Colorants Chemistry �

Volume 2: in Painting, Art and Inks

Author Dr. rer. nat. Ingo Klöckl St. Leoner Strasse 16 68809 Neulußheim Germany [email protected]

ISBN 978-3-11-077700-0 e-ISBN (PDF) 978-3-11-077712-3 e-ISBN (EPUB) 978-3-11-077729-1 Library of Congress Control Number: 2022949958 Bibliographic information published by the Deutsche Nationalbibliothek The Deutsche Nationalbibliothek lists this publication in the Deutsche Nationalbibliografie; detailed bibliographic data are available on the Internet at http://dnb.dnb.de. © 2023 Walter de Gruyter GmbH, Berlin/Boston Cover image: nikamata / iStock / Getty Images Plus Typesetting: VTeX UAB, Lithuania Printing and binding: CPI books GmbH, Leck www.degruyter.com

�

For all those who, through curiosity, have discovered or may discover their interest in the fascinating field of chemistry

Foreword Nothing shows us the beauty of our world as vividly as its colors. For our distant biological ancestors, there were predominantly smells. However, at some point in our long evolution into modern humans, we dared to swap the dull magic realm of scents for the bright precision of our eyes. And yet colors are also a magical realm that holds many a secret. Unlike shape, density, or surface texture, color is not an inherent property of an object but only our perception of how the object reflects or absorbs visible light. Moreover, our eyes show us only a tiny fraction of the immense spectrum of electromagnetic radiation that fills our universe. The wavelengths, and thus the frequencies, of this spectrum span 16 orders of magnitude—from the 10 to 20 km long radio waves of some military transmitters to the gamma rays of imploding galaxies, which are only a thousandth of a nanometer short. Life on our planet mainly registers wavelengths between 300 and 1000 nm. This range includes the ultraviolet, with wavelengths below 400 nm, which unlike many insects, we cannot see; the range from blue to green to red, that is, from 400 to about 750 nm, which means light to us; and finally, infrared rays, with wavelengths above 800 nm, which some animals perceive as light but we perceive only as heat. Ultraviolet was probably the first color that life on our planet saw. This spectral part of sunlight meant danger, as it destroyed many biological building blocks. Cells developed a sensor for ultraviolet and blue light that controlled the direction of rotation of their flagella to avoid these dangerous rays. Since these flagella act as propulsion propellers, the cells could now not only see the harmful short-wave light but also avoid it. A descendant of this blue light sensor is still found today in many primitive protozoa. This ingenious blue light sensor probably also served the cells as a construction manual for a solar collector, thanks to which they could feed on the energy of sunlight. Cells shifted the blue sensor’s absorption to yellow-orange to capture as much of the sunlight’s energy as possible. The cells coupled this solar collector to a system that converted the captured light into chemical energy. With its help, the cells could now power energy-hungry processes such as growth, cell division, and movement, or synthesizing fat, sugar, and proteins. This primitive photosynthesis is still found today in some singlecelled organisms that thrive in the salt-rich margins of the Dead Sea or spoiled cured fish. Ultimately, however, this form of photosynthesis proved to be a dead end because it did not efficiently convert light from the sun into chemical energy. When later cells used chlorophyll as a solar collector, ushering in modern photosynthesis, photosynthesis that evolved from the blue light sensor remained limited to a few primitive single-celled organisms. So, life learned very early to see the world in two colors—blue and yellow-orange. And now that it had seen color, it no longer wanted to do without it. With their extensive and information-rich genetic material, the complex modern cells created three, four, or even five different variants from the primitive blue light sensor, which opened up a vast and differentiated color spectrum for them. Even more, these modern cells were able https://doi.org/10.1515/9783110777123-201

VIII � Foreword to couple the signals of these different color sensors separately to increasingly complex nervous systems. Our eyes are equipped with five different light sensors, all closely related chemically and probably descended from the primordial blue light sensor mentioned earlier. One of these sensors is not used for vision but for the daily calibration of our “circadian” body clock. Another sensor is found in the rod cells of our retina. This sensor is susceptible to light and, therefore, we use it in dim light. However, this high light sensitivity comes at a price because our retinal rods do not detect color or fine detail. In bright light, we use three color sensors in the cone cells of our retina—one for blue, one for green, and one for red. These sensors are not very sensitive to light, but they show us fine detail— and color. Since each of these three color sensors can detect about a hundred different intensities of color, and our brain compares the signals from the three sensors, we can see not just three but one to two million colors. Older animals, such as insects and birds, have up to five different color sensors and cannot only distinguish many more colors than we humans can, but some can also see ultraviolet or infrared light to which we are blind. When the first mammals evolved, they mainly hunted at night, leaving some of their color sensors to atrophy, leaving only two of them. Almost all mammals—such as dogs, horses, cats, and cows—therefore see only about 10000 different colors—about the same as “color blind” humans. Only when intelligent apes wanted to distinguish ripe from unripe fruit against the background of multicolored leaves did they again develop a third color sensor, which allowed them to see the world in a new blaze of color. So, humans and our close relatives, the great apes, are the only mammals that can see millions of colors. In this impressive book, Ingo Klöckl describes the magic realm of colors from a chemist’s perspective. The synthesis of modern dyes with intoxicating color depth and impressive stability was one of the great triumphs of nineteenth and twentieth century chemistry, and the development of rewritable digital data carriers or catalysts for lightdriven water splitting suggests that the time of color chemistry is far from over. Ingo Klöckl describes the bewildering variety of dyes available today and gives us detailed information on how they can be produced, categorized, and compared with each other. This book is a masterpiece, a true magnum opus that reveals to us in each chapter a new wonder from the world of colors. The wealth of information it imparts to us is almost mind-boggling, yet it is an exciting read for anyone who is no stranger to chemistry. The book also builds a most welcome bridge between science and art, which have become increasingly distant from each other in recent centuries, forgetting their common roots. May not only natural scientists but also painters and art scholars pick up this book and lose themselves in the magical realm of colors. Basel

Dr. Gottfried Schatz†

Foreword to the English edition Dear readers, painting scientists, and researching painters, when the German edition appeared, I never expected it would appeal to so many fellow human beings since the balancing act between painterly observations and chemical-physical theories presupposes a profound understanding in many areas or at least interest. However, I was proven wrong, so this English edition will hopefully accompany and support your work in the vast, intricately interwoven field of art and technology. Unfortunately, the unhelpful division into natural science and humanities also divides the view of our world as the gods of the Greeks divided the spherical people. Bucklow’s work [10] showed me how a holistic, Platonically oriented understanding of the science of painting looked in the Middle Ages and embraced (al-)chemistry and painting equally. My deepest hope is that you will also see this bridging of art and science as a contribution to an overall humanistic understanding of the world, as was familiar to many great minds of science.

https://doi.org/10.1515/9783110777123-202

Acknowledgment My first and most important thanks go to my enchanting wife, Claudia, who once again had to spend a large part of her time with a fanatically lecturing author and who, through constant gentle urging, got me to finish the incubation of the book. Only this made the publication possible at all. Moreover, most important, she dedicated vast amounts of her time to translating the text into readable English, urging me to improve and clarify numerous statements. She introduced me to the secrets of translationoriented writing and the benefits of a concise language. Sadly, the author of the foreword, Dr. Schatz, passed away, to whom I will forever be grateful for the insightful discussions and his foreword, which so well expressed the magic of color. Furthermore, I would like to thank Mr. D. Widmer for important book recommendations and information on writing inks, Dr. S. Hunklinger for an engaging discussion on the color of semiconductors, Dr. T. Vilgis, Dr. B. Schneppe, and G. Bosse for a discussion of the topic of egg white binders and clarea, Dr. W. Müller for his feedback on the composition of acrylic paints, Dr. B. Born and Mrs. R. Ardal-Altun for a long, informative and entertaining telephone conversation on the production of artists’ papers, Dr. G. Kremer for many valuable suggestions, improvements and information from the practice of a paint manufacturer, Dr. K.-O. Schäfer and Dr. W. Thiessen for information on ink production, and Dr. G. Schatz for sending additional material. I would especially like to thank Dr. Kremer, and again Dr. Schatz, for their positive evaluation of the manuscript. They encouraged me, once as a practical color chemist and once as a versatile natural scientist, to pursue the book’s aim. They were right, as the friendly and positive letters from colleagues and experts prove, and I thank them for their interest and constructive comments. I would also like to thank the artists Mrs. S. Steinbacher and Mr. H. Karlhuber, with whom I learned old-master and contemporary painting practice and who were thus “question suppliers.” Finally, I would like to thank Dr. R. Sengbusch, Mrs. Bentkuvienė, and the production team of De Gruyter-Verlag as competent publication partners.

https://doi.org/10.1515/9783110777123-203

Table of contents for volume 1 Foreword � VII Foreword to the English edition � IX Acknowledgment � XI 1 1.1 1.2 1.2.1 1.2.2 1.2.3 1.2.4 1.3 1.3.1 1.3.2 1.3.3 1.3.4 1.3.5 1.4 1.5 1.5.1 1.5.2 1.5.3 1.5.4 1.6 1.6.1 1.6.2 1.6.3 1.6.4 1.6.5 1.6.6 1.6.7 1.6.8 1.7 1.7.1

Introduction � 1 Further (and underlying) literature � 5 Pigments as the basis of painting � 6 Pigments of the ancient and early medieval world � 6 Pigments from the late Middle Ages onward � 10 Pigments from Romanticism and Classicism to Classical Modernism � 12 Modern pigments � 14 Overview of pigments � 18 Color range white, black � 18 Color range yellow-orange-brown � 19 Color range red, purple � 21 Color range blue � 22 Color range green � 24 Paint systems, definitions � 27 Basic physical processes, spectra � 28 Emission colors � 29 Absorption color � 29 Color by absorption at a band edge � 33 RGB and CMY primaries, tristimulus theory, metamerism � 33 The interaction of light and matter � 37 Basics of dielectric materials � 37 Microscopic view: the oscillator model � 39 Macroscopic view: absorption � 44 Macroscopic view: Absorption by size-dependent collective excitations, surface plasmons � 45 Macroscopic view: transmission, refraction, dispersion � 51 Macroscopic observation: scattering, reflection, brilliance � 55 Consequences of absorption: metallic luster, metallic colors, bronzing � 62 Consequences of scattering: opacity, white pigments, and depth light � 66 Summary: physical factors influencing pigment properties � 70 Particle size � 71

XIV � Table of contents for volume 1 1.7.2

Crystal structure and particle shape � 72

2 2.1 2.2 2.2.1 2.2.2 2.2.3 2.2.4 2.2.5 2.2.6 2.2.7 2.3 2.3.1 2.3.2 2.3.3 2.3.4 2.3.5 2.3.6 2.3.7 2.3.8 2.4 2.4.1 2.4.2 2.5 2.5.1 2.5.2 2.5.3 2.5.4 2.5.5 2.5.6 2.6 2.6.1 2.6.2 2.6.3

The chemistry of color � 77 Chemical absorption mechanisms � 77 SC: band gap transitions in semiconductors � 82 Valence and conduction band � 84 Color � 86 SC-based chromophores � 88 Influence of lattice width and crystal structure, thermochromism � 92 Alloys, solid solutions, and color � 95 Manufacture of semiconductor alloys � 100 Doping and blue diamonds � 101 LF: splitting d orbitals in a ligand field � 102 Crystal field theory and ligand field theory � 104 Splitting of degenerated d orbitals � 104 Spectroscopic selection rules � 111 Ligand-field splitting in octahedrally coordinated complexes � 113 Influence of ligand field strength � 121 Distortion of the octahedral field, Jahn–Teller effect � 125 Tetrahedral coordination � 127 LF-based chromophores � 130 CT: Charge transfer transitions � 131 Ligand-to-metal transition and oxygen-to-metal transition � 134 Metal-to-metal transition (MMCT), intervalence transition (IVCT) � 138 MO: molecular orbital transitions � 143 VB and MO model, resonance structures � 148 Chromophore enlargement, bathochromic shifts � 150 Donor–acceptor chromophores � 150 Polyene chromophore � 164 Polymethine chromophores � 183 Other chromophores: Sulfide radical ions � 189 Laking and colored lakes � 190 Structure of the color lakes � 194 Practical procedure � 197 Hue shift � 199

3 3.1 3.2 3.3 3.4 3.4.1

Inorganic pigments � 205 Carbon pigments � 207 Copper pigments � 217 Ultramarine pigments � 227 Oxide and sulfide pigments � 231 Classical heavy metal oxides and sulfides � 231

Table of contents for volume 1

3.4.2 3.4.3 3.4.4 3.4.5 3.4.6 3.4.7 3.5 3.6 3.6.1 3.6.2 3.7 3.8 3.8.1 3.8.2 3.8.3 3.8.4 3.9 3.9.1 3.9.2 3.9.3 3.9.4 3.9.5 3.10 4 4.1 4.2 4.2.1 4.3 4.3.1 4.4 4.4.1 4.4.2 4.4.3 4.4.4 4.4.5 4.4.6 4.4.7 4.4.8

� XV

Iron oxide pigments, ocher � 238 Complex inorganic color pigments (CICP), mixed metal oxides (MMO) � 254 Cerium sulfide pigments � 266 Chromium oxide pigments � 268 Titanium oxides and zinc oxides � 270 Cadmium sulfide pigments � 274 Bismuth pigments � 278 Chromium pigments � 280 Chromate and molybdate pigments � 280 Chrome green (PG15, CI 77510); fast chrome green (PG48, CI 77600); zinc green � 284 Iron blue pigments (Prussian Blue, Berlin Blue, Milori Blue, Paris Blue, iron blue, PB27, CI 77510, 77520) � 285 Various metal pigments � 288 Calcium carbonates � 288 Lead white, flake white, Kremser white, Cremnitz white (PW1) � 289 White sulfates � 290 Miscellaneous colored pigments � 291 Glasses � 293 Glass coloring � 298 “Decolorization” of glass, color compensation � 302 Ancient glass coloring � 303 Frit colors � 305 Opaque glass � 307 Enamel � 307 Organic colorants � 309 Natural organic colorants � 310 Synthetic organic colorants � 312 Meaning of molecular structure � 315 Carotenoids � 317 Xanthophylls � 317 Flavanoids � 319 Origin in metabolism � 319 Classification � 320 Flavan-3-ols (catechins), flavan-3,4-diols, and flavanones � 321 Flavones � 322 Anthocyanins � 327 Neoflavones � 334 Quinone methides � 336 Chalcones and quinochalcones � 337

XVI � Table of contents for volume 1 4.4.9 4.5 4.6 4.6.1 4.6.2 4.6.3 4.6.4 4.6.5 4.7 4.7.1 4.7.2 4.7.3 4.7.4 4.8 4.8.1 4.8.2 4.9 4.10 4.11 4.11.1 4.11.2 4.11.3 4.11.4 4.11.5 4.11.6 4.12 4.13 4.14 4.15

Cause of color � 339 Xanthones � 341 Quinones � 343 Vat dyeing � 344 Natural quinones and naphthoquinones � 345 Natural anthraquinones � 346 Synthetic quinones � 353 Cause of color � 356 Indigoid colorants � 357 Natural indigoid colorants � 358 Synthetic indigoide colorants � 362 Dyeing with indigo and derivatives � 364 Cause of color � 364 Polymethine colorants: di- and triarylmethines, quinone imines � 365 Triarylmethine colorants � 367 Diphenylmethines, diarylmethines, indamine dyes � 378 Dioxazine pigments � 381 Phthalocyanine pigments � 382 Azo colorants (hydrazone colorants) � 387 Azo-hydrazone tautomerism � 389 The diazo component � 389 The coupling component � 390 Classification of azo pigments (hydrazone pigments) � 395 Cause of chromaticity, blue and green azo colorants � 412 Chronix toxicity, carcinogenicity � 417 Quinacridone pigments � 418 Perylene pigments � 421 Diketopyrrolo-pyrrole (DPP) pigments � 423 Azomethine, methine or isoindoline pigments � 424

Bibliography � 429 Index � 473

Table of contents for volume 2 Foreword � VII Foreword to the English edition � IX Acknowledgment � XI 5 5.1 5.2 5.2.1 5.2.2 5.2.3 5.3 5.4 5.5 5.6 5.7

Dyes for writing, painting, and drawing � 499 Types of bonds in the dye-substrate system � 501 Paper as a dye carrier � 503 Paper as ink carrier � 503 Paper dyeing � 504 Modification to paper compatible dyes � 506 Reactive dyes � 510 Direct or substantive dyes � 513 Mordant dyes (metal complex dyes) � 518 Cationic dyes � 522 Anionic or acid dyes � 524

6 6.1 6.1.1 6.1.2 6.1.3 6.1.4 6.2 6.3 6.3.1 6.3.2 6.3.3 6.4 6.5 6.6 6.7 6.7.1 6.7.2 6.7.3 6.7.4 6.7.5 6.7.6 6.7.7

Structure of paint systems � 529 Binders � 530 History � 530 Binder analytics � 534 Drying of binders � 536 Bonding types in the world of colorants � 538 Solvents � 541 Wetting agents and dispersants, grinding paints � 542 Wetting agents � 543 Dispersants � 550 Stabilization of dispersions, dispersants � 552 Thickener, rheology modifier � 556 Film-forming aids (coalescing agents) � 565 Other excipients � 566 Paper � 567 Structure and composition of raw materials � 567 Pulp from wood � 576 Composition and manufacture of paper � 589 Sizing and coating � 605 Calendering (satinage) � 613 Paper grades, general and industrial � 614 Special case artists’ paper � 616

XVIII � Table of contents for volume 2 6.7.8 6.7.9 6.7.10 7 7.1 7.1.1 7.1.2 7.1.3 7.2 7.2.1 7.2.2 7.2.3 7.3 7.3.1 7.3.2 7.3.3 7.3.4 7.3.5 7.4 7.4.1 7.4.2 7.4.3 7.4.4 7.4.5 7.4.6 7.4.7 7.4.8 7.4.9 7.4.10 7.4.11 7.5 7.5.1 7.5.2 7.5.3 7.6 7.6.1 7.6.2 7.6.3 7.7

Paper decay � 622 Aging-resistant paper � 624 Yellowing � 624 Paint systems in art � 627 Ceramics and their painting � 627 Classical ceramic painting � 627 Pigments of the cold-painting technique � 630 Ceramic enamel and glaze colors � 630 Stained glass � 640 Reverse glass painting � 642 Stained glass windows � 644 Stained glass � 646 Fresco (mural painting) � 649 Fresco-buono technique � 650 Lime-painting technique � 651 Fresco-secco technique � 651 Mixed techniques � 652 Pigment degradations � 652 Oil paint � 655 Basic composition of oil paints � 655 Types of oils � 656 Drying of oils, film formation � 660 Stand oils � 672 Effect of heavy metals, siccatives � 675 Linseed oil varnish � 676 Technical improvement of colorants and painting agents in the nineteenth century, paint tubes � 676 Resins, resin balsam, turpentine oil � 679 Other solvents: benzines, turpentine substitutes � 687 Varnish materials � 689 Pigment degradations � 691 Protein systems (poster paint, gouache paint, glue paint, size paint, distemper paint) � 703 Albumin as binder (whole egg, egg white) � 706 Collagen as a binder (poster, gouache, glue, size, distemper paint) � 709 Casein as binder � 713 Tempera � 717 Egg yolk tempera, pure egg tempera � 718 Egg tempera � 720 Fatty egg tempera, egg-oil emulsions � 720 Watercolors � 721

Table of contents for volume 2

7.7.1 7.7.2 7.7.3 7.7.4 7.7.5 7.8 7.9 7.9.1 7.9.2 7.9.3 7.9.4 7.9.5 7.9.6 7.9.7 7.10 7.10.1 7.10.2 7.10.3 7.11 7.12 7.12.1 7.12.2 7.12.3 7.12.4 7.12.5 7.13 7.14 8 8.1 8.1.1 8.1.2 8.1.3 8.2 8.2.1 8.2.2 8.2.3 8.2.4 8.3 8.3.1 8.3.2 8.4

Basic composition of watercolors � 721 Gum Arabic � 723 Gum tragacanth � 725 Ox gall � 726 Paper � 726 Alkyd colors � 727 Acrylic paints � 731 Basic composition � 732 Irreversible film formation � 734 Retarders � 738 Media, thickeners, gels, acrylic butter � 738 Wetting agents and dispersants � 739 Film formation aids � 740 Other additives � 741 Lithographic printing, lithography � 741 Lithographic crayons and inks � 744 Reprint and materials for reprint � 744 Printing inks for lithography � 745 Silicate paint � 745 Low binder systems: chalks and pencils � 746 Blackboard chalk � 746 Pastel crayons � 747 Pencils � 747 Colored pencils � 750 Paper � 750 Fingerpaint � 750 Intarsia art � 751 Inks � 753 Carbon inks � 755 Inks in antiquity � 756 Modern carbon inks � 760 Chemistry of carbonization, combustion, and sooting � 761 Chemistry of phenolic ink constituents � 775 Oxidation and polyphenols � 776 Hydrolyzable tannins � 779 Condensed or nonhydrolyzable tannins, proanthocyanidins � 781 Tannin-like tanning agents � 783 Inks based on natural materials, book illumination � 787 Colored natural inks, book illumination � 787 Brown inks � 792 Durable writing inks (iron gall inks) � 793

�

XIX

XX � Table of contents for volume 2 8.4.1 8.4.2 8.4.3 8.4.4 8.5 8.5.1 8.5.2 8.5.3 8.5.4 8.5.5 8.5.6 8.6 8.7 8.7.1 8.7.2 8.7.3 8.7.4 8.7.5 8.7.6 8.7.7 8.8 8.8.1 8.8.2

Chemistry of iron gall inks � 796 Color of iron gall inks � 797 Brown iron inks � 798 Excursion: the iron-phenol reaction � 800 Dye inks (fountain pen, felt-tip pen, ballpoint pen, inkjet printing) � 801 Function of components � 806 Colorants for fountain pen ink � 815 Colorants for felt-tip, fiber-tip, ballpoint pens � 818 Colorants for inkjet inks � 818 Paper, inkjet support materials � 826 Colorants for stamp pads � 832 Laser or copier toner � 832 Printing inks � 836 Inks � 841 Pigments � 843 Binders � 846 Rosin derivatives as binders � 849 Solvents � 852 Auxiliaries � 854 Paper � 855 Tusche � 855 Sepia, Natural Brown 9 � 857 Shellac � 858

Bibliography � 861 Index � 903

5 Dyes for writing, painting, and drawing In the subject of “dyes,” we risk entering a varied and wide-ranging field that can quickly lead us away from the book’s actual subject. Nevertheless, we want to focus on some basic properties of dyes since certain forms of expression in the visual arts, for example, printmaking and calligraphy, use pigments and dyes alike to achieve visible results. Later in this book, we will highlight printing inks that are pigmented or composed of dye pastes, (book) painting that works with (dye) inks and ballpoint pens, and the adhesion of watercolor colorants to paper or fabric. Albeit inadequately, we will address the often complex mixtures of natural dyes used in book illumination in ▶Section 8.3. Unfortunately, we have to limit ourselves to a selection of well-defined, more modern colorants relevant to artists’ materials, and we will not consider the history and chemistry of dyeing. This exciting subject is addressed by [13, 18–21, 109]. These works provide good introductions for readers interested in textile dyeing. In addition, encyclopedias such as [203] contain dedicated sections on this field of application. Due to its long history and huge economic significance, a true plethora of dyes are available today. To discuss them, we can classify them according to different characteristics, which cannot be strictly distinguished from each other and partly overlap. The following categories are relevant for this topic: – solubility (for arts, mainly in water, vegetable oils, turpentine oil, and solvents based on mineral oil) – dying technique – chemical structure Further classifications are possible and in use depending on the context, such as chemical structural types or dyes for special applications (leather, hair, stationery). Since water plays a significant role as a solvent in the arts, we first organize colorants according to their water solubility, progressing to the dying technique, and finally, to their chemical structure, ▶Figure 5.1: – Solvent dyes. They are colorants insoluble in water but soluble in other solvents. This property is caused by the absence of polar groups such as sulfonic acids, carboxylic acids, or ionic centers. Therefore, they are suitable for coloring plastics, oils, and waxes, and in the field of artists’ materials, for filling pencils and pens. – Disperse dyes. They are sparingly soluble in water and employed molecularly dispersed. First, they are adsorbed superficially on the substrate and then diffuse into it. However, their field of application is not the art sector but the dyeing of hydrophobic fibers made of polyester, nylon, or acrylic. – Development dyes. The color body forms only during dyeing from mostly colorless precursors on the substrate or near it. For example, vat dyes form from soluble colorless leuco bases, and mordant dyes from soluble complex-forming dyes. The resulting colorants are insoluble in water and applicable as artists’ pigments. https://doi.org/10.1515/9783110777123-005

500 � 5 Dyes for writing, painting, and drawing

Figure 5.1: Classification of dyes according to their solubility in water, dyeing technique, and chemical structure.

–

–

–

–

Reactive dyes. Their main feature is forming covalent bonds with the substrate during dyeing, thus acquiring high wash fastness. In order to achieve this, the colored structure contains reactive groups adapted to the substrate, such as cotton, wool, nylon, and paper. Anionic or acid dyes are water-soluble anionic compounds mainly used for nylon, wool, silk, acrylic fibers, paper, leather, food, and inks. They are addressed as direct dyes when they have a high affinity for cellulosic substrates. Direct dyes. Their main characteristic is “substantivity,” i. e., strong adsorption to cellulose and cellulose derivatives (cotton and paper) from an aqueous solution without further auxiliaries. They are anionic water-soluble dyes, and thus a subgroup of acid dyes. Cationic or basic dyes. They consist of water-soluble cationic compounds and are mainly suitable for paper, polyacrylonitrile, nylon, and polyester.

▶Table 5.1 summarizes the substrate’s adhesion mechanism for the indicated dye

classes. The English names of the classes correspond to a standard naming scheme for dyes according to the Color Index [1], e. g., – RR24 (Reactive Red 24) denotes a red reactive dye for inks. – DBk19 (Direct Black 19) denotes a black direct dye for inkjet printing. – BG4 (Basic Green 4) or AY23 (Acid Yellow 23) indicates green and yellow dyes for felt-tip pens and inks.

5.1 Types of bonds in the dye-substrate system

�

501

Table 5.1: Different dye classes and mechanisms to achieve the adhesion to the substrate [19, 20]. Class

Mechanism of adhesion

Disperse dye Vat dye Mordant dye Reactive dye Acid dye (anionic dye) Direct dye Basic dye (cationic dye)

Dipole-dipole interaction, dispersion forces Dipole-dipole interaction, dispersion forces Aggregation, dipole-dipole interaction, dispersion forces (ionic bonding) Covalent bonding Ionic bonding, dipole-dipole interaction, dispersion forces Ionic bonding, aggregation, dipole-dipole interaction, dispersion forces Ionic bonding, aggregation

5.1 Types of bonds in the dye-substrate system Before we look at individual dye classes, we first briefly summarize the mechanisms that provide adhesion of the dye to the substrate. They are discussed in more detail in ▶Section 6.1.4. In dye chemistry, we encounter mainly noncovalent bonding modes and physical intercalation of larger aggregations in fiber voids, ▶Table 5.1. Ionic bonding Apart from reactive dyes, where covalent bonding is decisive, ionic bonding plays a crucial role in the dyeing process. Examples are anionic acid dyes, forming ionic bonds with ammonium cations of proteins. Conversely, cationic dyes interact with carboxylate anions of protein fibers. Typical ionic sites of common fiber materials are listed in ▶Table 5.2. We see that protein fibers and some synthetic fibers naturally provide binding sites. At the same time, other materials such as high-quality paper tend to react nonionically and must be coated to improve adhesion. ▶Section 6.7.4 shows how this is accomplished with paper. Table 5.2: Typical ionic sites of common fiber materials, relevant for dyeings [19, 20]. Fiber

Cationic sites

Anionic sites

Wool Silk Nylon Cotton Paper containing wood Paper wood-free – glue-coated – polymer-coated – rosin acid metal salt-coated

Proteinic amino groups Proteinic amino groups Terminal amino groups None None None (Proteinic amino groups) Depending on polymer Cationic metal salts

Proteinic carboxyl groups Proteinic carboxyl groups Terminal carboxyl groups None (hydroxyl groups not acidic) Lignic acids and phenolates, glucuronic acids Glucuronic acid contents (Proteinic carboxyl groups) Depending on polymer None

502 � 5 Dyes for writing, painting, and drawing Dipole-dipole interaction A common factor in the dyeing process is the interaction of two permanent dipoles formed from covalently bonded atoms of different electronegativity. The electrons involved are permanently unequally distributed. Typical partial structures in dyes are −OH, −CO−, −NH−, or −CN. The hydrogen bond, a dipole-dipole interaction between hydrogen as a positive partner and electronegative elements (O, N), has approximately tenfold strength. It occurs in most groups of substances relevant for coloration. Dispersion forces, actual van der Waals forces All polymeric substrates (fibers of protein, nylon, and cellulose, and polyethene films) have nonpolar and uncharged regions that can interact with dyes via temporary dipoles. Therefore, extended nonpolar regions are created in a dye to mirror the substrate and attach the dye to it. As a rule, such molecule parts are elongated or planar in structure. We can observe the strength of this interaction in crystalline regions of polyethene, which has no special features such as polar centers or charges. Nevertheless, these crystallites show a high strength. Hydrophobic interaction Water shows a higher-order state at hydrophobic surfaces, as it aligns itself to the surface and forms hydrogen bonds with itself. When the hydrophobic groups come together, structured water molecules are released, and a lower-order state is reached. The entropy gain is the driving force of the process. Hydrophobic interactions support apolar dye regions’ adhesion to apolar substrates or the aggregation of dye molecules, like van der Waals interactions. Aggregation The formation of aggregates is not an actual type of chemical bonding, but it describes a vital adhesion mechanism summarily. It is based on the fact that dye molecules diffuse into the fiber cavities and form larger aggregates. Driving forces can be all of the previously mentioned interactions. Subsequently, the complexes are physically trapped in the fiber cavities. Aggregation is achieved in two ways: – The first method is to cool down a hot dye bath. The dye is structured to show a natural tendency to aggregate. Direct dyes have, e. g., extended nonpolar regions that exert van der Waals forces against neighbor molecules. Aggregation is the normal state at a lower temperature and is reduced by heating the dye solution for the dyeing process. After diffusion of the isolated dye molecules into the fiber, they aggregate upon cooling and are physically retained in the substrate. – The second method is forming a metal complex. Dye molecules form complexes with metal salts after diffusion into the fiber. By design, the complex comprises two or

5.2 Paper as a dye carrier

�

503

three dye molecules and is better retained in the fiber than a single dye molecule since all of the weak interactions mentioned above now act on it many times their strength. Furthermore, the complex is physically trapped due to its sheer size.

5.2 Paper as a dye carrier This chapter presents information about dyes, which essentially comes from the field of (textile) dyeing. In the beginning, textile dyes were also used to a large extent for the dyeing of paper (in the mass, on the surface, or as a drawing), and numerous findings originate from the commercial dyeing industry. Later, we will see how dyes specifically developed today for paper or inkjet printing fit into the existing classification. 5.2.1 Paper as ink carrier We will find the basic structure of the paper explained in detail in ▶Section 6.7. For now, a first glimpse shows us that there are two main types of paper: – Unbleached paper contains wood and has a high proportion of lignin in addition to the desirable cellulose and hemicellulose. Besides, this polymer of phenylpropane units has a high number of anionic substituents due to the preparation of the wood. – Bleached wood-free paper consists ideally of pure cellulose and hemicellulose. The chemistry of paper dyeing must therefore deal primarily with the properties of cellulose. It is significant that, unlike natural textile fibers such as wool, or synthetic fibers such as nylon, cellulose has no amino or amide groups, and thus no cationic centers. In addition, its hydroxyl groups do not react acidic, so we also would not expect anionic centers. However, as shown in ▶Section 6.7.3, a small portion of glucose is present as glucuronic acid, and acidic hemicelluloses remaining in the paper pulp contribute a more significant proportion of mannuronic and galacturonic acids to the paper’s anion activity. There are three stages to consider for dyeing: – First, the dye diffuses into the cavities of the paper fibers. – Second, the dye is adsorbed there. – Finally, the dye’s insolubility is increased to improve smear and water resistance by staining with metal salts and adding fixatives (▶p. 602) or binders. The coloring of the paper itself is usually not the artist’s responsibility, as the processing happens in the paper mill. The dye can be added to the pulp mass before making sheets or brushed or dipped onto the paper surface. Surface dyeing is better done with pigments, reducing the risk of bleeding. With this type of dyeing, the paper manufacturer can take measures to ensure good adhesion of the dyes on or in the paper pulp. Conversely, when drawing or writing on paper, the artist cannot influence the effectiveness of the three steps. In contrast to the textile dyer, neither can he choose reaction

504 � 5 Dyes for writing, painting, and drawing conditions that lead to good adhesion of the dye, nor can he use hot dye baths, and thus different solubilities or aggregation tendencies of the dyes. It is also impossible for him to create the necessary acidic or alkaline environment, add additives, or hotfix the dyeing. Therefore, it should come as no surprise that writing inks, in particular, are often not very water-resistant and smear once they get wet. Possibilities for improving the situation are: – Papers are coated with a high affinity to dyes. We will become familiar with coating materials, ▶Section 6.7.4. – Binders added to the ink increase smudge and water resistance by forming a protective film. Discussing the composition of inks, we will get acquainted with appropriate means, ▶Section 8.5. – Dyes are used to match the specific writing and drawing situation. We will see an example of a modified direct dye in the following. Due to high development costs, dyes are modified only for profitable markets such as inkjet or laser printing, not for artists’ inks. 5.2.2 Paper dyeing The mentioned standard works on dyeing always contain supplements on paper dyeing; more details can be found in [203, keyword “paper”], [13, Chapter 5.3], [178, Chapter 9.3], [177, Chapter 6.4], [179, Chapter 3.6.1], [1005], [205, p. 446ff]. In ▶Table 5.3, we see the main dye classes considered for use in paper, namely anionic and cationic direct dyes as well as basic dyes. ▶Table 5.4 contains examples of colorants currently offered for paper coloring. Table 5.3: Suitability of dye classes for paper dyeing [177, Chapter 6.4]. Characteristic

Cationic or basic dyes

Anionic or acid dyes

Direct dyes anionic

Direct dyes cationic

Molecule size

Small

Small

Large

Large

Charge

Positive

Negative

Negative

Positive

Affinity toward mechanical pulp pulp unbleached pulp bleached

+ o −

−2 −2 −2

− o +

o o +

Lightfastness

−

−

o

o

28 %

2%

50 %

6%

Wood-containing papers, packaging papers

Wood-containing and wood-free sized writing papers

Wood-free papers

Unsized or alkaline sized papers

Market share1 Application

1

[179, Chapter 3.6.1], the rest: pigments. Requires cationic fixative, ▶p. 602.

2

5.2 Paper as a dye carrier

�

505

Table 5.4: Example overview of pigments and dyes for paper coloring (surface and bulk) [981, 983, 984]. Colorants Yellow Orange Red Purple Green Blue Black Brown

AY1, AY17, AY23, AY36, AY42, BY2, BY28, BY29, BY37, BY40, BY51, BY57, BY87, BY96, DY11, DY44, DY118, DY147 AO7, AO10, AO51, AO56, BO1, BO2, BO31, DO15, DO26, DO102 AR1, AR14, AR18, AR52, AR73, AR88, AR97, AR119, AR131, AR151, BR12, BR14, BR15, BR18, BR46, DR6, DR16, DR23, DR80, DR81, DR236, DR239, DR254 AV12, AV17, AY49, BV1, BV2, BV3, BV4, BV5, BV8, BV10, BV11, BV14, BV16, DV9, DV35 AG1, AG20, BG1, BG4, DG26 AB1, AB7, AB9, AB92, AB113, BB1, BB3, BB4, BB7, BB9, BB26, BB41, BB99, BB159, DB15, DB71, DB80, DB86, DB199, DB218, DB290 ABk1, ABk2, ABk52, ABk172, ABk210, ABk234, DBk19, DBk155, DBk170, DBk186 BBr1, BBr4, BBr16, BBr17

Metal complexes for muted colors Yellow AY59, AY99, AY114, AY194, AY204, AY241 Orange AO74, AO86, AO142 Red AR183, Ar184, AR186, AR194, AR195, AR219, AR357, AR362 Purple AV90, AV92 Green AG104 Blue AB158, AB193 Black ABk2, ABk52, ABk58, ABk60, ABk194, ABk172 Brown ABr45, ABr355, ABr365, ABr369 Pigments Yellow Orange Red Purple Green Blue Black

PY1, PY3, PY12, PY13, PY14, PY17, PY65, PY74, PY83, PY174, PY191:1 PO5, PO13, PO16, PO34 PR2, PR3, PR8, PR19, PR22, PR23, PR48:1, PR48:2, PR48:3, PR49:1, PR49:2, PR53, PR53:1, PR57:1, PR60:1, PR63:1, PR112, PR146, PR170 PV19, PV23 PG7, PG8 PB15, PB15:1, PB15:3, PB15:4, PB29 PBk7

These classes are selected from the viewpoint of a paper manufacturer, for whom a high coloring efficiency is essential. In writing or drawing fluids, in contrast, it may well make sense to employ, e. g., acid dyes. For this application, poor adhesion, manifesting as a lack of smear or water resistance, can be tolerated or improved by adding binders to the writing fluid. Cationic dyes (basic dyes) are particularly suitable for wood-containing or unbleached wood-free papers since lignin and acidic hemicelluloses in the aqueous paper pulp have a high anion activity during production. The anions originate from uronic acids and artifacts due to preparation, such as phenolic hydroxyl groups and sulfonic acids [203, keyword “cationic dyes”]. The dyes can adhere well due to ionic bonds. However, their affinity to bleached paper is low, and anionic fixatives are needed here, ▶p. 602. Since the dyes are brilliant but not very lightfast, they are more suitable for

506 � 5 Dyes for writing, painting, and drawing low-quality wood-containing papers, wrapping paper, and packaging material. Examples are BY2, BO2, BR1, BV1, BV2, BV3, BV4, BV10, BB9, BB11, BB26, BG1, BG4, and BBr1. In contrast, anionic direct dyes show a high affinity to bleached paper. Their adhesion via hydrogen bonds and van der Waals forces is good. Cationic fixatives can support rich and deep colors if required in individual cases, ▶p. 602. They are suitable for coloring wood-free papers, such as writing papers, blotting papers, and napkins. Examples are DO102, DR239, and DB218. Cationic direct dyes are listed in the CI systematics under the cationic dyes but differ from them in their size and linear or planar molecular shape. They adhere via hydrogen bonds and van der Waals forces. The intentional introduction of cationic centers such as −C2 H4 N⊕ (CH3 )3 further increases substantivity by forming ionic bonds. Cationic direct dyes have a moderate affinity for bleached and wood-containing paper but a high affinity for wood-free paper with its anion activity. Therefore, they are applicable for unsized and alkaline-sized papers. Less used (commercially) are acid dyes since they show little affinity to cellulose and other paper pulps. Although they diffuse well into the cavities of the paper felt, they always require the use of a cationic fixative (▶p. 602), resin sizing, or a metal salt (aluminum) that can precipitate color lakes. They are used for wood-containing and woodfree, sized writing papers; examples are AO7 and AV17 (cationized). 5.2.3 Modification to paper compatible dyes The special requirements for paper dyeing are as follows: – high substantivity for the substrate cellulose – short dyeing time at low temperature, and thus good cold water solubility – improved lightfastness Developments related to cold water solubility and increased substantivity, especially of anionic direct dyes, follow several directions [13, Chapter 5.3] depending on the intended application: – Replacing a benzoyl group with the triazine ring and introducing additional hydroxyl groups via the bis-(2-hydroxyethyl)amino group. In the common dye DR81, this modification leads to the increased solubility and substantivity of DR253, ▶Figure 5.2. – Recharging, i. e., the introduction of positive charges. The substantivity toward anionic paper pulps can be increased in the manner of cationic direct dyes. The N,Ndiethylaminopropyl side chain is responsible for introducing cationic charges; its quaternary amine is counterbalanced as an acetate or lactate. From DR81, we thus arrive at BR111, ▶Figure 5.2, in which these side chains are coupled to the chromophore with cyanuric chloride. Phthalocyanines can also be made paper compatible in this way. The N,N-diethylaminopropyl side chain is introduced

5.2 Paper as a dye carrier

� 507

Figure 5.2: Possibilities of modifying a direct dye to increase its suitability for paper dyeing: Introduction of hydroxyl groups to form hydrogen bonds (DR253) and cationic groups to form ionic bonds (BR111) [178, p. 138].

by treating the phthalocyanine with chlorosulfonic acid and amidating it as a sulfonamide:

–

The use of the coupling component 2-amino-5-naphthol-7-sulfonic acid and its N-acetyl, N-benzoyl, or N-aryl derivatives for soluble and substantive azo dyes. Examples include DO102, DR239, DV51, or DB71:

508 � 5 Dyes for writing, painting, and drawing

– –

Employment of high molecular weight polykis azo dyes with the corresponding substantivity; examples are again DR81, DR253, and BR111 or DB71. The use of heterocycles as azo and coupling components in azo dyes such as DY28, DY137, and DY147:

5.2 Paper as a dye carrier

–

�

509

Bridging mono and diazo dyes with phosgene or cyanuric chloride to obtain large molecules with increased substantivity. Examples again include DO102 and DR239 as well as DY51 and DG26, in which a yellow and a blue dye have been combined to a green preparation.

Dyes with higher lightfastness are obtained mainly by metallization with copper. The copper complexes preserve the planar shape of the molecules, and thus the substantivity. 3,3’-Dimethoxy-4,4’-biphenyldiamine (dianisidine) has proven itself as a complexforming bridge, which forms metal complexes by demethylation. DB218 provides an example:

Further developments replace toxic components such as the dianisidine bridge (a benzidine derivative) with nontoxic bridges, such as, e. g., in DB273:

510 � 5 Dyes for writing, painting, and drawing The designers of inkjet inks are confronted with similar problems; see ▶Section 8.5.4 for the solutions adopted.

5.3 Reactive dyes Reactive dyes [19, 20], [203, keyword “reactive dyes”], [13] are water-soluble dyes that form a covalent bond with the substrate during dyeing, especially with the groups −OH, −SH, or −NH−, as they are present in many textile fibers made of cotton, wool, or nylon. Therefore, reactive dyes exhibit very high adhesion and washing fastness. They are mainly applied in textile dyeing but also in writing and inkjet inks. First experiments with this type of dye were carried out in 1895, and the first industrially important dye Supramin Orange R for wool came onto the market around 1938. It carries the reactive chloroacetylamino side chain −NH−CO−CH2 −Cl. From the 1960s onward, dye families with triazines, vinylsulfonic acids, and similar compounds as anchors followed in rapid succession. Chemistry According to their mode of operation, reactive dyes consist of two parts: a chromophore and a reactive group. During dyeing, this group couples with the fiber. The dye can carry other groups that increase the solubility of the precursor in the aqueous dye bath:

A sulfonic acid −SO3 H is often applied as a solubilizing group. Any organic chromophore can act as the color-active part of the dye. Often azo, anthraquinone, or phthalocyanine chromophores are used so that reactive dyes can achieve complete coverage of all hues. In contrast to other classes of colorants, large nonpolar or ionic molecular skeletons are not required since covalent bonding ensures adhesion adequately. Adhesion The reactive group that ensures an anchoring of the dye to the fiber is multiform and leads to different dye families. Frequently, triazines and pyrimidines or vinyl sulfonic acids are used, which can be halogenated in different ways:

5.3 Reactive dyes

� 511

The triazine anchor, e. g., is coupled to the dye via cyanuric chloride, ▶Figure 5.3. The selective reaction of the three chlorine atoms (the first reacts at 0–5 ℃, the second at 35–40 ℃, the third at 80–85 ℃) allows controlled introduction of the dye. During the dyeing process, the anchor reacts with hydroxyl or amino groups of the dyeing material by eliminating hydrogen chloride, forming an ether or amine bond. A similar reaction for cellulose (paper) would require alkaline conditions to convert the hydroxyl groups of the cellulose into anions.

Figure 5.3: Synthesis of a reactive dye with triazine anchor and coupling to the amino group of a fiber [203, keyword “reactive dyes”], [19].

512 � 5 Dyes for writing, painting, and drawing Examples Of the following examples of reactive dyes with triazine anchors, RR24 is used for red writing inks.

Anchors of vinylsulfonic acids form a carbon-carbon bond in an addition reaction, ▶Figure 5.4. For example, RR23 (red) is used in writing inks, RR180 (red) and RBk31 (black) in inkjet inks.

Figure 5.4: Synthesis of a vinyl sulfone-based reactive dye and coupling to the hydroxyl group of a fiber [203, keyword “reactive dyes”], [19].

5.4 Direct or substantive dyes

�

513

5.4 Direct or substantive dyes Direct or substantive dyes are water-soluble anionic dyes whose main characteristic is substantivity, which means a very high dye affinity to natural or processed cellulose (cotton, jute, viscose, paper) [13, 19, 20]. Direct dyes are among the oldest colorants. For example, since antiquity, carotenoids such as crocetin and bixin have dyed textiles yellow (▶Section 4.3.1), whereas carthamine from the safflower made red dyeings possible, ▶Section 4.4.8 [609]. Direct dyes require for the dyeing no further auxiliaries such as mordants, simplifying their use. The process runs in a neutral dye bath with NaCl or Na2 SO4 added. A disadvantage is poor washing fastness, which a post-treatment of the dyeing material improves; however, this does not quite meet today’s standards. Of interest to us is their usage as dyes for writing and inkjet inks on paper. Unlike other dye classes, direct dyes have a high affinity for lignin-free (bleached or wood-free) paper, so paper production and dyeing require no extra fixative. They are employed

514 � 5 Dyes for writing, painting, and drawing for coloring wood-free papers of all kinds, such as writing papers, blotting papers, and napkins. In paper chemistry, direct dyes are also called cationic direct dyes. According to the CI system, they are classified as cationic dyes (basic dyes). However, they differ from basic dyes in their size and linear or planar molecular shape and are thus closer to the direct dyes. Therefore, they are suitable for dyeing wood-containing paper in contrast to the anionic dyes. Chemistry The characteristic feature of direct dyes is a large, elongated, nonpolar molecule whose solubility in water is increased by anionic groups, e. g., sulfonic acids. They share the acid groups with acid dyes; in fact, they form a subgroup of the acid dyes. The transition between high molecular weight acid dyes and direct dyes is fluent. However, unlike acid dyes, they exhibit an extensive planar molecular shape, which provides the necessary substantivity. In terms of color range, direct dyes cover the entire spectrum. The examples from antiquity show that polyene dyes can serve as a base. The advent of the azo chromophore in modern times has given rise to a generation of direct dyes such as Congo red. They have undergone further improvements like the azo chromophore itself. Today, azo dyes are used for the most part; for the dark, blue, brown, and black shades, bis and polykisazo compounds and copper-azo complexes are employed. Copper phthalocyanines provide greenish-turquoise shades. Adhesion The adhesion mechanism was previously thought to be a formation of an ionic bond by the acid groups of the dye with the cationic centers of the substrate. Consequently, hydrogen bonds between polar groups of the dye and the substrate would contribute to adhesion due to the parallel orientation of dye and substrate. Hydroxyl groups of a cellulose chain show an example:

Some factors suggest that these are not the only causes of adhesion: Cellulosic substrates do not have the cationic positions at all, many vat dyes for cotton are not elongated but only planar, and hydroxyl groups of the cellulose form hydrogen bonds with each other

5.4 Direct or substantive dyes

�

515

rather than with a partner to which they are spatially insufficiently aligned [19, Chapter 14.7]. Since an extended, planar molecular shape plays a significant role in the degree of substantivity, today it is assumed that an interplay of the mentioned mechanisms with van der Waals forces and aggregation occurs [19, Chapter 14.7], [20, p. 87]. The high affinity of anionic direct dyes to bleached paper is mediated by hydrogen bonds and van Waals forces, ▶Section 5.2. We see below how incorporating the structural features from ▶Section 5.2.3 further improves the suitability for paper. Likewise, a cationic fixative (▶p. 602) can improve rich and deep colors in individual cases if needed. Cationic direct dyes adhere via hydrogen bonds and van der Waals forces; the cationic centers increase substantivity by forming ionic bonds. Since lignin specifically has the necessary anion charge, they are particularly suitable for use with wood-containing paper. Van der Waals and dispersion forces are only sufficiently strong for large dye molecules. For this reason, size is of decisive importance for substantivity and can serve as a criterion for differentiation from acid dyes. In the dye solution, hydrophobic interaction causes dye molecules to orient themselves parallel to molecular chains of the fiber so that forces can act between nonpolar regions of dye and substrate. It is assumed that cellulose, polar in nature, can also participate since its axial hydroxyl groups are more hydrophobic than equatorial hydroxyl groups or solvent water. The idea of aggregation is that individual dye molecules diffuse into the fiber cavities and are adsorbed there. Then, driven by the hydrophobic interaction and favored by their planar molecular shapes, they aggregate into larger units retained in the cavity. Since aggregation of the individual molecules at room temperature also occurs in the dye solution, the temperature must be increased during textile dyeing to dissolve the aggregates and allow diffusion into the substrate. The durability of the dyeing on textiles is less with direct dyes than with chemically bonded dyes, but a suitable finishing achieves an improvement. Such treatment can be performed with metal salts (preferably copper and chromium), whereby metal-azo complexes are formed with hydroxy groups of the dye [19, Chapter 14.5]. The increased molecular weight and decreased charge reduce the solubility and diffusivity of the complex significantly. The hue shift toward darker tones (▶Section 5.5) is a side effect, which is quite desirable for blue, brown, and black. The second possibility of post-treatment is to diazotize free amino groups of the primary direct dye. A new azo dye is synthesized directly in the substrate, which is also less diffusible and has a darker hue by coupling with a suitable coupling component. Examples An early representative of the direct dyes is Congo red, which is no longer in use due to its carcinogenic benzidine component:

516 � 5 Dyes for writing, painting, and drawing

Modern azo-based direct dyes are DBk19 and DR75, two dyes for writing inks. DBk19, DBk154, and DBk168, as well as DY86 and DY132, are also used in black and yellow inkjet inks:

5.4 Direct or substantive dyes

� 517

Important examples of direct-acting phthalocyanine dyes are DB86 and DB199, used in blue writing and inkjet inks:

The already excellent suitability of direct dyes for dyeing wood-free paper can be further increased by modifying their structure, ▶Section 5.2.3. For example, the azo dye DB218 is a large molecule of high substantivity, and it is metalized to increase lightfastness. DO102 and DR239 are built on 2-amino-5-naphthol-7-sulfonic acid as a coupling component, increasing the dye’s substantivity and cold water solubility in paper applications. In addition, the dye molecules are enlarged by linking two partial molecules with phosgene via a urea bridge, which also increases their substantivity:

518 � 5 Dyes for writing, painting, and drawing

5.5 Mordant dyes (metal complex dyes) Mordant dyes often consist of an acid dye, which forms with the cations of a metal salt, the so-called stain, a stable, insoluble complex, or color lake with high light and washing fastnesses [13, 19, 20, 46, 705, 706]. The laking that occurs in the process changes the color, possibly significantly, ▶Section 2.6. Staining dyes were already used for dyeing in antiquity. Classic examples are madder, carmine, and kermes (anthraquinone dyes), dyer’s woad and dyer’s broom (flavonoid dyes), redwoods, and bluewoods (iso and neoflavonoid dyes). All were laked with alum as an AlIII salt [609]. One specific dyeing with madder yielded Turkish red, which was highly valued, ▶Section 4.6.3. In modern dyeing chemistry, mordants significantly increase acid dyes’ light and washing fastnesses by anchoring the dye in the substrate through complexation. Also, the color shift opens up the range of blue, green, brown, and black. Mordant dyes are not applicable for the production of artists’ material. They are nevertheless interesting for us since, in past times, color lakes were obtained as pigments from natural dyes, ▶Section 2.6. Chemistry Examples from antiquity show that a wide variety of structures inducing color can serve as the basis for mordant dyes. Modern representatives are mainly based on an-

5.5 Mordant dyes (metal complex dyes)

� 519

thraquinone (red to purple) or azo compounds (green, blue, brown, black). Due to the metallization, the colors obtained are duller and darker than those of the original dyes, ▶Section 2.6.3, but also more lightfast. For the formation of the metal complex, the addition of metal salts is required, especially CrIII , SnIV , AlIII , CuII , or FeIII salts. The addition aims at – fixing the dye – changing or deepening the hue Both objectives are achieved by forming an insoluble metal-dye complex (color lake), whereby the bathochromic effect described occurs, ▶Section 2.6.3. The complex formation is only possible if the chromophore or its substituents can form coordinative bonds with the metal. Suitable groups are hydroxy and oxo groups, carboxylic acids, and diazo groups; common structural elements are [19, p. 260]:

These elements are realized mainly in anthraquinones and azo compounds. Examples are the tridentate azo dyes:

520 � 5 Dyes for writing, painting, and drawing Based on the coordination numbers 3, 4, and 6 of the metals and the bidentate and tridentate dye ligands, we can expect dye-metal complexes in the ratio of 1:1 and 2:1 [706]. To fully achieve the coordination number of the metal, secondary ligands such as water, hydroxyl anions, halide anions, or acid groups are applied. Adhesion The dye-metal complex can be fixed via bonds between suitable groups of the substrate and the metal, which, depending on the metal and the binding partner, have a more covalent, coordinative, or ionic character, as illustrated by some examples of 1:1 and 2:1 complexes (dye to metal ratio):

Meanwhile, there is no confirmation of this plausible theory, and in the case of some 2:1 complexes, the necessary free valence at the metal would not be present. It is, therefore, assumed that the adhesion is due to ionic bonding, dipole-dipole interactions, and dispersion forces already effective in the underlying free dye [19]. In addition, the complex has a significantly reduced diffusion rate compared to the free dye since its size means that more interactions with the substrate occur than per isolated dye molecule. Furthermore, the complex is physically trapped due to its sheer size. For staining, two procedures have been established. They differ in the order the reagents are added: – Traditional staining process: soaking of the substrate with the mordant, then adding the dye – Staining process with mordants: dyeing of the substrate with suitable dyes, then adding the mordant Commercially, the second method almost exclusively is used today. The applied method achieves dark colors such as black. Traditional staining process For staining, according to the first method, the dyeing material is immersed in a solution of metal salt such as alum, AlCl3 , or FeCl3 , and saturated with it. In this process, the

5.5 Mordant dyes (metal complex dyes)

� 521

metal cations form salts or complexes of more or less ionic bonding character with acidic groups (hydroxyl, carboxylic acid, or sulfonic acid groups) of the fibers [207, p. 484], [208, p. 518], [209, Chapter 34.6.5]. Subsequently, the mordant is fixed with hot water vapor, and any valencies of the metal cations that remain free are hydroxylated. In addition to the metal hydroxides that are ionically or complexly bound to the fiber, metal oxide hydrates also form, which are distributed in the fiber. Then the dye solution is added to the stained dyeing materials. Dye anions replace hydroxide ligands in equilibrium reactions, and subsequently, the insoluble metal complex is formed on and in the fiber. Ionic and complex bonds anchor the complex to the fiber:

After-chroming This process employs metal salts as mordants. In the case of chromium mordant, it is called “after-chroming”, but the term got used for this process in general. In this process [19, Chapter 13.7.3], [20, p. 85], the substrate is first dyed with a suitable dye (e. g., acid dyes for wool or disperse dyes for nylon). In the following step, by adding the mordant, the metal complex is created:

In this order, it is unlikely that the metal cations will form many ionic bonds with the substrate, and thus contribute to fixation. As the name suggests, CrIII salts are very popular in practice as mordants because intense dark shades (blue, brown, black) are obtainable. Dichromates are used as metal salts; CrIII is derived from them by reducing groups in the substrate or sodium thiosulfate. Since contamination of wastewater and dyeing materials with the toxic hexavalent

522 � 5 Dyes for writing, painting, and drawing CrVI could be possible, the share of chromium mordants slowly decreases in countries with environmental protection regulations. In the course of replacing chromium and cobalt with nontoxic metals, iron complexes with red, purple, blue, and black colors have been found [707, 708].

5.6 Cationic dyes Cationic or basic dyes are water-soluble compounds that are electrostatically fixed to anionic groups in the substrate [13, 19, 20], [203, keyword “cationic dyes”]. Their colors are often brilliant and intensive. The first cationic azo dye, vesuvin, was described as early as 1863 and was obtained by coupling m-phenylenediamine to m-phenylenediamine. Chrysoidin followed in 1875. Mauvine is significant for the development of the dyeing industry, and thus for today’s large chemical corporations since its discovery opened up the era of synthetic dyes. The emergence of synthetic fibers also increased the value of cationic dyes since their main application is the dyeing of nylon, anionic polyacrylics, and polyester fibers or leather. They show a high affinity for wood-containing paper, so they are frequently used for paper, inks, and stamping inks. An advantage is their color brilliance, but their lightfastness is usually low, making them only suitable for cheap wood-containing papers, wrapping or packaging materials, or for applications where they are not exposed to light for long periods. Chemistry The variable chemistry of cationic dyes applies hemicyanines from diazo compounds, triarylmethines, cyanines, thiazines, oxazines, and acridines. The chromophores carry cationic centers, e. g., ammonium groups, and form salts with anions of simple acids (halide, acetate, oxalate, or sulfate). Due to a large number of coloring structures, basic dyes cover the entire color spectrum. They are named “basic dyes” because the dye cations form insoluble “basic” precipitates dye⊕ ⊖ OH with alkali hydroxides. Adhesion Cationic dyes form ionic bonds with anionic positions of the substrate, e. g., carboxyl groups. Their suitability for paper dyeing (▶Section 5.2) is due to their high affinity for wood-containing and unbleached wood-free aqueous paper pulps. Lignin and acid hemicelluloses in these pulps exhibit a high anion activity that binds the cationic dyes during production. However, their affinity to bleached paper is low, and auxiliaries are needed, which are admixed to the paper pulp as fixatives (anionic polymers, ▶p. 602) or added as an anionic component during sizing, ▶p. 605.

5.6 Cationic dyes

� 523

Examples Cationic dyes in inks for felt-tip pens are often based on the triphenylmethane skeleton, ▶Section 4.8.1 at p. 370, less frequently on the diphenylmethane skeleton:

Azo dyes, acridines, and cyanines are also suitable ink dyes for felt-tip pens:

524 � 5 Dyes for writing, painting, and drawing

For coloring paper in pulps, BR1, BO2, and BBr1 are applicable; BR1 is also a suitable ink for felt-tip pens:

5.7 Anionic or acid dyes The group of acid dyes comprises water-soluble compounds carrying anionic substituents. These increase water solubility and impart ionic bonds to the substrate [13, 19, 20]. Appropriate substrates for acid dyes are fibers with amino groups (polyamides, wool, silk) or other substituents that form cations by protonation, leather, and treated paper. Due to their affinity for paper, they are used to a lesser extent for paper coloring but more commonly in writing and inkjet inks. Acid dyes with low molecular weight are unsuitable for cellulose; with a high molecular weight, they change into direct dyes. Chemistry The history of acid dyes began around 1876 with naphthol orange and later Eosin (AR87), which is still a dye of red ink today. Acid dyes are based on all of today’s chromophores. Azo chromophores cover the color range from yellow to red, purple, and brown; disazo compounds achieve blue and black. The resulting large structures are already close to direct dyes. Anthraquinone chromophores cover the range from purple-blue to green,

5.7 Anionic or acid dyes

�

525

and copper phthalocyanine chromophores that of light turquoise. For lower lightfastness requirements, triphenylmethines are used for blue to green and xanthenes for red to purple. Chemically, acid dyes are compounds with molecular weights of 300–1000 g/mol carrying anionic substituents (sulfonic acids, carboxylic acids). In particular, the sulfonic acid group is readily introduced by sulfonation of the structure. The anionic groups increase the solubility and form ionic bonds with the substrate, which in the case of acid dyes of low molecular weight, achieve adhesion to the substrate. Adhesion The name “acid dye” is derived from the acidic environment of the dye bath, with pH values of less than 2 (sulfuric acid) to more than 6.5 (ammonium acetate). The processes involved in dyeing a protein fiber such as wool or a synthetic fiber such as nylon are as follows: – The amino acids of the proteins are present as zwitterions, according to their isoelectronic points. – Amino acids or the C and N termini of the nylon chains are converted in the acidic dye bath into nondissociated carboxylic acids and ammonium salts of the added acid. – The acid anions are replaced by dye anions, which ionically adhere to the fiber.

Since cellulose has no cationic charges, acid dyes show no affinity toward untreated paper made from plant fibers, ▶Section 5.2. Therefore, adhesion to paper depends on its properties, which can be adjusted by a suitable paper pulp or coating composition. Possible fixatives (aluminum sulfate, cationic polymers) are presented on ▶p. 602, suitable sizing agents (resin sizing) in ▶Section 6.7.4. The ionic bonding described primarily contributes to the fixation of small dye molecules. With increasing molecular weight and more extensive nonpolar regions, acid dyes adhere to the substrate additionally via dispersion forces and dipole-dipole interactions and transition seamlessly into direct dyes. ▶Table 5.5 shows the influence of molecular weight. Examples The excellent adhesion of acid dyes allows them to become alternatives in the dyeing industry for cases where the migration fastness of disperse dyes is too low in some synthetic fibers. AY17/AR37/AB45 and AY29/AR57/AB40 are primary color triplets, with

526 � 5 Dyes for writing, painting, and drawing Table 5.5: Influence of molecular weight Mr of acid dyes on various dye properties [19, Chapters 13, 13.4], [20, p. 81]. Mr low

Mr high

Substantivity for wool, nylon Diffusion in fiber ▶migration fastness Hydrophobicity Washing fastness Adhesion

Low High Low Low Low Ionic bonding

Solubility Sulfonic acid groups

High 1–3

High Low High High High Hydrogen bonding, dipole-dipole interaction, dispersion forces Low 1

which knitwear and nylon tights can be dyed in all the required shades without allergens. Some of them, such as AY17 and AR37, are used for inkjet and writing inks:

5.7 Anionic or acid dyes

� 527

Another primary color triplet AY23/AR52/AB9 is used together with Food Black 2 in inkjet inks:

The following dyes are examples of yellow, orange, red, and blue inkjet and writing inks. Further examples (AY73 and AR87) were given with the triphenylmethane dyes, ▶Section 4.8.1. AG27 and AV41 provide examples of green and purple acid dyes, respectively:

528 � 5 Dyes for writing, painting, and drawing

6 Structure of paint systems The following chapters are dedicated to the actual artistic techniques. Colors are an essential part of each technique, as varied as the techniques and their results. By “colors” we do not mean the visible hues, but the paint systems, i. e., the paints (the color paste essential for oil and acrylic painting), pencil leads, printing inks, or colored solutions to create a work of art. Colors differ depending on the type and technique of painting, but there are nevertheless similarities in their structure. We will therefore address general characteristics here, and in subsequent chapters, we will focus on the specifics of certain painting techniques. More can be found in [47, 74, 75, 97, 98, 117–122, 187, 188], Tadros [221] explicitly considers the role of colloids in paints, and Tadros [222] describes the theoretical basis of wetting processes. Paints are generally composed of several components: – colorants (pigments, sometimes dyes) and fillers – binders (film former) – solvents – auxiliary substances The latter support the main constituents in their function or perform particular tasks. They are only contained in paint to a minimal extent in terms of quantity. Auxiliary substances can be: – wetting agents and dispersants – film-forming aids – thickeners and rheology modifiers – defoamers, siccatives, preservatives The most critical components, also in terms of quantity, are colorants, fillers, binders, and solvents. Pigments that give the paint color, body, and materiality can vary depending on the technique, but we can usually employ a pigment in several techniques. We have discussed colorants in ▶Chapters 3, 4, and 5. ▶Section 1.6.8 already made us aware that pigments and fillers can only have the desired opacity if their refractive index is sufficiently high concerning air, water, and binder. In addition to paints, media and binders are essential products of artists’ supplies. Partly, they share some components with paints, but in a different composition since they provide only one aspect of a paint: – binder – solvent – defoamer, preservative, film-forming aid, matting agent, gloss enhancer

https://doi.org/10.1515/9783110777123-006

530 � 6 Structure of paint systems

6.1 Binders Binders permanently fix pigments to the support or ground by forming a film that traps them (and also most of the other paint components). The film can be formed physically by solvent evaporation and retention of the film-former on the substrate or chemically by cross-linking the molecules of the film former. Binders, however, also provide essential optical properties of a paint system along with the film. These include film thickness (▶ deep light, transparency, chalky-matte appearance), gloss, and color depth. In addition, the binder determines usage properties such as film hardness, film resistance, and abrasion resistance. However, binders can significantly influence the processing properties during the application, depending on their proportion in the paint. These properties include, above all, the open time or drying time, i. e., the time during which the paint can still be manipulated or processed. Whereas paints for coatings and paints should only be “open” during the application itself, and then rapid drying is desired, the artist requires a wide range of fast, medium, and long-drying paints. Following this essential requirement, a wide range of binders exists for artists’ materials, ▶Figure 6.1. Another critical factor is the viscosity, which is a binder’s property. If necessary, it must be modified by adding solvents (thinners) or thickeners. After a historical overview and an outlook on the problems of binder analysis, we will deal with the physical-chemical fundamentals of drying and the forces that lead to bonding. Binders are macromolecular by nature to provide the desired fixing and protective film around the pigments. Usually, they have molecular weights in the range of 500–30000. Binders with low molecular weights have the advantage of lower viscosity during processing but must polymerize on drying to form a durable film. ▶Figure 6.1 roughly indicates the mass range in which the essential binders can be classified: Most binders are polymeric by nature; drying oils and oil-modified alkyd resins are low molecular weight but polymerize on drying. Resins are low molecular weight compounds that do not polymerize on drying.

6.1.1 History Binders have a rich past dating back to Neolithic times [111, 117, 120, 128, 921]. Some milestones of chemistry are indicated in the following outline:

6.1 Binders

� 531

Figure 6.1: Overview of the leading chemical binder classes. Blue background: chemically drying. Light gray: predominantly physically drying. White: purely physically drying [100]. Left: low molecular weight compounds. Right: high molecular weight compounds.

532 � 6 Structure of paint systems 47000 BC

Possible casein paint (South Africa, ocher dissolved in milk)

20000 BC

Natural fresco (calcium bicarbonate-bound ocher drawings), egg white, blood

7000 BC

Fresco secco (Çatalhöyük)

3000 BC

Gums, such as gum Arabic and cherry gum (watercolors, fresco secco, soot ink (carbon))

3000 BC

Proteins (glues, such as skin and fish glue), soot ink (carbon), papyrus

2000 BC

Fresco buono (Crete)

1500 BC

Proteins (glue, egg) (watercolors and tempera, fresco buono and secco), (vegetable gum with honey, casein, starch, tragacanth, glue, egg white, egg tempera), glazes (to absorb colorants)

1000 BC

Proteins (egg tempera)

500 BC

Resins, fresco, encaustic (panel painting on wood)

200 BC

Parchment

0

Nut oil, gums, milk

100

Paper

600

Earliest mention of oils with pigment

800

Book illumination, gum and albumen bound

850

Egg tempera (icons)

1000

Linseed oil, gum Arabic

1100

Early example of an oil binder in Denmark

1250

Egg tempera (duecento panel painting)

1400

Oil painting with turpentine oil in Early Netherlandish painting (distillation from Arabia), walnut oil, linseed oil, watercolor (gum-bound), gouache (gum and glue-bound)

1500

Shellac (by India trade)

1500

Oil painting in late Renaissance art

1600

Lacquers

1700

Low-fusing glaze for onglaze paints

1877

Polymethacrylic acid (Sittig, Paul)

1901

Acrylic polymers (Röhm)

1905

Cellulose ethers (Suida)

1912

Polyvinylchloride and polyvinylacetate (Klatte)

1926

Polyvinylalcohol (Herrmann)

1930

Oil-modified alkyd resins (Kienle), acrylic dispersions

▶Figure 6.2 shows crucial phases from an artistic point of view. Early cave paintings

have been faithfully preserved in a fresco-like technique, though probably unintentionally, by calcareous water. The pigment, which at first adhered to the rock wall only by pure adsorption or inclusion in fat, was washed over by the calcium hydrogen carbonate contained in the dripping water and was therefore enclosed in a lime casing by calcination:

6.1 Binders

� 533

Figure 6.2: Rough chronological overview of the essential paint systems [72, 73, 117, 120, 128]. Top, blue: paint systems. Bottom: binders.

The reaction proceeds similar to the setting of a fresco binder (▶Section 7.3), which consists of slaked lime and also sets to lime:

This true fresco is deliberately used from the second pre-Christian millennium onward. It is later perfected in the Hellenic and Roman periods before experiencing further flourishing in the Middle Ages and Baroque. Initially, however, in the earliest human settlements, loam/clay and gypsum plasters were used as wall coverings and painted with watercolors in a fresco secco technique. Their binders are based on polysaccharides (gums) from plant or tree saps (fig milk, gum Arabic) or glue from proteinaceous substances (egg whites, casein, skin, bone). The gum-bound paints are refined in modern times to today’s watercolors and gouaches; the protein-bound paints are precursors of today’s poster paints and other glue paints. Tempera was already developed in antiquity, using aqueous (gum, glue) and nonaqueous (resin, oil) components as emulsion binders. Gum, glue, egg, and lime remained the main binding agents until the Middle Ages. Then, besides mural paintings, book illumination with watercolors emerged as a vital form of artistic expression. The origins of European panel painting lie between the turn of the first millennium and the Renaissance. Tempera with egg (egg tempera) is its dom-

534 � 6 Structure of paint systems inant binding agent, as we can see from the typical appearance of numerous masterpieces, e. g., of the early Italian painting of the Duecento and Trecento. The addition of drying oils to egg tempera leads to oil painting. Within a few hundred years, it became the dominating technique thanks to the Early Netherlandish painting in the fifteenth century and significantly changed the art’s appearance. Paintings before 1505 were predominantly executed in egg tempera, and only isolated areas were worked in walnut oil [929, 930, 932, 933, 935–942]. After 1500, artists used oil almost exclusively in paintings. At the beginning, walnut oil was employed, later rather linseed oil, until around 1700, linseed oil (occasionally poppyseed oil) was predominantly applied. Higgitt and White [931] illustrate the differentiated parallel use of both media in Italy, e. g., oil for glazes. Detailed studies of individual artists such as Dürer and Tintoretto are given in [922–924, 943]. The latter shows that, e. g., Van Dyck used walnut oil and linseed oil side by side because of their respective advantages; the lighter and less yellowish walnut oil for sensitive color tones such as white, yellow, or blue; for colors such as black, which dried with difficulty, he also used stand oils or boiled oils. A unique feature of the eighteenth and nineteenth century was megilp, painting agents consisting of a mixture of drying oils, lead compounds, and mastic varnish. They showed a pronounced thixotropic behavior [934]. While they could be painted smoothly for glazes, they were also suitable for impasto, which requires a paint that is buttery and “stands up.” Since they led to severe cracking and yellowing, they were applied more and more rarely. Well-known users were Reynolds, Turner, and Wilson. Oil painting retained its supremacy until the last century. In the field of coating technology, from the nineteenth century onward, the oils were ester-modified and led to alkyd colors, which then in the twentieth century became popular also in the panel painting. Alkyd paints, however, could not displace oil binders. So far, the last development impulse created acrylate-based dispersions, which were initially also used for coatings. From around 1950, acrylic dispersions became available for artistic painting. They rapidly advanced to dominant binders besides oils since they were inexpensive, water-soluble, low-odor, easy to process, and adherent to many surfaces. However, since acrylic film differs considerably from oil film, it remains to be seen to what extent the painting properties and the considerably different appearance of acrylic paintings will be able to compete with oil painting (including its advantages and disadvantages) in the long run.

6.1.2 Binder analytics When analyzing art objects, we encounter particular problems in the case of binders, which often make it difficult or impossible to determine the exact agents used. Most colorants have a characteristic structure that facilitates analysis or at least classification. Problematic colorants are usually bio-organic, such as lakes, which fade, degrade, and

6.1 Binders

� 535

have similar structures, so individual representatives are difficult to identify. These difficulties are even more pronounced concerning binders before the last century: they are primarily of natural origin, have complicated or polymeric structures (proteins, polysaccharides), belong to a class of substances with numerous similar representatives, and are often modified or degraded over long periods. A detailed analysis has only recently become possible using methods such as chromatography (GC, HPLC, TLC) and mass spectroscopy. Furthermore, one-dimensional and two-dimensional NMR spectroscopy has allowed for significant advances; here, 1 H-NMR and 13 C-NMR are especially suitable methods. [101, 102, 921, 925–928] give insights into the analytics of binders and show possible solutions. Since individual substances are hardly representative, characteristic ratios of dominant substances or classes must be determined for each binder. Samples have then to be compared with these ratios. For oil-based binders, information can be obtained on the relative amounts of fatty acids in the oil’s glycerides. Bonaduce et al. [928] list examples often used to identify a drying oil. These are the ratios palmitic acid/stearic acid, azelaic acid/suberic acid, or azelaic acid/sebacic acid. However, the figures obtained strongly depend on the pigments employed and the painting’s age, i. e., on processes during the oil’s drying. Suitable for identification are also hydrolysis or oxidation products. In the case of egg tempera, instead of the fatty acid ratios, the amount of azelaic acid can be determined compared to palmitic and stearic acid. Egg-containing binders also have cholesterol content. Nondrying lipids indicate binders such as animal glue. Glues, as well as tempera, are characterized by their protein content. The relative amounts of amino acids then permit conclusions about the exact nature: high contents of hydroxyproline or glycine suggest collagens, the absence of hydroxyproline in the case of high proline and leucine contents indicates casein, the presence of leucine and aspartic acid point to egg or egg white. Unfortunately, glues are binders easily destructible. Identifying plant gums such as gum Arabic, gum tragacanth, or gums from pome fruit in painting specimens is challenging and doubtful. Therefore, only a few investigations on art objects are available so far. Plant gums, e. g., the three gums mentioned before, are effluxes from the endosperm of some seeds, which consist of high molecular weight polysaccharides. The main components of polysaccharides are aldopentoses, aldohexoses, and uronic acids. The presence or absence of key sugars indicates the origin of gum. After the hydrolytic breakdown of polysaccharides into monosaccharides, GC/MS can be employed to analyze the gum base. However, the analytical results often show deviations from reference materials. Little is known about aging and related changes in polysaccharides, including their interactions with the carrier materials, environmental influences, or bound pigments. In [927], patterns of key sugars are proposed that are stable to aging and pigment effects, thereby providing a decision tree based on specific sugars such as mannose, xylose, or fucose; in this way, the underlying gum can be identified in proper order.

536 � 6 Structure of paint systems Acrylic binders [99] can be clearly distinguished from binders of natural origin because they contain many synthetic components. These include, e. g., polyethylene glycols, and residual monomers of polymerization. 6.1.3 Drying of binders To understand binders, we need to imagine a painting’s drying process. Depending on the paint system, this involves processes that pursue two objectives: – removal of the solvent that was necessary to transform the powdered pigments into paintable colors – creation of a solid bond between the pigments and the support or ground, e. g., by embedding the pigments in a transparent binder film or by glueing We can achieve both objectives in two ways by: – chemical drying – physical drying In chemical drying, after or during the solvent’s evaporation, the binder polymerizes and forms a chemically different solid substance binding the pigments to the substrate. In most cases, a crystal-clear polymer film forms, embedding the pigments. An example is oil paint, in which linseed oil cross-links to an insoluble polymer when exposed to air and light. In physical drying, the solvent evaporates. However, the pigments and a dissolved, nonvolatile binder remain. The binder adheres to the substrate, binds the pigments to the substrate, or encloses them in a film-like layer. The binder does not change chemically in the process. Resinous painting agents such as dammar varnish provide an example. The solvent (turpentine oil) evaporates rapidly, and the nonvolatile resins remain on the support or ground, binding the pigments. Some paints even dispense with binders altogether, relying on the pure adhesive forces of pigments. Both variants are often combined to achieve better application properties of the system. Thus, e. g., in the technique of the Old Masters, on oil paint layers, a white highlight with aqueous egg tempera is applied. This tempera layer dries physically, its water evaporating quickly. As a result, this layer can be painted over, and its oily fraction dries chemically together with the other oil layers over time. In addition to applicationrelated advantages, there are also advantages to the painting technique. For example, the distinct oily and watery systems lead to sharply defined, hair-thin drawings, as we can impressively see in fur and hair in Dürer’s paintings. Chemical drying Chemically drying binders form the basis for some of the best-known traditional painting methods. These include:

6.1 Binders

– – –

� 537

drying fatty oils calcium hydroxide oil-modified alkyd paints

Drying fatty oils comprise, e. g., linseed oil, walnut oil, or poppy seed oil. In this classical oil paint system, the oil molecules cross-link covalently under oxygen absorption. As a result, they build a three-dimensional network in the form of a thick glass-clear film, ▶Section 7.4. Calcium hydroxide is the binder of the genuine fresco. The chemical drying process is carbonatization, the formation of calcium carbonate with carbon dioxide from the air, ▶Section 7.3. Oil-modified alkyd paints are a new development. They dry chemically due to the oil content. Physical drying Physically drying binders have also been in service since historical times: – plant gums such as gum Arabic or gum tragacanth as a base for antique tusches and inks, water-based colors, and modern watercolors – animal glues (proteins), e. g., glutin glues (collagen) from skins and bones, egg white from egg, casein from milk for inks and watercolors – resins such as dammar or mastic as a component of varnishes and resin oil paints – acrylic dispersions as the latest extension of painting agents for acrylic paints Over time, binders of animal glues polymerize through chemical changes, cross-link, and thus, in some cases, undergo chemical drying. In contrast to chemical drying, the binder does not change in purely physical drying. The processes that lead to a stable film or an adhesive layer are based on secondary, intermolecular interactions such as electrostatic and van der Waals forces, which we consider in detail in the next section. Most binders in this category are polymers, which provide a large number of such weak, secondary bonds; for this reason, extended, insoluble polymer networks form. In ▶Figure 6.3, we see essential steps of physical filming: – In the dry binder, the molecules form polymer coils, ▶Figure 6.3(a). – In an ideal solvent, the polymer coils disentangle and the molecules are stretched until they are completely enveloped by the solvent. In contrast, in an unsuitable solvent, the polymer coils predominantly remain intact, ▶Figure 6.3(b). – Evaporation of the solvent starts the film formation: – The formerly isolated binder molecules mutually approach, either by their thermal migratory motion, or by the solvent’s evaporation, increasing the binder concentration, ▶Figure 6.3(c). In case of a good solution, large regions of the stretched molecules can optimally overlap, fixing themselves via intermolecu-

538 � 6 Structure of paint systems

Figure 6.3: Schematic process of physical drying of polymeric binders. White box: support. (a): The powdered binder comprises isolated polymer coils. (b): In solution (blue), polymer coils unfold and binder molecules stretches. (c): The expanded molecules interact due to evaporation and thermal movement. Evaporation increases the binder’s concentration and, therefore, interaction probability. (d): Partially crystalline or intermolecularly linked regions form in the film.

–

lar interactions. For a poorly dissolved binder, the individual polymer coils only slightly interact. Parts of different polymers become entangled and interlock tightly by intermolecular forces, forming quasi-crystalline regions, ▶Figure 6.3(d). The formation of hydrogen bonds provides additional stabilization. As we will see, such a process is irreversible once certain energy thresholds have been overcome, thus leading to films that can no longer be dissolved. The polymers can similarly interact with the support so that the evolving cross-links also become fixed to the support, forming a solid film. For a poor solution, the individual polymer coils remain loosely coupled. A fragmented film of inferior stability results, which can readily disintegrate into individual coils, ▶Figure 6.3(d).

6.1.4 Bonding types in the world of colorants We have already addressed the role of secondary bond types in physical drying, which leads to forming a compact film. However, now is the time to explore the mechanisms in the pigment/dye-binder-substrate system responsible for film formation and adhesion of pigment or dye. We will mainly encounter noncovalent bonds, ▶Table 5.1. They do not

6.1 Binders

� 539

link the atoms of a molecule, but molecules among each other and have low strengths. Most interactions of colorants with their environment are therefore weak, ▶Table 6.1. Table 6.1: Relative strengths and energies of secondary bond types in colorants and binders. Bond type or strength Ionic bond Covalent bond Hydrogen bonds Dipole-dipole Dispersion force Dipole-induced

Relative strength [19, pp. 42]

Energy [kJ/mol] [220, p. 35]

700 400 40 5 5

590–1050 60–700 50 20 42 2

Ionic bonding Ionic bonding depends on both bonding partners’ electrostatic attraction of opposing electrical charges. Typical examples from chemistry are crystalline salts, whose crystal lattice is built up of anions and cations. Interesting for our topic is ionic bonding when used to bind basic, acid, and mordant dyes to the substrate, ▶Chapter 5. Noncovalent or van der Waals interactions The very weak noncovalent, attracting van der Waals interactions also act between uncharged and nonpolar atoms and molecules. Since they diminish rapidly with increasing distance between the partners, typically with the sixth power, we usually cannot notice their effects on a large scale. They occur wherever particles cluster together and we observe adhesion effects, i. e., effects associated with spatial proximity of molecules (on a small scale) or particles and extended surfaces (on a large scale). – Fine powders tend to form clusters as the particles aggregate. – Dyes and geckos adhere to their support with the help of their extended molecular or body surfaces. – Glues, which do not react chemically, adhere to the surfaces through noncovalent interactions. – Physical film formation in proteins or latex dispersions is achieved by aggregation of the binder molecules. Under the term “van der Waals interaction,” we summarize three very similar interactions, although frequently no clear distinction is made between them (as it is often unnecessary): – the dispersion interaction (London force) between two polarizable molecules, i. e., two induced dipoles – the Keesom or dipole-dipole interaction between two dipoles

540 � 6 Structure of paint systems –

the Debye interaction between a dipole and a polarizable molecule, i. e., an induced dipole

Often the term van der Waals force refers to the London dispersion interaction, which is also the dominant one of the three forces. Origin of dipoles Common to all three forces is a charge separation (dipole formation), which occurs in both partners and induces dipole moments. Permanent dipoles are evoked by the different electronegativities of the atoms, and thus charge densities in the molecule, e. g., in compounds that contain oxygen or nitrogen (hydroxyl and amino groups, carbonyl groups, nitriles). A well-known example of dipole-based interactions is the hydrogen bonds. Temporary dipoles emerge because electrons (considered in the particle picture) are localized in very tiny time intervals, and a charge separation occurs femtosecond by femtosecond, even if the molecule is neutral on time average (spontaneous polarization). They occur, e. g., in the interaction of nonpolar proteins in casein paints (▶Section 7.5.3) or the adhesion of dyes to nylon fibers and polyethene films (▶Chapter 5). Dipoles of both types also induce a charge separation and a dipole moment in their neighborhood: the original dipole A generates an electric field by which particle B is polarized. The resulting dipole B also generates an electric field, which interacts with that of A and leads to mutual attraction if B aligns itself to A and the fields can overlap constructively. Orbital interpretation of the interaction An interpretation of the van der Waals interaction in the framework of the orbital interaction theory is outlined in [213, Chapter 3]: the interaction of a spatially localized occupied orbital of one partner with an unoccupied, spatially extensive orbital of a second partner leads to an energetic lowering of the occupied orbital, and thus to an energy gain. With decreasing distance, the lowering progresses further until occupied orbitals of both partners interact and develop a repulsive force. Range of interaction In the molecular range, van der Waals interactions can be described by a potential that decreases with the sixth power of the distance between the particles, i. e., it decreases rapidly and has a range of a few nanometers: Va (r) = −

k r6

In the Lennard–Jones potential, these interactions represent the attracting term.

(6.1)

6.2 Solvents

� 541

In the order of magnitude of colloidal particles (a few hundred nanometers), the attracting forces should already have decreased to an imperceptibly small value. However, an important effect occurs here: every single atom can interact to a certain extent additively with all atoms of the partner particle so that we observe a focusing of the dispersion force that is called long-range van der Waals force or Hamaker force. Interestingly, it only decreases approximately quadratically with distance [218, p. 268]: VaH (r) = −

k r2

(6.2)

Its range extends to the order of 100 nm, so this force contributes decisively to the dynamics in dispersed systems; as we will see later in the example of acrylic dispersions, ▶Section 7.9.1. Hydrophobic interactions The term “hydrophobic interaction” is often used to describe an entropic phenomenon: The tendency of hydrophobic groups to agglomerate in water or aqueous solutions and the resulting reduction of contact area between the hydrophobic groups and the water. The cause of this aggregation can be described entropically. A thin layer of water molecules evolves at the interface of a nonpolar particle to the water. They are more highly ordered than the free water since all their hydrogen bonds are directed toward the free water. This state implies a low entropy and a limitation of the mobility of these water molecules. Nonpolar molecules that collide with each other through random thermal motion reduce the interfacial layer, release some previously ordered bound water molecules, and the entropy of the solution, including the aggregates, increases. The process is irreversible since entropy can only ever increase in closed systems. As a result, more and more particles aggregate, and the aggregates grow in size. In addition, van der Waals interactions between the nonpolar particles occur during aggregation, leading to an enthalpy gain. We note, however, that this effect becomes strong only when the particles are very close to each other. The real driving force is the entropy gain due to the release of solvent molecules from the hydrophobic interfaces.

6.2 Solvents Solvents are only necessary during the application phase and escape after applying the paint as it dries. Together with the binder, they provide specific processing properties. Viscosity is the eminent property determined primarily by the binder but can be modified by adding solvents or thickeners. We will discuss solvents in individual paint systems if necessary.

542 � 6 Structure of paint systems

6.3 Wetting agents and dispersants, grinding paints Wetting agents and dispersants play an essential role in producing or grinding paints and their storage. They support the wetting of pigments with binders and stabilize the pigment dispersions obtained by preventing flocculation, aggregation, or sedimentation of pigments. As a result, we obtain an even surface of the painting layer with uniform gloss, transparency, and other optical properties. Anyone who, as a beginner, has ever tried to manually mix a color probably received a hint concerning the importance of grinding. The author bore this wisdom in his mind when trying to produce oil paint. Stirring the pigment into the linseed oil was unproblematic, and soon the author had produced a thick paste that looked like oil paint, smelled like it, and could well be picked up with a brush. To his horrified astonishment, however, the color on the canvas literally turned to dust and left only a dirty stain on it. What had happened? Stirring the pigment into oil had produced a seemingly thick paste but had not created any bond between the pigment and the oil, i. e., no bond between the binder and the pigment particles had been created. Chemistry provides explanations and tools to produce high-quality colors, but first, we need to take a closer look at the elementary steps of grinding [190, Chapter 4], [221, 222], [185, Chapter 4.3.2]: – step 1: wetting of the pigment powder – step 2: fragmenting of aggregates and agglomerations and dispersing of the primary particles – step 3: long-term stabilizing of the dispersed particles Initially, the pigment is present as a powder in the air. However, for the successful production of color pastes, the powder must be thoroughly wetted with the binder, i. e., the air-pigment interface must be replaced by a water-pigment one. The adsorption energy of the solvents is often not sufficient to spontaneously provide the energy required to form the new interfaces. Therefore, we bring mechanical energy into the system and support the wetting by grinding. Then, even a pigment supplied by the manufacturer as a powder consists of large aggregates of molecules, viewed microscopically. In the production of pigments, we do not directly obtain isolated molecules since these tend to form compact aggregates of many molecules. This tendency is particularly evident in planar organic molecules, which stack flat on top of each other. The weak interactions holding the aggregates together generate large forces because of the spatial proximity. Consequently, the aggregates are very stable. However, weak interactions are also active at the supramolecular level. Pigments that are not synthesized directly but are extracted from minerals and are finely ground form larger complexes, the agglomerations, through weak interactions. These are not two-dimensional but are linked via corners and edges of the particles and show weaker cohesion. Instead, they have a large number of cavities containing air bubbles. Their interfaces to air, which must be wetted, are thus considerably more extensive than those visible at first glance.

6.3 Wetting agents and dispersants, grinding paints

� 543

By grinding, we disintegrate these agglomerations and aggregations, which are already wetted by the solvent, into small fragments (primary particles). The mechanical energy that we supply by grinding is largely retained as surface energy in the greatly enlarged surface of the pigment dispersion. Finally, dispersing is the process of countermanding the attractive forces within the aggregations and forming durable new interfaces between the pigment and the solvent. A dispersion tends to flocculate, since the dispersed pigment particles tend to reaggregate. Consequently, we must repress this tendency and stabilize the pigment dispersion to produce a durable paint paste.

6.3.1 Wetting agents Additives can support all steps: Wetting agents such as surfactants, reduce the interfacial tension, and thus the energy required to form the interface with the solvent in step 1. In addition, dispersants and stabilizers help separate the primary particles obtained in step 2 during grinding and keep them from reaggregating. In the long term, they thus maintain the dispersed state in step 3. Theoretical treatment The theoretical treatment of the wetting process shows that the essential quantity necessary for good wetting of the pigment is the contact angle θ. θ is the angle at which a drop of the binder forms with the pigment surface (▶Figure 6.4), the equation of Young applies [185, Chapter 4.3.2.3], [221, Chapter 3], [222]: γp = γps + γs cos θ

or

cos θ =

γp − γps γs

(6.3)

The γp , γs , and γps are the surface tensions of pigment (p) and solvent (s) or the interfacial tensions between both, respectively. The smaller θ is, the better the wetting: Spontaneous wetting occurs for θ = 0, and the solvent spreads spontaneously over the entire surface of the pigment, ▶Figure 6.4(a). For 0∘ < θ < 90∘ , the pigment wets well, ▶Figure 6.4(b) left. For 90∘ ≤ θ < 180∘ , the pigment wets poorly, and the solvent beads off, ▶Figure 6.4(b) middle and right. In the case θ = 180∘ , the pigment is not wettable at all; the solvent forms closed drops, ▶Figure 6.4(c). Spontaneous wetting occurs because the system gains energy. We can divide the work required to wet or disperse a pigment with the surface A into three steps and calculate for each the energy conversion, ▶Figure 6.5. In the adhesion, the pigment surface comes into contact with the solvent and binder mixture. The immersion describes the complete dipping of the pigment into the liquid. The spreading describes the detachment of the pigment from the interface with air. The energy expended for each of these steps can be described in terms of the respective interfacial energies:

544 � 6 Structure of paint systems

Figure 6.4: Influence of the contact angle θ on the wetting of a pigment surface (white box) with a solvent (blue).

Figure 6.5: Model describing three steps in the wetting of a pigment (white box) by a solvent (blue) [190, Chapter 4]. (a): Adhesion (contact between the pigment and solvent). (b): Immersion (complete dipping of the pigment in the liquid). (c): Spreading (detachment of the pigment from the interface with air).

Wa = γps − (γs + γp ) = −γs (cos θ + 1)

(6.4)

Ws = (γps + γs ) − γp = −γs (cos θ − 1)

(6.6)

Wi = 4γps − 4γp = −4γs cos θ

Wd = Wa + Wi + Ws = −6γs cos θ

(6.5) (6.7)

We see that adhesion always occurs automatically (Wa < 0, energy gain), immersion only when θ < 90°, and spreading only when θ = 0°. In the other cases, much effort is necessary to achieve complete wetting. However, we also recognize that this situation can be improved by reducing θ. Using ▶equation (6.3), we see that γs must be reduced for this purpose: ⏟⏟⏟γ⏟⏟p⏟⏟ =const

= ⏟⏟γ⏟⏟⏟ ps⏟⏟ + ≈const

cos ⏟⏟ ⏟ ⏟⏟⏟θ⏟ ⏟⏟⏟γ⏟⏟s⏟⏟ changeable should: change from −1 to +1

(6.8)

In the equation, γp is a constant that depends on the pigment, and γps is approximately constant if we assume that a wetting agent essentially changes the properties of the solvent. Both parameters cannot be changed or can only be changed at the expense of a modification of the chemical structure of the pigment. To change θ from 180° to 0°, i. e.,

6.3 Wetting agents and dispersants, grinding paints

� 545

to change the cosine from −1 to +1, we have to decrease γs , the surface tension of the solvent. Using ▶equation (6.7), we can also say that we obtain a smaller contact angle for the same dispersion work by this measure. Unfortunately, we cannot independently change θ or γs as purely energetic considerations suggested. They are also related to the depth l, to which the solvent will penetrate in a given time. l represents the solvent’s penetration into the cavities of the pigment powder. The equation of Washburn relates l to high pore size r, low viscosity η, and high surface tension γs : l=√

k ⋅ r ⋅ γL cos θ 2η

(6.9)

As a result, although we could now take a coarse pigment powder, we would have to find an acceptable compromise between good wetting and fast wetting, as is often the case when formulating paints and inks. Reducing the surface tension of a solvent is the typical task of a surfactant. Due to the similarity in structure, dispersion stabilizers can also act as wetting agents, significantly the sterically active dispersants. Frequently used are, e. g., Aerosol OT® or branched sodium dodecylbenzenesulfonate. Surfactants We divide surfactants or emulsifiers according to their charge into anionic, cationic, or nonionic compounds [203, keyword “surfactants”], [966]. Cationic surfactants do not occur in the painting environment. However, we will see examples in the chapter on the composition of inks, ▶Section 8.5.1 at p. 813. Anionic and nonionic compounds, on the other hand, are frequently used in formulations for paints [185, Chapter 4.3.2.3]. Generally, they exhibit a chain-like structure with one hydrophilic and a hydrophobic end, each of which shows a high affinity for either water or the polymer or pigment particle:

546 � 6 Structure of paint systems At suitable concentrations, surfactants completely envelop the particles in a hedgehoglike form (▶Figures 6.8(a) and 6.9(a)), and thus support the wetting of the particles. At the same time, they can help keep the particles in the dispersed state: ionic surfactants, due to their electrical charge, and nonionic surfactants, due to their steric claims, prevent the particles from forming larger units. In the next section, we will address this effect to function as a dispersant. As ionic surfactants, sulfates and sulfonates of polyethylene glycols etherified with fatty acid alcohols or alkylaryl sulfonates are applicable. Sulfonate groups are also introduced with the help of succinic acid esters (sulfosuccinates). Linear or branched higher alcohols (C7−18 ) are employed. Counterions are alkali, ammonium, or alkanolamines (triethanolamine):

For nonionic surfactants, similar substances are employed. Instead of the sulfate or sulfonate group, however, they carry hydroxyl groups:

The formerly essential alkylphenol ethoxylates are no longer used for environmental reasons. Furthermore, betaines also belong to nonionic surfactants. These inner salts are derived from the aminoacetic acid or the amino acid glycine:

6.3 Wetting agents and dispersants, grinding paints

�

547

Common surfactants available commercially are listed as examples in ▶Figure 6.6. The exemplary betaines are derived from stearic acid and “cocoic acid,” a mixture of higher fatty acids derived from palm oil.

Figure 6.6: Examples of ionic and nonionic surfactants: alkyl and alkyl aryl sulfates and sulfonates, sulfosuccinates, and polyglycols. The last two compounds are nonionic betaines.

Surfactant production Glycol alcohols (polyethylene glycols, PEG) The solvents and surfactants mentioned here and the humectants mentioned later are often established on glycol alcohols and their alkyl ethers. These compounds have favorable properties and can be easily prepared on a large scale, while their properties can be varied in many ways. The starting point for the preparation is oxirane (ethylene oxide), obtained from ethene in a catalytic process:

548 � 6 Structure of paint systems

The resulting ethylene oxide is used mainly in the production of polyester plastics such as polyethene terephthalate (PET) and sizably as a solvent and antifreeze. Glycol alcohols are synthesized as follows:

If we apply alcohols instead of water (R is an alkanol or alkylphenol), we obtain ethylene glycol alkyl ether or ethylene glycol alkyl phenyl ether. In both cases, the reaction can usually be conducted so that only or predominantly monoglycols, diglycols, or higher glycols form. In addition, if methyloxirane or propyl epoxide are applied instead of oxirane, we also obtain significant polypropylene glycols. Sulfates, sulfonates Anionic surfactants based on sulfates and sulfonates are also cheap large-scale industrial products. Alkane sulfonates are produced via sulfochlorination:

To prepare the alkylbenzene sulfonates, the corresponding alkylbenzenes are produced by Friedel–Crafts alkylation from alkenes and reacted with fuming sulfuric acid:

6.3 Wetting agents and dispersants, grinding paints

� 549

Alkoxyalkyl sulfonates can be prepared from alcohols and alkylglycols in an ether formation reaction with chloroethanesulfonic acid:

Alkyl sulfates can also be readily produced from fatty alcohols by reaction with fuming sulfuric acid, chlorosulfonic acid, or sulfur trioxide:

550 � 6 Structure of paint systems

6.3.2 Dispersants Dispersants stabilize dispersions. Dispersions represent the important case of physically drying binders that are not solutions. In general, dispersions are systems that comprise finely dispersed particles in a homogeneous medium without forming genuine solutions. Many well-known multiphase systems fall under the term: mist (liquid finely dispersed in gas), fume (solid in gas), suspension (solid in liquid), foam (gas in liquid), emulsion (liquid in liquid). In connection with artists’ paints, we encounter dispersions in acrylic paints (polyacrylate suspensions in an aqueous solvent, ▶Section 7.9) and inks, ▶Chapter 8 and ▶Section 8.5.4 at p. 823. Hunter and Tadros [218, 221, 222] provide detailed information about the exciting physics of disperse systems. To understand how stabilization of dispersion paints and film formation work, we need to know which processes take place in the dispersion when particles approach. As a practical example, we will look at an acrylic paint dispersion in which the particles are acrylate polymers. The noncovalent interactions involved were already addressed in ▶Section 6.1.4. We assume that the particles are electrically charged, either already given by the chemistry of the particles or by the addition of charged dispersing agents, ▶Section 6.3.3. The interactions between the particles of the acrylic dispersion are described by a theory called DLVO theory after the first authors Deryaguin, Landau, Verwey, and Overbeek. A comprehensive summary is given in [218, Chapter 9], [219, 221]. ▶Figure 6.7(b) shows the interaction potential Vt (r) between two particles of an acrylate-latex dispersion for three electrostatic repulsion forces of different strengths. Vt (r) is a function of the distance between the particles. According to the DLVO theory, we can separate the potential curve into several parts, which are easier to describe, ▶Figure 6.7(a): – The dispersion or van der Waals forces have an attractive (negative) potential, which in the form of the Hamaker force reaches a range of about 100 nm and shows qualitatively an approximately inverse square dependence from the distance: Va (r) = − –

c r2

(6.10)

The similar electric charge of the particles evokes a repulsive electrostatic potential, which drops exponentially for spherical particles: Vr (r) =

c′ −kr e k

(6.11)

6.3 Wetting agents and dispersants, grinding paints

�

551

Figure 6.7: Total potential between two particles of an exemplary dispersion as a function of distance r. Depending on the magnitude of the (electrostatic) repulsion, the primary minimum, the primary maximum, and the secondary minimum can be distinguished. If the system is in the primary minimum, a nonresolvable film is present, while the secondary minimum describes the stable dispersion. The primary maximum represents the potential barrier preventing coalescence and stabilizing the system. For this barrier to be strong enough to prevent coalescence under surrounding conditions, the repulsion must be sufficiently strong (light blue graph). If it is too weak, the system will coalesce on its own as it can constantly lose energy with smaller r (red graph).

–

At very small distances, particles feel the Born repulsion VB (r) caused by the interaction of the orbitals with each other, which shows a very strong dependence on distance and acts only on short distances: VB (r) =

c′′ r6

(6.12)

552 � 6 Structure of paint systems Since quadratic functions increase more than exponential ones, the attractive potential Va (r) is dominant for small and large r, i. e., the attraction between the particles predominates in these ranges. If the repulsive potential Vr (r) is small, then Va dominates even for any distance, resulting in a permanent attraction. The dispersion is, in this case, unstable and coagulates because it can gain energy (Vt becoming steadily more negative) with an approach of the particles (r becoming smaller). The coagulation stops only with the closest approach of the particles due to the Born repulsion becoming effective. Only for a sufficiently strong, repulsive potential Vr , there is a specific range of r, in which the repulsion predominates and builds up a barrier Em against a closer approach of the particles, the so-called primary maximum. This barrier can be overcome by introducing energy, e. g., thermal energy from Brownian motion. Vt becomes negative again at more considerable distances and forms the secondary minimum; at smaller distances, the primary minimum. What does the potential curve Vt (r) mean for acrylic dispersions? If particles approach each other sufficiently (r large), the van der Waal forces initially prevail until an equilibrium is reached at (3) between the electrostatic repulsion and the attraction. This point characterizes a stable dispersion. At a further approach, the electrostatic repulsion prevails, and the energy increases up to a maximum Em (2). This quantity represents the barrier against coalescence and is a significant number that depends on the surrounding conditions. For example, at high salt concentration, it is lower; at uncharged polymer particles, it does not exist. Suppose we overcome this barrier by external forces and bring the particles closer together. In that case, we reach the stable final position (1), characterized by the van der Waals forces being very strong at such short distances. The dispersion is now irreversibly coagulated and cannot be regenerated (repeptized) again because now not only the energy between zero and primary maximum has to be applied, but also between the primary minimum and maximum. The particles lose their identity at this point and start to fuse, beginning the film formation, ▶Section 7.9.2. 6.3.3 Stabilization of dispersions, dispersants As we have seen before, pigment or polymer dispersions are only stable under certain conditions. While coalescence and film formation are desirable in (1) after painting, the system must be previously stabilized at (3) as a dispersion to avoid flocculation, that means during storage and processing of the acrylic paints, [218, Chapters 2, 5, 9], [221, Chapter 4], [185, Chapter 3.2, 4.3.2.4]. For stabilization, we need to prevent the reversion (3 → 1) to the thermodynamically more stable flocculation state through kinetic barriers. We can choose between several ways:

6.3 Wetting agents and dispersants, grinding paints

–

–

–

� 553

Electrostatic stabilization: We add electrolytes to the dispersion with a high affinity for the polymer and surround it with a shell of similar electrical charges of the same type. The charged polymers repel each other. Using this method, we increase the barrier Em . Steric stabilization: We add protective colloids, which, section by section, alternately have high polymer or water affinity. The hydrophilic sections are designed to be spatially demanding. In this way, the polymers acquire a bulky or “hairy” surface to keep the particles at a distance. Again, we increase Em by adding repulsion terms. Addition of surfactants (emulsifiers): We add these to reduce the surface tension of the polymer particles significantly. The surface energy of the polymer particles contributes to the total energy of the polymer system according to UO = γ ⋅ A

(6.13)

Herein, γ is the surface tension, and A is the surface area. The smaller these particles are, the greater their surface area and surface energy. The system can reduce its total energy by reducing the surface area and forming a few large particles from many small ones in the dispersion. We perceive this as coagulation, flocculation, phase separation, or precipitation of solids. In this process, the polymers approach each other, and the van der Waals forces provide for further mutual adsorption of the particles. The surfactant significantly reduces the surface energy of the system after ▶equation (6.13) and also the possible energy gain in flocculation. Therefore, despite this energy gain, the particle system cannot overcome the barrier Em on its own. Electrostatic stabilization The electrostatic stabilization of dispersions is based on the coverage of the polymer particles with the same electrical charge by adding ionic compounds, ▶Figure 6.8. Simple compounds have a hydrophobic adhesive group and a hydrophilic end; they thus correspond to the ionic surfactants we discussed in ▶Section 6.3.1 and show that these dispersants can also act as wetting agents simultaneously. Examples of surfactants include alkyl polyethoxy sulfonates, alkyl polyethoxy sulfates, alkyl benzene sulfonates with chain lengths n between 10–30, or polyphosphoric acid. Polymer surfactants have several hydrophobic and hydrophilic sections. An example is polyacrylic acid PAA, used as potassium, sodium, or ammonium salt, which obtains hydrophilic and hydrophobic sections by copolymerization with acrylic acid esters. Such polymers have a stabilizing effect partly also because of their steric demands. In the case of polymer dispersions, it is possible to introduce charges by copolymerization with suitably charged monomers in the polymer chain itself. One monomer is acrylic acid, anionically charged in a suitable environment (acrylate anions). The hydrophobic ends of the surfactants are adsorbed on the polymer (e. g., via van der Waals interactions), while their hydrophilic (charged) ends protrude into the

554 � 6 Structure of paint systems

Figure 6.8: Electrostatic stabilization of polymer or pigment dispersions. The particles are covered with ionic surfactants in a hedgehog-like manner. The symbol ⊖ symbolizes charged hydrophilic heads or chain sections; zigzag sections symbolize hydrophobic hydrocarbon chains.

aqueous solvent and build up the charge layer. To compensate for the charge, the counterions outside the charged polymer particles form an oppositely charged cloud so that we have an electrically charged double layer with a diffuse flow to the outside. The electrostatic repulsion drives similarly charged clouds, and thus also polymer particles apart and stabilizes the dispersion in this way. Since external factors easily influence the electrical conditions, we must not add electrolytes such as salts, acids, or bases to such a stabilized dispersion without further consideration because the charge distribution and layer structure would be changed or destroyed. The consequence would be a coalescence of the dispersion. Steric stabilization This method of dispersion stabilization is based on coating the polymer with nonionic surfactants or protective colloids, ▶Figure 6.9. Alkyl polyglycols are used as surfactants; with a chain length n between 10–50, they are significantly larger than ionic surfactants. Again, the hydrophilic parts of these molecules show affinity to the aqueous phase, while hydrophobic parts are oriented towards the polymer (e. g., due to van der Waals interactions) and cover it densely. This protective layer is thin compared to the particle: while pigment and latex particles have a size of about 0.1–10 µm, the protective layer is about 20 nm thick. Here again, we can use simple or polymeric stabilizers. Two driving forces are discussed as possible mechanisms for this type of stabilization; both prevent the dispersed polymer particles from approaching each other too closely:

6.3 Wetting agents and dispersants, grinding paints

� 555

Figure 6.9: Steric stabilization of polymer or pigment dispersions: The dispersed particles are covered in a hedgehog-like manner with nonionic surfactants or protective colloids. The dark dots ∙ symbolize hydrophilic heads or chain sections; zigzag sections symbolize hydrophobic hydrocarbon chains.

–

–

When two particles approach each other, the layers of the stabilizers permeate, which restricts the conformational mobility of the stabilizer molecules. Due to the increasing order, the entropy decreases, and the system’s internal energy increases. The level of this energy threshold is the barrier against aggregation of the polymer particles. As the stabilizer layers permeate, the concentration of the stabilizer increases, and solvent molecules are displaced from the overlapping space. If the stabilizing agent has a high solubility in the solvent, solvent molecules try to dissolve it again to avoid unfavorable stabilizer-stabilizer contacts and establish stabilizer-solvent contacts instead. As a result, the solvent develops an osmotic pressure, which acts as a separating force.

Protective colloids In contrast to surfactants, protective colloids are high-molecular compounds but also have hydrophilic and hydrophobic sections. Therefore, instead of many small surfactant molecules, some of these large molecules of the protective colloid surround the polymer particles. Examples include polyvinylalcohol (PVOH), poly-(vinyl-2-pyrrolidone) (PVP), cellulose ethers such as hydroxyethyl cellulose (HEC), and polyacrylates such as polymethacrylic acid (pMAA), ▶Figure 6.10. Depending on the ratio of the monomers, copolymers based on acrylic acid/acrylic acid ester can be regarded as polymer surfactants or protective colloids. Spinelli [862] shows by using the example of a dispersing aid for modern pigmented inkjet inks how block copolymers p(A-B) assemble from hydrophobic (A) and hydrophilic (B) blocks. The

556 � 6 Structure of paint systems

Figure 6.10: Examples of protective colloids (nonionic compounds). Polymethacrylic acid exemplifies a polymeric surfactant.

hydrophobic A block p(MMA/BMA/EHMA) contains methyl methacrylate (MMA), butyl methacrylate (BMA), or ethylhexyl methacrylate (EHMA) as comonomers, which bind to the hydrophobic pigment or particle. The hydrophilic B block is formed by including acrylic acid (AA) or methacrylic acid (MAA) into a block of the same monomers, B=A/AA/MAA. This hydrophilic region protudes into the aqueous phase. Polyacrylates can be flexibly adapted through their composition and size of the A and B blocks. For example, spatially demanding polymers with wide hydrophilic loops for steric stabilization, or charged regions for ionic stabilization can form.

6.4 Thickener, rheology modifier Thickeners and rheology modifiers adjust the viscosity and flow properties of paint according to the application requirements. Typical agents are inorganic substances (mainly silicates) or organic polymers (polyacrylates, polyvinylpyrrolidone, polyurethanes, and cellulose derivatives). Kittel [185, Chapter 4.3.1] extensively depicts rheology in connection with paint, Hester and Squire [855] depict the thickening mechanism for aqueous latex systems. Two processes achieve a thickening effect: – Nonassociative thickeners or hydrogeling agents (▶Figure 6.11(a)) increase the viscosity of aqueous systems. They have a thickening effect because they structure the aqueous phase, and thus increase their hydrodynamic volume. The dipole moment of the water molecules is exploited. Water molecules are adsorbed on the thickener’s polar groups and also form hydrogen bonds. Above a critical concentration, entanglement of polymer coils occur, enhancing the viscosity.

6.4 Thickener, rheology modifier

�

557

Figure 6.11: Two ways of thickening a solution. White circles: polymer particles; black dots: pigments.

–

The resulting gels are often structurally viscous since the hydrogen bonds break when the shear force is sufficiently large. As a result, they become highly fluid when subjected to external forces (brushing, spraying, stirring) but regain their viscosity as soon as the external force decreases. If this change in viscosity does not occur immediately with the onset or end of force but rather with a delay, we speak of thixotropic behavior. Associative thickeners (▶Figure 6.11(b)) are hydrophilic polymers with hydrophobic subregions. In contrast to hydrogelling agents, they do not involve the solvent but the other constituents of the paint dispersion, such as pigments or binder polymers. While the thickener polymer disentangles in the water, the hydrophobic regions are adsorbed onto other thickener polymers, pigments, and binder polymer particles, thus forming a network that includes all components except water. The increase in viscosity is caused by particle bridging.

Nonassociative thickeners (hydrogeling agents) Nonassociative thickeners structure the aqueous phase, and thus increase their hydrodynamic volume, ▶Figure 6.11(a) [185, Chapter 4.3.1.2.1.1]. At the same time, hydrocolloids can act as protective colloids. Nonassociative thickeners are polymers of high molecular weight. Examples are: – cellulose ether – other polysaccharides (starch, guaran, xanthan) – homopolymers of unesterified acrylic acid p(AA)

558 � 6 Structure of paint systems Cellulose-based nonassociative thickeners Cellulose-based nonassociative thickeners are cellulose first converted into sodium hydroxylate by boiling with sodium hydroxide solution. Subsequent treatment with chloroethane, ethylene oxide, propylene oxide, or chloroacetic acid leads to ethers of ethanol, propanediol, or acetic acid [185, Chapter 4.3.1.2.1.1]. Etherification occurs preferentially at the primary hydroxyl group C6 , followed by C2 and C3 . The hydroxyethyl ethers formed in the reaction of ethylene oxide with primary hydroxyl groups can further react to form polyethoxy ethers [856]. Commercially, cellulose ethers are present in products such as Klucel® E or Tylose® : – methyl cellulose (MC) – methyl hydroxyethyl cellulose (MHEC, Tylose® MH [990]) – methyl hydroxypropyl cellulose (MHPC, Tylose® MO [990]) – hydroxyethyl cellulose (HEC, Tylose® H, HS, HA [990]) – hydroxypropyl cellulose (HPC, Klucel® [988]) – sodium carboxymethyl cellulose (NaCMC) or sodium carboxymethyl hydroxyethyl cellulose (NaCMHEC) As the formula structures show, only some hydroxyl groups of the cellulose are etherified, the exact number determining the properties of the individual products. Typically, 0.8–1.3 of the three free hydroxyl groups are etherified:

6.4 Thickener, rheology modifier

� 559

The modifications of cellulose shown are necessary to convert it into an appropriate water-soluble form. In its normal state, cellulose is insoluble in water because several cellulose chains can be clustered together in a rope-like manner, ▶Section 6.7.1 at p. 569. Hydroxyl groups reinforce the cohesion by forming numerous hydrogen bonds, giving the strands high strength. Due to this ability to form microcrystalline adducts, cellulose is a part of many natural structural materials such as wood, cell walls, or stale bread. Etherification converts the water-insoluble cellulose into a soluble form as the ethers block hydroxyl groups. This process prevents the formation of hydrogen bonds and the clustering of strands into insoluble adducts. Cellulose ethers increase the viscosity of the aqueous phase by adsorbing water molecules along their chains. Thus, their hydrodynamic volume expands considerably. The viscosity expands with the molecular weight of the cellulose ethers and concentration and falls with temperature. Cellulose tangles can be disentangled by high shear forces and aligned along the direction of the force so that cellulose-thickened solutions are strongly shear thinning. As a result, they become more fluid when subjected to external forces, such as during painting, rolling, spraying, or stirring. High degrees of polymerization enhance this property. Starch Starch paste also serves as a thickener [185, Chapter 4.3.1.2.1.1.2]. Since amylose tends to form crystallites on cooling, starch is modified with ethylene oxide or propylene oxide. The resulting hydroxyethyl starch (HES) or hydroxypropyl starch (HPS) is readily watersoluble and stable.

560 � 6 Structure of paint systems Polyacrylic acid-based nonassociative thickeners Homopolymers of free acrylic acid (PAA) or methacrylic acid (PMAA) can also act as nonassociative thickeners [185, Chapter 4.3.1.2.1.2]. Polyacrylic acid adsorbs water molecules along its chain and, in this way, increases its hydrodynamic volume. By copolymerization p(AA/MAA-EA) of polyacrylic acid with acrylic acid esters such as ethyl acrylate (EA), the polyacrylic acid is partially hydrophobically modified, and this controls the extent to which ambient water is structured.

ASE 60® is a commercial, partially hydrophobically modified polyacrylic acid based on a copolymer of methacrylic acid with ethyl acrylate p(MAA-EA), ▶Figure 6.12 [987]. ASE stands for alkali soluble emulsion and is an acid, thin emulsion of undissociated polyacrylic acid in the delivery form. It reacts during neutralization with a substantial increase in viscosity. Carboxylic acid groups are transformed into carboxylate groups. The resulting homogeneous charge causes a disentangling of the acrylic polymer, which adsorbs water via hydrogen bonds in a wide area, thus forming an extended wide-meshed gel. Therefore, polyacrylate thickeners are strongly pH-dependent and exhibit a maximum effect at medium pH values.

Figure 6.12: Swelling of nonassociative thickeners.

6.4 Thickener, rheology modifier

�

561

Neutralizing agents for polyacrylic acid are salts of strong bases (alkali hydroxides, ammonium hydroxides, or alkali carbonates) or organic amines (triethanolamine, diisopropylamine, 2-amino-2-methylpropanol-1) [986]:

Ammonium salts or amines with high vapor pressure are preferable to alkali salts since they can evaporate during the film drying; they partially evaporate while the free carboxylic acid reforms. On the other hand, alkali anions remain permanently in the film and lead to constant redissolving. Cross-linked polyacrylic acid thickener ASE thickeners can cross-link with bifunctional dienes, an example being the Carbopol® EZ-2, frequently available in artists’ supplies. It is polyacrylic acid (PAA) with small parts of acrylate components, which is cross-linked with polyalkenyl ether or divinyl glycol (hexa-1,5-diene-3,4-diol), ▶Figure 6.13 [986].

Figure 6.13: Cross-linked polyacrylic acid (PAA). Thick lines: PAA chains connect through bridges. Thin lines: PAA chains form large-scale networks. The carboxylate groups structure the water through hydrogen bonds (hydrogel formation). White circles: other components of the paint (pigments or particles of a latex dispersion), which are adsorbed on the polyacrylate polymers or held in the meshes of the network.

Cross-linking of the polyacrylic acid chains is achieved by replacing one acrylic acid unit with a bifunctional diene that can become part of two chains and links them:

562 � 6 Structure of paint systems

Other cross-linking monomers include trimethylolpropane diallyl ether (TMPDAE), dipropyleneglycol diacrylate (DPGDA), and allyl methacrylate:

Due to the cross-linking, the particles are no longer physically soluble but only colloidally. They can swell considerably, carbopoles to a thousand times their original volume. A high water-binding capacity accompanies this increase, so we often find crosslinked polymers in products such as diapers or absorbent gritting material. Associative thickeners Associative thickeners are water-soluble polymers of low molecular weight, but with numerous hydrophilic groups and distinct hydrophobic regions [185, Chapter 4.3.1.2.1.3]. They thus resemble polymer surfactants used to stabilize a dispersion. Their hydrophilic sections extend into the aqueous phase, and hydrophobic sections become adsorbed on pigments or polymer particles, ▶Figure 6.11, right. Gel formation occurs through hydrophobic interaction between alkyl chains, leading to increased effective molecular mass and a wide-meshed network. In contrast to hydrogel-forming agents, this gel does not involve the solvent, but the other particles of the ink, such as pigments or dispersed binder polymers. To obtain associative thickeners, we can copolymerize numerous hydrophilic polymers that serve as thickeners with hydrophobic monomers. Commercially available products on the market can be identified by the designation “hydrophobically modified”: ether-urethanes (▶ HEUR), polyethers (▶ HMPE), polyacrylamides (▶ HMPAM), polyacrylic acids/ASE (▶ HASE), and hydroxyethyl cellulose (▶ HMHEC). Hydrophobic elements are introduced by incorporating small amounts of “hydrophobically modified monomers” (HMM) at both ends of the polymer chain or randomly scattered along its length. Usually, alkyl polyglycols, alkyl aryl polyglycols, or

6.4 Thickener, rheology modifier

� 563

fatty alcohols are hydrophobic carriers introduced as acrylate or methacrylate into a polyacrylic acid, or they form cellulose ethers with cellulose derivatives. Exemplary monomers are C16−22 -alkylated compounds [856, 858, 859, 861, 863–865, 989]:

HASE thickener Hydrophobically modified ASE thickeners (HASE) are copolymers p(AA/MAA/MS/Itaconic acid-EA-HM) of acrylic or methacrylic acid (ASE) with a hydrophobic, long-chain alkyl comonomer (HM) and possibly further acrylic acid components such as ethyl acrylate, butyl acrylate, or methyl methacrylate [185, ch. 4.3.1.2.1.3.4], [856]. HM is an alkyl polyglycol or alkyl aryl polyglycol that is introduced as a (meth)acrylate or maleinat into the chain, which may also contain maleic acid or itaconic acid:

An example from commerce is Tafigel® AP with the structure p(AA/MAA-HM), where HM denotes the typical hydrophobic monomer alkyl-polyethoxy-acrylic acid ester [989]. The proportion of hydrophobic comonomers is about 20 %.

564 � 6 Structure of paint systems Due to the composition, HASE thickeners have a comb-like shape, the hydrophobic chains forming the teeth, ▶Figure 6.14(a). Like ASE, the hydrophobically modified product must be neutralized with alkaline solutions to initiate the swelling process. Besides the associated electrostatic repulsion, the swelling is supported by the fact that the glass temperature of the polymer drops sharply in contact with water. This drop considerably increases the mobility and radius of movement of the polymer chains, which is perceived macroscopically as swelling.

Figure 6.14: Structures of associative thickeners used in paints.

HEUR thickener Hydrophobic polyurethanes are also used in water-based paints [185, Chapter 4.3.1.2.1.3.1]. This HEUR type includes products such as Tafigel® PUR [989]. Their structure is an ABA block copolymer with polyethylene glycol as the B block, sectionally linked to hydrophobic urethane units (A block), ▶Figure 6.14(b). A long-chain hydroxyl or amino compound is chosen as the alkyl chain, as the following example shows [866]:

6.5 Film-forming aids (coalescing agents)

� 565

HM*C thickener If we provide cellulose ethers with hydrophobic units, we arrive at HM*C types [185, Chapter 4.3.1.2.3.5], [856]. An example is HMHEC hydrophobically modified with randomly distributed alkyl chains. Typically, 5 % of the hydroxide groups are alkylated, and the chain length of the alkyl residues varies between 10–24:

6.5 Film-forming aids (coalescing agents) Film-forming aids or coalescing agents intend to optimize the film formation of binders based on a polymer dispersion under specific application conditions or even make them possible in the first place [190, Chapter 6.2]. If a polymer dispersion is used as a binder, as with acrylic paints, a perfect film formation is only possible above a specific minimum temperature, the so-called minimum film-forming temperature, MFT. The last step of film formation is the interdiffusion (depicted in more detail in ▶Section 7.9.2), in which polymer particles are welded together by the exchange of polymers, thus forming the homogeneous film. This phase takes place after drying. Toward the end of the actual drying, capillary forces increasingly occur, which are the more significant, the thinner the remaining water films between the polymer particles become. These forces press the polymer spheres against each other, whereby the spheres must be increasingly hexagonally deformed. In addition, by fusing individual particles to form larger units, the particles can reduce their surface energy, as the surface area of the fused units is smaller than that of the numerous individual particles. The hardness or deformability of the polymer spheres influenced by the temperature during filming opposes this endeavor. Therefore, filming is only possible at or above the MFT; only then do the polymer particles become sufficiently deformable to coalesce. The polymer’s MFT and glass transition temperature TG are closely related; hard polymers lead to a high MFT, soft ones to a lower one. For standard artist acrylic dispersions, e. g., the MFT is approximately at room temperature or slightly below. In order to allow processing at lower temperatures, soft polymers with low TG can be applied, but these result in soft films and are therefore not suitable for the desired purpose. Film-forming aids temporarily lower the MFT to such an extent that a uniform film formation is possible even at lower temperatures. They are initially dissolved in the

566 � 6 Structure of paint systems aqueous phase and diffuse into the polymer particles, reducing their hardness and increasing their deformability, i. e., act as plasticizers. The film-forming aids gradually escape from the film during drying. The film hardens as the agent evaporates and regains its original hardness determined by the TG . This temporary softening allows harder polymers, which produce a film with high abrasion fastness, a more uniform surface, and consistent color reproduction due to the improved filming. These auxiliaries, by nature, are moderately hydrophilic solvents that evaporate more slowly than water and must have a particular affinity for both water and polymer. Therefore, high-boiling polar products with alcohol, ester, ether, or keto functions are preferred. In addition, frequently used are esters of lower alkane carboxylic acids, and alkyl ethers of ethylene glycols and propylene glycols, ▶Figure 6.15.

Figure 6.15: Examples of film-forming aids (coalescing agents) commonly used in paints [190, Chapter 6.2].

6.6 Other excipients Defoamers prevent foaming during processing and support air escape during the drying of the binder film. Fatty acid esters, metal soaps, mineral oils, silicone oils, and siloxanes are used. Siccatives support the drying process of a chemically drying binder and promote its radical polymerization. Metal soaps with cobalt, manganese, calcium, zinc, or barium are used. In ▶Section 7.4.5 at p. 675, we learn about applying a siccative using the example of oil painting. Preservatives prevent infestation of the moist and often nutrient-rich colors (binders!) by microorganisms.

6.7 Paper

�

567

6.7 Paper As one of the earliest artificial writing materials for texts and paintings, papyrus was used already 3000 BC in Egypt [777]. Paper has not such a long tradition; depending on the continent, it is longer (Asia) or shorter (Europe). Paper as we know it was invented in China in the year 105; in Europe, we find papermills from 1144 (Spain) and 1390 (Nuremberg) onward. The fundamental manufacturing processes and materials have changed little for an extended period. However, in the last few decades, chemistry has produced numerous specialty papers through selective surface coatings and paper pulp treatment, including writing paper to filter papers to absorbent, wet-tear resistant, or oil-resistant paper. At first glance, these finishing techniques seem unspectacular for the chemistry of painting, but they prove to be a treasure trove [820–822, 824, 825], [218, p. 301], [1005]. [173–178], [203, keyword “paper”], [182, 183, 830] consider the production, properties, and chemistry of paper in detail. Chemically, the paper consists of densely matted fibers and can form sheets of various thicknesses. Depending on the raw materials the fibers are extracted from, we obtain high-quality papers for writing and drawing, cheap ones for mass printing, cardboard papers, or high-quality rag paper. The various products differ in the type of fibers and the number of manufacturing and finishing steps. ▶Figure 6.16 illustrates the essential steps of paper production: – Extraction of the fibrous materials from the raw materials, e. g., of pulp (lignin-free cellulose) from wood, or conversion of the raw materials into a suitable fibrous form. – Production of the paper pulp from the fibrous materials to scoop the paper. As a rule, a mixture of different fibrous materials is used to obtain the desired paper grade. – Addition of additives to control dry and wet strength, fixatives to fix certain chemicals on the paper, and flocculants to facilitate processing. – Addition of special additives to control surface tension (sizing in the pulp) and colorize (coloring in the mass). – Scooping (traditionally by hand) or screening (by machine) the sheets from the pulp to produce the paper web. By pressing off the water with felts, the surface gains a texture (rough, matte, ribbed) and, optionally, a watermark. – If necessary, surface treatment for surface coloring of the paper and control of the parameters smoothness and absorbency. – If necessary, calendering (rolling) to achieve smoother, shimmering surface qualities. 6.7.1 Structure and composition of raw materials Paper-building fibers can come from a variety of sources. In principle, using many raw materials such as straw would be possible. However, in the countries of origin of the paper, papyrus plants were a natural choice. In the European region, cellulose fibers from

568 � 6 Structure of paint systems

Figure 6.16: Overview of the main raw materials for paper fibers and the main processing steps during paper production [173–178], [203, keyword “paper”], [830].

rags, or textile cotton waste, established themselves as a raw material for centuries. However, woods replaced them as the primary paper source in the last two centuries. Today, synthetic fibers are the choice for tear- and water-resistant specialty papers. For high-quality papers, however, rags are still used today. Recycled papers contain recycled paper products such as market deinked print (MDIP) or deinked pulp (DIP, recovered paper with ink removed), recycled bleached kraft (RBK, recycled bleached Kraft paper), old corrugated container (OCC, used cardboard boxes), or old newspaper (ONP). However, such raw materials are not considered for long-life artists’ papers. The selection of raw materials and processing methods before preparing the pulp determines the essential parameters of the paper. The most important raw materials are woods, cotton, and synthetic fibers; in the Middle Ages, rags from textiles containing cellulose. Approximately 43 of the pulp needed today is produced chemically, 41 by mechanical means. Of the chemical pulps, 90 % are produced by the Kraft process, 10 % by

6.7 Paper

� 569

the sulfite process. Cotton fiber pulp is indispensable for producing high-quality artists’ papers (and banknotes) but is insignificant in quantity. The most critical target parameters for paper are strength and printability. Softwoods (spruce, fir) yield longer fibers (2.5–4.5 mm) than hardwoods (beech, birch, oak, 0.7–1.6 mm) and give the paper higher strength, whereas fillers reduce it. Conversely, the printability of hardwood pulp is better because the short wood fibers compensate for surface irregularities more adequately than long fibers. Specific processing steps such as sizing, coating, and calendering improve the surface properties. Wood The most widely available renewable raw material, wood, consists of approximately 40–45 % cellulose, 25–35 % hemicelluloses, 20–30 % lignin, and small amounts of other substances such as waxes or fats, depending on the type of wood, [173], ▶Table 6.4. The most valuable part of the wood includes cellulose and hemicelluloses; in the case of cheap papers, lignin is also essential for increasing the paper mass. However, lignin degrades over time under the influence of light to various colorless and yellowish compounds, leading to the well-known rapid yellowing of wood-containing papers, ▶Section 6.7.8. Cellulose The essential component of paper fiber is cellulose, a chain-like molecule of poly-(βD-1,4-glucopyranosyl-glucopyranoside) [173]:

In cotton fibers, the chains contain about 1500 glucose units; in spruce wood, about 1500; and in processed pulp, about 700. The β linkage of the glucose molecules results in a complete turnaround of the chain itself every two glucose units. This glucose pair is called cellobiose:

Specific functional groups of cellulose are significant for paper chemistry and dyeing: – Per glucose unit, one primary and two secondary alcoholic hydroxyl groups are contained. These groups are responsible for the main properties of cellulose in pa-

570 � 6 Structure of paint systems

–

–

per chemistry, as they can bridge several cellulose chains or fibers via hydrogen bonds. Carboxyl groups replace approximately 1 % of all primary hydroxyl groups due to growth-related irregularities. This small proportion of carboxyl groups is also vital for paper chemistry, as it allows cellulose to dissociate in the aqueous medium. The resulting polycarboxylate exhibits anion activity and influences the binding of auxiliaries, sizing and coating, and dye affinity. Aldehyde, keto, or (glycosidic) hydroxyl groups can occur per terminal glucose unit instead of the standard hydroxyl groups. Cleavage of the terminal glucopyranose ring is also possible:

The alcoholic hydroxyl groups and the ring oxygen cause the high strength that cellulose fibers can develop: as soon as cellulose strands are oriented in parallel, they can stiffen considerably due to numerous hydrogen bridges, as ▶Figure 6.17 shows schematically [280, Chapter 4.4.16]. Hydrogen bridging is possible only in quasicrystalline, i. e., ordered regions of cellulose since a distance of the binding partners of about 0.25 nm is necessary for forming the bridge. In amorphous (disordered) regions, the chains are too far away from each other, and the density of the hydrogen bonds is considerably lower. In the same way, the cellulose chains on the surface of the fibers are bridged. Consequently, they form the desired paper felt in the pulp during the paper sheet’s scooping process. The hydrogen bonds are thus responsible for the dry strength of the paper. A higher paper strength requires denser bridging, longer fibers, or the addition of synthetic polymers that act as bridges (dry-strength additives). The mechanism explains the considerably lower wet strength of untreated paper compared to the dry state: Water molecules move between the cellulose chains and interrupt the direct bridging of the chains. As a result, they act as a gliding layer, and the close cohesion of chains and fibers is lost, thereby most of the strength. Therefore, if desired, wet strength must be ensured by adding cross-linking auxiliaries (wet-strength additives). Water ingress is particularly possible in amorphous areas, which can be exploited when producing specific water-soluble cellulose derivatives that act as glues and rheology modifiers. Sterically demanding substituents keep the cellulose chains artificially

6.7 Paper

� 571

Figure 6.17: Schematic representation of the bridging of two cellulose chains via hydrogen bonds. Left: in the dry state. Right: in the wet state. Blue: In the latter state, water molecules approach, push between the chains and disconnect them. In this way, the number of connections between cellulose chains or fibers, and thus the strength decreases.

separated from each other and prevent the formation of crystalline regions. The substituents are introduced by methylation or esterification of hydroxyl groups, ▶Section 6.4 at p. 558. The aging processes of cellulose, which lead to the decay of the paper and occur particularly in the acidic environment, will be discussed in ▶Section 6.7.8. Hemicelluloses Hemicelluloses are heterogeneous polysaccharides consisting of up to 200 monosaccharides linked by 1 → 4- and 1 → 3-glycosidic bonds and branched to a small extent, ▶Table 6.2 [173, Chapter 5], [176, Chapter 7.4], [268, 269]. The hexoses glucose (Glc), mannose (Man), and galactose (Gal), as well as the pentoses xylose (Xyl) and arabinose (Ara), appear as monomers. As functional groups, they possess primary and secondary hydroxyl groups, which are practically not dissociated but influence the properties of the hemicelluloses by forming hydrogen bridges. In xylans, the uronic acids glucuronic acid (GlcA), galacturonic acid (GalA), and 4-O-methyl-glucuronic acid (4-OMe-GlcA) appear, which are largely dissociated by their pKa of 4–5 at a pH value of 7 and contribute strongly to the anion activity of pulp. The linear glucomannans consist of the main chain of (1 → 4)-linked β-D-Manp and β-D-Glcp and carry side chains of β-D-Xylp or acetyl groups:

572 � 6 Structure of paint systems Table 6.2: The main hemicelluloses in wood and their composition [173, Chapter 5]. Wood type

Hemicellulose

Proportion %

Monomer

Molar Proportion

Bond

Softwood

Galactoglucomannan

5–8

β-D-Manp β-D-Glcp α-D-Galp O-Acetyl

3–4 1 1 1

�→� �→� �→�

Softwood

Glucomannan

10–15

β-D-Manp β-D-Glcp α-D-Galp O-Acetyl

3–4 1 0.1 1

�→� �→� �→�

Softwood

Arabinoglucuronoxylan

7–15

β-D-Xylp 4-OMe-α-D-GlcpA α-L-Araf

10 2 1.3

�→� �→� �→�

Larch

Arabinogalactan

3–35

β-D-Galp L-Araf

6

� → �, � → � �→�

Hardwood

Glucuronoxylan

15–35

Hardwood

Glucomannan

β-D-Arap

2–5

β-D-Xylp 4-OMe-α-D-GlcpA O-Acetyl β-D-Manp β-D-Glcp O-Acetyl

� � � �

�→�

10 1 7

�→� �→�

1–2 1 1

�→� �→�

The highly branched galactans have the main chain of (1 → 3)-linked β-D-Galp and side chains of β-D-Galp and α-L-Araf :

The acidic xylans possess a backbone of (1 → 4)-linked β-D-Xylp monomers and carry short side chains of 4-OMe-α-D-GlcpA, α-L-Araf or acetyl groups. They contribute decisively to the anion activity of pulp through their carboxyl groups:

6.7 Paper

�

573

Hemicelluloses are present in large quantities in raw or processed wood and are the primary source of the anion activity of the paper pulp, and thus responsible for technological effects, among others: – They influence the water absorption and swelling behavior of the paper. – They bind auxiliaries. Essential for us is the binding of cationic dyes. – They form a swollen viscous mucilage, which acts as an adhesive and considerably strengthens the fiber mass during drying. Hemicelluloses are among the valuable constituents preserved during wood processing whenever possible since their content of uronic acids makes them anionic polyelectrolytes. Unfortunately, since they are degraded via hydrolytic 1 → 4 cleavages more rapidly than cellulose, they are also one of the causes of paper aging. Lignin Lignin is a hard, highly polymeric phenolic substance that, as an extensive network, fills large parts of the wood structure as a fundamental matrix. Carbohydrates (cellulose, hemicelluloses) are embedded in this matrix. Wood gains structural strength from the compression-resistant lignin, while the tear-resistant and flexible carbohydrate fibers contribute to the high tensile strength like a concrete reinforcement. Basic building blocks of the high polymer amorphous polycondensate are C3 −C6 elements, so-called phenylpropanes [173, 762, 763], [275, Chapter 2.1]. The main building blocks are

In lignin, these building blocks are linked in various ways through carbon and ether bonds. The formula shows a possible section:

574 � 6 Structure of paint systems

It contains typical elements such as ether bridges, alcoholic and phenolic hydroxyl groups, and aryl-α-carbonyl structures, which are essential for the chemistry of the pulp and the finished paper. The aim of obtaining the so-called chemical pulp is to reduce the lignin content as far as possible since lignin forms complex colored quinoid compounds through oxidation in air. These are the primary source of yellowing phenomena in cheap woodcontaining (meaning lignin-containing) papers, ▶Section 6.7.10. As a hydrophobic and water-insoluble compound, lignin also prevents the formation of hydrogen bonds between cellulose fibers, reducing the paper’s strength. Color of wood The question of the nature of wood color is of practical importance since it directly influences the color of the pulp [174, Chapter 3.3, 3.4]. Wood has a yellow-brown color from the beginning, caused by specific chromophoric structures within the lignin network. In addition, wood, bark, and pulp can also darken by phenolic structures oxidatively reacting with oxygen to form quinones or with metal cations to form metal complexes. The primary chromophore in wood is coniferylaldehyde, which occurs at a frequency of 5 per 100 phenylpropane units in lignin. Due to the absorption maximum of 400 nm in the solid form, this compound absorbs blue light and imparts a yellowish color to the wood. α-keto structures, o- and p-quinones, and dienones also contribute to the color.

6.7 Paper

�

575

Free phenolic hydroxyl groups, traces of o- and p-quinones, and catechol structures can be converted simply to the corresponding quinones if oxygen is available (autoxidation). Typical structures include:

Quinones react with oxygen and water in a neutral or alkaline solution to chromophores that are more intensely colored. With other phenols, e. g., from water-soluble bark substances, further intensely colored structures can be obtained via autoxidation and phenol condensation, ▶Figure 6.18. At temperatures of 140–170 ℃ maintained for TMP production, even more chromophores with conjugated carbonyl and double bond structures form.

Figure 6.18: Formation of intensively colored structures from wood in an alkaline environment by autoxidation and phenol condensation [174, Chapter 3.4].

Cotton, linters Cotton fibers are seed hairs of certain members of the family malvaceae, i. e., the mallow family [184, Chapter 9]. The fibers used for textiles come from the highly visible white fiber tuft surrounding the mature seeds. Depending on the plant, these fibers are 22–36 mm in length and are readily spinnable to produce yarns and, eventually, fabrics. Each cotton fiber represents a single, long cell that develops in the surface layers of the boll. The matured fiber is dead and consists of a hollow, twisted cell wall tube.

576 � 6 Structure of paint systems The length of the fiber and its twists give cotton the typical grip that distinguishes it from other fibers. Chemically, cotton consists of > 95 % cellulose; the cotton fibers converge into polymer tangles, giving them high strength. This property also makes them extremely valuable as a raw material for artists’ papers where tear-resistance and durability are required. For artists’ papers, however, not only the long cotton fibers of the textile industry are eminent, but also a second type of fiber that grows inside the white fiber tuft directly at the boll tip. These are called linters. Their fibers are very short (12–15 mm), sometimes pigmented light brown, and have different chemical and physical properties. Thus, they are coarser, stiffer, and about twice as thick as textile cotton fibers. Consequently, they are not suitable for textile production but indispensable for pulp production.

6.7.2 Pulp from wood As shown above, paper can be made from various raw materials. The raw materials are first turned into a mash called the pulp, which shows typical properties according to the raw material and the manufacturing process. Then, depending on the desired paper grade, the pulp is processed into paper or blended with other pulp types. In many cases, the choice falls on wood as the cheap sole or primary raw material of the paper. After debarking the logs, there are two basic methods for processing the logs into pulp, the mechanical comminution of the entire wood log or the chemical isolation of the cellulose. In both cases, recovering the cellulose fibers contained in the wood is the objective of pulp production. The pulp exhibits specific properties characteristic of the production method, ▶Table 6.3. Table 6.3: General properties of mechanical and chemical pulp [174]. Mechanical pulp (MP, wood pulp)

Chemical pulp (CP, cellulose pulp)

High energy input

No external energy required

90–98 % yield based on wood

≈ �� % yield based on wood, approx. 50 % dissolved content

Contains all wood constituents except slightly soluble carbohydrates Fibers are stiff and unchanged (▶ body, volume,

Contains essentially cellulose, lignin is greatly reduced

Contains fibers, fiber fragments, and cell wall fragments (fines)

Contains essentially cellulose fibers

bending stiffness)

Less light in color, but opaque and smooth

Fibers are more flexible and can form quasi-crystalline regions (▶ strength) Light in color, solid

6.7 Paper

� 577

6.7.2.1 Mechanical pulp, wood pulp In mechanical pulp (MP) production, mechanical energy separates the cellulose fiber bundles, fibers, and fiber fragments from the wood matrix (lignin) and the cell walls [174]. The focus here is on preserving the amount of wood pulp to a large extent to increase the yield of paper pulp with adequate strength and brightness of the mash. The mechanical comminution can be done by grinding the logs or by refining, ▶Figure 6.19. Grinding wood, GW pulp By grinding the logs, we obtain wood pulp of the type ground wood (GW/GWD) and stone ground wood (SGW). During grinding, cellulose fibers and wood cell structures are torn off as a whole or as fragments. The result is a high proportion of fine fiber and cell wall fragments, the so-called fines. Pressure ground wood pulp (PGW) is produced similarly, but the logs are preheated at 100–140 ℃ and then pressurized ground. The grinding process is suitable for softwoods (spruce, fir) and hardwoods (beech, aspen). Refining wood, RMP wood pulp More gentle than grinding is crushing the wood into chips and separating the fibers by refining between roughened discs. This so-called refiner mechanical pulp (RMP) contains longer fibers, increasing the paper pulp’s strength significantly when the applied force is adjusted to comminute the wood gently. The thermo-mechanical pulp (TMP) is cautiously produced by treating the chips with hot steam at 115–155 ℃ and refining under pressure. Chemi-thermomechanical pulp (CTMP) is produced even more gently. Here, the chips are additionally impregnated with diluted sodium sulfite solution to dissolve the lignin and facilitate the release of the long fibers. Softwood is treated with 1–5 % Na2 SO3 solution at 120–135 ℃, hardwood with up to 3 % Na2 SO3 - and 1–7 % NaOH solution at 60–120 ℃. The chemical treatment reduces the yield by dissolving wood substances, especially resins. Other types, effects on fiber content Other pulp types differ from TMP and CTMP in applying pressure at the processing stages and combining heat and chemical treatment. Thermo-refiner mechanical pulp (TRMP) softens wood chips only with hot steam. Pressure refiner mechanical pulp (PRMP) is comminuted under increased pressure; as a result, it exhibits increased strength. In chemical mechanical pulp (CMP), the softening of the wood pulp takes place through higher concentrated sodium sulfite (5–10 %), and the optical properties deteriorate strongly. Chemical refiner mechanical pulp (CRMP) applies only mild chemicals for softening. In all cases, the thermal or chemical pretreatment influences the fracture behavior of the fibers during grinding or refining. With pretreatment or increased temperature, the fracture zone shifts to the lignin-rich zone of the middle lamellae, especially when

578 � 6 Structure of paint systems

Figure 6.19: Main steps in the production of mechanical wood pulp and its common types (bold: most common types) [830], [174, Chapters 2–4, 10].

6.7 Paper

�

579

the temperature becomes higher than the softening temperature of the lignin (for softwood such as spruce, about 125–145 ℃, for hardwood, about 20 ℃ less). As a result, fewer fines and more long fibers are produced, increasing the strength at the expense of opacity. The fines are considered an advantage of mechanical pulp because they are responsible for the opacity of the paper pulp and their good surface properties. The widespread size distribution of the fines causes large fluctuations of the refractive index and high scattering of light, i. e., high opacity. The high fiber stiffness in the mechanical pulp provides papers of low density or high volume, body, and high bending stiffness. If many stiff, unmodified fibers are present, many fines must be available to act as a binder virtually. A significant disadvantage of fines is that the lignin substances make the paper susceptible to light and aging. In practice, the paper composition depends on the application. Mechanical or chemical pulp or both are selected to combine the advantages of both pulp types. Impregnation/sulfonation In the production of CTMP, wood is treated at elevated temperature with a 1–5 % solution of sodium sulfite Na2 SO3 , which dissolves resin out of the wood. Consequently, by softening the lignin structure, easier separation of the fibers is achieved. The reaction proceeds at a neutral or slightly alkaline pH value within minutes [174, Chapter 3.5]. With a higher concentrated sulfite solution (5–10 %), chemimechanical pulp (CMP) is formed. As a strong nucleophile, the sulfite anion attacks α,β-unsaturated carbonyl structures in lignin, as they are present in coniferylaldehyde or o- and p-quinones. As a result, sulfonic acid groups add to the double bond or the quinone cores, increasing the water solubility of the lignin, ▶Figure 6.20.

Figure 6.20: Addition of the strong nucleophile SO2⊖ 3 to α,β-unsaturated carbonyl structures and o- or p-quinones under the formation of sulfonic acids [174, Chapter 3.5]. The sulfonic acids increase the water solubility of lignin.

580 � 6 Structure of paint systems Bleach The last step of pulp preparation is bleaching. In contrast to chemical bleaching, mechanical pulp bleaching must be carried out with the greatest possible care to preserve the lignin substances. This bleaching aims to convert chromophores in the wood into colorless forms, either to achieve uniform brightness or to increase the overall brightness. The brightness of the mechanical pulp is initially 60–63 % ISO; by bleaching, up to 80 % ISO can be achieved. The bleaching process is reversible, i. e., after some time, a coloration may occur again. Therefore, mechanical pulps are intended for applications more in the newspaper and magazine sector, not for fine office or even artists’ papers. As a result, wood pulp grades are available such as bleached CTMP (BCTMP), special bleached CTMP softwood (BCTMPSW), or bleached CTMP hardwood (BCTMPHW). Mechanical pulp bleaching is carried out reductively with sodium dithionite Na2 S2 O4 in a neutral environment or oxidatively with alkaline hydrogen peroxide H2 O2 [174, Chapter 3.6]. If the aim is only a uniform brightening of the pulp with dithionite, the reagent can be added to the pulp. When bleaching with peroxide, the reaction is carried out at 60–70 ℃ for 1–2 h in a separate stage. In a bleach with dithionite, only certain structures such as o- and p-quinones are reduced, ▶Figure 6.21. Therefore, only partial bleaching occurs, and a yellowish coloration remains.

Figure 6.21: Bleaching of mechanical pulp using dithionite S2 O2⊖ 4 [174, Chapter 3.6]. Dithionite can only reduce o- and p-quinones to colorless compounds.

The bleach with alkaline peroxide, on the other hand, allows a very high degree of bleaching. The reaction must be carried out carefully because the pulp reacts readily to dark substances by phenol oxidation in the alkaline environment. HO⊖2 is a strong nucleophile and attacks electron-deficient structures such as coniferylaldehyde and conjugated carbonyl compounds. The reaction breaks down these structures, leading to smaller aromatic aldehydes and formic acid, ▶Figure 6.22, top. Hydroquinones can form if free hydroxyl groups are present, ▶Figure 6.22, bottom.

6.7 Paper

� 581

Figure 6.22: Reactions in bleaching with alkaline peroxide [174, Chapter 3.6]. The strong nucleophile HO⊖2 attacks phenylogue double bonds and conjugated carbonyl structures. As a result, the lignin network breaks, and aldehydes and small molecules such as formic acid emerge; in either case, the chromophoric structure is destroyed. If free phenolic hydroxyl groups are present, hydroquinones can also occur.

The chromophores degrade in this type of bleaching. For example, the color-intensive quinones are degraded to carbonyl acids but can form colored hydroxylquinones in a competitive reaction toward the end of the bleaching reaction, when the concentration of the hydroperoxide ions HO⊖2 is low:

6.7.2.2 Chemical pulp The objective of chemical pulp (CP) production is to release the undamaged cellulose fibers from the wood’s lignin matrix and remove the lignin to avoid aging and yellowing effects. The fibers obtained by this method are more flexible than those of mechanical pulp, and the fibers can adhere well to each other, thus giving the pulp a high strength. Chemical pulp is, therefore, suitable for thick paper, sack paper, and cardboard paper. Pulp is the substance remaining when all compounds except cellulose and hemicelluloses are removed from the wood, especially lignin. In practice, the cellulose content of pulp is 60–85 %, besides a small amount of hemicelluloses and fiber wall lignin. Depending on the wood used (softwood, hardwood), the growing region (northern, southern), and the type of bleaching, different types of pulp are produced and sold under individual names by pulp manufacturers, ▶Figure 6.23.

582 � 6 Structure of paint systems

Figure 6.23: The main steps in the production of chemical pulp (cellulose) from wood, as well as the leading types of pulp traded worldwide [830], [174, Chapters 5, 6]. Bold: most common types.

6.7 Paper

� 583

The type of wood (softwood, hardwood) and the region in which the trees grow (north, south) significantly influence the properties, as they determine the exact morphology of the wood cells, and thus the cellulose fibers. Softwood yields tear-resistant long fibers that assemble well to form a uniform surface or dense quasicrystalline areas. Therefore, softwood pulp such as NBSK is suitable as a reinforcing pulp that can be added to a pulp mixture to give it strength. Papers become pliable and soft but tearresistant, and they do not have a large volume and are dense. Softwood pulp from southern regions provides fluffier qualities suitable for hygiene articles such as kitchen towels or diapers. On the other hand, hardwood pulp has short fibers that increase the paper’s stiffness, smoothness, and opacity. Since this pulp does not form as many quasicrystalline dense areas, a paper’s bulk and absorption capacity are increased. Lignin is the most reactive substance, followed by hemicelluloses and finally by cellulose. This fact is considered for pulp production [174], [177, Chapter 3]. In practice, two processes in particular have become established: Kraft cooking with NaOH and Na2 S (so-called white liquor) and sulfite cooking with H2 SO3 . From a chemical point of view, bond fractures aim to create lignin fragments in chemical pulp production. Then, charged functional groups are added to keep them in solution to separate the cellulose fibers. However, since the degradation processes are not 100 % lignin specific, the desired carbohydrates are likewise gradually attacked and degraded. Therefore, the process must stop when there is still some lignin left to avoid excessive loss of carbohydrates, especially cellulose. At this point, unbleached Kraft pulp (UKP) is present with the typical brown color of packing paper. After cooking, oxidative delignification with oxygen again significantly reduces the lignin content remaining from cooking before final bleaching, resulting in bleached Kraft pulp (BKP), ▶Figure 6.23. Kraft cooking Kraft cooking is the dominant process for producing chemical pulp [174, Chapter 5]. Reagents of the process are hydrogen sulfide anions HS⊖ for fragmenting the lignin and hydroxide anions HO⊖ for dissolving the fragments. In the nineteenth century, Kraft cooking served only to produce very thick unbleached papers, and sulfite cooking was the preferred process. The reason was the dark color of Kraft pulp. During the Second World War, however, a process was developed to efficiently bleach Kraft pulp with chlorine dioxide. Kraft cooking became the predominant process with this method and its superior technological advantages. The reaction runs in three phases. In the first phase, lignin and carbohydrates with comparable reactivity are degraded and dissolved. In the second phase, about 90 % of lignin is selectively extracted. The remaining lignin portion can only be further removed in the third phase at the expense of large parts of the carbohydrates (hemicelluloses) since carbohydrates exhibit high reactivity again. Kraft cooking must therefore stop when the last phase is reached. ▶Table 6.4 displays typical proportions before and after Kraft cooking. The result is light brown pulp, generically referred to as unbleached Kraft

584 � 6 Structure of paint systems Table 6.4: Exemplary amounts of cellulose, hemicelluloses, and lignin in untreated wood and after Kraft cooking [174, Chapter 6.5]. Fiber Cellulose Glucomannan Xylan Other carbohydrates Lignin

Pine Wood/%

Kraft pulp/%

Birch Wood/%

Kraft pulp/%

41 17 8 5 27

38 4 5 – 3

40 3 30 4 20

34 1 16 – 2

pulp (UKP) or, more specific, unbleached Kraft softwood pulp (UKSP) or unbleached Kraft hardwood pulp (UKHP), ▶Figure 6.23. For extraction, the lignin network must be fragmented and provided with functional groups that hold the fragments in the solution until the pulp is separated. Fragmentation occurs with HS⊖ in an alkaline solution on the main bonds of the individual phenylpropane units, the so-called β-O-4 structure. ▶Figure 6.24 represents the primary reaction. Degradation of carbohydrates via the peeling reaction The carbohydrates present in the wood suffer an approximate 10 % loss of substance during Kraft cooking due to the peeling reaction and the alkaline hydrolysis [174, Chapter 5.6]. The peeling reaction occurs on polysaccharides bearing substituents at the 4-position, such as glucomannans, xylans, and cellulose. From the reducing end, a C6 unit is cleaved from the cellulose through ring-opening and β-elimination. The remaining cellulose chain, shortened by one unit, carries a new reducing end and can again undergo the peeling reaction. The cleaved C6 unit rearranges to the stable isosaccharinic acid, which remains in the solution:

The result of the reaction is depolymerization. At a specific point in the degradation, the residual polymer goes into solution. In particular, glucomannans can be depolymerized entirely and dissolved with a polymerization degree of about 100 units. Because of its length, cellulose with a degree of polymerization more than ten times higher, despite

6.7 Paper

� 585

Figure 6.24: Main reaction in the Kraft process for dissolving lignin: cleavage of the β-O-4 structure by hydrogen sulfide HS⊖ . In the alkaline environment, equilibrium between the β-O-4 structure I and the quinone methide II is favored. In the presence of HS⊖ , a further equilibrium is established between the quinone methide II and a thioalcohol III, which corresponds to the educt I, but is more nucleophilic. In a nucleophilic substitution of a phenol unit by the thioalcohol, a thiirane IV and a phenol form irreversibly. Thus, the lignin network breaks at this point. Elimination of sulfur from the unstable thiirane leads to products such as coniferyl alcohol [174, Chapter 5].

depolymerization, shows no significant dissolution loss due to the peeling reaction. On xylans, the peeling reaction is limited by the substituents. Degradation of carbohydrates via alkaline hydrolysis At the maximum process temperature, alkaline hydrolysis of cellulose increasingly appears as a side reaction [174, Chapter 5.6]. By unspecific attack of a hydroxide anion, cellulose is strongly fragmented. ▶Figure 6.25 shows an exemplary reaction course, but the hydrolysis proceeds in a heterogeneous manner.

586 � 6 Structure of paint systems

Figure 6.25: Cleavage of a cellulose chain by alkaline hydrolysis [174, ch. 5.6]. The chain is fragmented nonspecifically, giving rise to a reducing end. The reaction can be run repeatedly, fragmenting the cellulose significantly.

Oxidative delignification and final bleaching of Kraft pulps The small amount (2–5 %) of residual lignin after Kraft cooking gives the pulp its typical brown color, as we know from packing paper [174, Chapters 9, 10]. To produce highquality white paper, exhaustive delignification and final bleaching to the desired brightness must now occur. A series of oxidations and extractions achieve this until an almost lignin-free pulp is obtained. In principle, the process is based on the oxidative ringopening of phenols and their conversion into soluble carboxylates:

An efficient oxidizing and bleaching agent was chlorine, which disproportionates in water to form hypochlorite ClO⊖ . After a process for producing chlorine dioxide ClO2 was found around 1940, the process sequence CEDED (chlorine, alkaline extraction, chlorine

6.7 Paper

�

587

dioxide) could lead to a fully bleached sulfate pulp. The CE-stage is the initial, actual oxidation stage, which performs the delignification, while the D-stage achieves the final whitening. Around 1970, oxygen slowly replaced chlorine as an oxidizing agent (O-stage) because of the discovery that small amounts of magnesium salts could protect the pulp from unwanted oxidative degradation. An O-stage can remove about 50 % of lignin. At the same time, chlorine dioxide gradually replaced chlorine. This development was accelerated by discovering the highly toxic dibenzodioxins, dibenzofurans, and several lower chlorinated aliphatics and aromatics in paper mill effluents. By about 1990, therefore, all C-stages were rapidly replaced by bleaching stages based on oxygen (O-stage), chlorine dioxide (D-stage), or hydrogen peroxide (P-stage). Since the desired target parameters, low residual lignin content, and high brightness cannot be achieved in a single step, several stages are combined to form process chains or bleaching sequences in practice. Typical sequences for the nowadays strongly demanded chlorine-free bleached pulps elemental chlorine-free ECF and total chlorine-free TCF are ECF: OD (OP)DD,

TCF: O Q(OP)Q(PO)

The initial OD or O-stages support oxidative delignification, in which most residual lignin is destroyed, followed by the final bleaching or whitening toward the target value. Typical process parameters are for the – O-stage: O2 gas, NaOH aq., Mg salts, 30–60 min at 90–100 ℃ and pH 10–11 – P-stage: H2 O2 , 1–3 h at 80–110 ℃ and pH ≈ 11 (transition metals must first be removed in a Q-stage by complexation) – Q-stage: EDTA or DTPA, 5 min–2 h at 50–90 ℃ and pH 4–7 – D-stage at the beginning of a bleaching sequence (D- or OD-stage): ClO2 , 30–45 min at 65–75 ℃ and pH 2–3 (ClO2 efficiently degrading lignin; no bleaching is achieved in the process) – D-stage at the end of the bleaching sequence: ClO2 , 3 h at 70–80 ℃ and pH 3.4–4.5 (ClO2 efficiently lightening the lignin-poor pulp) The result is light-colored pulp, which is generally called bleached kraft pulp (BKP) and marketed as bleached softwood kraft (BSK) or bleached hardwood kraft (BHK). Depending on the species and origin of the wood, further sales types are derived from these basic types, ▶Figure 6.23. If bleaching is carried out without elemental chlorine or completely chlorine-free, the pulp is additionally classified as ECF or TCF pulp. Sulfite process Digestion of the wood material is carried out in the sulfite process with CaSO3 , MgSO3 , Na2 SO3 or (NH4 )2 SO3 under acidic conditions [174, Chapter 5.11]. The central reaction is the sulfonation of a phenylpropane unit with SO2⊖ 3 , increasing the solubility with each

588 � 6 Structure of paint systems additional sulfonic acid function. In the strongly acidic pH range, a high degree of sulfonation can be achieved; possibly, acidic hydrolysis of ether bonds in lignin and carbohydrates occurs as a side reaction. The result is raw pulp, marketed as unbleached softwood sulphite (USS) for softwood, ▶Figure 6.23. Specifically, sulfonation starts with an initial protonation of a benzyl alcohol group II, followed by elimination of alcohol and addition of the sulfite anion, producing a sulfonic acid III, ▶Figure 6.26(a).

Figure 6.26: Lignin-dissolving reactions in the sulfite process [174, Chapter 5.11]. In the acidic pH range, the reaction sequence starts with the protonation of a benzyl alcohol group, followed by the elimination of alcohol, and finally, the addition of a hydrogen sulfite anion HSO⊖3 .

6.7 Paper

� 589

In the alkaline pH range, the phenolic hydroxyl groups of I are involved in the reaction, ▶Figure 6.26(b). The alkaline catalyzed elimination of an alcohol leads to the quinone methide II, which adds a sulfite anion and becomes the sulfonic acid III. Through the attack of a further sulfite anion, the lignin network of III is broken, and a disulfonic acid IV and a new phenol end group form. Acidic hydrolysis of the carbohydrates also occurs during sulfite cooking, especially in the case of glucomannan and xylan losses of up to 70 % are possible. Oxidative delignification and final bleaching of sulfite pulps Residual lignin also remains in the pulp obtained during sulfite cooking. However, the pulp is lighter in color than raw Kraft pulp and more accessible to bleach. For TCF grades, e. g., sequences are used such as (EOP)Q(PO) ZEP (EO)P (Z-stage: ozone bleaching, E-stage: alkaline extraction). The result is light-colored pulp sold as bleached softwood sulphite (BSS) or bleached hardwood sulphite (BHS), ▶Figure 6.23. If the bleaching is done without elemental chlorine or is completely chlorinefree, the pulp is additionally classified as type ECF or TCF. 6.7.3 Composition and manufacture of paper As a starting product of paper production serves a pulp of the fiber material mentioned before [175, 176], [177, Chapter 6.1], [822]. Depending on the desired paper grade, the mash contains rags or linters, mechanical pulp, or cellulose. The paper pulp shows a more or less sizeable anionic charge, which is essential for further processing and the fixation of substances. The causes of the charge are [178, Chapter 3.3]: – the small natural proportion of carboxyl groups in cellulose – the natural proportion of acidic hemicelluloses, which accounts for a large part of the activity – the high proportion, depending on processing, of phenolic hydroxyl and sulfonic acid groups in the lignin content – a variable proportion of degradation products of cellulose formed during bleaching The raw material mash, which has a solids content of about 0.1–1.5 %, is poured through a gap onto sieve-like screens that rapidly move under the discharge gap. In the screens, most of the water flows off, the solids content increases to approximately 20 %, and the fibers become matted. This process is supported by subsequent rolling and pressing between several rollers, removing the residual moisture. During pressing, felts or stencil

590 � 6 Structure of paint systems grids can imprint a texture, e. g., a rough, matte, linen-like, or ribbed one. In producing high-quality artists’ papers, round screens are also used. The result is raw, highly absorbent paper with an uneven surface that, in general, will then be sized, coated, and possibly calendered. Depending on the intended use of the paper, additives in the paper pulp adapt the properties of the paper. These include – fillers – binders – retention aids – dry-strength additives for improvement of stability in the dry state, wet-strength additives for the same in the wet state – dispersants to support a homogeneous pulp – pigments, dyes, and fixatives, if bulk coloring is desired – glue, if bulk sizing is desired It is possible to add the necessary chemicals directly to the pulp (bulk sizing and coloring) rather than in a subsequent separate stage (surface sizing and coloring). Many additives discussed below have more than one function in the papermaking process; ▶Table 6.5 contains an overview. Fillers Fillers are finely ground mineral and synthetic substances added to the paper pulp. They change the properties of the paper to a large extent [177, Chapter 6.1], [179, Chapter 2.2.1]. An important use case is to increase the quantity of paper by adding cheap fillers to reduce the product’s price. Increasingly important is the improvement of properties such as – optics of a paper sheet (brightness/whiteness, opacity/transparency in wet and dry state, tinting, or coloring) – structural condition (dimensional stability, weight) – surface texture (smoothness, uniformity, printability, writability) Suitable materials are kaolin Al4 (OH)8 (Si4 O10 ), natural calcium carbonate (chalk, calcite, limestone, ground calcium carbonate GCC), or precipitated calcium carbonate (PCC), talcum Mg3 (H2 O)(Si4 O10 ), silicon dioxide in various forms, calcined alumina Al2 O3 /SiO2 , and other aluminum-magnesium silicates, barite or synthetic barium sulfate, gypsum, anhydrite, alumina Al(OH)3 and Al2 O3 ⋅ H2 O. In addition to light-colored fillers, white pigments such as titanium dioxide are often added to the white paper to increase its brightness and opacity further. This admixture is particularly necessary for papers that must be opaque white, even in wet or oily environments. Furthermore, with white pigments, high scattering is desirable. For pigments and fillers alike, the following applies: the higher the refractive index to air, water, and

+++

+++

+++

PEI

pDADMAC PEO

+

PVOH-PVOAc

PVAm-PVF

+++

PAAE resin

+++

+++ (Mr medium, Q medium) +++ (Mr medium, Q medium)

+

PAM anionic PAM cationic

+++

Cellulose – CMC

+++ + +

+++

+++

Starch – cationic – anionic – neutral

Strength additive

Gums

Binder

Polymer

+++ (Mr high) +

+++ (Mr high, Q high)

+++ (Mr high)

+++ (Mr high, Q low-medium) +++ (Mr high, Q low-medium)

+++

+++

+++

Retention aid

+++ (Mr low, Q high)

+++ (Mr low, Q high)

+++ (Mr low, Q high)

+

+

+

Fixative

+++ (PVOAc high)

+

Sizing agent

+++ (Mr low, Q high)

Dispersant

Table 6.5: Functional ranges of various polymer additives in the context of papermaking. +++: frequent use, +: also used, Mr : molecular weight of the polymer, Q: charge of the polymer.

6.7 Paper � 591

592 � 6 Structure of paint systems binder, the more opaque the pigment, ▶Section 1.6.8 at p. 67. Since fillers such as chalk or kaolin, as well as the pulp, have a refractive index of n ≈ 1.6, they are opaque white only in the air (n = 0) but not in aqueous, oily, or resinous media, whose refractive indices are also around 1.6. For wet opaque papers, therefore, highly refractive, although more expensive, white pigments such as anatase (n = 2.5) or rutile (n = 2.7) are necessary. Colorants Colorants (color-strong pigments or dyes) can be added to the paper pulp to color the whole mass. Organic pigments are transparent; inorganic pigments have an opaque effect due to their scattering capacity. They offer insolubility, lack of migration tendency, and fastness to solvents, which originate from printing inks. Typical inorganic pigments are iron oxides (yellow, orange, red, brown, black), chromium(III) oxide (green), soluble iron blue (blue, in the paper pulp), insoluble iron blue (blue, as a coating), and mixed oxide pigments (many colors), in addition to organic azo and phthalocyanine compounds, and carbon black [203, keyword “paper”]. Current assortments of colorant suppliers contain mainly organic azo and phthalocyanine compounds. The binding of the colorants to the fiber takes place in several ways: – mechanical retention through the paper felt – adsorption on the fiber by van der Waals forces and ionic bonding – entrapment by other particles (colloids such as aluminum hydroxide or resin precipitates) – flocculation, i. e., by binding to the fiber using a retention aid The dyeing of paper and the dyes suitable for this purpose are discussed in detail in ▶Section 5.2. Adhesion mechanisms between the dye and the paper fiber are the subject of the following sections, separated by dye class. Brighteners raise the whiteness of the paper by fluorescence. Alternatively, ultramarine blue compensates for a yellowish tint of the paper pulp. Retention aids At the production beginning, the pulp is a very dilute, aqueous mixture with about 0.1–1.5 ̇% solids (cellulose, fillers). Then it gradually is concentrated by sieving and pressing until it becomes a moist paper felt. In the process, most of the constituents can be lost with the water through the comparatively coarse-meshed screens (0.1-millimeter range). Since some fillers are expensive, manufacturers avoid removing them from the paper pulp during filtration with the wastewater by adding retention aids, [822], [177, Chapter 6.7], [826, Chapter 14], [180, p. 76ff], [181, Chapter 17]. Together with the anionic cellulose on the one hand and the fillers, on the other hand, these agents form flocs. They are easier to settle or filter in the wires, and thus remain in the paper to a greater extent. Initially, the partners are only loosely held together by ionic bonds. However, polymeric bridging agents attach themselves more tightly to the partners and draw the particles together until they also come into contact and form bonds. If the paper pulp contains

6.7 Paper

� 593

Figure 6.27: Polymers used as retention aids in papermaking [822], [177, Chapter 6.7], [826, Chapter 14], [180, p. 76ff], [181, Chapter 17].

fines, these initially form small mobile “sticky” balls, which bind the larger particles together. Chemically, retention aids can have different compositions: – They consist of silica or silicate-based microparticles made from water glass. – They can be built from aluminum salts and inorganic compounds such as aluminum sulfate, sodium aluminate, poly aluminum chloride (PAC), and bentonite. For the production of acid-free paper today, chalk (CaCO3 ) is preferred. – Polymers are applicable, forming long bridges between partners and becoming chain-like with higher molecular weight; ▶Figure 6.27 shows examples. Some

594 � 6 Structure of paint systems polymeric retention aids such as polyDADMAC or PEI can also act as fixatives or strengthening additives; see below. However, unlike these, they have a high degree of polymerization, and thus can form the necessary bridges between the partners. In addition, for fixation on the fiber, they are usually weakly cationic: – cationic starch, cationic galactomannans (guarans), carboxymethyl cellulose (CMC) – polymeric diallyl dimethyl ammonium chloride (polyDADMAC) – polyacrylamides (PAM) with high molar mass (10 million), used prevalently cationic modified, also neutral or anionic – polyethylenimine (PEI), neutral or cationic – polyamideamine (PAmA), polyethylene oxide (PEO) – polyvinylamine (PVAm), due to production by incomplete hydrolysis also as a copolymer with polyvinylformamide PVAm-PVF Anionic polymers such as anionically modified polyacrylamide are used less frequently. However, they are effective because they can form strong hydrogen bonds to cellulose. The aluminum salts frequently used as flocculants until the last century, mostly aluminum sulfates, hydrolyze slowly in the pulp and even later. In the process, they form sulfuric acid, so the paper becomes acidic. This acidification is of great concern from an archival point of view since acids promote paper decay, ▶Section 6.7.8. Therefore, calcium salts such as chalk (CaCO3 ) are employed today. They remain in the paper as an alkali reserve to slow future acidification when used in excess. polyDADMAC is a polymer of diallyl dimethyl ammonium chloride (DADMAC) and possesses a permanent cationic charge through which it adsorbs to anionic fibers and particles:

Polyacrylamides are neutral by nature but are usually cationic modified by copolymerization with acryloxyethyl trimethyl ammonium chloride or trimethyl ammonium propylacrylamide. Anionic PAM is obtained by copolymerization with acrylic acid. Despite its charge, which corresponds to cellulosic fibers, the adhesion is good since the amide form strong hydrogen bonds with cellulose. Polyethylenimine represents a polymeric secondary amine with a high number of charged sites. Therefore, the effective charge of this agent is higher, the lower the pH value is

6.7 Paper

� 595

Strengthening additives The composition of paper significantly determines its strength. If we have a choice, we can use longer cellulose fibers, add fewer fillers, and avoid acidic pH values to increase the strength properties of the paper. If this is not possible, additives will help: Dry-strength additives increase dry strength, tear strength, and abrasion fastness in dry paper and allow the use of lower-quality pulps [176, Chapter 6], [178, Chapter 5], [177, Chapter 6.3], [179, Chapter 3.6.5], [826, Chapter 16], [181, Chapter 13]. Wet-strength additives increase the strength in the wet state [176, Chapter 7]. The additives are necessary for combinations of properties that would otherwise be difficult to obtain, such as high tear strength at high porosity, where bonding would not be an option, or tear resistance in the wet state. A wide range of substances is applicable, ▶Figure 6.28: – cationic modified starch – unmodified starch, starch paste, modified starch (oxidatively or enzymatically degraded), anionic starch (*) – vegetable gums – anionic (*) and cationic polyacrylamides (PAM) – soluble cellulose derivatives such as CMC (*) – synthetic polymers such as polyethyleneimines (PEI), polyvinylamine-polyvinylformamide copolymers (PVAm-PVF), or polyvinylalcohol-polyvinylacetate copolymers (PVOH-PVOAc) The starred agents (*) require the addition of cationic fixatives. The purpose of the additives is: – to maintain existing bonds between the cellulose chains and fibers (e. g., hydrogen bonds) by preventing the fiber from swelling when wet and losing the close contact between the cellulose strands – to form further (water-resistant) bonds, ideally of a covalent nature The additives diffuse into the fibers and between the cellulose chains to protect existing bonds. There, they intertwine and cross-link with themselves and the carbohydrates, fixing the chains, even in the presence of water. In addition, the additives react with hydroxyl or carboxyl groups of the carbohydrates to form new, covalent bonds. All agents have in common a polymeric character with long water-soluble and preferably cationic chains to build bridges between cellulose fibers and cross-link

596 � 6 Structure of paint systems

Figure 6.28: Polymers used as strengthening agents in papermaking [178, Chapter 5], [177, Chapter 6.3], [179, Chapter 3.6.5], [826, Chapter 16], [181, Chapter 13]. The structure of starch is shown only schematically.

6.7 Paper

�

597

them. Bonding to cellulose occurs through van der Waals forces, hydrogen bonds, and ionic bonds. Starch is one of the oldest agents for strengthening paper and is still widely used today. However, unmodified starch cannot be applied since native starch does not show any adhesive effect and does not adsorb to the fiber material. The reason is the structure of the starch molecules. In native starch, they form compact grains insoluble in water, resulting in a thin suspension with no adhesive effect since the supply of starch molecules cannot be set into effect. Starch is, therefore, first opened up. In purely physical terms, hot steam achieves this opening up, which causes the starch granules to swell and burst due to water absorption so that the released starch molecules gelatinize. In the process, part of the starch, amylose, dissolves, and the viscosity of the suspension considerably increases while crystalline parts of the starch structure melt. In essence, hydrogen bonds between glucose molecules are cleaved, ▶Figure 6.17 [280, Chapter 4.4.4.14]. The exact process occurs in the thickening of sauces: Starchy thickeners must boil before they develop their adhesive effect. The following applications result (▶Table 6.6): – use of the (neutral) starch paste – production of cationically modified starch – production of anionically modified starch Neutral or anionically modified starch bound, for the most part, only reversible to the cellulose fiber. Cationically modified starch possesses the most significant strengthening effect; approximately 1–5 % of the primary hydroxyl groups are cationized. It is irreversibly adsorbed on the cellulose fiber; besides hydrogen bonds, mainly ionic bonds between starch cations and the anion activity of the cellulose are involved in the binding. The effectiveness of starch resides in the fact that it provides new binding sites after its binding. In addition, it effectively increases the number of fiber-fiber hydrogen bonds, which require sizeable spatial proximity of the partners (typical distances are 0.3 nm). This proximity is rarely achieved in the coarse fiber felt, but starch can fill the wide gaps between the fibers with a bonding matrix and cross-link them. Quaternary ammonium salts or protonated tertiary amines act as cations, which are introduced into the starch molecule by reactive monomers such as N,N-diethylaminoethyl chloride, or 2,3-epoxypropyl trimethylammonium chloride:

Charge

None None

Cationic

Anionic

None

Anionic

None None None None

Modification

Native – gelatinized

Cationic

Anionic

Etherified

Oxidized

Degraded (hydrolyzed) – thin boiling – thermal – pyrolytic – enzymatic High–medium Medium–low Medium–low Medium–low

High, possibly medium

High

High

High

High High

Molar mass

Table 6.6: Starch and key technical modifications [277, 278, 280].

Lower Lower Lower Lower

Lower

Lower

Lower

Lower

High High

Viscosity

Mild acid HCl diluted H� PO� , H� SO� , HCl dilute Heat, dry, some acid Enzymes

NaOCl, H� O� , MnO⊖�

Ethylene oxide, propylene oxide

NaH� PO�

2,3-Epoxypropyl trimethylammonium chloride, N,N-diethylaminoethyl chloride

Reagent

Glucose, maltose, higher saccarides

Example

598 � 6 Structure of paint systems

6.7 Paper

� 599

The direct use of the high molecular weight neutral starch paste encounters processing problems due to its high viscosity. Consequently, further processing steps are required, leading to products with lower viscosity and better suitability for machine application [277, p. 50ff, Chapter 6.1], [278]. The treatment with enzymes, heat, or dilute acids hydrolytically degrades starch to medium and small fragments (dextrins). Exposure to sodium phosphate leads to phosphoric acid starch esters of anionic character. Thermally assisted oxidation with hydrogen peroxide, hypochlorite, ammonium persulfate, or permanganate converts the hydroxyl groups at C6 , C2 , and C3 (possibly opening up the glucopyranose ring) into aldehydes or anionic carboxyl groups, depending on the pH value. Starch can significantly degrade during the process depending on the reaction procedure’s settings [277, 829]. The adsorption of neutral or anionic starch onto the fiber material, and thus the stiffening effect increases if starch couples to the fiber with cationic polymers such as protonated polyvinylamine [179, Chapter 3.4.2]. Plant-derived gums such as guaran and locust bean gum are predominantly used. The basic structure of these gums, with a molecular weight of 200 000–300 000, is a chain of mannan with short side chains of galactose [280, Chapters 4.4.4.9, 4.4.4.10]. They are employed for unbleached Kraft papers. The hydrophilic gums are similar in structure to cellulose. Adsorbed through van der Waals forces and numerous hydrogen bonds to the fibers, they have a strengthening effect by increasing the contact between the fibers after drying, reinforcing the fiber-fiber bonds, and improving their formation. To a certain extent, gums also have a cross-linking effect. Since natural plant gums are nonionic, they are not optimally adsorbed on the fiber. Therefore, cationic modified derivatives are preferred. Polyacrylamides (PAM), which we have already met as retention aids (see above), contain about 10 % ionic monomers in addition to acrylamide. Cationic PAM are copolymers with ammonium salts such as acryloxyethyl trimethyl ammonium chloride or methosulfate H3 COSO2 O⊖ , as well as DADMAC, 3-acrylamido-3-methylbutyl trimethylammonium chloride, and vinylbenzyl trimethylammonium chloride. Anionic PAM are obtained as copolymers with acrylic acid and require a cationic fixative to bind them to the anionic fibers. Possible fixatives include aluminum sulfate or cationic polymers such as polyamidoamine-epichlorohydrin resins (PAAE resins, see below), serving as wet-strength additives.

600 � 6 Structure of paint systems PAM have a strengthening effect, as they offer new, flexibly positioned binding sites for fiber-fiber hydrogen bonds when the distance between the fibers is too considerable for forming direct hydrogen bonds. Hydrogen bonds between the amide group and cellulose hydroxyl groups are even more potent than those between cellulose hydroxyl groups themselves:

The acrylate chain is long enough to provide the necessary binding sites for the desired strength and bridge the distance between fibers. However, it is not so long that it bridges fibers widely itself. However, bridging is possible; PAM then acts as a retention aid. Soluble cellulose derivatives such as CMC also act as fixatives through their long chains and require a cationic fixative like PAAE resin. Polyvinylamines (PVAm) form from polyvinylformamide (PVF) by hydrolysis so that they can be used alone or as copolymer p(VAm-VF). PVAm forms hydrogen bonds with cellulose via the amino group and has a cationic effect in acidic conditions as an ammonium salt. PVAm can also be used as a retention aid or a fixative, depending on the electrical charge and chain length. Polyethylene imines (PEI) can also be used as retention aids or fixatives, depending on its electrical charge and chain length. Polyvinylalcohols (PVOH) form from polyvinylacetate PVOAc by hydrolysis and act as copolymer p(VOH-VOAc). The higher the remaining proportion of PVOAc, the more hydrophobic the polymer and the higher its suitability as a sizing agent. PAAE resins belong to a class of polymeric compounds that are used, among other things, as wet-strength additives [176, Chapter 7]. These additives consist of a polymeric backbone that is cationic functionalized. Three types of polymers with amine functions functionalized with epichlorohydrin are chiefly used: Poly(amido amine)

Poly(alkylene polyamine)

▶ PAE/PAAE

▶ PAPAE

▶ PAE/PAAE

Polymer amine

Reactive group

▶ APE

3-Hydroxy-azetidinium chloride Glycidyl-ammonium, (2,3-epoxypropyl)ammonium

Functionalization of the secondary amino groups of the polymer with epichlorohydrin leads to equilibrium in an aqueous solution between the open-chain amino chlorohydrin form and the cyclic azetidinium chloride form:

6.7 Paper

� 601

The azetidinium form has the necessary cation charge and a high ring tension, making it very reactive. The reaction of tertiary amino groups of the polymer yields cationic glycidylammonium compounds. PAE or poly(amidoamine)-epichlorohydrin resins, PAAE are the most important additives of this group. They are prepared from epichlorohydrin and a polyamidoamine; the latter is derived from a dicarboxylic acid and a polyalkylene polyamine. Common components are adipic acid and ethylenediamine or diethylenetriamine:

PAAE resins are cationic, possessing charged azetidinium groups due to epoxidation. The resins are reactive due to the high ring tension and can form covalent bonds (▶Figure 6.29) with – other polymer chains (self-cross-linking) – cellulose chains, especially the carboxyl groups (cross-linking) – water

602 � 6 Structure of paint systems

Figure 6.29: Reaction possibilities of a polymer amine-epichlorohydrin (ECH) resin in the azetidinium form [176, Chapter 7]. Cross-linking opportunities are particularly interesting to the function as a strengthening agent (wet-strength additive).

After the paper is scooped, the cross-linking products contribute to the strength in both dry and wet states. PAPAE resins (poly(alkenylpolyamine)-epichlorohydrin) are products of the direct reaction of polyalkenyl polyamines with epichlorohydrin; APE resins are those of the reaction of amine polymers with epichlorohydrin. Fixatives Fixatives constitute an important part of the pulp [203, keyword “paper”], [177, Chapter 6.8],[179, Chapter 3.6.1], [826, Chapter 14], [180, p. 74ff]. Their task is to permanently fix ionic fillers and dyes to the fiber by forming ionic bonds. This function is crucial when, e. g., the dye does not have a natural affinity for the fiber, has too low an affinity, as is the case with acid dyes, or if the dye charge has incorrect polarity. Such dyes must be specifically fixed to avoid premature washout during the manufacturing process. Since the substances to be fixed are generally anionic, cationic fixatives are needed. They can take effect in two ways. In the first case, the anionic fiber is recharged by cou-

6.7 Paper

� 603

pling the fixative to the fiber, giving the fiber a net positive charge on the outside. This way, a dye anion can couple to this cationic charged fiber. In the second case, the dye anion is first recharged by forming a cationic complex with the fixative, which then couples to the anionic fiber:

Fixatives (▶Figure 6.30) are small molecules with a higher charge density than the substance to be fixed. Therefore, they have low molecular masses and a high number of charges. Cationic fixatives are – aluminum salts, especially Al2 (SO4 )3 or poly aluminum chloride PAC, sodium aluminate NaAl(OH)4 , formerly alum – cationic polymers such as polyDADMAC, polyacrylamido epichlorohydrin resins (PAM resins), and polyamidoamine-epichlorohydrin resins (PAAE resins), retaining their charge regardless of the pH value – polyvinylamine (PVAm) and polyethylenimine (PEI), which are charged in the acidic pH range – dicyandiamide/formaldehyde resins We are already familiar with them as retention aids and strengthening agents (see above). Some polymeric fixatives such as polyDADMAC, PVAm, or PEI can also act as retention aids (see above). However, unlike these, fixatives have a low degree of polymerization and high charge. Examples of the less commonly used anionic fixatives commercially offered as polymers are [203, keyword “paper”]: – methylene-bridged condensation products of arylsulfonic acids and hydroxyarylsulfones (a commercial product is, e. g., MESITOL® ) – condensation products of naphthalenesulfonic acid with formaldehyde, forming lakes with the dye [827, p. 5] (a commercial product is, e. g., Tamol® ) Condensation products of hydroxyarylsulfones and arylsulfonic acids with formaldehyde have a similar structure to the resol resins. Due to the reactivities of the reactants

604 � 6 Structure of paint systems

Figure 6.30: Polymers used as fixatives in papermaking [203, keyword “paper”], [177, Chapter 6.8], [179, Chapter 3.6.1], [826, Chapter 14], [180, p. 74ff]. Some compounds carry inherent charges, while others attain charge at appropriate pH.

(sulfonic acids have a meta-directing effect, phenol groups ortho-/para-directing), we can expect structures similar to the following ones (the dots marking the reactive sites):

6.7 Paper

� 605

Aluminum sulfate, formerly alum, is the best-known agent for fixing resin glues. It hydrolyzes in an aqueous solution to form aluminum ions and sulfuric acid:

A pH value of around 3.5 is established during the reaction. These acidic conditions lead to paper decay (▶Section 6.7.8) and are increasingly avoided today. Polyethylenimine is a polymeric secondary amine with a high number of charged sites. The effective charge of this agent is higher the lower the pH value is

Dicyandiamide resins are made from dicyandiamide, formaldehyde, and ammonium chloride [828, p. 21] and have a permanent cationic charge:

6.7.4 Sizing and coating The plain paper obtained after pressing and drying has a rough surface and is interspersed with numerous capillary cavities in the fiber felt. In conjunction with hydrophilic fibers, these cavities lead to high water absorption and absorbency in plain paper. Therefore, it can be used as blotting or filtering paper in this form, but not yet for writing, printing, or drawing. For such papers, the writing or printing inks or colors must be prevented from bleeding. Furthermore, to obtain a sharp and clean appearance of the printed or written typefaces, they are surface-coated to achieve a defined degree of absorbency, ink affinity, smoothness, and opacity [177, Chapter 6.2], [188, 822], [181, Chapters 16, 20]. Two processes are often combined:

606 � 6 Structure of paint systems

Figure 6.31: Influence of sizing on aqueous writing fluids (blue).

–

–

Sizing was already applied to the first papers since it renders the surface hydrophobic, reduces capillarity and absorbency, equalizes pore size, and increases the smoothness of the paper. ▶Figure 6.31 depicts the process. Untreated paper (a) has high absorbency and quickly absorbs the writing fluid. The fluid runs over a wide area and bleeds out. Since narrow capillaries strongly attract liquids, differently-sized capillaries in the paper felt lead to feathering and blurred edges. The paper is partially hydrophobic in the ideal state, and the ink stands well and sharply on the paper (b). As a result, the capillary sizes are equalized by the sizing agent’s partial filling of the pores, and the feathering is reduced. Excessive hydrophobicity results in repulsion (c): the writing fluid is repelled from the paper and forms tiny isolated droplets. Such behavior may be desirable for specialty papers such as oiled or grease-proof paper. Sizing also seals the fiber-fiber binding sites and increases or stabilizes the strength of the paper. Coating is a surface finishing carried out with a particular coating color. The aim is a closed, smooth surface with uniform pore size, a high degree of whiteness and opacity, and a certain feel due to fillers and colorants. The coating also increases writeability or printability.

In both processes, fiber cavities in the paper felt, which act as capillaries, are controlled; large voids are closed with fine-grained fillers or sizing agents, and small ones are brought to as uniform a size as possible. A minimum absorbency must remain so that writing or printing ink can penetrate the paper to achieve adequate adhesion. The hydrophobic coating prevents the excess penetration and bleeding of the predominantly aqueous or hydrophilic inks. In the case of copying paper, the hydrophobic coating ensures good adhesion of the hydrophobic toner. The coating can also be enhanced with chemical auxiliaries to adhesion to the writing, drawing, or printing inks.

6.7 Paper

� 607

Sizing agents For the hydrophobic modification of paper, we can resort to sizing agents, which often simultaneously perform the functions of retention aids, fixatives, or strengthening additives. Their history is as old as that of paper: Rice starch was already applied as a sizing agent 2000 years ago. Around 1280, sizing with animal glue and alum developed, from which the name of the process was derived. It remained the leading process until 1806. Around this time, the innovation of resin soap sizing was introduced; it was not until 1950 that modern polymers began to appear. Modern agents are presented in ▶Figure 6.32 [175, Chapter 14], [177, Chapter 6.2], [822], [180, p. 63ff], [179, Chapter 3.6.4], [223, Chapter 5.2], [181, Chapters 16, 20]:

Figure 6.32: Agents used for paper sizing [175, Chapter 14], [177, Chapter 6.2], [822], [180, p. 63ff], [179, Chapter 3.6.4], [223, Chapter 5.2], [181, Chapters 16, 20]. Traditional agents such as gelatin or animal glue are still used for artists’ papers. MS: maleic acid, MSA: maleic anhydride.

608 � 6 Structure of paint systems – – –

–

tree resins (bulk sizing) reactive monomers such as alkylketene dimer AKD, alkenyl succinic anhydride ASA (bulk and surface sizing) polymers such as styrene-acrylate copolymers, styrene-maleinate copolymers, or polyurethanes of toluene diisocyanate and glycerol monostearate (surface, but also bulk sizing) starch paste and cationic, oxidized and enzymatically, thermally, or acidically degraded starch, ▶Table 6.6 at p. 598 (surface sizing, less used today)

Starch derivatives, however, are used more often as strengthening agents. The resins are alkaline saponified balsamic resins (▶Section 7.4.8), i. e., alkali salts of the resin acids abietic acid, levopimaric acid, and dehydroabietic acid. Some resin acids can participate as a diene in a Diels–Alder reaction with maleic anhydride and react to a modified resin acid, which contains two additional carboxyl groups for anchoring after hydrolysis of the anhydride:

The resin acid anions are precipitated by AlIII salts (aluminum sulfate, poly aluminum chloride, alum) [826, Chapter 17]. Significant to the reaction is a series of complexes that aluminum forms in an aqueous solution:

6.7 Paper

� 609

In the required acidic pH range, resin acid anions build a cationic aluminate complex ionically adsorbed on the anionic fiber (ABA = abietic acid anion). Zerler and Kästner [177, Chapter 6.2] and Roberts [178, Chapter 7.3] elaborate this further:

According to [175, Chapter 14.3.2], polynuclear AlIII complexes may also be involved in binding the cellulose to resin acids, e. g., a trinuclear complex such as structure II:

Polymers can be cationic or anionic modified. For bulk sizing, cationic polymers are preferred because they adhere ionically to the fiber material. For surface sizing, both polarities are in use. Polymers act by forming a mosaic of hydrophilic and hydrophobic regions on the paper, thus increasing the wettability. When used as copy paper, the hydrophobic areas can improve the adhesion of the toner resin. As hydrophobic polyacrylates, copolymers of methyl methacrylate, n-butyl acrylate, and styrene p(MMA-BA-styrene) are employed. As in acrylic paints, they form a film and adhere via electrostatic and van der Waals forces to the paper’s surface. Anionic polymers are obtained with small amounts of acrylic acid or maleic anhydride; cationic polymers with tertiary or quaternary amino compounds. A well-known example of a cationic monomer is acryloxyethyl trimethyl ammonium chloride:

610 � 6 Structure of paint systems

Due to the possibility of combining all components in varying amounts, we obtain a range of polymeric sizing agents: Anionic:

p(AA-MMA-BA-styrene) p(AA-styrene-maleic acid) p(AA-styrene-maleic acid monopropylester) p(styrene-maleic acid-maleic acid monoisopropylester-diisobutylene)

Cationic:

p(MMA-BA-styrene-acryloxethyl trimethylammonium chloride) p(styrene-acrylonitrile-acrylamide)

The transition to styrene-maleic acid copolymers p(styrene-maleic acid) is smooth. This polymer, possibly with olefins, acrylic acid, and acrylic acid derivatives as further comonomers, provides an anionic polymer with alkaline pH used for surface sizing. Polyurethanes made from diisocyanates and hydrophobic diols are also employed, which can be charged with anionic or cationic monomers. An example is a polyurethane made from toluene diisocyanate and glycerol monostearate, which can be cationic modified. In addition, cross-linking with epichlorohydrin yields an anionic product. The hydrophobic properties of alkylketene dimer (AKD) and alkenyl succinic anhydride (ASA) are due to large hydrocarbon-like sections, which in the case of AKD are derived from C8−22 -fatty acids and in the case of ASA from alkenes formed by isomerization from 1-alkenes:

6.7 Paper

� 611

AKD and ASA react with hydroxyl groups of cellulose to form a cellulose ester. Then they adhere covalently to the paper through the ester bond:

Coating colors With or without the sizing, a further coating, the coating color, can be applied to the paper [175, Chapter 16], [203, keyword “paper”, Chapter 9.2], [179, Chapter 3.6.9], [223, Chapter 5.3], [180, p. 86ff]. Their purpose is to give a smoother surface and a certain look and feel to the coated paper through fillers. The necessity of the surface smoothness results from the fact that the dimensions of the paper fibers used are a few millimeters in length and 50–100 µm in width. In this way, halftone dots from 300 dpi printing already are the same size as or even smaller than the fibers, so unsmoothed paper limits the possible resolution severely. White pigments improve the whiteness and brightness of the paper. Colored pigments are used for coloring; however, about 95 % of paper dyeing is done directly in the pulp. It is possible to dispense with coating and add the necessary fillers and pigments directly to the paper pulp prior to pressing and drying, thus eliminating the need for a separate step if the chemistry of the auxiliaries permits this. A typical coating color contains (▶Figure 6.33): – fillers and pigments as main ingredients – dispersants for stabilizing the filler, and pigment dispersion – binders to bind the substances to the paper – cellulose- or polyacrylate-based thickeners – other auxiliaries such as wet-strength additives or defoamers Fillers and pigments are mineral or synthetic substances. Whether a substance is considered a pigment or a filler depends considerably on its refractive index. Low-refractive substances serve as fillers. Commonly used are kaolin Al2 O3 ⋅ 2 SiO2 ⋅ 2 H2 O, calcined clay Al2 O3 /SiO2 , natural and precipitated calcium carbonate CaCO3 (ground calcium carbon-

612 � 6 Structure of paint systems

Figure 6.33: Constituents of a coating color: wetting agents, dispersants, binders, and thickeners [175, Chapter 16], [203, keyword “paper”, Section 9.2], [179, Chapter 3.6.9], [223, Chapter 5.3], [180, p. 86ff].

ate GCC, precipitated calcium carbonate PCC), talcum MgO⋅4 SiO2 ⋅H2 O, barite or barium sulfate (blanc fixe) BaSO4 , aluminum hydroxide Al(OH)3 , gypsum CaSO4 ⋅ 2 H2 O, or titanium dioxide TiO2 [179, Chapter 2.2.2]. Wetting agents and dispersants support the grinding of the above mentioned substances and dispersion in the solvent. ▶Section 6.3 examines the underlying processes in more detail.

6.7 Paper

� 613

Classic agents are sodium or ammonium salts of polyacrylic acid or polyphosphoric acid Na5 P3 O10 . Due to their charge, they have an electrostatically stabilizing effect. Polymers such as polyacrylamide (PAM), starch, or polyvinylalcohol (PVOH) form protective colloids around the particles and have a sterically stabilizing effect. Both effects are present in carboxymethyl cellulose CMC and products from lignosulfonic acids and sulfonic acid-containing aromatic formaldehyde resins, which act electrostatically and sterically. Some compounds also act as binders, retention aids, or fixatives, see above. A wide range of natural substances can be applied as binders. They include starch, casein, soy proteins, and cellulose. Starch and cellulose must be modified to achieve their full effectiveness; possible candidates are oxidized starch, hydrolyzed starch (including ester and ether derivatives), and carboxylated and etherified cellulose (CMC, HEC). Fully synthetic binders have become important. The most significant are styrene acrylates (SA) and styrene-butadiene latex (SB). Styrene acrylates are copolymers of butyl acrylate with styrene or vinyl acetate p(BA-styrene-VAc). Styrene-butadiene latices are used as carboxylated copolymers (XSB); the carboxylic acids are derived from small amounts of acrylic acid, methacrylic acid, and maleic acid p(styrene-butadiene AA/MAA/maleic acid). Polyvinylalcohol or polyvinylacetate is also included in binders. Thickeners adjust the viscosity of the coating color to achieve optimum processability. Typical thickeners are polyvinylalcohol (PVOH) and nonassociative thickeners based on cellulose or polyacrylic acid, especially carboxylated cellulose CMC, polyacrylic acid copolymers of acrylic acid/methacrylic acid with methyl acrylate, ethyl acrylate, methyl methacrylate, or acrylamide p(AA/MAA-MA/EA/MMA/acrylamide). From the field of associative thickeners, representatives of hydrophobically modified polyurethanes (HEUR), polyacrylic acids (HASE), and celluloses (HMHEC) are employed. We have described the mode of action and structures of thickeners in ▶Section 6.4. 6.7.5 Calendering (satinage) After dewatering with light pressure through felts, the raw paper is naturally rough or slightly textured through the felts, but in all cases, uneven. Sized and coated paper is also matte after the treatment with glue or coating. The term “calendering” describes operations in which the (moist or already dried) naturally rough matte paper web is passed through various roller systems (calender) and thereby more or less firmly pressed [175, Chapter 15]. Calendering aims to achieve a more even surface suitable for writing or printing on it with rigid printing plates or showing a higher gloss. Objectives are: – to strengthen the surface to reduce dusting and tearing of fine fibers. – to reduce the porosity of the paper, lessening the absorbency and run-off of ink. As a result, the sharpness of a drawing or printed image is increased. Since this reduces the local inhomogeneity of the refractive index, the amount of scattered light produced by the paper and its brightness decreases while transparency increases.

614 � 6 Structure of paint systems –

–

To decrease surface roughness in the range of millimeters to the size of fiber diameters is essential for printing methods that use rigid printing plates. They require sufficient contacts of paper and printing plate, such as rotogravure or copperplate printing. In the case of copperplate printing, artists also like to use soft paper, into which the printing plate is impressed deeply. to reduce surface roughness in the micrometer range to obtain glossy papers. A smooth surface induces gloss with a homogeneous refractive index, on which hardly any diffuse reflection or scattering occurs.

Roller systems may consist of two or many rolls, hard or soft, cold or heated. Typical systems include: – many hard rollers (up to 17) heated up to 100 ℃ (supercalender) to produce very smooth paper grades such as WFC, LWC, and SC (roto-engraving quality) – two soft heated rollers for LWC (offset and newspaper printing) – four soft heated rollers for SC, LWC (offset quality) – many soft rollers for smooth SC and LWC types and glossy WFC (all roto-engraving quality) Calendering cannot be considered independently but only in combination with sizing and coating, as we will see in the example of artists’ papers.

6.7.6 Paper grades, general and industrial Depending on the raw materials, pulp or cellulose types used, and the type and number of processing steps, a paper manufacturer can produce many different paper types, which in turn permit different application types. Each step uses its specific conceptual taxonomy, ▶Figure 6.34. The classification according to the material basis takes into account the origin of the pulp and is ordered by increasing paper quality: wood-containing papers—pulp papers—rag paper: – Wood-containing papers are largely made from mechanical pulp, i. e., wood pulp or cut or ground wood, ▶Section 6.7.2.1. However, since wood fibers are torn and are very short in this type of pulp production, the paper has little mechanical strength. Therefore, for higher-quality papers for magazines and print inserts, particular types of pulp are added as a counteracting measure. These consist of longer-fibered cellulose and serve as reinforcement. Papers containing wood are cheap and often of poor quality. They frequently contain fillers and additives to improve their properties, which we can recognize when burned, receive ash volumes of about 14 %. – Pulp papers or wood-free papers consist of pure pulp, i. e., essentially pure cellulose obtained from chemical pulp from which undesirable woody components such

6.7 Paper

� 615

Figure 6.34: Four levels in papermaking, from raw material to application. All levels have their specific conceptual taxonomies, often used side by side. Raw materials are applied to produce intermediates standardized for wood and cellulose pulps, and traded worldwide. The industry combines intermediates into pulp mixtures processed into many grades of paper. The paper grades are also often standardized for multiple applications.

–

as lignin have been chemically removed, ▶Section 6.7.2.2. Basically, cellulose remains in the structure in which it is already present in the wood, i. e., in the form of long, well-preserved fibers. Compared to wood-containing paper, the fibers are long (1–3 mm), stable, and matte well. Since wood-free papers contain no additives, they yield practically no ash. They are applied, among other things, as high-quality writing papers, papers for graphic work (commercial printing), filter papers, and restoration work. Drawing and layout papers are also mostly pulp papers. Rag papers were initially made from rags consisting of textile fibers of flax (linen), cotton, or hemp. The fibers are long, uninjured, and matte very well into durable, age-resistant papers, so artists often used them as a base for drawing and painting. Rag papers contain only a few additives. They are still the preferred choice for highquality artists’ papers, or, at least specific long-lasting pulp papers are. Since the late Middle Ages, the availability of rags has been a limiting factor in paper production, so cotton fibers have been used directly to produce rag paper without going via textiles. The fiber raw materials yield very pure virgin cellulose fibers, as we can see when burning. The typical ash volume is only about 2 % of the paper volume [820].

Four main groups exist according to the type of application: – Graphic papers include all types of printing paper: press products, stationery, office (copy) applications, and forms. They are available as roll or format paper. – Packaging papers comprise paper, board, and paperboard, e. g., wrapping paper, folding boxboard, or corrugated base paper.

616 � 6 Structure of paint systems –

–

Specialty papers including types of papers and boards for technical and specialty applications, e. g., wallpaper, filter, or cigarette papers. Of interest to us are the various types of artists’ papers as well as those for inkjet application. Hygiene papers include tissue papers (made of pulp) and crepe papers (made of wood pulp) for toilet paper, kitchen rolls, and others.

The type of coating also characterizes papers: – Natural papers are uncoated papers. Their natural porosity makes them less suitable for reproducing images but ideal for writing, office, copying, and laser printing papers. To control the ink flow, they are sized to varying degrees. A light pigmentation below 5–10 g/m2 , e. g., with chalk for smoothing the surface and increasing brightness, is not considered a coating. Natural papers come in different varieties: – Machine-smooth papers that have not undergone surface treatment other than processing into paper. – One-side smooth papers are glossed by a smoothing cylinder but not pressed. – Satinized papers are more glossed by a calender under pressure and elevated temperature. Multiple calendering or a supercalender results in an even smoother surface. Artists’ papers are essentially uncoated. – Coated papers have a smoother and more uniform surface than uncoated papers. They are therefore ideal for high-quality reproduction of images with brilliant colors. They include: – Double-sided coated papers, such as LWC (lightweight coated), art paper, and picture paper. – One-side coated papers mainly for packaging, such as label paper or folding box board (type designations are SBB, SUB, FBB, WLC). ▶Figure 6.35 provides an overview of crucial graphical paper grades commercially available today in a quality-value relation. ▶Table 6.7 contains details of essential pa-

per grades. All art printing and artists’ papers are strictly wood-free and contain rags in higher grades. The following section will look at artists’ papers in more detail. 6.7.7 Special case artists’ paper What has been said so far about basic paper materials and properties applies in principle to commercial paper products such as writing paper, letterpress paper, catalog paper, and most importantly, artists’ papers. However, the latter represents a niche with no standardized paper grades. The many different grades of paper that are important for the artist result from the interaction of a few parameters: – surface sizing

6.7 Paper

� 617

Figure 6.35: Modern commercial graphic paper grades. The abscissa symbolizes the quality of the product (brightness, surface, printability) and the ordinate the “value” [832]. Left: The numbers indicate the grammage in g/m2 . Light blue: uncoated paper. White: coated paper. The artists’ papers are shown in ▶Figure 6.36.

– – – – –

pressing during dewatering, calendering for surface texturing material or pulp mixture coating (artists’ papers are, in general, natural papers, so they are not coated) carefully determined machine parameters (temperature, press pressure, etc.) clean water, preferably spring water

We will discuss specialty papers for a particular art technique in connection with a paint system, but the parameters mentioned already allow some general statements. In addition to the material, surface sizing, and pressing during dewatering or calendering are the main factors determining the properties, ▶Figure 6.36 schematically shows the relationship between these two parameters. ▶Table 6.8 compiles typical properties of artists’ papers. Sizing Artists’ papers are consistently sized in the pulp to ensure strong cohesion of the fibers and achieve the necessary mechanical resistance. As a rule, AKD is employed as a neutral size. AKD may react acidically by hydrolysis, but artists’ papers are buffered with 2–4 %

Art paper

– HWC, HWC-WF – WFC, WFU -

+++

+

+++ (CaCO� )

-

++ (alumina) ++ (alumina) ++

2–3× coated (high quality)

BS, SS, uncoated, possibly calendered

Worked to bulk by short fibers

Strong satin finish Light satin finish, coating 8–10 g/m� 2x coated

Many short wood fibers (birch), few long wood fibers (pine), 2–3× coated with CaCO� /alumina (standard coating), possibly plastic for high gloss. Single coated for matte paper, multiple coated and satin for glossy paper 80–250 +++ +++ (CaCO� ) Art book, high quality brochures 80–250 +++ +++ (CaCO� )

+++

+ ++ ++

Possibly bleached and pigmented to reduce transparency

Image printing paper

70–250

+++ ++ ++

-

-

Misc.

60–90

49–60

Magazine paper – SC – LWC – MWC

+++

+

Filler

75–160 + +++ Many short wood fibers (birch), few long wood fibers (pine)

34–45

Directory paper DIR

+++

Chemical pulp, Kraft

Copying, office, letter and writing, woodfree, WFU

42–49

Newsprint paper SNP, INP

Mechanical pulp

Book paper

Grammage [g/m� ]

Paper grade

Table 6.7: Overview of important commercial graphic paper grades and their composition or manufacturing characteristics [176, Chapter 9.5], [180, p. 101], [179, p. 98]. Brighter grades contain more chemical pulp. BS: bulk sizing, SS: surface sizing to control ink flow.

618 � 6 Structure of paint systems

6.7 Paper

� 619

Figure 6.36: Schematic representation of the influence of surface sizing and pressing with dewatering or calendering on the application properties and areas of use of artists’ papers. All papers are uncoated and consist of cotton, and lower qualities are made partly or entirely of cellulose. Light blue: application properties achievable by sizing and surface.

calcium carbonate CaCO3 as an alkali reserve to keep the pH value to 7.5–9.5 and to be considered resistant to aging according to ISO 9706, ▶Section 6.7.9. Combined with the alkali reserve, this results in a neutral-alkaline sizing system. In addition, traditional sizing with animal glue (gelatine) is also typical. Depending on the intended use of the paper, surface sizing can follow in order to set essential application parameters: – A surface sizing increases the surface strength and resistance to mechanical effects such as: – the movement of an eraser or a hard pencil (drawing papers) – the rubbing off of masking rubber or rubbing crepe or the tearing off of adhesive tape (watercolor papers) – the brush movements when correcting or removing color (watercolor papers) – A weaker surface sizing slows down, and a stronger one stops the water or ink absorption of the paper: – The open time is prolonged, i. e., the paint or ink remains correctable, smudgeable, or mixable on the paper with other colors (watercolor, layout, and fineliner, acrylic painting papers).

80–250

180–920

170–250

75–220

Bristol

Calligraphy, pen & ink drawing

Graphics, illustration

+++

+++

+++

+++

−

−

−

−

See above

CaCO�

CaCO�

CaCO�

CaCO�

See above

In general SS, heavily calendered. SS and calendering increase strength against erasing and form a wet solvent and ink barrier.

Strong satin finish, possibly coated. Colors and markings stand well and brilliantly.

SS, very smooth and white, colors stand well and brilliant. Clear markings.

SS increases strength against erasing, glued 2x if necessary. Fine-grained surface facilitates pigment abrasion of pencil, charcoal or chalk. High volume facilitates retention of abraded pigments.

Soft, worked on volume, in general no SS, absorbent to help solvent whisk away. Surface rough, matte, satin, suitable for all techniques. Satin especially for woodcut, screen printing; soft and voluminous especially suitable for gravure and planographic printing.

Rough, matte, satin finish. SS/calendering increases strength against corrections, rubbing crepe, tape.

Drawing, pastel

See above

See above

130–320

See above

Print

See above

200–640

Watercolor

Other

– simple – high quality – highest quality

Filler

Uncoated, in general BS, SS if applicable, natural gray, matte or satin finish. For chemical pulp types, addition of > �,� % CaCO� acc. to ISO 9706 to slow down aging, designation “aging resistant” +++ − CaCO� Study quality ++ ++ CaCO� − +++ −

Rag (cotton)

General

Chemical pulp (pulp)

Grammage [g/m� ]

Application

Table 6.8: Overview of artists’ paper and its composition or manufacturing characteristics [1004]. Bulk sizing (BS) and surface sizing (SS) control ink flow, increase the mechanical resistance, and act as a water, solvent, and ink barrier. SS works in conjunction with calendering. Environmentally-friendly ECF or TCF bleached types are used as high-quality pulp.

620 � 6 Structure of paint systems

6.7 Paper

� 621

–

–

–

There is a water and color barrier that makes the paper suitable for low viscosity and alcohol-based media (watercolor, acrylic, layout papers) and prevents the bleed-through of low-viscosity ink (bristol, layout papers). A surface sizing reduces or prevents ink soaking and bleeding: – Colors stand brilliantly on the paper (watercolor, layout, and bristol paper). – A sharp drawing emerges (pen drawing, layout, bristol paper). Conversely, the lack of surface sizing allows solvents or water to absorb quickly into the paper so that prints dry rapidly (rotogravure, lithographic papers) and large amounts of water are absorbed and released in a controlled manner (watercolor paper).

Surface texture The surface of the artists’ papers is defined when the water is pressed off the pulp using various felts for pressing, producing rough (more structured) or matte (less structured) surfaces. A fine-grained or even smooth surface can be achieved by, i. e., hot pressing. The effects complement those of sizing: – A smooth surface increases the surface strength and the resistance against mechanical effects; it slows down the paper’s water or ink absorption and reduces or prevents ink from absorbing and running (see above). – Conversely, a matte or rough surface allows solvents or water to be quickly absorbed into the paper so that prints dry quickly (rotogravure, lithographic papers) and large amounts of water are absorbed and released evenly in a controlled manner (watercolor paper). – A matte or rough surface produces a higher abrasion of powdery or solid paint systems like pencils or pastels. It can better fix the abraded pigment particles in the paper’s pores. Materials Today, the highest-quality artists’ papers consist of 100 % cotton; cheaper grades comprise mixtures of cotton and chemical pulp or up to 100 % chemical pulp paper for study papers. Some grades are also made from annual cellulose-producing plants such as bamboo, blended with small amounts of cotton to make a mixed media paper. In the Middle Ages, cotton came from rags, i. e., textile rags, but was soon obtained directly from the cotton plant due to the scarcity of this resource. It forms long outer seed hairs and short inner seed hairs, ▶p. 575. Artists’ papers are made from both hair types; however, some manufacturers prefer the inner, short, soft, and clean seed hairs, called linters. During papermaking, the long fibers of textile cotton can become knotted and form webs, disturbing the uniformity of the paper. The knotting and felting process could also occur during later painting and rubbing with a brush, resulting in undesirable “paper sausages.” The paint application would then make the disturbances in the paper visible. Linters fibers produce an open volume paper that is soft but robust and is a moisture buffer for aqueous solvents (water, alcohol-based fineliners).

622 � 6 Structure of paint systems In pulp-containing papers, in general, pulp types from softwood, which are soft and pliable, are mixed with hardwood pulps, which are strong and durable, to obtain optimum paper properties. A critical material factor is the achievable volume of the paper. In a pulp with long fibers, the fibers can be well parallel and form dense quasicrystalline areas with high strength but keep the paper volume low. In pulp with a high proportion of short fibers, fibers can lay crosswise and form an open-pored paper with a 50–100 % higher volume than a solid long-fiber paper for the same basis weight. High volume implies pleasant hand feel, stiffness, and a general sense of value. For some printing techniques, the volume of the paper is also essential from a technical point of view to absorb the binder and solvent of the printing ink. For example, the absorption and release of water through the paper are paramount when working with water-based inks. Softwood pulps from conifers such as spruce are typical sources for long fibers; short fibers come from hardwood pulps (birch, eucalyptus) or linters.

6.7.8 Paper decay Paper is easy to make, but under certain circumstances, it decays just as readily, as shown by the works threatened with decay today [823–825]. Especially the groundwood pulp portions of cheap papers rapidly decay to low molecular weight products. Furthermore, even higher-grade papers sooner or later show signs of decomposition since cellulose as a polyether is slowly hydrolytically cleaved by acids, ▶Figure 6.37(a). Cellulose is also attacked oxidatively, with the primary hydroxyl group at C6 oxidized to the carboxylic acid. The oxidation increases the paper’s acidity even without adding acid. It stabilizes the intermediate of the protonated ether leading to the cleavage of the ether bond, ▶Figure 6.37(b). Protons can come from various sources, one of which is acidic inks. Historic iron gall inks are a classic problem, but modern dye inks can also react acidic. Papers sized with tree resins and alum (i. e., the majority of papers from the last two centuries) contain sulfuric acid from the hydrolysis of excess alum:

Another source of decay appears in graphic works, in which green copper pigments were used. As [421] delineates, copper ions efficiently catalyze the depolymerization of cellulose. However, the exact course of the reaction is not yet fully clarified; in the case of gold plating or metal plating with copper-containing metal alloys, various oxidation states of copper come into action, via which atmospheric oxygen is employed for cellulose oxidation.

6.7 Paper

� 623

Figure 6.37: Basic reactions in paper decay [823].

Ancient papers made before around 1850 show comparatively less decay because rags were used in their manufacture. As already mentioned, these provide longer fibers, which, when matted, can form crystalline regions of densely packed and ordered cellulose chains, thus significantly increasing the mechanical strength. In contrast, woodcontaining papers of later times are little interwoven due to their short, broken fibers and disintegrate at only a few breaks in the cellulose chains. On top of that, the chains are coated with nonpolar lignin, which strongly impedes the formation of crystalline regions.

624 � 6 Structure of paint systems 6.7.9 Aging-resistant paper Especially for cultural assets such as paintings, prints, and drawings on paper as support, it is of utmost importance that the support lasts as long as possible and does not fall victim to the described decay phenomena or animal (insects) and plant (mold) pests. While this was supported in earlier times by the choice of rag as the paper base, today, we have to pay close attention to how they are composed and what resistance the manufacturer gives them. In Germany, e. g., two sets of standards exist parallel and with slightly different objectives. The national standard DIN 6738 “Paper and board, Service life classes” defines four quality or life classes (LDK). Samples are tested with simulated aging tests: exposing papers to a temperature of 80 ℃ and relative humidity of 65 % for up to 24 days. Then the difference between mechanical paper properties before and after aging is determined. In order to be classified in the highest class LDK 24-85, the papers must still have at least 85 % of the property values, based on the difference. They may then be called aging resistant. The international standard ISO 9706 “Information and documentation—Paper for documents—Requirements for permanence,” on the other hand, defines quality characteristics that a paper must meet in its original state. These include specific mechanical properties, a certain alkali reserve (usually as a calcium carbonate buffer CaCO3 ), a high oxidation resistance, and a slightly alkaline pH of 7.5–10. Oxidation resistance, expressed as a kappa number, is tolerated to a maximum lignin content above this number but below 1 %. Artists’ papers are in general certified by manufacturers in accordance with ISO 9706 as aging resistant to museum standards. In a direct comparison of the two sets of standards, official state archives and archive administrations regard only ISO 9706 as suitable for the permanent protection of cultural property [1006]. It is stated that the ISO 9706 sets stricter standards for the paper than DIN 6738, which itself also speaks of life span estimates, and on top of this, refers in a note to the criteria of ISO 9706. Thus, the lignin content according to DIN may be much higher than according to the ISO and the proportion of acid waste paper. To summarize, several practical and regulatory approaches exist for assessing paper resistance against aging, so the suitability of paper, treatment, and storage must be evaluated per case. Regardless of the certifications of the paper, storage conditions significantly determine its actual preservation.

6.7.10 Yellowing Papers containing wood show signs of yellowing under the influence of light or heat [174, Chapter 3.8], [824]. The higher the wood content, the greater the yellowing. This discoloration limits the usability of mechanical pulp for certain bright paper grades.

6.7 Paper

� 625

The cause of yellowing is the lignins that remain in the wood portion. Functional groups in the lignin can absorb light from 300–400 nm. In addition, some aromatic rings can also absorb short-wave light directly. As a result, new chromophores absorb in the UV and near-visible range and appear yellowish, e. g., aryl-α-carbonyl and quinone structures. The reaction proceeds via excited carbonyl groups as photosensitizers and oxygen as an oxidant. Carbonyl groups form from alcoholic hydroxyl groups of phenylpropane units and phenolic hydroxyl groups:

The carbonyl groups can continue to react and build complex structures. In yellowing, we distinguish between a fast and a slow phase. In the first, fast phase, simple phenols with catechol or hydroquinine structures are converted into quinones:

Various reactions occur in the subsequent phase, such as the nonspecific oxidation of aromatic rings with free hydroxyl groups and of benzyl alcohol groups:

626 � 6 Structure of paint systems The resulting radicals can react with oxygen to form oxygen functions or with active hydrogen atoms of other organic compounds. An example of the formation of intensively colored quinones is the following:

During this process, carbon and ether bonds in the lignin break. Besides the yellowing, the paper structure weakens, and the paper becomes brittle. We can see here the eminent significance of the wood-free paper and the removal of the wood components for a paper-based work of art.

7 Paint systems in art The previous sections have presented all of the building blocks of various essential paint systems in art. In this section, we can now take a closer look at pigments, dyes, and specific examples of binders and auxiliary materials.

7.1 Ceramics and their painting The artistic coloring of ceramics is one of the earliest artistic expressions after cave painting and reached a high level already in the millennia before the birth of Christ. Therefore, we will enter the field of paint systems and binders by considering ceramic painting first [41, 500]. In contrast to mural or panel painting, the high firing temperatures present conditions that only a few inorganic and no organic pigments can withstand when painting ceramic objects. Nonetheless, ancient artisans applied successfully two possible solutions: – ceramic painting with highly refractory pigments before firing the object – cold painting in the cold, ready-fired state after firing Concerning the first solution, in the case of glazed ceramics (i. e., not classical ceramics), we can further distinguish between – underglaze colors applied to the body before the actual glaze; they then undergo the high-temperature firing process – onglaze colors applied to the finished glazed objects as a low-fusing decorative glaze and fired at considerably lower temperatures 7.1.1 Classical ceramic painting Ceramic painting with highly refractory colors has accompanied the development of ceramics since the earliest times. The pigments are applied to the raw ware and fired with it, which means that they must withstand the necessary high temperatures of several hundred to about 1200 ℃ undamaged so that only a limited number of inorganic pigments come into question. In antiquity, these were essentially heat-stable iron and manganese oxides, calcium carbonates, and clays, providing black, brown, red, and white colors. However, due to the complex chemistry of iron oxides, the color space could only be opened up gradually, as the development of ceramics shows, ▶Figure 7.1. Iron-reduction technique The earliest technique used for painting is the iron-reduction technique, which gives the body a black coating (black glaze): https://doi.org/10.1515/9783110777123-007

628 � 7 Paint systems in art

Figure 7.1: The early development phases of ceramic hot painting in the Mediterranean [41, 500].

–

–

An oxidizing atmosphere due to full firing and sufficient oxygen supply achieves high firing temperatures necessary for sintering ceramics. Iron compounds in the starting materials (alumina, ocher) are oxidized to red iron(III) oxide. In a second step, the objects are reduction-fired by reducing the oxygen supply and adding combustible materials, thus forming black iron(II,III) compounds. The result is a consistently black-colored ceramic since the red iron(III) oxides are also reduced.

Iron-reoxidation technique, polychrome firing Further development of the iron-reduction technique made it possible to use the different colors of iron oxides in different oxidation states to produce multicolored ceramics. Therefore, the process contains a further step for this purpose: – Steps 1 and 2 remain the same as in the iron-reduction technique. – Upon renewed oxidizing atmosphere in the course of cooling, certain portions of the black surface are easily attacked by oxygen, and consequently, reoxidized to red iron(III) oxide. Crucial to the technique’s success is precisely determining the parts to be reoxidized. The early reoxidation technique exploits the fact that oxygen less readily attacks thicker applied iron-rich painting slips. If the reoxidation stops quickly enough, only thin areas are oxidized and show a red color, while thick painting layers are still black or show different shades of brown. By varying the thickness of the layers, artists were thus able to create subtle color transitions between red and black and achieve sculptural effects.

7.1 Ceramics and their painting

�

629

The matured later reoxidation technique uses paints of different chemistry and provides safe results. Potassium-rich clays serve as black paint, which sinter more readily than the body (or paint), which should remain red. This material creates a dense mass that is not attacked by oxygen and is impermeable. The potassium content can be selectively increased by adding potash or other potassium-rich aggregates. Manganese-black technique The manganese-black technique uses black iron-manganese spinels to produce the black color. This technique is younger than the iron-reduction one and requires the presence of manganese-containing starting materials such as umber, which contain manganese(IV) oxide MnO2 ⋅ nH2 O. In one step, the black color and the red iron(III) oxide are oxidatively produced since the low-valence manganese spinels formed in the heat are stable and retain their black color. Pigments of the hot-painting technique Black pigments Three main pigment types achieve black color: – Iron oxide black consists of iron oxides, preferably magnetite Fe3 O4 , maghemite γ-Fe2 O3 , hercynite FeAl2 O4 , and mixed crystals of these oxides, formed during the firing process, partly with the clay of the body. Depending on the exact composition, the black color is altered to brown, reddish-brown, or yellowish-brown by increasing proportions of hematite. Surprisingly, the brown and colorless iron compounds maghemite and hercynite can also contribute to the black hue if spinel formation between them and magnetite or impurities causes oxidation states II and III to be present simultaneously. An IVCT transition FeII → FeIII then induces the color as in magnetite. – Manganese black comprises a less common group of black iron-manganese spinels containing manganese in oxidation states II and III: the mixed spinel (Fe, Mn)2 O3 , hausmannite Mn3 O4 , jacobsite MnFe2 O4 , and bixbyite Mn2 O3 . The manganese contents are strongly fluctuating and indicate that manganese is partly contained only as an impurity, partly also intentionally added to increase the manganese content, and thus the color depth. The MnII and MnIII compounds are formed from the different manganese earths by oxygen released during firing. – Carbon occurs in the form of graphite on ancient ceramics but cannot provide opaque painting. Finely dispersed carbon black is better suited than graphite. However, it has the disadvantage of being very sensitive to oxidation when fired, which is why few examples of this technique exist.

630 � 7 Paint systems in art White pigments Only a few compounds can be used as white pigments for hot painting: – The Kamares pottery uses talcum Mg3 (OH)2 Si4 O10 , which is transformed in heat to protoenstatite MgSiO3 . Since the process preserves the foliated structure of the talcum, the white painting layer is not very abrasion-resistant and hardly usable for utilitarian purposes. – Calcite CaCO3 is best used as a fine-particle carbonate such as chalk or marl (calcite with light, low-iron clay). The carbonate’s dissociation into CaO and CO2 during firing is compensated by subsequent carbonate reformation with carbon dioxide from the air. – Bright clay minerals with kaolinite Al2 (OH)4 Si2 O5 decomposing to metakaolinite Al2 O3 ⋅ 2 SiO2 form the white ground or white color on the famous vases of the Greek period. However, since clay components have a discoloring effect, high demands are made on the purity of kaolin clays, so this white pigment came into use relatively late. Red pigments The only heat-stable red pigment of ancient ceramics is hematite α-Fe2 O3 . Iron-rich clays thus include, by nature, red color in the base material of ceramic objects. Ocher was often added to increase the iron content in the base material, intensifying the color. Besides red ochers, yellow ochers can also be used as a starting product since the iron oxide hydrate is converted to hematite during the firing phase.

7.1.2 Pigments of the cold-painting technique In cold painting, decorative colors are painted on the finished ceramic only after firing. Since high temperatures are no longer applied, all pigments for mural or panel painting can be used; ▶Table 7.1 lists pigments of antiquity. The cold painting extends the color palette into the critical color space of yellow, green, and blue. White is formed by the ground, mostly a kaolin, lime, or gypsum slurry primer. Since cold painting is not cofired and does not form a dense coating, it lacks the high resistance against environmental influences and abrasion of hot painting.

7.1.3 Ceramic enamel and glaze colors The limited range of ceramic hot painting and the somewhat wider range of cold painting remained almost unchanged from the invention of ceramics to modern times. In antiquity, only rarely, but later more and more frequently, glazes protected ceramic products, which could be self-colored or allow decorative hot painting. In close connection

7.1 Ceramics and their painting

�

631

Table 7.1: Pigments more commonly found in ancient ceramic cold painting [41, 440, 495]. Pigment

Composition

Yellow Goethite* Jarosite

α-FeOOH (K, Na)Fe� (SO� )� (OH)�

Red Hematite* Cinnabar* Madder*

α-Fe� O� HgS Organic

Green Malachite* Copper hydroxychlorides (atacamite)* Egyptian green*

Cu� (OH)� CO� Cu(OH)Cl, Cu� (OH)� Cl Copper silicate glass

Blue Cobalt blue* Egyptian blue*

CoAl� O� CaCu[Si� O�� ]

White, black, brown Calcite* Dolomite Huntite Gypsum, anhydrite Carbon black, vegetable black* Iron and manganese oxides*

CaCO� CaMg(CO� )� CaMg� (CO� )� CaSO� ⋅ � H� O, CaSO� C (Fe, Mn)(Fe, Mn)� O�

*

The pigment was also commonly used as an artists’ pigment.

with the technique of colored glass, enamel paints emerged early on, which were also suitable for glass painting, ▶Section 7.2.3 at p. 648. Hopper [44] depicts an entertaining and exciting history of the development of glazes and their coloring; [42, 43, 45] supply details of color mixing. Modern ceramic painting applies enamel colors, which form colored glazes on the body or an existing underglaze. They consist of three components [204, keyword “Keramische Farben”]: – So-called frits furnish the actual glaze. The frits absorb the color bodies during melting and coat them with a firmly adhering layer after solidification. They are thus the binder of ceramic colors and are similar in structure to silicate glass. The earliest frits consisted of lead silicate, obtained by applying quartz powder and lead oxide. Today, lead, lead-boron, lead-aluminum, lead-alkali, lead-earth alkali, or soda-lime silicates are employed. – Colored bodies that impart color to the frits or subsequent glazes (see below). – So-called fluxes are low-melting silicate glasses that improve the glaze’s melting behavior and the substrate’s wettability. Today, lead-boron, alkali-boron, and alkaliboron-aluminum silicates are most commonly in use.

632 � 7 Paint systems in art Based on the melting point of the glaze frits, we distinguish two series of ceramic enamel colors: – Underglaze colors include frits melting from 900–1100 ℃. Frits form the actual glaze of the object and obtain their coloration from colored bodies. The earliest finds of color-glazed objects date to around 1500 BC in Egypt. – Onglaze colors are usually based on lead-boron silicates; their frits melt from 500 ℃. These colors are painted over a previously applied actual glaze and are used to colorize, not protect, the object. Their melting temperature must be low enough not to damage the existing glaze. Due to the necessary knowledge of glaze preparation, onglaze colors have been available only since the eighteenth century, the age of European porcelain. 7.1.3.1 Glazes As the name suggests, glazes represent another type of glass besides the every-day household glassware. They are vitreous coatings for ceramic materials that protect the object and provide hardness, gloss, and color. Their composition is highly variable, and they often have lower melting temperatures than the body itself to ensure optimal application and uniform coating of the object. Not to be confused with glazes are glassy coatings on metals, i. e., enamels (▶p. 307). However, enamel has a different structure due to the required adhesion to a foreign material (metal). As the following examples show, glazes, unlike glass, are more variable and complex in structure and differ significantly depending on the application: Terracotta lead glaze [43, 45] A lead glaze for terracotta ceramics contains essentially lead and SiO2 : Red lead 80 %, kaolin 10 %, quartz 10 %. Another composition with borate content is red lead 40 %, kaolin 5 %, quartz 30 %, borax 18 %, potash feldspar 8 %, CaCO3 2 %. Architectural ceramics glaze [43, 45] Glazes for construction ceramics can consist of sodium zirconium silicates: Na4 SiO4 34 %, glass 11 %, ZrO2 30 %, ZnO 15 %, SiO2 5 %, kaolin 5 %. Majolica glaze [43, 45] Majolica wares are coated with lead feldspar glazes: Lead white 47 %, ZnO 2 %, CaCO3 3 %, feldspar 18 %, kaolin 9 %, SnO2 9 %, SiO2 13 %. Stoneware glaze [43, 45] These may be lead glazes (PbO ⋅ 2SiO2 70 %, CaCO3 5 %, feldspar 10 %, kaolin 15 %), or borate glazes (borax 23 %, chalk 6 %, SiO2 25 %, feldspar 33 %, red lead 14 %). Porcelain glaze [43, 45] Glazes for fine porcelain are based on aluminosilicates: SiO2 50–80 %, Al2 O3 0–20 %, CaO 5–25 %. Another variant is feldspar-based: SiO2 13 %, feldspar 53 %, dolomite 26 %, kaolin 6 %.

7.1.3.2 Colorants for glazes Glaze colorants are exposed to temperatures up to 1000 ℃ during ceramic firing. Due to the high temperatures during sintering, only a few colorant types are suitable for ceramic colors [16, 439]:

7.1 Ceramics and their painting

– – – –

� 633

cadmium sulfoselenides stabilized as inclusion pigments metals colloidally dispersed in the glaze matrix metal oxides dissolved in the glaze inorganic mineral phases with the addition of coloring metal cations (mixed oxides, complex inorganic pigments CICP), including the essential color bodies based on zirconia (ZrO2 ) and zirconium (ZrSiO4 )

Inclusion pigments, cadmium sulfoselenides We already know them as excellent pigments for the artist. However, they are also crucial ceramic pigments, as they open up the essential color range of yellow, orange, and red. In the form employed by the painter, cadmium pigments are not suitable for use as ceramic paints, as they are far too thermally unstable. However, inclusion in a crystalline matrix stabilizes the pigments against the conditions of the sintering process (inclusion pigment). The most commonly used matrix is zircon ZrSiO4 , which forms in a solid-state reaction from ZrO2 and SiO2 with the addition of mineralization aids. In a second step, the zircon-containing phase grows as zircon on crystals of cadmium sulfoselenide. The zircon phase is liquid, thanks to the mineralizers. The sulfoselenide is prepared separately:

The complex manufacturing process makes this pigment type expensive. Like the cadmium sulfoselenides, other colorants can be stabilized by a hightemperature resistant layer; the best-known example is zirconium iron pink ZrSiO4 ⋅ Fe2 O3 , which we will discuss in the mixed oxides section below. Colloidal metals dispersed in the glaze These induce color due to free electrons that perform collective oscillations in the metal particle. With suitable dimensions of the metal particles, the oscillation can resonate with visible light and absorb the light, which leads to color. We have described these so-called surface plasmons in ▶Section 1.6.4. This type of coloring is not relevant to the painter and belongs to the domain of ceramic artists and glassblowers, ▶Section 3.9.1.2.

634 � 7 Paint systems in art The most important metal for purposes of coloring is gold. It yields a deep pink, red, or purple color, the purple gold. Such colored glass is called gold ruby glass. Elemental gold particles are produced via a redox reaction with SnII salts:

The reaction takes place in a slurry of kaolin or clay to immediately separate the tiny gold particles from each other and keep them colloidal. Adding AgCl results in redder color, and adding CoO results in purple. Silver, platinum, and copper are other metals suitable for this type of coloration. Transition metal oxides These are dissolved in a glassy matrix, coloring it through an LF splitting of the metal’s d electrons. This process is the same as the ion coloring of glass, ▶Section 3.9.1.1, and provides an effortless way of tinting glazes. However, the coloring of glass fluxes with metal oxides has some disadvantages: – Some metal oxides decompose to release oxygen, which can cause defects in the glaze when it escapes. – The conditions in the kiln influence the position of all redox equilibria, among other things, the oxidation state in which an added metal cation is present. The oxidation state directly determines the hue. – The color of the metal cations depends strongly on the coordination chemistry at the metal cation. The chemical composition of the glass largely determines the coordination chemistry. The size of the anions present decides which coordination polyhedra can be formed around the metal cation, and thus the coordination number at the metal cation. As ligands, the anions directly confine the ligand field strength that takes effect at the metal cation, and thus the color achieved. – The presence or absence of other metals influences the compounds that form in the glass mass, i. e., which color-active metal complexes appear. – Before unintentional color distortions occurs, the number of impurities that can be tolerated must be determined. In addition to the technical problems, the prediction of glaze color is difficult, specifically when several coloring metals occur together. Detailed impressions of the complex appearances are given in [43, 45].

7.1 Ceramics and their painting

� 635

Mixed metal oxides They are suitable color bodies, since the conditions of their formation at 800–1400 ℃ are similar to those of the sintering process, ▶Section 3.4.3. Suitable colorless host lattices are spinel, pyrochlore, olivine, garnet, phenazite, and periclase. ▶Table 7.2 offers examples of pigments for enamel colors and mass coloration. [204, keyword “Keramische Farben”], [206, keyword “colorants for ceramics”], [439, 1007] provide an overview. Numerous details and Recipes for representing the colored bodies can be found in [42, 43]. LF transitions lend color to this type of colorant, ▶Section 3.4.3. Colorants such as Naples yellow or cobalt blue have long been part of the artists’ palette, and modern mixed oxides have also found their ways there, such as cobalt green PG26, spinel brown PY119, chrome tin pink PR233, or Victoria green PG51. Many other mixed oxide colorants in the tables, such as nickel silicate green PG56, are only used in ceramic applications. Green color bodies are rare as always. Today in the industry, most green ceramic colors are obtained by mixing yellow and blue color bodies, e. g., zirconium vanadium blue and zirconium praseodymium yellow, or cobalt-chromium spinel with cadmium sulfide or Naples yellow. White color bodies are achieved by adding opacifiers, i. e., substances that do not dissolve in the glaze but become finely distributed as particles. These scatter the light and, therefore, appear white. Suitable opacifiers are the high-temperature resistant oxides Sb2 O3 , As2 O3 , CeO2 , SnO2 , TiO2 , ZrO2 , and ZrSiO4 . A group of mixed oxides accounting for more than 50 % of the world production of ceramic pigments is based on the host lattices zirconia ZrO2 (naturally as mineral baddeleyite) and zircon ZrSiO4 . Problematic are the very high melting temperatures of zirconia (2710 ℃) and zircon (2550 ℃). Zirconia pigments Zirconia ZrO2 , which occurs naturally as mineral baddeleyite, serves as a host lattice. The introduction of V2 O5 leads to yellow pigments:

Small amounts of In2 O3 yield intensive yellow; other oxides vary the hue between greenish-yellow and orange-yellow. However, trivalent cations with a compatible ionic radius must be available apart from VV to maintain charge neutrality. Suitable candidates are GaIII , InIII , or YIII . The formation of VIII is then suppressed, and pure orangeyellow is obtained. Vanadium zirconia colors are stable in glazes and ceramic colors up to 1350 ℃.

636 � 7 Paint systems in art Table 7.2: Pigments in modern ceramic glazes (hot painting) [42, p. 81], [43], [206, keyword “colorants for ceramics”], [40, 439]. Pigment Yellow Naples yellow PY41 Zircon praseodymium yellow PY159* Tin vanadium yellow, vanadium yellow PY158 Nickel titanium yellow PY53, nickel tungsten titanium yellow PY189 Chromium titanium yellow PBr24* , chromium tungsten titanium brown PY163* Zircon vanadium yellow PY160, also with indium and yttrium Zirconcadmium yellow Zirconcadmium orange Nickel niob titanium yellow PY161, chromium niob titanium brown PY162 Red Aluminum chrome pink PR230, aluminum manganese pink PR231* Chromium tin pink, chromium tin violet PR233 Zircon iron pink PR232 Zirconcadmium red Yttrium red Aluminum iron red Hematite PR101 Aluminum chrome red PR235 Zinc iron chrome brown PBr33*

Mineral

Composition

Pyrochlore Zircon Cassiterite Rutile

Pb� Sb� O� (Zr, PrIV )SiO� (SnIV , SnII , VV )O� (Ti, Ni, (Sb, W))O�

Rutile

(Ti, Cr, (Sb, W))O�

Baddeleyte

(Zr, (V, In, Y)III , VV )O�

Zircon Zircon Rutile

ZrSiO� ⋅ CdS ZrSiO� ⋅ Cd(S, Se) (Ti, (Ni, Cr), Nb)O�

Corundum

(Al, Cr)� O� , (Al, Mn)� O�

Sphene, cassiterite Zircon Zircon Perovskite Corundum Hematite Spinel Spinel

Cr� O� in CaO ⋅ SnO� ⋅ SiO� or SnO� , participation of CrIV ZrSiO� ⋅ Fe� O� ZrSiO� ⋅ Cd(S, Se) Y(Al, Cr)O� (Al, Fe)� O� Fe� O� Zn(Al, Cr)� O� (Zn, Fe)(Fe, Cr)� O�

Green and blue Cobalt zinc aluminate blue PB72* Cobalt blue PB28* , cobalt turquoise PB36, cobalt green PG26 Nickel silicate green PG56 Victoria green PG51 Zircon chrome green, zircon copper blue Nickel chrome green, zinc chrome green Cobalt titanium green PG50 Zircon vanadium blue PB71 Cobalt silicate blue PB73 Cobalt zinc silicate blue PB74 Chrome green only*

Spinel Spinel

(Co, Zn)Al� O� CoAl� O� – Co(Al, Cr)� O� – CoCr� O�

Olivine Garnet Zircon Spinel Spinel Zircon Olivine Phenazite Corundum

Ni� SiO� � CaO ⋅ Cr� O� ⋅ � SiO� (Zr, Cr, Cu)SiO� (Ni, Zn)Cr� O� Co� TiO� ZrSi�−x O� : V�⊕ x Co� SiO� (Co, Zn)� SiO� (Cr, Al)� O�

Gray Tin antimony gray PBk23 Iron cobalt chromium black PBk27 Titanium vanadium antimony gray PBk24 Cobalt pewter gray

Cassiterite Spinel Rutile Spinel

Sb� O� ⋅ SnO� (Co, Fe, Ni, Mn)(Cr, Fe)� O� (Ti, V, Sb)O� Co� SnO�

7.1 Ceramics and their painting

�

637

Table 7.2 (continued) Pigment

Mineral

Composition

Brown Manganese chrom antimon titanium brown PBr40 Manganese niob titanium brown PBr37 Iron chromite brown PBr35* Zinc iron chromite brown PBr33 Manganese zinc chromite brown PBr39 Nickel ferrite brown Zinc ferrite brown PY119 Iron titanium brown PBk12

Rutile

(Ti, Mn, Cr, Sb)O�

Rutile Spinel Spinel Spinel Spinel Spinel Spinel

(Ti, Mn, Nb)O� Fe(Fe, Cr)� O� (Zn, Fe)(Fe, Cr)� O� (Zn, Mn)Cr� O� NiFe� O� (Zn, Fe)Fe� O� Fe� Ti� O�

Black Copper chromite black PBk22 Iron cobalt chromite black PBk27 Manganese ferrite black PBk26 Cobalt ferrite black PBk29 Chromium nickel ferrite black PBk30 Cobalt nickel zircon black only*

Spinel Spinel Spinel Spinel Spinel Zircon

CuCr� O� (Co, Fe)(Fe, Cr)� O� (Fe, Mn)(Fe, Mn)� O� (Fe, Co)Fe� O� (Ni, Fe)(Cr, Fe)� O� (Zr, Co, Ni)SiO�

*

Pigments which can be used in ceramic bulk coloration (e. g., architectural ceramics).

Zircon pigments The host lattice of the zircon ZrSiO4 has gained great importance since the middle of the last century, as it has met the needs of the construction and sanitary industries concerning high-temperature-resistant colorants [441, 442]. Through the development of zirconium vanadium blue ZrSiO4 : V4⊕ [443], the zircon praseodymium yellow (Zr, PrIV )SiO4 [448], and the zirconium iron pink ZrSiO4 ⋅ Fe2 O3 [449], a primary color triplet was provided that is not only temperature-resistant but also shows very pure colors. However, the red pigment is not a mixed oxide pigment but an inclusion pigment like the ceramic cadmium sulfoselenides. Since no commercially suitable metal produces the red color in the zirconium lattice, it was necessary to use the colorant hematite for the primary red color. It must be sealed in an inclusion pigment in zirconium silicate [441]. The high-temperature zirconium silicate protects iron oxide from changes during firing. All three colorants are stable up to 1350 ℃ in all types of ceramics. However, the color depends on parameters such as the ratio of V2 O5 to NaF, the purity of the raw material, the particle size of the materials, mixing details, and the reaction atmosphere. The coloring power of the blue pigment depends on the amount of vanadium that can be introduced into the color body. Adding lead to praseodymium pigments intensifies the hue, and adding Ce2 O4 shifts the hue to orange. The exact position and oxidation state of vanadium in the zircon vanadium blue were controversial for a long time. Among others, the oxidation state IV and the derived

638 � 7 Paint systems in art possible substitutions SiIV → VIV or ZrIV → VIV were discussed, which would yield the formulas Zr(Si, VIV )O4 or (Zr, VIV )SiO4 . Today, the prevailing view is that VIV occupies an irregular tetrahedral site called “16g” in the zircon, and the pigment is then to be addressed as ZrSi1−x O4 : V4⊕ x [444–447]. Charge neutrality is achieved by omitting one Si4⊕ cation per V4⊕ . Accordingly, the color is due to LF transitions in the V4⊕ cation, specifically 2 T2 → 2 E. Due to the distortion of the tetrahedron around the 16g site, the Td symmetry is broken. All 3d orbitals are energetically different and form two groups, 2 E (ground state) and 2 T2 (excited state), which allow the electron transition, ▶Figure 7.2. Slight differences in the color of the produced pigments and laboratory samples, which varies between greenish and bluish turquoise, are explained by the lower crystal field in the industrial pigment, 4⊕ in which V is highly concentrated so that V4⊕ x clusters instead of isolated V cations are involved.

Figure 7.2: Splitting of the 3d orbitals of V in a tetrahedral ligand field or the further splitting in a distorted tetrahedral field of the 16g interstitial site in zirconium vanadium blue without considering terms or LS coupling [445]. The two orbital groups allow a transition 2 T2 → 2 E.

The zircon matrix is built up from ZrO2 and SiO2 in a solid-state reaction and calcined with vanadium, praseodymium, or iron salt to obtain the above base colors:

7.1 Ceramics and their painting

� 639

Vital is the addition of mineralization aids that allow mineral formation. Alkali or alkaline earth salts, especially NaF, are used. The processes are complex and not yet entirely verified, but an impression of the probable process is given in ▶Figure 7.3.

Figure 7.3: Role of mineralization aids exemplified by the formation of zircon vanadium blue [442, 443]. Above 600 ℃, the alkali halides form volatile silicon halides and liquid alkali silicates. These combine with the above 600 ℃, likewise liquid zirconium-vanadium compounds to form V-doped zircon above 750 ℃. The role of the mineralizing agents is thus to convert the refractory zirconium and silicon oxides into liquid or volatile form to facilitate mass transfer and mineral formation.

640 � 7 Paint systems in art It is necessary to use pure starting materials, making the pigments more expensive. The price is driven up by separating the small amounts of ZrO2 from zircon sand, which is necessary to obtain pure ceramic colors. A new process forms the colorants directly from zircon sand without a detour via zirconia. The treatment of the starting material zircon with alkali compounds leads to alkali zirconate silicates, which are decomposed by acid to form a zirconia-silica mixture. If so much SiO2 is added that equimolar amounts of ZrO2 and SiO2 are present, the zircon lattice forms in subsequent calcination [442]:

Colloid metals are not acutely or chronically toxic. However, finely divided copper, cobalt, manganese, or nickel oxides must be handled following occupational safety and health regulations. Zirconia and zircon are considered to be noncritical. The potentially toxic heavy metals enclosed in the mineral matrix behave differently than the free metals and metal salts since their bioavailability is significantly reduced in the insoluble matrix.

7.2 Stained glass Artistic creations around or with glass can lure us away from our actual painting subject and lead us far into craftwork, so we will only briefly examine this area since the chemical basics of glass (▶Section 3.9) and its coloration (▶Section 3.9.1) are already familiar to us. More detailed accounts are provided by the literature [27, 28, 35, 38, 39, 122]. Artistic expressions in this context are: – reverse glass painting, ▶Section 7.2.1 [87, 88] – actual stained glass, ▶Section 7.2.2 – tesserae of colored glass parts The basis is always colorless or colored glass, which underwent a long history of development, as essential stages display [38, 122]:

7.2 Stained glass

1500 BC 1200 BC

Egypt Mesopotamia

400 BC 0 800 12th/13th century 14th century

Rome Iran, Iraq Northern Europe Venice

14th century 15th century

Northern Europe Venice

16th century 17th century

Venice Northern Europe

17th century

England Germany France Everywhere

18th century 19th/20th century

� 641

Cu colloid staining Bronze Age red glass, Cu colloid Naples yellow, yellow glazes Celtic pottery, metal colloids (Cu and Cu� O) Red mosaics of Cu and Au stained glass Iridescent lustre glazes on ceramics Cu colloid staining Color by metal oxides (green, red: Cu; yellow, red: Fe; purple: Mn; blue: Co) Green Forest Glass (potassium silicate glass) Cloudy and milk glasses, opal glasses, iridescent glasses. Mosaic glass: ZnO in lead glass (white); enamel glass: As� O� opalescent lead glass Cristallo (soda lime glass, color compensated with Mn) Bohemian chalk glass (potash-lime glass color-compensated with Mn); Au colloid stain Lead crystal glass Gold ruby glass Rhinestones (lead glass with: Sb, Tl, metal salts) Colorations by using a variety of metals and compounds

As early as the Bronze Age and antiquity, glass artisans could produce coloring metal colloids by precipitating metallic copper and gold in the glass. If the glass was destined for utilitarian or jewelry purposes, it was also possible early on to refine the surface by applying an iridescent layer (luster). These multicolored iridescent surfaces are realized by thin refractive layers on glass or ceramics. In the Islamic Middle Ages, alkali and alkali-lead glazes were applied to ceramic bodies. In these glazes, metal colloids (copper and silver) with 10–15 nm particle diameters were arranged in one or more (usually two) ordered layers. For production, copper or silver salts (sulfates, nitrates) are applied onto the glazed ceramic and baked in a kiln with a reducing atmosphere. During the process, the metal cations diffuse through ion exchange into the interior of the molten glass, where they are reduced to the metal and grow into particles of the desired size:

The metal particles’ size, shape, and organization depended on the temperature control during the firing and were very different, resulting in various color phenomena. This technique reached the Mediterranean area (Venice) and Spain, then Europe. Even in modern times, luster is created by finely dispersed metals, as two recipes show:

642 � 7 Paint systems in art Luster glaze, Tiffany 1884 2 parts SnCl2 , 1 part BaCO3 , 21 part SrCO3 , 1 part Cu(NO3 )2 evaporate. Precipitating vapor on glassy surfaces gives an iridescent surface. Luster glaze, Tiffany 1884 Reduce metal resinate (Bi, Pb, Ag, Cu) to metal with smoke fire and melt into glass. Results in lustre glaze.

7.2.1 Reverse glass painting Reverse glass painting progressed with the development of decorated glassware, stained glass windows, and stained glass [87–89]. The following overview indicates essential stages: 2200 BC 100 BC

Syria Syria, Rome

Later 13th century 14th century 15th/16th century

Italy Germany Germany

16th century Since the 16th century Renaissance

Italy Venice

Baroque, Rococo 18th/19th century

Alpine region, France

Gold etching Fondi d’oro (glass pane, gold leaf with etched drawing, ground color) Silver foil, colored etching, and painting techniques with oil paints, casein, gouache, watercolor, tusche, inks, resin paints Inlay panels in reverse glass painting or gold etching Glaze reverse glass painting with gold ground Black/sepia drawing on the gold ground or overpainted with glazes and painterly elements (crack in distemper, surface color in oil) Reverse glass painting after famous paintings for the wall Religious motifs and high baroque art Graphic art recedes in favor of painterly elements; gold leaf recedes, oil paints without contour Urban workshops with a commercial mode of manufacture New flourishing by folk and religious representations

Unlike stained glass, in which enamel or enamel paints are applied to the glass and fused in, reverse glass painting is a cold-color technique, i. e., no melting or baking required. The colors adhere to the glass through suitable binders and are protected by the front glass pane and the support. Reverse glass painting works by applying paint to a piece of glass and then viewing the image by turning the glass over and looking through the glass at the image. It generally has a multilayer structure: glass – (metal foil) – color layer (opaque to glazing) – (base color) – (metal foil) – support (board, metal ground). Several variants of reverse glass painting have developed, all following this structure but differing in the use of the color layers and metal foils, ▶Figure 7.4.

7.2 Stained glass

� 643

Figure 7.4: Top: Layered structure of the different stained glass techniques. In the case of gold, silver, and color etching, the drawing is etched into the upper gold or silver foil or a dark color layer; a white or light base color or metal foil shines through the drawing. In amelioration, the drawing is also buried in the upper gold foil; a transparent color shines through the drawing and appears shiny metallic through the backed metal foil. In eglomisé, the drawing is buried into the upper gold foil or a black color layer; a black base color or metal foil shines through. In stained glass, transparent or opaque layers of paint become visible according to their layered structure. Below: Appearance of the layered structure.

Color or metal etching In the case of etching reverse glass, the glass is painted in a single color (usually with a dark one, then called color etching) or covered with a metal foil (usually gold or silver, then called gold or silver etching). Next, the drawing is scratched (etched) off the color layer or metal foil with coarse and fine tools. Subsequently, the image is painted with a contrasting color such as white or gold or backed with metal foil (gold, silver, other metals). When viewed from the front, the background color or metal foil then shines out from the dark base color and achieves the image effect. The metal foil can be glued with clarified egg white, gum Arabic, soaked quince pits, or a mixture of linseed oil, nut oil, and possibly mastic resin. The metal foil’s formation of luster and color was discussed in ▶Section 1.6.7. For covering large areas in the painting, opaque paints are used, e. g., oil paints, waterproof drying paints, or poster tempera. Water-soluble paints must be protected with a fixative. For the background color, e. g., poster paint can be used. Amelioration The amelioration is a metal etching technique painted with transparent color lakes and then backed with metal foil (silver, tin, brass). The foil gives the transparent colors a

644 � 7 Paint systems in art shiny metallic appearance. The color lakes are bound with larch turpentine, pine resin, or mastic resin and thinned with alcohol or essential oils. Their coloring is carried out with transparent colorants. Eglomisé The glass is covered with metal foil or opaque paint, and the etched design is made visible by dark background colors or backed with metal to lend luster to it. Colored reverse glass painting The reverse glass painting is an actual painterly technique. It consists of a black outline drawing, executed as a contour with fine or broad lines. On the contour, the color layers of the painting follow one after the other, which can be applied in glazing, semi-covering, or covering manner. The coverage by the glass leads to a luminous color effect. The time-saving water-oil technique was frequently used in history; for the outline drawing, water-soluble inks are applied to which ox gall can be added to improve adhesion to greasy glass surfaces. Sometimes a primer with gelatin or egg white was used. A variety of colors is applicable: acrylic paint, casein paint, opaque dispersions of synthetic resins, gouache, tempera, ink, or tusche. In watercolors, the respective contemporary pigments were rubbed with gum Arabic or isinglass glue as a binder. Examples of such pigments are madder lake, gum gutter, verdigris, and Prussian blue (Bavarian Forest, Silesia, eighteenth century) and white lead, red lead, vermilion, Prussian blue, ocher, brown earth, sap green, and pine-chip soot (Austria, nineteenth century) [87]. Oil paints were preferred for the surface color and painterly elements, which can be diluted with linseed oil, nut oil, and turpentine oil. A fast-drying binder is boiled linseed oil (linseed oil varnish), which simultaneously has good adhesion to glass.

7.2.2 Stained glass windows The impressive stained-glass windows of medieval cathedrals and public buildings prove that painting on and with glass was increasingly appreciated as a form of art in its own right. Historical landmarks of the emergence of painterly elements on colorless or monochromatic glass in Europe can be found in the next section, dedicated to the actual stained glass. The initial sacral and later public-civic stained glass windows are based on the available solid-colored glass described in the introduction to this chapter and ▶Section 3.9. The huge windows of gothic cathedrals consist of greenish wood ash glass (forest glass). However, with its high potassium content, this glass, unfortunately, is unstable and causes significant restoration problems today. The more transparent soda glass was

7.2 Stained glass

� 645

used only in cases in which the greenish tint of the glass impaired a hue. In the fifteenth century, forest glass was further developed into wood ash-lime glass and soda-lime glass with increased proportions of lime and soda, which was made possible by improved furnace technology and change of composition. The melting temperature could be increased from about 1100 ℃ to 1300 ℃, resulting in a more homogeneous, purer, thinner, and more transparent glass. With the help of refining agents such as arsenic and antimony compounds apart from the already known manganese ones, glassworks were also able to increase the clarity of the glass. These factors combined produce a material with a previously unknown clarity and light transmission [27, 28, 86]. For the production of medieval glass windows, the finished panel glass was stained according to state of the art by metal cations and metal colloids as described in ▶Section 3.9.3. We often find the ingredients mentioned in ▶Table 7.3, but all agents mentioned in ▶Table 3.11 except calcium antimonate were employed for glass staining. Table 7.3: The most important coloring components of medieval church glass (stained glass windows) [28, Chapter 10], [621]. In addition the coloring agents described in ▶Section 3.9.3 were used. Color

Components

Emerald green

Wood ash lead glass with copper oxide (CuO) in oxidizing atmosphere. Later soda lime glass with CuO, FeO, and Fe� O�

Red Blue Yellow

Purple, incarnate

Wood ash glass (later soda lime glass) with copper in reducing atmosphere, gives Cu� O (pigment) or Cu� (colloid)

CoO in soda ash glass or soda lime glass with lead (as wood ash glass is too greenish). Later soda lime glass with FeO and MnO, turquoise with CuO and iron

Iron polysulfides (“carbon amber”) in wood ash glass, produced from sulfur/carbon. Later soda lime glass with colloidal silver and (for brown yellow) MnII /FeIII MnIII in wood ash glass

The following steps follow the preparation of the colored panel glass: – Cut (solid-colored or colorless) glass panels into pieces according to the drawing. – Break glass pieces cleanly into shape. – If necessary, paint with black and silver solder (see next section), and bake the color onto the painted glass in the furnace. – Apply lead, i. e., set the finished glass pieces in lead tape. – Solder the lead bands. – Cement the edges of the lead bands with a mastic made of chalk and linseed oil [37].

646 � 7 Paint systems in art 7.2.3 Stained glass The actual, painterly-decorative glass work is as old as the material itself, as we see in the following overview [122], [27, Chapter 7], [28, 86, 620]. After decorating glass and ceramic objects with enamels in antiquity, a phase of decorating and later painting on glass vessels (hollow glass painting) followed in the Roman period. The colorations obtained in this process are based on the ion and annealing coloration mechanisms mentioned in ▶Section 3.9.1. In the early Middle Ages, a painterly technique became established, associated with the mosaic-like picture design by monochromatic glass pieces. The sober austerity is accompanied by graphic elements such as dark opaque contours and hatchings, leading through a more unrestrained application to semiopaque or glazed layers and tonal modeling with high plasticity. Finally, the strict graphics recedes in favor of free painting. Stepping out of the sacred framework, panel painting on glass develops. Black solder drawings, silver solder, sanguine, or iron red solder Painting in the strict sense of the word on flat surfaces was initially done in the early Middle Ages on colorless or polychrome glass with black solder paint. It was prepared from a copper and iron oxide mixture, a low-melting colored lead glass, water, vinegar or turpentine oil, and a binder such as gum Arabic. The black solder, thickly rubbed with turpentine oil, serves to draw the outline, the contours of the picture, and graphic elements such as hatching. Aqueous black solder allows the colored glass to be painted thinly over the entire surface and softens the luminosity of the glass in this area. Bristly or fine brushes can add structures to the brownish-greyish paint, and highlights can be brought out again. The artist can draw delicate structures, facial features, and folds in robes and objects with thicker black solder. After the painting is finished, the glass panel and its black solder drawing are fired at about 600 ℃. During this process, the glass components of the solder melt and firmly bond the metal oxides to the glass panel. However, the temperature is not so high that the metal oxides dissolve in the glass ionically but instead form a transparent to opaque brown-gray mass that helps attenuate transmitted light according to the painted structure. Black solder, after Theophilus [27, Chapter 7.6] Copper oxide, green glass, “sapphire-colored glass” (probably lead glass), wine, or urine Black solder [28, Chapter 10.3] Iron oxide, copper oxide, lead glass Black solder [36] Iron or copper oxide, lead glass, vinegar or turpentine oil, gum Arabic

The role of vinegar is explained by the fact that lead(II) oxide in lead glass is readily attacked by acetic acid at higher temperatures. Consequently, the lead glass becomes etched and porous; the metal oxides can penetrate the glass more easily [620].

7.2 Stained glass

Predynastic

1470 BC

� 647

Ancient Green and blue glazes on quartz pottery with CuII . Yellow opacities with Egypt, Pb/Sb and Pb/Sn, white ones with Ca� Sb� O� , black ones with FeIII , purple Mesopotamia ones with MnIII , red ones with Fe� O� Egypt

Yellow enamel, yellow glass (Naples yellow)

500 BC

Babylon

Yellow enamel tiles

6–8 century

Egypt

Silver yellow

8th/9th century

Europe

Colored windows in Carolingian and Longobard churches and palaces; mainly green, besides yellow, blue, and brown. Black solder for drawing and painting

9th century

Iraq, Middle East

Colorfully decorated glass with enamel paints, luster enamel; silver yellow

841

Painting on glass

12th century Europe

Beginning of forest glass production (wood ash glass)

Around 1250 Europe

Need for large-scale stained glass due to the new construction of Gothic cathedrals, e. g., at Cologne, Regensburg, Strasbourg, Freiburg

13th century Europe

Early Gothic black solder painting, 1134 grisaille painting in Cistercian monasteries

12th to 14th century

Syria

Colorfully decorated jars with enamel paints, chandelier enamel

13th/14th century

Europe

High phase of glazing of Gothic cathedrals

1300

Europe

Reddish-brown paint for contours and incarnate

1300

Europe

Silver solder for yellow glass

14th century Europa

Etching in black solder, modeling with brushes (stippling). Black solder residue clouds the colored glass

15th century Europe

Panel painters as stained glass artists; the emergence of landscape depictions

15th century Venice

Enamel with opaque colors on low-iron, colorless glass

15th century Venice

Faience, opaque paint on pewter white glaze, enamel painting on glass, opaque paint on colored glass

15th/16th century

Harder and purer wood ash lime glass

Europe

15th century Europe

White and yellow enamel paint for tinting and shading, red glazes (sanguine or iron red) from Fe� O� , glazing to opaque, brown to yellow, reddish-brown to flesh-colored

15th century Europe

Fine drawing, hatching, and similar graphic techniques provide chiaroscuro, tonal values, plasticity. In the late Gothic more painterly treatment, lead rods recede, black and silver solder with etching and etching techniques. Enamel or enamel colors on colorless or monochrome glass

16th century Venice

Enamel painting on clear glass, opaque color

Germany, Austria, Venice

Cold painting on glass, fine painting on glass with transparent colors (black solder, silver solder, iron red, manganese violet, first onglaze colors for porcelain)

16th century Europe

Residual scratching out the paint with metal brushes (scratch dabs) results in brilliant highlights

17th century Venice

Milk glass and iron red (sanguine)

648 � 7 Paint systems in art Silver solder, which appeared later in Europe, has a transparent character. It consists of silver chloride and sulfide, possibly Sb2 S3 , ocher and clay, water, and a binder such as gum Arabic or oil. As soon as it is applied (usually covering a large area) to the glass panel, it is fired at about 600 ℃. During this process, colloidal silver forms by reduction with FeII ions. As a result, the glass becomes transparent and luminous and bright lemon yellow to yellow-orange, depending on temperature control and firing time. Silver solder [27, Chapter 7.7] Silver powder, antimony(III) sulfide, clay, or ocher or loam, water, sticky substances

For painting incarnate served the iron red or sanguine of Fe2 O3 , with which red glazes could be achieved. Depending on the composition and the conditions of application and firing, the glass painters could paint glazing to opaque layers and colors from brown to yellow-brown and red-brown to a nude color. Enamel paints The developing enamel painting resides on enamel paints, which are very similar to the enamels and ceramic paints mentioned in ▶Section 3.10 and ▶Section 7.1.3 [27, Chapter 7], [28, 29]. Some ancient Egyptian-Mesopotamian works from predynastic times show green and blue glazes on quartz ceramics (faiences) colored with copper. Yellow opacities with Pb−Sb and Pb−Sn, white ones with Ca2 Sb2 O7 , black ones with FeIII , purple ones with MnIII , red ones with Fe2 O3 , and the colorants mentioned in ▶Section 3.9.3 complete the palette [28, Chapter 2–4]. Actual enamel paints appeared in the High Middle Ages in the Islamic sphere of influence and are based on halophyte ash lead glass for white, yellow, green, and black, and halophyte ash glass for blue and red respectively. Twelfth-century enamel paints from the Middle East, e. g., show the following compositions: Blue CoII , black FeII,III , white SnO2 , yellow PbSnO3 , and green CuII [28, Chapter 9.7]. Medieval glazes on glass from the Islamic region around 1300 show the composition mentioned in ▶Table 7.4. Table 7.4: Composition of medieval glazes from the Islamic region, around 1300 [29]. Colorless base glass White Blue Red Pink Purple Yellow Green Brown Black

Halophyte ash soda lime glass, MnO� for color compensation Soda glass, opacified with SnO� and Ca� (PO� )� White enamel, stained with lapis lazuli or CoII Soda lime glass, Fe� O� as hematite, or lead glass with Fe� O� White enamel and hematite Blue and red colors with lapis lazuli and hematite Lead glass, lead-tin yellow Lead glass, lead-tin yellow or lead antimonate, copper oxide Lead glass, chromite (Mg−Fe−Al chromium oxide) Lead glass, chromite (Mg−Fe−Al chromium oxide), copper

7.3 Fresco (mural painting)

� 649

Enamel paints are mixtures of easily melting glass (flux), usually lead glass and coloring metal oxides or metals mixed in water and gum Arabic or oil and turpentine to form the paint. The paintability improves by adding egg, honey, or sugar syrup. Binders and additives are not critical since they burn when heated. The additives listed in ▶Table 3.11 were already used for glass coloring in historical enamel paints. As more and more layers of paint were superimposed with the development of painterly technique, aqueous and oily painting media were often used alternately so that the following layer of paint could not wholly or partly dissolve existing ones. The final fixation of the color took place only at the end of the firing. At about 600–700 ℃, the enamel color fuses by melting the glass with the base glass. White mosaic glass (▶Section 3.9) was used as the basis for the initially opaque colors, and transparent colors were obtained by a lead glass containing boric acid. In contrast to enamel colors for metal coating, glass enamel colors contain less alkali to adapt the thermal expansion to the base glass.

7.3 Fresco (mural painting) In addition to ceramic painting, the fresco is one of the oldest techniques already executed in antiquity, ▶Figure 7.5 [121, 128, 129, 131, 132].

Figure 7.5: Schematic temporal use of the different techniques of mural painting [120, p. 195].

650 � 7 Paint systems in art From ancient literature to modern linguistic usage, all kinds of mural paintings are often called frescos. In addition, there is the so-called lime painting. Therefore, we must conceptually distinguish the genuine fresco technique (fresco buono) from fresco secco. The fresco technique requires that the layers of paint are applied to a wet (freshly laid) lime plaster, bond with it, and dry together to form a homogeneous layer. The secco techniques are distinguished from this, in which the paint is applied to dried plaster. From ancient times until today, paints bonded with gum, glue, or tempera paints were used for the secco technique. Many mural paintings from the eleventh century are bound in the milk of lime, hide glue, casein, or linseed oil [54, 121, 509–513].

7.3.1 Fresco-buono technique The genuine fresco technique is ancient [120, p. 195]. Already 1500 BC excellent mural paintings were executed as fresco buono by the Minoan culture on Crete, some of which show surrealistic-looking undersea landscapes. The technique was widespread in the Aegean and was widely used by Etruscans and Romans until the fourth century. It continued in Byzantium in late antiquity and later reexported to Italy. The Trecento extended it with secco techniques, making possible a sophisticated painting technique with a higher degree of realism. In the Southern Alpine region, this tradition continued until modern times. In the Northern Alps, the buono technique does not have a tradition; it was first introduced in the Romanesque period, culminating in the illusionistic ceiling paintings of the Baroque period. The rediscovery of the true fresco occurred in the nineteenth century by the Nazarenes. In contrast to the practical execution, which requires high craftsmanship from the painter, the chemistry of fresco buono is comparatively simple [121, 944]. It comprises two steps; the first is carried out industrially in producing lime mortar. It produces calcium oxide (burnt lime) through burning limestone (calcium carbonate) at high temperatures. Subsequently, the calcium oxide is turned into calcium hydroxide (slaked lime) by immersion in water:

The actual painting process begins with the painter or plasterer who uses sand and slaked lime to create a layer of plaster that serves as a base for painting, and at the same time, as a brilliant white color. The painter mixes the pigments with water or lime milk and paints wet on the wet plaster. Since the plaster is wet during painting, water or

7.3 Fresco (mural painting)

�

651

lime milk combines effortlessly with its moisture, and the applied pigments sink into it. There they are enclosed by lime solution. During drying, the slaked lime sets by absorbing carbon dioxide from the air under recarbonatation, in which silicate components of the sand also participate:

The pigments are enclosed in a solid but thin, translucent lime layer. This protection against the outside world explains the stability of this type of painting. The high speed at which the lime sets is somewhat tricky for practical work. Fresco painters, therefore, divide the mural surface into “giornata” (Italian for “days”), i. e., smaller sections that can be plastered and painted within a day. If the setting process is already advanced or even completed, the painting can no longer be executed al fresco. The only ways out are to paint in secco technique on the dry plaster, continue in lime paint, or knock off the hardened layer of plaster and replaster the section. 7.3.2 Lime-painting technique Besides the fresco buono technique, since 1500 BC, the so-called lime painting spread in the Aegean, Rome, Byzantium, and the Southern Alps area [120, p. 195]. In Asia Minor and north of the Alps, it was a mural painting technique used in all epochs. Until the Romanesque epoch, it was even the only one known north of the Alps. In contrast to buono painting, the plaster substrate for lime painting is moist or even dry, i. e., already set. A whitewash of slaked lime is then applied, in which the colors can then be painted wet-on-wet. Like the al fresco technique, the colors are bound by the setting lime, enclosing the pigments with a layer of lime. Thus, the bonding is much better than the secco techniques, where the paint adheres superficially to the plaster but less than the fresco buono technique. The appearance of a lime painting is duller, more graphic, and two-dimensional than a fresco, which acquires a subtle shimmer and depth through the alignment of pigments and lime crystals. 7.3.3 Fresco-secco technique Painting al secco is any painting executed on dry plaster. Depending on the binder, there are different variants:

652 � 7 Paint systems in art –

–

Binding agents of glue or casein painting is animal or vegetable glue from, e. g., protein, animal hide, fish bones, casein, starch, or gum Arabic. This technique is attested already around 3300 BC in Egypt. In tempera painting, an emulsion is used as a binder. Water-soluble components are casein, animal glue, or plant gums. Water-insoluble components are oils, resins, or waxes. The origin of tempera is assumed in the northern Europe of the thirteenth century, in England or northern France. From there, it spread to the south but could not displace the fresco buono technique in Italy.

7.3.4 Mixed techniques In mixed media paintings, the artist paints more or less extensive sections of a fresco al secco. This approach may be necessary to make corrections, add finishing touches and details, or save time. However, a painting is only expressly denoted as executed in mixed technique if the painter has performed a considerable part of the fresco al secco or has added the secco technique for artistic intent. 7.3.5 Pigment degradations Pigments can change in undesirable ways due to their chemistry or environmental interactions. Especially mural paintings, which often date back to the Middle Ages, late antiquity, or even the early period, i. e., 400–4000 years of age, show an abundance of damage patterns that can be traced back to such unwanted pigment degradations. Some of these are: – total loss of coloration, leaving gray or black colors – browning of red lead or lead-tin yellow – blackening of lead white or vermilion – greening of blue copper pigments – loss of yellow, purple, and blue colors in mandorlas or rainbow representations Degradations also affect drawings, watercolors, and panel paintings, although usually not to the same degree as mural paintings, ▶Section 7.4.11. Unfortunately, the level of knowledge is still low; only a few systematic studies of a chemical nature are available. According to previous research, pigments containing copper, lead, and mercury, are particularly affected [481]. Since antiquity, there has been and still is a discussion about whether certain pigments are generally unstable in lime binders or intrinsically unstable or whether weather conditions, moisture, and atmospheric gases cause degradations. Pliny the Elder declares, e. g., azurite, orpiment, realgar, cinnabar, red lead, lead white, Stil de grain, indigo, carmine, and madder lake to be unsuitable pigments for fresco. Knoepfli [121] explains that it was not the lime sensitivity of azurite, which posed the problem, but the

7.3 Fresco (mural painting)

� 653

introduction of chloride ions. Also, the transformation of azurite into malachite had not been proven. Lead white would become brown by oxidation or by sulfide formation, but not due to lime intolerance. Furthermore, cinnabar was sensitive to light and heat, not to lime. On the contrary, lake pigments and specific copper, lead, mercury, or arsenic pigments were especially sensitive to lime. The situation is further complicated because, over the millennia, a large number of different names circulated for the same pigments, or the same names denoted different pigments, names had fallen into disuse, or the chemistry of the designated pigments was or is wholly or partly unclear. Earlier restorers often only described the changes visually without chemical analyses of the original pigments, the degradation products, and the environment. [481] systematizes the degradations according to cause, among other things: – The production phase lays the foundations for the durability of a pigment in the paint system. Future events depend on the purity of the raw pigments and the additives used in their production. – The painting phase influences the pigment through the presence of other pigments and the type of inclusion or separation in the painting layer. This structure of the painting layer enables or prevents reactions of the pigments with each other and with environmental conditions such as atmospheric gases. – During the weathering phase, the pigments are exposed to various salts (chloride, sulfate) from the soil and masonry, moisture, and atmospheric gases such as CO2 , SO2 , or O3 . Lime can thus transform into gypsum and coat the pigments with gypsum crusts. These few examples should illustrate that the chemistry of pigment degradations on walls or panel paintings is complex, so we can only depict exemplary processes here. Reactions of copper pigments Blue copper pigments can turn green and degrade to complete blackening:

Blue copper carbonates convert to green copper chlorides by introducing chloride ions in this process. The necessary chloride ions may come from a red lead, in which chloride was always present due to the production process. In industrial time, factories releasing Cl2 - and HCl-containing gases are a regional chloride source. Under the influence of an oxidizing agent, copper pigments can turn into black oxide in the final stage of decay. Conversely, green copper pigments can turn blue:

654 � 7 Paint systems in art

The sulfate anion necessary to form the blue, basic copper sulfates can be derived from gypsum moieties or SO2 from the air. Green copper pigments may also blacken as a result of oxide formation:

Reactions of lead pigments In the case of the lead-containing pigments, mainly changes in the orange-red red lead used until the late Middle Ages were investigated. Phenomena observed are the browning of red lead, which is typical of Gothic paintings, blackening, or fading:

The black or, when mixed with red pigment, brown color impression is caused by black lead dioxide, while a decolorization is caused by transformation into white or colorless lead salts. Lead dioxide also causes the blackening of lead-tin yellow, and the formation of colorless sulfate is responsible for the decolorization of massicot:

7.4 Oil paint

� 655

Reactions of vermilion Vermilion can undergo blackening by converting the trigonal red to the cubic black modification. The phase transformation proceeds under the action of light and heat:

Subsequent reactions can lead to colorless lead salts, e. g., mercury(I) chloride.

7.4 Oil paint Oil painting as we know it dates back to the late Middle Ages and has become the standard technique for European panel painting since about 1500. This success is undoubtedly due to several characteristics that other paint systems did not possess or came to possess only in modern times: – The slow drying of the oil implies a long open time of the paint, i. e., the image remains moist and changeable for a long time. – Fine color gradations are possible by smudging and blending. The earlier tempera technique could only achieve this effect by stroking. – Oil paint can be applied both thinly glazed and plastically thick. – The colors do not change during drying. – The physical film shows the clear surface structure (brushwork) and promotes deep light (pure, intensive colors) instead of scattered light (matte chalky colors). 7.4.1 Basic composition of oil paints Oil paints consist of pigments embedded in a drying oil, usually from linseed. Therefore, we can rub simple oil paints according to this basic recipe: Oil paint [74, Chapter 9], [75, p. 178] 1–3 parts of pigment, 1 part linseed oil or another drying oil (for water-dilutable paints, modified oil together with emulsifier), possibly fillers, and siccatives; for resin-oil paints, a resin (dammar, mastic, acrylic resin). Keep the amount of oil at a minimum; to be more precise, tabulated values of the oil absorption indicate how much oil is required for each pigment; see [75, p. 65ff] for examples.

656 � 7 Paint systems in art

Mix pigment and binder with a flexible palette knife or spatula to a stiff paste. Ground the paste with a muller. Work in more pigment if suitable. Be careful not to inhale hazardous or toxic pigments and avoid direct body contact!

The literature cited, which deals explicitly with self-producing oil and other paints, give detailed practical tips and procedures. Oils suitable for oil painting are drying fatty oils (see the next section), which dry due to their unsaturated character if oxygen is added with cross-linking of the oil molecules and form a transparent, viscous mass, ▶Section 7.4.3. Fillers are included in expensive or strongly coloring pigments. White pigments or cheap calcium carbonates can lighten the shade or stretch the pigment. Siccatives support and accelerate the drying process by their catalytic action, ▶Section 7.4.5; they are compounds with metal as an active component, e. g., cobalt or zinc. Some pigments, such as cobalt blue, already contain these elements. In addition to their color effect, they also have an accelerating effect on drying. Grinding oil paint is not a simple process. At the first attempt, novices in painting produce a paste that resembles oil paint and can be picked up with brushes but crumbles and becomes dusty when applied to the canvas. The reason lies in the energetic barriers that occur during the interaction of pigment particles and the binder, which we have to overcome mechanically or with auxiliaries. ▶Section 6.3 depicts the process of grinding, [74, Chapter 9], [75, p. 187] explain the procedure for homemade paints. 7.4.2 Types of oils Drying fatty oils are responsible for the properties that characterize oil painting: The formation of a durable transparent film with embedded pigments allows us to work with surface light and deep light. Reason enough to take a closer look at these crucial substances [280, p. 581ff]. We can divide fatty oils into nondrying and drying oils. Nondrying oils do not polymerize but cannot dry by evaporation due to their high molecular weight. Therefore, they are of no importance as painting material and would even destroy the paintings due to their persistent softness. The so-called semidrying oils such as soybean or sunflower oil, highly esteemed in the kitchen, also dry too slowly or insufficiently to be considered painting material. However, the drying oils are distinguished because they remain liquid after application, allow to work a la prima, then dry chemically, i. e., polymerize, and form a film. The time required for drying depends on the type of drying oil used. In sum, these oils are the fundamental substances for the oil painting technique. 7.4.2.1 Drying fatty oils Drying oils are obtained by pressing plant seeds or nuts. The most significant ones are linseed oil (from seeds of the linseed plant, whose fibers provide us with linen), poppy

7.4 Oil paint

� 657

Table 7.5: Composition of essential vegetable oils in percent [280, p. 589]. The oils commonly used in the art environment are highlighted in bold type. Fatty acid 16:0 (palmitic acid) 18:0 (stearic acid) 20:0 (arachidic acid) 18:1 (� ) (oleic acid) 18:2 (�,�� ) (linoleic acid) 18:3 (�,��,�� ) (linolenic acid)

Canola

Wheat germ

Maize germ

Linseed

Poppy seed

Walnut

Safflower

Soy

Sunflower

4

17

10.5

6.5

9.5

8

6

10

6.5

1.5

1

2.5

3.5

2

2

2.6

5

5

0.5

0

0.5

0

0

1

0.5

0.5

0.5

63

20

32.5

18

10.5

16

12

21

23

20

52

52

14

76

59

78

53

63

9

10

1

58

1

12

0.5

8

0.5

seed oil, safflower oil, and walnut oil. As a glance at ▶Table 7.5 confirms all of these oils contain high amounts of polyunsaturated compounds responsible for polymerization. They are bound in the oil as esters with glycerol:

The table shows that the classic linseed oil has the highest content of triple unsaturated linolenic acid. As we shall see in a moment, all reactions leading to oil drying begin earlier and proceed faster with an increasing number of allyl groups. Linseed oil, therefore, has particularly pronounced drying properties. In addition to the ability to polymerize, the double bonds add a further property to the molecule, namely the lowering of the melting point. Since the double bonds have a (Z)-configuration, the fatty acid molecules become more curved as the number of double

658 � 7 Paint systems in art bonds increases. This shape prevents the formation of crystalline regions in the molecular structure. Oils are therefore liquid, while fats, which consist mainly of saturated or less unsaturated fatty acids, are solid or semisolid (→ fat hardening by hydrogenation for margarine production). 7.4.2.2 Linseed oil variants Besides raw linseed oil, several variants are manufactured and offered for commercial and artistic use [74, p. 75], [75, p. 169]. Each exhibits particular properties suitable for different fields of application in oil painting, ▶Table 7.6. Table 7.6: Variants of raw and treated linseed oil, in order of decreasing superiority and suitability for oil painting [75, p. 169]. Oil

Treated oil

Refined oil

Linseed oil, cold-pressed Binder Grinding and paint vehicle Paint medium Yellowing, slow-drying

Stand oil Glazing and leveling paint medium nonyellowing, fast-drying

Alkali-refined oil, varnish oil Binder, varnish Grinding and paint vehicle Paint medium Color-stable

Sun-refined or bleached oil Varnish ingredient Leveling paint medium Bleached, fast-drying Linseed oil, hot-pressed not recommended

Blown oil, boiled oil not recommended

Cold and hot-pressed linseed oil Linseed oil can be obtained in several ways. Cold pressing provides the traditional, very pure linseed oil [74, p. 75], [75, p. 169]. After some aging time, mucilaginous sediment settles out, and the oil is ready for use after filtering. It is valued as the highest-quality oil for painting. However, the oil yield increases through extraction with hot water vapor, which delivers an oil contaminated with steam and water-soluble substances. Therefore, it must first be purified or “refined” before being suitable for artistic use and is regarded inferior to cold-pressed oil before purification. Refined linseed oil, varnish linseed oil Hot-pressed linseed oil must be purified (“refined”) before it is applicable for oil painting [74, p. 75], [75, p. 171]. Commercially, the oil is mixed with sulfuric acid, water, and,

7.4 Oil paint

� 659

eventually, bleaching agents. The treatment removes mucilaginous and yellowing components. Subsequently, all traces of acid and water are removed. More modern treatment employs superheated steam instead of chemicals; alternatively, alkali instead of acid can be used to purify the oil, yielding alkali-refined oils with a low acid number, suitable for varnishes and painting. Varnish linseed oil denotes an alkali-refined linseed oil that “do not break”, i. e., oil that can be heated rapidly to 260 °C without flocculating. This type of oil is suitable for the production of clear varnishes due to its color stability, widely available and can be employed instead of cold-pressed linseed oil for general application. Varnish linseed oil is not to be disturbed with linseed oil varnish, ▶Section 7.4.6. Stand oil Stand oil is linseed oil, heated to 275–300 °C for several hours under strict exclusion of air or oxygen [74, p. 75], [75, p. 172], [185, Volume 1]. The result is a heavy, viscous oil with a viscosity of about that of honey. For use, it is thinned with several parts of turpentine. Stand oil is less suitable as an actual binder (although it has not lost its binding power) or as a vehicle for grinding oil paint. However, artists highly value it as a significant additive to glazing and painting mediums, varnishes, oil paints, and tempera emulsions, increasing viscosity and gloss and introducing pleasant flowing properties. Its principal characteristic is its capability of leveling, i. e., to dry to a uniform, smooth, enamel-like film free of brush marks. It imparts this property also to mediums and paints when added. Simultaneously, its wetting power and dispersive properties decrease, and drying speed, leveling, and protective properties increase. While the heating process was carried out in open kettles in former times, the resulting product was inferior due to oxygen access. Nowadays, high-quality stand oil is manufactured industrially, using specialized equipment and high vacuum or a CO2 atmosphere. Lighter and heavier grades of viscosity exist, but there is no particular target viscosity. Stand oil is partially polymerized but not oxidized. Diels–Alder reactions between different triglycerides form the polymers, cross-linked via carbon bridges, ▶Section 7.4.4. Especially drying oils with conjugated double bonds polymerize rapidly at high temperatures since these already have a diene structure. Blown oil Blown linseed oil is a result of blowing air at moderate temperature through linseed oil or other drying oils [74, p. 76], [75, p. 173]. The high oxygen supply accelerates the processes of oxidative drying, and the oil polymerizes as it does when exposed to air after the painting is finished. The thickened, viscous oil is prepolymerized and cross-linked by ether and peroxo bridges. Since cross-linking has already taken place, the blown oil no longer has any binding power and is inferior to raw linseed oil or stand oil. It is not

660 � 7 Paint systems in art recommended for permanent painting, and painters hardly ever use it. Instead, it is intended for coatings and commercial paints. Sun-refined or bleached oil In the Renaissance, a rapidly drying, bleached oil was made by frequently and vigorously shaking linseed oil with water and exposing this mixture to intense sunlight for several weeks while allowing the access of air [74, p. 75], [75, p. 173]. The length and intensity of this treatment were manifold and not concise. Before use, the resulting oil was filtered to remove gelatinous impurities and separated from the water. By this treatment, the linseed oil becomes thick and bleached, partially oxidizes, and polymerizes by cross-linking with oxygen bridges. It is an intermediate state between stand and blown oil, in which the processes of regular drying are partially anticipated and, therefore, sometimes regarded as similar in quality to blown oil. Nevertheless, it was used with good results in the past. It is suitable for clear varnishes, glazes, and painting medium purposes. Boiled oil Boiled linseed oil is linseed oil heated with siccatives until it achieves a slightly thickened consistency [74, p. 76], [75, p. 173]. It is not recommended for permanent painting and intended for coatings and commercial paints.

7.4.3 Drying of oils, film formation One of the most striking phenomena of drying oils is their ability to form a transparent, solid film, which embeds and protects the pigments. For this film formation to take place, we must expose the fresh oil painting to both oxygen and light. To understand what happens during this process and why usually harmful environmental parameters of light and air are necessary, we need to look at several interconnected processes [189, 293, 294], [280, p. 175ff], [892–904]. Many decades of chemical research have shown that hydroperoxides formed from fatty acids by oxygen addition play a prominent role in these processes. Several phases of drying group the processes: – Induction phase. Since all drying reactions are by nature radical, the inherently present antioxidants first appear. Only when their capacity is exhausted, the actual drying begins. – Formation of the hydroperoxides. This phase provides the reactive species from the all-cis acids under cis-trans isomerization and thus determines the rate of oil drying. – Decomposition of hydroperoxides. The formed hydroperoxides decompose relatively readily during the drying phase, which takes weeks to months. The decomposition leads to two types of reactions:

7.4 Oil paint

� 661

–

–

Cross-linking. The radicals formed during hydroperoxide decomposition crosslink molecules and lead to the desired film formation by building a carbonoxygen network. – Cleavage. The hydroperoxides decompose while the oil components undergo cleaving. As the cross-linking and molecular size is reduced in the process, the quality of the film decreases. Hydrolysis of the glycerides. In the months and years after drying, the glycerol esters react to moisture and form glycerol and free acids by hydrolysis. Since hydroperoxides are always formed by atmospheric oxygen, slow cleavage and degradation reactions of the oil components occur.

During the oil drying process, the proportions of certain compound classes show a typical time course: The number of double bonds decreases with time, the amount of cis bonds steadily diminishes, and that of the trans bonds increases at first and then also decreases. The amount of hydroperoxides increases to a maximum and then falls, while the amounts of aldehydes, ketones, ethers, and peroxo-ethers increase. Polymeric ethers and peroxo-ethers are the desired results of film formation, while aldehydes, ketones, and the resulting carboxylic acids and alcohols are signs of film degradation. Furthermore, in more detail, we will consider the basic processes shown summarily in ▶Figure 7.6. In doing so, we will encounter the question of which of several possible hydrogen atoms is radically abstracted. Looking at the binding energies of various types of hydrogen [280, p. 177], we recognize that the bis-allyl hydrogen is the weakest bound since the remaining bis-allyl radical is the most stabilized with respect to mesomerism:

Formation of radicals and hydroperoxides The radicals at the center of all reactions are formed either directly or through hydroperoxides, which decompose into radicals: – Autoxidation. The homolytic cleavage of a hydrogen atom from a fatty acid forms an alkyl or allyl radical, which is then oxidized to the peroxo-radical and maintains the chain reaction. This process is very slow at room temperature and occurs only with unsaturated fatty acids. – Lipoxygenase reaction. In living cells, the enzyme lipoxygenase generates hydroperoxides from fatty acids. Since all paint oils are produced from plants, even fresh oil always contains a small number of hydroperoxides.

662 � 7 Paint systems in art

Figure 7.6: Basic reactions in the drying of oils. Initiation phase: formation of the starting radicals by autoxidation, lipoxygenase reaction, or photo-oxygenation. Chain reaction phase: chain reaction between C-radical and peroxo-radical (also part of autoxidation). Cross-linking phase: formation of peroxo and ether bridges. LOX: lipoxygenase; dashed arrows: photo-oxygenation; bold blue arrows: chain reaction; red arrows: cross-linking reactions.

–

Photooxygenation. Irradiation with light accelerates the oxidation of the fatty acids and the formation of hydroperoxides by a factor of one hundred. This speed advantage is the reason why we expose our paintings to light and air during drying to promote the drying of the oil layer.

Autoxidation The autoxidation or direct oxidation of fatty acids by the ordinary triplet oxygen

7.4 Oil paint

� 663

is spin-forbidden because the lipids have a singlet ground state. However, this barrier is overcome by the triplet oxygen attacking a fatty acid radical to form a peroxy radical. This peroxy radical subsequently changes into a hydroperoxide by abstracting an (allylic) hydrogen atom from another fatty acid molecule. This reaction is part of the chain reaction and the rate-determining step:

The initial C-radical can form in several ways: – by abstracting a hydrogen atom directly from the fatty acid (e. g., thermal). Only the most weakly bonded hydrogen atoms can be used for this purpose, which must be (at least) in an allyl position:

–

by decomposing hydroperoxides into alkoxy, hydroxy, and peroxy radicals under the influence of light, heat, or metal ions. The primary radicals abstract hydrogen from a fatty acid and generate the C-radical:

The hydroperoxides are either already present in the natural material or created from fatty acids by photooxidation or by catalysis from lipoxygenase (see below). ▶Figure 7.7 shows that the mesomerism of the intermediates gives rise to numerous

isomers; in all cases we obtain cis-trans and trans-trans-products from the natural allcis-compounds. As the temperature increases, the proportion of trans-compounds increases. Radicals formed from a bis-allyl system such as the one of linoleic acid are approximately forty times more reactive than simple allyl radicals. They preferentially form products, substituted at both ends of the bis-allyl system. The oxidation products shown can also be observed with triple unsaturated fatty acids. In these, however, the three double bonds do not act together, activating even more strongly than two double bonds, but behave like two isolated bis-allyl systems. As a result, terminal cis-double bonds remain in some products, which can form cyclic

664 � 7 Paint systems in art

Figure 7.7: Isomerization possibilities during autoxidative oil drying induced by mesomers of the allyl radical.

peroxides. The radical can either form a hydroperoxide or further react to form bicyclic compounds:

7.4 Oil paint

� 665

Lipoxygenase reaction

Lipoxygenase is an enzyme that directly yields hydroperoxides from cis fatty acids.

There are several types of this enzyme, each of which targets specific positions, and thus yields clearly defined hydroperoxides:

The resulting hydroperoxides can, by decay, provide the radicals necessary for the chain reaction, as already shown for autoxidation.

Photo-oxidation

Through the photo-oxidation, we circumvent the spin barrier of direct oxidation mentioned before by using singlet oxygen as the reactant, which we can obtain from triplet oxygen by irradiation:

Spin inversion can either be direct or via a sensitizer, which is activated in the first stage:

666 � 7 Paint systems in art

As a result of both variants, singlet oxygen can now readily attack double bonds of the fatty acid in a cyclo-addition. In contrast to autoxidation, bond cleavage and linkage occur in a concerted reaction:

We also obtain a complex mixture of isomers during photo-oxygenation since hydrogen atoms on both sides of the double bond(s) can become the hydroperoxide-hydrogen, ▶Figure 7.8. Since no free radical occurs, the product mixture is composed differently than in the case of autoxidation. From natural all-cis compounds, we obtain trans- or cistrans isomers. With increasing reaction time and temperature, the equilibrium shifts to the side of the all-trans compounds. Symmetric hydroperoxides are not preferred. Furthermore, six-membered cyclic peroxides can form in a concerted reaction:

The hydroperoxides formed in this process can also decay and provide radicals for the chain reaction. The chain reaction At the core of oxidative oil-drying is the chain reaction of an alkyl radical with oxygen forming a peroxy radical, which removes a hydrogen atom from another molecule of fatty acid, thus regenerating an alkyl radical and forming a hydroperoxide:

In sum, one hydroperoxide forms from triplet oxygen per run.

7.4 Oil paint

� 667

Figure 7.8: Isomerizations caused by photo-oxygenation during oil drying.

Compared to the alkoxy or hydroxyl radical, the peroxy radical is much less reactive and abstracts only the most weakly bound hydrogen atoms of the reaction partners. The binding energies of differently bound hydrogen atoms show that the target of the peroxy radical must again be preferably an allyl or bis-allyl hydrogen. Since the resulting O−H bond possesses an energy of about 376 kJ/mol, it becomes clear that saturated fatty acids at room temperature cannot be attacked by the peroxy radical and that unsaturated fatty acids are required for the drying process. Decomposition of hydroperoxides, start of film buildup We would not obtain a film if only isolated molecules of fatty acid hydroperoxides were formed in oxidative drying. However, the hydroperoxides decompose relatively readily into alkoxy radicals, which are available for polymerization reactions:

668 � 7 Paint systems in art Heavy metals, heat, and light promote decomposition (▶Section 7.4.5) and increase the reaction rate considerably. Peroxy radicals are formed due to the higher energy barrier only when radical starters are added to the reaction mixture. Two hydroperoxides can also react with each other to form peroxy radicals and alkoxy radicals:

However, this reaction only occurs when hydroperoxide concentration has become sufficiently large. Polymerization of decomposed products The first components of the resulting oil film are dimers, which form by recombination of the various radicals present:

The recombination of two peroxy radicals leads to the unstable tetroxide I, which decomposes under oxygen release into oxygen-containing fragments of various natures. By abstraction of an (allylic) hydrogen atom from a hydrocarbon, C-radicals can emerge, which provide further opportunities for recombination:

Adding a radical to an unsaturated hydrocarbon forms high molecular weight polymers, retaining the radical character, and the addition continues as a chain reaction, ▶Figure 7.9, pathway I. Since radical attacks occur preferentially at allylic positions, typical

7.4 Oil paint

� 669

Figure 7.9: Reaction pathways in the polymerization of oil films: radicals add to olefins and transfer the radical character to dimers, oligomers, and others. Recombination with different radicals leads to oxygenated polymers.

linkage patterns with one to two oxygen atoms in the bridge emerge from all the above reactions, ▶Figure 7.9, pathway II. Each intermediate radical can combine with hydroxyl radicals, oxygen, or hydrogen donors to form various oxygen-containing products (alcohols, ketones). The film-building polymerization is favored by a high content of unsaturated fatty acids and high oxygen availability, as is the case during the drying of oil paintings. At room temperature, peroxo bridges form preferentially, followed by ether bridges. However, C-C bridges only form in appreciable quantities at higher temperatures, at which point the peroxides decompose, and further reaction possibilities come into play. We exploit this in the hot polymerization process to obtain stand oil, ▶Section 7.4.4. Decomposition of hydroperoxides, degradation and side reactions The radical species formed during the decomposition of the hydroperoxides undergo manifold reactions in addition to polymerization. These reactions are mostly undesirable, as they lead to low-molecular volatile decomposition products, which are often aggressive, such as carboxylic acids, esters, and alcohols. In the food sector, we consider these decomposition products to be distinctly unappetizing and associate terms such as “rank,” “old,” and “fishy,” with them. As long as the oil film of paintings stays exposed to atmospheric oxygen, hydroperoxides form slowly but steadily, and their decomposition promotes further degradation of the oil film.

670 � 7 Paint systems in art ▶Figure 7.10 provides an overview of these reactions. The shown reactions begin in the first phase of drying oils, the oxidative drying, while the content of hydroperoxides and their radical decomposition products is high. However, they continue as long as a painting remains exposed to atmospheric oxygen. Consequently, the already hardened oil film is degraded by β-scission and Norrish scission [906].

Figure 7.10: Side reactions during the drying of oils: formation of various oxygen derivatives from the fatty acids (possibly with cleavage of chains) and disproportionation of hydroperoxide radicals.

β-scission is a mechanism that cleaves bonds adjacent to a radical:

7.4 Oil paint

� 671

Norrish scission attacks carbonyl compounds. Peroxy bridges of the film are homolytically cleaved by irradiation, yielding alkoxy radicals, which react further to form ketones, alcohols, aldehydes, and carboxylic acids. Ether bridges and C-C-bridges are oxidatively cleaved and yield esters, alcohols, and ketones. The resulting radicals can combine with hydroxyl radicals or oxygen to form various oxygen derivatives. The transfer of the radical function according to the vinylogy principle leads to other products:

▶Figure 7.10 also shows the origin of the yellowing of oil films: Several conjugated dou-

ble bonds in α-position to a carbonyl group have a slightly yellowish color. Carbonyl compounds form by recombining two peroxo radicals via an unstable tetroxide or by βand Norrish scission. Besides the desired polymerization and the formation of low molecular weight cleavage products under the breakage of the double bond, more side reactions such as radical cyclization and radical epoxidation consume double bonds, ▶Figure 7.11. The cyclization leads, under dimerization, to cyclohexene. Epoxidation often follows a radical attack on a double bond. At higher temperatures, as is common in boiling stand oil, cyclizations do not occur radically but as [4+2]cycloaddition or Diels-Alder cyclization, ▶Section 7.4.4. Hydrolysis of glycerides, pigment soaps The reactions do not yet entirely stop after the oil film has thoroughly dried, which may take several months. Over the following years and decades, the glycerol esters are hydrolytically cleaved, releasing glycerol and the (cross-linked) fatty acids [905]. Some figures give an impression: after 2 years, 20 % of the glycerides were hydrolyzed, and after 200 years, about 80 %. Hydrolysis is vital for film quality since cross-linking in the film decreases, and the resulting low-molecular substances either leave the film or act aggressively as free acids. The hydrolysis is promoted by traces of moisture and basic metal oxides, as they occur in pigments such as lead white. A well-known phenomenon is the increasing transparency of paint layers in which lead white has been converted into lead carboxylates (“lead soaps”). As organic lead salts, these are no longer semiconductors and are no longer opaque white but transparent. Copper ions can also create copper salts with the released carboxylic acids, including the formerly known, deliberately glazed copper green colors, ▶p. 225.

672 � 7 Paint systems in art

Figure 7.11: Other side reactions caused by a radical attack on double bonds, leading to their decrease.

7.4.4 Stand oils Thickened or bodied oil is a viscous linseed oil type that we can prepare by heating linseed oil in the absence of air (in a vacuum or a nitrogen atmosphere) to 250–300 ℃ and keeping this temperature for several hours, ▶Section 7.4.2.2 at p. 659. During this process, nonoxidative polymerization leads to a thick oil containing numerous cyclic compounds and fatty acid oligomers [103, 189, 894, 907–913, 928]. In the reactions, about 2 of the double bonds disappear. In contrast to blown oils, a residue remains for regular 3 oxidative drying after application. Large portions of the naturally present antioxidants are consumed during heating and cannot delay drying later. Due to pre-polymerization, stand oils dry more slowly than regular oils and absorb less oxygen. This aspect is advantageous because the increase in the volume of the oil film during curing is not as substantial, and the tendency to crinkle is reduced. The product spectrum of the stand oil is diverse and not yet fully understood. Apart from the great heat, a significant difference from oxidative polymerization is the almost complete absence of oxygen, and thus, hydroperoxides. The initially present hydroperoxides are rapidly cleaved, and the peroxo-bridged dimers already formed also decompose to lower oxygen dimers (R−O−R → R−O−R, R−R). The reaction is dominated by (allyl) radicals formed by thermal dissociation:

7.4 Oil paint

� 673

As a result of the radical formation and the possible mesomeric transfer of the radical function into the neighborhood, a shift of isolated double bonds to a conjugated position can occur. This modification changes the initial relative amounts of fatty acids, which can complicate the analysis and identification of drying oils. The product mixture is not, as in cold film formation with air participation, determined by oxygen bonds but by carbon bonds formed from the C-radicals: Radicals can recombine or add to double bonds. The addition preserves the radical character and leads to oligomers. Polymerization via radical reactions The carbon radicals II or III, mostly allyl radicals, formed at high temperatures do not compose hydroperoxides but recombine or polymerize. The recombination of two radicals leads to a carbon-linked dimer:

When the carbon radical adds to a double bond, a new radical forms, which maintains a chain reaction that leads to polymers:

Polymerization via Diels-Alder reactions In addition to radical reactions, a Diels–Alder reaction can also lead to polymerization by cross-linking two unsaturated fatty acids to form a dimer, ▶Figure 7.12. If the oil contains polyunsaturated fatty acids, further Diels–Alder reactions can form oligomers with the remaining double bonds. Similarly, the double bond of the resulting cyclo-hexene can undergo another Diels–Alder reaction, resulting in trimers and oligomers. In practice, however, at most trimers are formed.

674 � 7 Paint systems in art

Figure 7.12: Example of a Diels–Alder reaction during stand oil boiling, leading to dimers. Possibly additional Diels–Alder reactions lead to oligomers. Fatty acids without conjugated double bonds such as linolenic acid must first thermally isomerize before they can function as dienes. The Diels–Alder product still has free double bonds and can act as a dienophile for further Diels–Alder reactions.

The diene component of the Diels–Alder reaction requires conjugated trans-oriented double bonds. Therefore, the unsaturated acids of linseed oil cannot enter the reaction until isolated double bonds as in structure I have been isomerized to the conjugated system IV. Isomerization can occur as shown above through thermally formed radicals II and rearrangement to form III and IV, including also cis-trans isomerizations. The reaction of II to III is favored because III is stabilized concerning mesomerism by the conjugated double bond. The conversion from isolated to conjugated double bonds is slow and rate-determining in the production of stand oil. However, if the oil contains conjugated fatty acids from the outset, stand oils can be produced quickly and easily: Oils with conjugated

7.4 Oil paint

� 675

double bonds can be produced at as low as 250 ℃, whereas oils with isolated double bonds require about 300 ℃ [189]. Other low molecular weight products Secondary heat treatment products are monocyclic and bicyclic fatty acids, which form by intramolecular ring formation. The following compounds may give an impression; the amount and position of double bonds and bridges are variable:

Polyunsaturated fatty acids can also form cyclohexadiene during heating. As diene components, they lead to oligomers in a Diels–Alder reaction:

7.4.5 Effect of heavy metals, siccatives Salts of heavy metals such as iron, copper, cobalt, or lead have such a marked accelerating effect on drying in oil painting that they are now commercially available as so-called siccatives. Early oil painters were aware of this phenomenon and used paints containing these metals as part of the pigment, first and foremost lead white. Since Old Master glaze painters applied it in the underpainting and all other stages of the painting process, a drying accelerator was thus evenly distributed throughout the work. Later, cobalt- or manganese-containing paints took over this role. The effect of the metal salts is based on the decomposition of the hydroperoxides present to alkoxy and peroxy radicals, which initiate the chain reaction of film formation:

676 � 7 Paint systems in art

The metal cation is regenerated in the second step and acts as a catalyst. We need a metal for this reaction, which possesses two oxidation states. In the first step, the reduced form is effective. The theoretically possible direct oxidation of the oleic acids by the oxidized form of the metal without the participation of hydroperoxides is very slow and does not occur during oil drying:

7.4.6 Linseed oil varnish Related to fatty oils and siccatives are products referred to as linseed oil varnish. Unfortunately, the term has no strict definition. It partly refers to a series of more or less defined linseed oil products, partly to the result of oxidative drying of a linseed oil product, i. e., the film. The linseed oil varnish is offered for various applications, such as protective coating for garden or wood furniture, coatings, and as a base for commercial paints and printing inks. In every case, it is more intended for coatings and commercial paints than for artists’ paints. Linseed oil varnish refers mainly to boiled linseed oil to which siccatives have been added for faster drying. There are also “linseed oil varnishes” on the market, which consist of (nonboiled) “linseed oil” mixed with turpentine and drying agents or “quickdrying linseed oil.” The common understanding seems to be that of a boiled or nonboiled linseed oil enriched with drying agents.

7.4.7 Technical improvement of colorants and painting agents in the nineteenth century, paint tubes In connection with oil painting, a product was developed that we encounter everywhere today: the resealable metal tube for adhesives, foods, and countless other items. The improvement originated in 1841 when it took the form of a tin tube and replaced the (pig) bladders used until then for storing paint [604]. From the nineteenth century onward, it became usual for most artists not to produce their oil paints by themselves, as this had been common the centuries before. Instead, a new trade was established by paint dealers and paint manufacturers from the nascent chemical industry or who worked as artists. They produced, distributed, and

7.4 Oil paint

� 677

sold all kinds of pigments and prepared them as ready-to-use paint. The first beginnings of these ready-to-use paints existed already from the seventeenth century, but at all times, the artists had the problem of storing the paint, wherever it came from, in a suitable container if they did not want to make or buy a small amount of paint for each session. The requirements consisted of an airtight seal to prevent premature curing due to oxidation and the delivery of small partial quantities. Until the early nineteenth century, bladders were the principal containers, tightly sealed at their opening. To retrieve the paint, they were punctured, the paint squeezed out, and the hole was closed with a needle or a button. There were some disadvantages: some colors could decompose or change, oxidation occurred after the first opening, smearing color around the hole was the typical case, and bursting of the bladder when squeezing out the paint could never be ruled out. Professional painters did not question these disadvantages. The need for alternatives increased only in the nineteenth century when more and more amateurs were engaged in painting and expected easy handling of the painting material. In 1804, the English paint maker J. Rawlinson suggested fixing a cylinder in the opening of the bladder and retrieving the paint through it. In 1822, J. Harris, an art teacher from Plymouth, presented a tinned syringe that allowed the easy and clean squeezing out of the paint through its nozzle. The experts recognized the benefits, but the devices were too expensive. In 1840, W. Winsor, a painter and cofounder of Winsor&Newton, a company still significant today, patented glass syringes. The colors were easily discernible through the glass, and the glass cylinders could be sealed by lacquered or impregnated membranes for long-term storage. For use, the membranes were punctured and fitted with nozzles and stamps. Winsor&Newton announced this container in 1840, which was well received but also aroused competition. [605] displays some of the containers. As a result, in 1841, T. Brown sold the solution with Brown’s patent collapsible color tubes, which is still the standard for storing oil paints: the metal tube made of tin. However, the actual inventor is J. G. Rand, an American painter, craftsman, and inventor who came to London where he punched the tubes from tin sheets and cold-formed the tin discs into a tube by sudden pressure. London paint merchants and makers immediately recognized these tubes as the definitive solution but did not like to see any other name than their own on their paint tubes. According to [604], e. g., Winsor&Newton worked on their tubes and agreed only after a public advertisement dispute with Rand that he would supply the tin tubes for the company. Shortly afterward, he also became a supplier for other paint manufacturers. The malleable metal tube was quickly used to store all sorts of goods and was made from other soft metals such as lead, but its origin was the search for a suitable container for oil paints. Therefore, it was made of tin since tin, unlike other soft metals such as lead, does not react with pigments, a fact that Rand knew as a painter. Reports of early lead tubes in painting should therefore be regarded with care because they often refer to other quickly found applications.

678 � 7 Paint systems in art Today, tubes for colors are made of aluminum and are protected on the inside by stoving varnish. Since aluminum could not be produced purely until 1825, and due to the complex manufacturing process in the early nineteenth century, this material was as expensive as gold. Therefore, it was not yet significant for the early tubes. Only in the second half of the nineteenth century did the price of aluminum fall. Metal tubes as a prerequisite for plein-air painting? Ease of handling was not the primary consideration in the invention of the paint tube, but rather its usefulness as a tightly closing container for color preservation. The invention of the collapsible metal tube is sometimes credited for giving an impetus to the open-air painting of the Impressionists (from the 1870s onward). However, according to [604] and [606], contemporary chemical developments around oil painting may also be significant. The commercially offered tube paints of this time exhibited considerably improved drying behavior, mainly by adding manganese and cobalt compounds as siccatives. New pigments with pure colors such as cobalt blue, emerald green, viridian, and chrome yellow were bound to inspire the work of landscape painters. As binder analyses show, Impressionist artists used the available, improved painting media. These include prepolymerized stand oils added to purchasable paints and the commercial availability of fast-drying, paste-texturing painting media (resin-oil mixtures), which were in demand for impasto effects in the depiction of foliage and landscapes. Finally, it must be taken into account that from the nineteenth century onward, many other aids such as canvases, primers, portable easels, and brushes were buyable and made it increasingly easier for the plein-air painter. It fits into this more differentiated picture that, at first, portrait painters, rather than plein-air artists used tube paints. In 1840, Winsor&Newton advertised the cleanliness and low odor of the tube paints and not their suitability for outdoor use. In France, tube paints were produced by Lefranc only from 1859 on, and in 1854, E. Delacroix still bought paints in bladders. The taking of bladder paints was standard among French painters. Moreover, 5 to 10 centimes were charged more for the expensive tubes, with an average paint price of 25 centimes. Furthermore, the idea of plein-air painting was by no means new; it had already formed the basis of the painterly activities of the Barbizon School in the 1820s to 1850s, as paintings by Corot and Rousseau show. The real impetus came from the age of enlightenment, which induced British artists to seek pictorial truths on natural locations and produce oil sketches there, namely by R. Wilson, T. Jones, J. W. M. Turner, J. Constable, or R. P. Bonington. In France in the early eighteenth century, representatives were A.-F. Desportes, followed by P.-H. de Valenciennes, C.-J. Vernet, J. Coignet, A. Enfantin, J. B. C. Corot, finally the Barbizon painters and the Impressionists. Callen [607] guides very nicely through the history of plein-air painting. These landscape painters had always used colors in bladders even when working outdoors and had overcome diffi-

7.4 Oil paint

� 679

culties with the available material with their skills. Undeniably, tube paints decisively simplified the outdoor work for nineteenth century artists. Today it is impossible to imagine the artist’s workshop without tube paints.

7.4.8 Resins, resin balsam, turpentine oil Apart from fatty oils and pigments, most of the materials we use for oil painting belong to the large group of terpenes or isoprenoids. Solvents such as turpentine oil, varnish resins such as dammar and mastic, and paint additives such as Venetian turpentine are based on isoprene.

With a repeated linkage of C5 -isoprene units, according to ▶Figure 7.13, nature builds bodies up to C45 . These parent bodies are subject to reaction possibilities such as cyclization, methyl shift, and hydroxylation, which yield a plethora of compounds. The low molecular weight C5 representatives are volatile, highly aromatic compounds. Many flavors and fragrance compounds belong to them and span a wide range, from fresh lemons to fennel, camphor, cinnamon, or conifer aroma. The next higher C15 compounds are hardly volatile and constitute aroma and bitter compounds, e. g., in grapefruit. As molecular weight increases (C20 and above), we arrive at the resin acids. They serve plants as wound closure, insecticide, or saponin, and we will look at them in more detail in a moment. Even higher oligomers from C40 are important as yellow and red carotenes, which are plant colorants in foliage, fruits, and vegetables, ▶Section 4.3. While plants synthesize all levels, humans primarily produce triterpenes (steroid hormones and bile acids). C10 - to C30 -compounds are of interest as painting materials. They are produced by many plant families and occur mainly in two forms: – Resin balsams are viscous to solid masses deposited from living trees and consist of di and triterpenes (resin acids, C20−30 ). The resin acids are dissolved in mono or sesquiterpenes (C10−15 ). – Deposited or fossil resins consist of partially polymerized resin acids and terpenes. Amber is a beautiful example in the truest sense. Carpenters value the fossil resins as components of valuable furniture varnishes. The resin balsams provide the painter with important solvents and varnish resins [185, Volume 1], [103]. Specific trees are cut to obtain them, and the sap that emerges is collected [949]. The sap consists of two fractions:

680 � 7 Paint systems in art

Figure 7.13: Biosynthesis of isoprenoids from isopentenyl pyrophosphate (IPP) and dimethylallyl pyrophosphate (DMAPP) [274, p. 89], [273, 275]. The paths relevant to painting that lead to solvents (turpentine oil), resins, and carotenoid colorants are indicated.

– –

Liquid monoterpenes. The resin balsam of conifers (pines, spruces, firs, larches) provides turpentine oil. Solid di and triterpenes (resin acids, balsamic resin). We can derive rosin from conifer balsam, the dammar resin from certain Indonesian tree resins, and the mastic from pistacia species.

The resin is initially liquid when it flows out but rapidly thickens as soon as the most volatile monoterpenes have evaporated. The nonvolatile balsamic resins remain behind

7.4 Oil paint

� 681

and partially crystallize so that the sap becomes cloudy. We can obtain the two main components in pure form by distillation and subsequent purification. Since the distillation technique has been known in Europe since the sixteenth century, oil painting developed further only with the availability of this solvent. 7.4.8.1 Turpentine oil Turpentine oil represents the purified, volatile distillate from the crude resin of conifers. Pine species are used almost exclusively, as these trees proliferate, are widely distributed, and produce high resin yield (up to about 3 kg per year), lowering the turpentine oil cost. The composition of turpentine oil varies greatly with its origin, usually consisting of monoterpenes of approximate relative abundance [185, Volume 1], [103, 963, 965]: α-pinene > β-pinene > Δ3 -carene > limonene > myrcene > β-phellandrene

The amount of α-pinene can be as high as 70 %. Due to its structure, turpentine oil is nonpolar but more polar than pure hydrocarbons due to its oxygen functions. Therefore, we can use it as an efficient solvent for fatty oils (linseed oil) and natural resins (rosin, dammar, mastic). It is also applicable for dispersing waxes. High-grade turpentine oil is distilled several times and evaporates without residue. The double bonds can give rise to oxidation and polymerization reactions (resinification) during storage but without a regular film being formed. 7.4.8.2 Rosin, dammar, mastic The solid components remaining after the distillation of the volatile resin oils from the resin are the actual balsamic resins. As a result, we obtain several well-known products based on the resin’s origin, ▶Table 7.7. Balsamic resins consist of a complex mixture of

682 � 7 Paint systems in art Table 7.7: Botanical origin of important balsam resins [103, 963]. Type

Product

Coniferae (Pinaceae) Pinus (pine) Abies (fir) Larix (larch)

Common or Bordeaux turpentine, rosin Strasbourg turpentine, Canada balsam Venetian turpentine

Leguminosae Hymenaea, Copaifera

Copal resins, copaiba balsam

Dipterocarpaceae Hopea, Shorea (winged fruit plants)

Dammar resin

Anacardiaceae Pistacia (pistachio)

Mastic resin

resin acids with di, tri, or tetraterpene skeletons and a proportion of fatty acids and fatty alcohols [963, 965]. Pinaceae resins They are best known to us in terms of their composition. Although it is subject to variation, we can indicate the following rough proportions: pinus species: Levopimaric acid/pallustric acid > iso-pimaric acid > (neo-)abietic acid > dehydroabietic acid, pimaric acid, sandaracopimaric acid abies species: Abienol > abietic acid ≥ levopimaric acid/pallustric acid > neo-abietic acid larix species: Larixyl acetate > iso-pimaric acid > levopimaric acid/pallustric acid > (neo-)abietic acid ≥ epimanool/larixol

During distillation, isomerization of the double bonds occurs so that the original composition changes, and we, e. g., no longer find levopimaric acid in the rosin but more abietic acid. Carboxylic acids with abietane and pimarane skeletons dominate the pinus resins. They do not show a high tendency to polymerize and form soft resins.

7.4 Oil paint

� 683

Abies and larix balsamic resins also contain compounds with the bicyclic labdane skeleton:

The labdane skeleton of abies species can polymerize and lead to the hardening of the resin over time. Polymerization occurs via the diene structure [964], primarily as a 3,4addition and to a minor extent as a 1,4-addition, since the internal double bond is sterically shielded:

Leguminosae resins They form the basis of copal resins and are also based on the labdane skeleton. In addition, there are soft, nonpolymeric resins (copaiba balsam) and hard resins (copal resins) based on labdadiene in which polymerization has taken place through the conjugated double bonds. Remarkably, these resins contain almost only the enantiomers of the constituents of the pinaceae species. However, since many of the resins were studied before the era of GC/MS, our knowledge is less precise than that of conifer resins:

684 � 7 Paint systems in art

Dammar resins They are built up from triterpenes. A nonpolar, alcohol-insoluble fraction (β-resene) consists of poly-cadinen, and the remainder (the polar, alcohol-soluble, and partially acidic fraction, α-resene) contains hydroxy- and oxo-derivatives of tetracyclic and pentacyclic triterpenes with the skeletons of dammarane, oleanane, ursane, and hopane [103, 948, 949, 951, 963]:

7.4 Oil paint

� 685

Dammar resin is sparingly soluble in water and other solvents but dissolves in turpentine oil and melts at temperatures above 80 ℃. It has been a classic varnish resin since the nineteenth century; in ▶Section 7.4.10, the role of varnish is delineated in more detail. Mastic resin This resin was already known in antiquity. It was obtained from trees of the species pistacia, native to the Mediterranean region, especially the island of Chios, and thus occurs in the surroundings of the ancient cultures. Similar in composition to dammar resin, it contains high proportions of compounds with the euphane and oleanane skeleton, as well as one polymer (cis-1,4-poly-β-myrcene) [103, 951, 955, 963]:

Mastic resin has a melting point of around 100 ℃ and is slightly soluble in water and other solvents but readily in turpentine oil. This solution has been used as a resin varnish since the seventeenth century, ▶Section 7.4.10.

686 � 7 Paint systems in art 7.4.8.3 Aging, oxidation of resins The protective effect of varnishes with dammar or mastic resin is controversial since the easy oxidizability of the terpene ketones leads to numerous low molecular weight degradation products over time [948, 950–953]. ▶Figure 7.14 shows two pathways that could cleave the terpene ring systems. Both follow the typical chain reaction of radical oxidations:

Figure 7.14: Typical reactions during oxidative degradation of dammar and mastic resin varnishes. The figure shows a typical partial structure of triterpenes and possible subsequent reactions.

The initial radicals preferentially form adjacent to double bonds or carbonyl groups. Recombination or the formation of new double bonds breaks the radical chain. As a result, alkenes, aldehydes, carboxylic acids, and carbonyl groups are created. In the mastic resin, up to six oxygen atoms are introduced into a molecule in this way. Typical oxidation products are

7.4 Oil paint

� 687

Interestingly, oxidation occurs rapidly and independently of light exposure; only the reaction rates vary. Thus, after 7 weeks in the light, similar amounts of degradation products were found as after 31 weeks in darkness. As a result of oxidation, the skeletons of the triterpenes become reduced, modified, and highly functionalized. Polymerization of the intermediates hardly takes place due to the rapid further reaction of the radicals to carbonyl groups. As a result, the oxygenated compounds are hydrophilic and often acidic, leading to the well-known solubility of aged varnishes in alcohols. Significant for the conservation of old paintings are brittleness and yellowing of the varnish caused by the decomposition of the resin components; altered or faded colors are the consequences. Yellowing occurs especially when the varnished paintings are stored in the dark; this can be caused by light-induced oxidation and bleaching of the colored degradation products by sunlight. The coloration of the degradation products is caused by more or less extended double bond systems in conjugation with carbonyl groups, as they emerge during autoxidation, ▶Figure 7.15. Many of the disadvantages mentioned should no longer occur in modern varnishes based on cyclohexane or similar substances. However, their very short modern product cycles do not yet allow any statements over a more extended period, ▶Section 7.4.10. 7.4.9 Other solvents: benzines, turpentine substitutes Today, in addition to vegetable balsams such as turpentine oil, we can also buy solvents based on mineral oil. These include “wound gasoline,” “wash gasoline,” nitro thinner, and turpentine substitutes. These terms refer to liquid mixtures of aliphatic and aromatic hydrocarbons that result from gasoline production in petroleum refineries. The crude gasoline fraction (naphtha, boiling point 30–220 ℃) is further separated by fractional distillation, giving us different groups of mineral spirits [914]. Aromate-free aliphatic solvents Aromate-free aliphatic solvents consist of lower aliphatic compounds and have low boiling points and flashpoints. They are classified according to their boiling range and can be obtained chiefly freed of aromatics by fractional distillation:

688 � 7 Paint systems in art

Figure 7.15: Emergence of colored, quinoid, or conjugated structures during oxidative degradation of resin varnishes [956, 957].

– – – –

petroleum spirit 40–60 ℃: n-pentane, 2-methyl-pentane, cyclopentane, and cyclohexane petroleum spirit 60–80 ℃: n-hexane petroleum benzine 100–140 ℃: iso-octane, cyclohexane to cyclooctane, n-hexane, n-heptane petroleum spirit 140–165 ℃: n-octane to n-decane, iso-octane, iso-decane, cyclooctane, cyclononane

Aliphatic solvents containing aromatics These solvents are mixtures of aliphatics and aromatics: – naphthabenzine 100–140 ℃: n-hexane to n-nonane, iso-heptane to iso-nonane, cycloheptane, cyclooctane, toluene, xylenes – crystal oil 135–180 ℃: n-octane to n-undecane, iso-octane to iso-undecane, cyclooctane, cyclononane, xylenes, mesitylene, cumene, and other aromatics – turpentine substitute, turpentine oil substitute, white spirit 140–200 ℃: n-octane to n-dodecane, iso-nonane to iso-dodecane, cyclooctane to cyclodecane, tri- to pentamethyl-benzenes Today, they are also offered dearomatized by hydrogenating the aromatic components.

7.4 Oil paint

� 689

Solvents rich in aromatics They consist almost exclusively of aromatics, and thus have excellent dissolving power but, unfortunately, also high toxicity: – 160–182 ℃: tri and tetramethyl benzenes, cumene.

7.4.10 Varnish materials Before we consider the types of varnishes used, we must keep in mind what function a varnish should perform: – A varnish gives the painting more surface gloss, evens out microscopic irregularities in the painting layer, and provides a smoother surface. Consequently, the amount of light is reduced, which would otherwise be diffusely reflected and scattered by fine inhomogeneities, ▶Section 1.6.8. Instead, there is an increased depth of light and directional reflection, which manifests as surface gloss. – The varnish adds depth of color and color clarity to the image due to the depth light. In addition, the reduced scattering causes more light to pass through the painting layers and emerge as colored depth light, and the colors appear darker and richer, ▶Section 1.6.8. – The varnish gives the image more sharpness of detail because the varnish encloses all pigment bodies in a transparent body with a uniform refractive index. The incident light is thus refracted uniformly into the varnish and not to variable degrees at different pigment grains. – Layers of varnish protect the painting from environmental influences. [958] illuminates these points in more detail and discusses the advantages and disadvantages of varnishes. Few information sources about the early varnishes exist. However, known and available materials always served as a basis. ▶Figure 7.16 indicates essential key data [945]. We know that early varnishes consisted of oils thickened or fused with various resins to form thick layers with a high tendency to yellowing and browning. Then, in the seventeenth century, the first spirit-based varnish appeared, which could be applicable from today’s point of view: mastic dissolved in turpentine oil, which became the crucial painting varnish in the nineteenth century and early twentieth century [959]. Mastic also shows a slow yellowing. From 1827, dammar resin played a role, but it was little appreciated in the English and French regions. It was not established until the late twentieth century, especially in German-speaking countries. It yellows less than mastic. Both resins are initially soluble in turpentine oil, the solubility progressively decreases with aging, and the solubility in alcohol increases.

690 � 7 Paint systems in art

Figure 7.16: The development phases of varnishes [945].

Natural resin varnishes The most critical application of dissolved balsamic resins in painting is as a varnish, which came into practice in the seventeenth century. At that time, turpentine oil distillates became available, needed to dissolve the resins. The solvent evaporates from the resin solutions a short time after application, and the remaining balsamic resins spread thinly on the painting, forming a first solid, protective film. The resin film initially dries up purely physically, and cross-linking hardly occurs in the further course [948, 950–953]. The actual oxidative-chemical drying of the fatty oils takes place in and under the resin film in the course of the following weeks. We have already learned about the chemistry of dammar and mastic resins in detail in ▶Section 7.4.8 at p. 679 connected with balsamic resins and touched upon the question of the aging of resin films. Numerous investigations [948, 950–953] show that the protective function of a natural resin film diminishes after a time and finally ceases to exist. A detailed investigation into the general suitability of dammar resin as varnish is given in [949]. Synthetic polymer varnishes Synthetic polymers are repeatedly mentioned as an alternative to the natural resin varnishes with their recognized disadvantages. In Koller and Baumer [946], some high polymer materials of the twentieth century (acrylates, polyesters) are discussed, but not as an alternative to the conventional resin varnishes. In this way, e. g., the low-molecular natural resins dry without tension because the small molecules can arrange themselves freely as the solvent evaporates. On the other hand, polymers are hindered in their mobility and remain in a “frozen” state of tension after evaporation, so the addition of plasticizers is necessary. In the case of polyacrylate polymers, copolymers can be used instead.

7.4 Oil paint

� 691

Cyclohexanone varnishes (ketone resins) Cyclohexanone varnishes belong to low-molecular materials, but they are also negatively evaluated in [947]. Their mechanical and film-forming properties, redissolubility, and tendency to embrittlement also do not meet the requirements of a varnish. Nevertheless, they are valued for the restauration of Old Master’s resin varnishes [954]. The chemical structure of these varnishes, which have found a certain distribution since 1952, is not fully known; their resin basis forms two types of oligomers with the approximate structures shown in ▶Figure 7.17 [947, 954]. Both types are built by continued condensation from cyclohexanone or methyl-cyclohexanone, splitting off water molecules and forming single or double bonds. One molecule incorporates up to 12 monomers.

Figure 7.17: The two types of cyclohexanone resins and the synthesis of type II resins [947, 954].

7.4.11 Pigment degradations We have already discussed the undesirable change in pigments affecting the color over long periods based on damage to mural paintings, ▶Section 7.3.5. Oil paintings, drawings, and watercolors are stored in a more protected manner. Therefore, the damage is often not so pronounced. Nevertheless, there are several undesirable changes here as well, of which we can only look at typical representatives, such as – the (total) loss of yellow lakes (visible in areas of mixed green leaving blue trees and plants) – the (total) loss of red lakes (visible in pale faces and dull robe fabrics)

692 � 7 Paint systems in art – –

the (total) loss of blue color from Prussian blue or smalt (visible in silvery-gray skies and changed tonality of the painting) the aging of the linseed oil medium (visible in the vanishing of the painting subject into hard-to-read, formless semidarkness)

Color changes are caused by pigment or binder degradations, by the reaction of pigments with each other or with binders. In the following, we will look at examples for which a basic level of knowledge is already available. Overall, however, pigment and paint damages are a current research topic in art technology; see, e. g., [389, 390]. 7.4.11.1 Reactions of iron pigments While iron oxide pigments are very stable, the iron blue pigment (Prussian blue) is a different case. Shortly after introducing this intrinsically valuable blue pigment at the beginning of the eighteenth century, the tendency of the pigment to fade became apparent. Among painters, this phenomenon was perceived. Why they used the pigment regardless has been outlined in ▶Section 1.3.4. Consequently, we found examples of this in the works of R. Wilson, A. Watteau, Canaletto around 1730, T. Gainsborough around 1750, or Tiepolo around 1750, notable, especially in bleached-out parts of the sky, which are nowadays too light, turning greenish or whitish. Börner, Kirby, and Saunders [431–433] confirm the tendency of Prussian blue to fade due to photochemical reactions. Systematic investigations showed that fading only occurs with white mixtures, not pure pigment. Unfortunately, the pigment can hardly be used in isolation due to its high coloring power. Partly responsible for the poor durability of the early pigment were obscure manufacturing processes and their parameters. They were determined by trial and error, their influence on the pigment quality was unclear, and the addition of fillers was not traceable. The modern pigment is listed as moderate to durable, except in very thin glazes or mixtures with a high content of white. Good durability is the result of a controlled production from pure starting materials. 7.4.11.2 Reactions of cobalt pigments Smalt, a cheap cobalt blue glass, was used to supplement the limited blue range for a long time. Unfortunately, while cobalt blue pigments are very stable, the same can not be said of smalt. Consequently, it soon became apparent that the glass was prone to bleaching. The result is visible, e. g., in paintings by P. Veronese, B. Bruegel, or J. Beuckelaer, all from around 1560–1570, or T. Gainsborough around 1750 [426, 552, 559]. Optical damage includes total vanishing of blue sections, graying and discoloring of delicate bluish-purple shadows, and yellow-brown discoloring of the medium. When used in skies and to tint shadow areas, these changes often alter the entire tonality of the painting. According to [426–428], color bleaching occurs due to the structural loss of the potassium glass, the fundamental matrix of smalt. In smalt, cobalt is present as a tetrahedrally

7.4 Oil paint

� 693

coordinated CoII cation whose LF transitions are symmetry-allowed and intense due to tetrahedral symmetry (CoO4 chromophore). However, as soft glass, potassium glass is notoriously susceptible to moisture and acids, which slowly dissolve out the alkali and alkaline earth cations, especially potassium in the case of smalt. Potassium stabilizes the tetrahedral structure so that after the migration of K, the chemical environment of CoII changes, and the coordination number increases up to the octahedral coordination. However, this means that a symmetry-forbidden LF transition is now responsible for the color, which is considerably weaker:

Alterations in the medium’s color are accompanied by complex changes triggered by the formation of potassium soaps. 7.4.11.3 Mercury pigment reactions Blackening of vermilion is a well-known phenomenon [395]. Red vermilion can be stable for hundreds of years, while in other paintings, it can form a silver-gray or black crust that severely disfigures the painting’s content. Examples span all centuries, from G. del Ponte around 1420, P. Uccello around 1440, B. Gozzoli around 1460 to J. Jordaens around 1620, or A. Cuyp around 1650. Investigations showed that impurities from the wet production favor the blackening; nevertheless, the more stable vermilion from the dry production also shows blackening, sometimes next to well-preserved parts in the same painting. In most cases, black and white degradation products were found side-by-side. It has been shown that the white decomposition product mercury(I) chloride (calomel) emerges in cooperation with chloride from the air or dirt, light, and moisture. Earlier attempts formulated reactions such as these, additionally yielding black mercury sulfide:

Newer experiments suggest that the reaction proceeds via kenhsuite and corderoite up to elemental mercury [389, 396]:

694 � 7 Paint systems in art

In addition to the purity of the cinnabar, painting methods have a decisive influence on whether it blackens or not. Examples show that cinnabar has been preserved where it was protected by additional layers of paint, which either reduce the amount of light or form a barrier against, e. g., chloride ions. 7.4.11.4 Reactions of cadmium pigments The bright cadmium yellow pigments were frequently used in the nineteenth and twentieth century by impressionists. Nowadays, in some important artworks, a discoloration can be observed, e. g., in works by Van Gogh, Ensor, or Matisse [389]. The mechanism behind is a photo-oxidation of cadmium sulfide to cadmium sulfate in the presence of oxygen and moisture. Depending on the environmental conditions, manifold other cadmium compounds may be produced:

The reaction is not yet completely understood, some of the compounds found may be residues or additives from the manufacturing process. Also, the influence of the crystal structure and stoichiometry on stability is not yet clear. 7.4.11.5 Reactions of arsenic pigments Realgar is a bright orange-red pigment, which in some paintings shows significant change to a yellow powder, e. g., in J. Tintoretto’s Gonzaga cycle, whose expressive yellow highlights on fabrics and in flames initially showed milder orange tones [549]. Wallert [408] contains a formulation for a transformation of realgar to yellow orpiment:

7.4 Oil paint

� 695

However, according to [60], orpiment is thermodynamically more unstable, so the transformation must be formulated with yellow para-realgar as the final product:

In this mineralogically well-known transformation, the weak As−As bonds in the realgar break, and free arsenic forms, destabilizing the realgar structure and combining with the surrounding arsenic and sulfur atoms to form the powdery structure of para-realgar. Despite the same molecular formula, para-realgar has a different structure than realgar. Arsenic (white arsenic(III) oxide), also mentioned in the sources, can be formed as an oxidation product. 7.4.11.6 Reactions of lead pigments – Changes in binder Burmester and Krekel [549] describe a change in a picture cycle by J. Tiepolo, affecting the binder and effective over large areas: the vanishing of the picture subject into a semidarkness where contours or colors are difficult to discern. This example is only one of the countless other paintings of the most diverse painters and epochs, e. g., the Dutch of the seventeenth century such as Rembrandt, A. van der Neer, J. van Goyen, J. Steen, F. Hals, J. Vermeer, N. Berchem, or also V. van Gogh 1888. Affected are oil paintings from the fifteenth century to the twentieth century; on canvas, paper, wood, or copper; even layers of painting in egg tempera or varnish layers can show this type of damage. The mentioned visual changes include physical damages to the painting material, which may occur in different combinations: microscopic holes of 100–200 µm size distributed over the painting, streaky dark spots, incrustations, and spalling of parts of the painting layer. Facing many of these disturbing, damaged paintings, restorers, technicians, and chemists attempted explanations already at the beginning of the twentieth century. The chemist, A. P. Laurie, explained the increased transparency of whole layers of paint with an age-related increase in the refractive index of the linseed oil medium. Fresh linseed oil has a refractive index of about 1.48, which increases to about 1.6 with aging (▶Table 1.17 on p. 53). A pigment’s opacity depends on the difference between its refractive index and that of the medium, ▶Section 1.6.8. Since the refractive index of, e. g., malachite, verdigris, bone black, and smalt is close to that of the fresh linseed oil (▶Table 1.20 on p. 71), initially covering layers of these pigments can appear transparent today even with a slight increase in the refractive index of the medium. Lead and zinc soaps This explanation is correct for the low refractive index pigments mentioned. However, it does not explain why painting layers with high refractive pigments such as white lead,

696 � 7 Paint systems in art whose refractive index even in aged linseed oil is still clearly above the medium, are affected. At the beginning of the twentieth century, chemist, A. Eibner, argued that pigments and siccatives release lead cations. These form low-refractive lead soaps (lead salts of fatty acids) with free fatty acids of the linseed oil and thus cause transparency. Subsequently, it was also observed that damages were caused by other lead compounds such as lead-tin yellow or red lead. In the late twentieth century, the subject of “lead soap” and, more generally, “metal soap” became the subject of broader research dealing with the phenomena and damage patterns of aging of painting layers. The interim results display the painting layer as a system full of dynamics [915–920]: – The refractive index of fresh linseed oil of 1.48 increases to about 1.6 during aging, making low refractive pigments transparent or translucent. – Lead soaps consist mainly of stearic acid, palmitic acid, and azelaic acid lead salts. They form amorphous voluminous aggregates, migrate to the surface and breach it. This increase in volume leads to physical changes in or on painting layers, e. g.: Peeling and flaking, lifting, micro-perforation, cratering, efflorescence, haze, and incrustations. If underpainting is causative, the damage may be local or picture-wide if the metal soaps form in the primer. – Lead soaps lead to significant visual changes: paint layers containing lead white, imprimaturas, and primers become transparent, ▶Figure 7.18. Transparency emerges by lowering the refractive index from 1.9–2.1 for lead white to 1.50–1.55 for the formed lead soaps. This value is close to linseed oil and makes saponified lead white practically transparent since scattered light is partly or entirely suppressed. The accompanying visual change depends on the color of the affected layer and the layer structure as a whole: – Underlying layers become discernible through shape and contour-giving upper layers, primarily when low opaque pigments such as smalt are used in skies. Underpainting and pentimenti shine through. The image becomes difficult to discern due to loss of form and disappearance of the intended light-dark contrast. – White grounds or light imprimaturas disappear optically and let the often dark or brown-gray deeper layers shine through, or even the picture carrier itself, e. g., dark irregular wood. Finally, the painting subject vanishes into the ground. – Thick layers of lead white are less vulnerable than thin ones. All effects work together to affect a complex pattern of damage. So far, mainly soaps with the participation of lead or zinc cations have been identified, although it is difficult to distinguish metal cations from a pigment or a siccative. For example, damage patterns with zinc soaps can be found in images of the nineteenth century and twentieth century when zinc white was intensively used. The source of lead cations is mainly lead white, but also lead-tin yellow, red lead, or siccatives. Copper and potassium soaps occur to a

7.4 Oil paint

� 697

Figure 7.18: Increase in the transparency of whole painting layers with lead soaps [915, 916, 919]. The change becomes visible when no white painting layer exists under the layer that has become transparent. The depth light that now appears makes the lower painting layers or even the entire primer visible. If the primer has become saturated with oil, it also turns transparent. Besides, the often dark primer creates a general semidarkness. In addition, the deeper painting layers no longer fit the motifs visible at the top, resulting in a lack of contours and poor legibility. Superficial dirt and yellowing binders exacerbate the problem. ▶Section 1.6.8 describes the consequences of scattering phenomena. (a): Freshly dried painting. The primer in the example is calcium carbonate, n = 1.57 (white). Imprimatura and painting layer consist of aged linseed oil, n = 1.60, lead white n = 1.9, yellow ocher n = 2.3, and a blue pigment. The color effect is as intended by the painting layer and the imprimatura. (b): Aged painting. The primer has absorbed the linseed oil. The difference in the refractive index has disappeared, and the primer has become practically transparent. Lead white has been transformed into lead soap and is in linseed oil also transparent, n ≈ 1.50. Light can penetrate unobstructed to the image carrier and strongly distort the visual impression as dark depth light.

lesser extent. As fatty acids, mainly palmitic and stearic acids, were found, to a small extent, also azelaic acid as an oxidative degradation product, ▶Figure 7.19. The fatty acids are provided by hydrolysis of the triacylglycerols (TAG) of the oil medium. Investigations on Dutch pictures of the seventeenth century show dark discolorations, which trace the grooves of the wooden substrate. The lime base has accumulated thick layers in these grooves and has absorbed oil from the medium. In addition to the darkening mentioned above, the primer has become transparent by matching the refractive index to oil. It is assumed that such thick layers of a porous lime primer soak up oil like a sponge. During the aging process, the necessary free fatty acids are formed by hydrolysis at high concentrations at specific points and cause localized damage. This example shows that the concrete damage patterns are strongly dependent on the structure of the painting layers and can thus be assigned to an underlying school, epoch, or artist group.

698 � 7 Paint systems in art

Figure 7.19: Formation of transparent lead soaps from fatty acids and lead cations. The fatty acids originate from the hydrolysis of triacylglycerides of linseed oil or the oxidative cleavage of fatty acids (azelaic acid), the lead cations from lead-containing pigments, typically lead white, lead-tin yellow, or leadcontaining siccatives [915–917].

The exact processes are still the subject of research. Casadio et al. [915, Chapter 3] and Hermans [920] summarize the results. According to them, in a first step, aged linseed oil with soap-forming pigments represents an ionomer, a cross-linked anionic polymer with carboxyl groups from hydrolysis and cleavage reactions. Metal cations released from the pigment diffuse through the ionomer until dynamic electrostatic-chemical equilibrium has been established, and the carboxyl groups are bound to metal cations I:

7.4 Oil paint

� 699

The partners in these metal carboxylates are not permanently bound, and the metal cations can diffuse and form new loose coordinative bonds. In a second step, hydrolysis of the esters (triacylglycerides) contained in drying oils occurs, either spontaneously by moisture or by the involvement of diffusing metal cations. Cations such as PbII can act as Lewis acids and be coordinately bound by carbonyl groups of the esters. Due to their electron-withdrawing properties, the bound metal cations weaken the ester bond, and thus facilitate hydrolysis by moisture. The metal cations can bind to the resulting fatty acid carboxylates (FA) and form long-lasting metastable systems. In the third step, soaps can form rapidly when the equilibrium is disturbed, and a more significant part of free fatty acid carboxylates becomes available, either by hydrolysis or by adding, e. g., beeswax or aluminum stearates. Saponification proceeds rapidly and irreversibly since the metal soaps formed are insoluble in linseed oil (structure II, see above). After the formation of crystallization nuclei, free fatty acid carboxylates and metal cations diffuse to the nuclei and enlarge them to macroscopic aggregates. The exact processes depend on the concentrations of all the partners (such as fatty acid carboxylates and metal cations) and the diffusion, hydrolysis, and crystallization rates. Ultimately, the cleavage processes leading to the free fatty acids are complex and depend on conditions in the painting layer and environmental parameters. For example, high humidity leads to photolysis of fatty acids into shorter dicarboxylic acids such as azelaic acid, while higher temperatures accelerate hydrolysis to saturated fatty acids. Other metal soaps Especially in connection with smalt, potassium soaps could also be found, formed when the glass deteriorates and K migrates into the medium where it reacts with fatty acids. This process is assumed responsible for the brown discoloration of the medium and superficial clouding. Finally, copper soaps have long been known as reaction products of copper pigments with oil. Further damage patterns caused by lead pigments In addition to the damage patterns mentioned above, others include the formation of delicate white veils or dense white crusts of complex salt mixtures when smalt is present. The salts have been identified as lead potassium sulfate and calcium oxalate. Cations of lead and potassium are derived from smalt potassium glass, PbII also from lead white, the necessary sulfur from the atmosphere. The formation of the lead soaps promotes this reaction, as it facilitates the dissolution of lead white and promotes mobilization of lead cations:

700 � 7 Paint systems in art

Red lead can bleach by the reaction of photo-excited Pb3 O4 that reduces PbIV to PbII , induce decarboxylation of the oil binder, and form white lead carbonate from the released CO2 [389]:

7.4.11.7 Reactions of red and yellow lake pigments Frequent and conspicuous examples of the degradation of organic pigments appear in the area of yellow and red lakes [401–403, 549, 559]. The yellow lakes include laked flavonoids such as Stil de grain from rhamnus berries or yellow resins such as gamboge. We have already addressed the importance of yellow lakes for green mixtures and their problems, ▶Section 1.3.5. Typical, distinct visible changes occur in paintings over time caused by the problems mentioned there. Eye-catching is part of painted foliage or whole forests and landscapes, as well as fruit, which in extreme cases appear gray-blue or streaky-brown today, as in paintings by J. v. Huysum, J. Steen, M. Hobbema, C. Lorrain, P. Lastman 1618, A. Cuyp 1650, J.-B. Geuze at the end of the eighteenth century. Here are the yellow lakes of the green mixture wholly decomposed. Yellow lakes lose color and leave behind white material from the substrate and unknown decomposition products, making the painting appear blanched, chalky, or cloudy. Fortunately, the changes mentioned above are not ubiquitous. The majority of Old Master paintings do not show signs of degradation at all or only to a moderate extent, without noticeably impairing the effect of the painting. It is, therefore, a goal of today’s research to clarify the exact circumstances under which damage occurs. Stable yellow pigments are lead-tin yellow (opaque), Naples yellow (opaque), yellow ocher, and in more modern times, chrome yellow. The red lakes include madder lake, crimson, and lac dye. Effects of fading of red lakes are detectable in paintings from the fourteenth century on, e. g., by J. di Cione 1370, L. di Monaco 1414 (lac dye), B. Daddi, A. Gaddi, and other Florentine and early Italian paintings of the fourteenth century and later the fifteenth century, by Tintoretto 1580, Campana 1590, J. Reynolds 1760 (cochineal lake), or T. Gainsborough 1770 (cochineal

7.4 Oil paint

� 701

lake). While yellow lakes are often used in landscape and nature depictions, deteriorated red lakes result in pale white faces, lacking freshness, liveliness, modeling, and dullcolored clothing and draperies, which in the circles of the noble patrons were hardly worn like this. In the case of red lakes, influencing variables and their effects have already been systematically investigated [401]. However, the question of the substrate or the metal used for laking, in general, is difficult to answer. Before the nineteenth century, the substrate of lake pigments was white and translucent in the binder, usually aluminum oxide hydrate Al2 O3 , AlOOH, Al(OH)3 , or Al2 O3 ⋅ n H2 O. Often a Ca salt such as CaCO3 was added instead of or in addition to the precipitating reagent, forming a Ca Al salt. Excess of the Ca salt would reach the final product as filler material. Additions of clay or other white inert material were also common, so the final aluminum content could have been very low, possibly resulting in a correspondingly low adhesion of the colorant to the substrate. On top of that, Ca instead of Al in the complex influences the lake’s achieved color. Various influencing factors were identified and their effects investigated: – structure of colorant – extraction method – substrate – light – painting medium The influence of the structure of the colorant is illustrated regarding anthraquinones. The stability of anthraquinones to light decreases with increasing substitution. Brazilwood (brazilin) is even less lightfast than the most labile anthraquinone. Madder lake (vegetable anthraquinone) is more stable than carmine (animal anthraquinone). This results in a stability order: Modern alizarin carmine (pure synthetic alizarin) is more stable than madder lake in laboratory production (natural alizarin, accompanying substances, controlled production, and substrate) is more stable than madder lake (natural alizarin, accompanying substances, variable production, and substrate) is more stable than Kermes carmine (natural carminic acid, accompanying substances, variable production, and substrate) is more stable than Cochineal carmine (natural carminic acid, accompanying substances, variable production, and substrate) is more stable than Lac dye, brazilwood lake (natural colorant, accompanying substances, variable production, and substrate).

With flavonoids, it is currently difficult to make an exact statement; however, lakes from rhamnus berries or quercitron are more stable than woad extract. Also, the extraction method influences the stability. The decomposition to light is faster if the dye is extracted from wool (old textiles) than if the colorant is extracted directly from the original material. A fractional extraction could take place with wool

702 � 7 Paint systems in art as the competing substrate. There are weaker bonds between colorant and substrate if wool fibers and mordant are also present in the system. However, the color change in the painting is the same in both extraction processes. The nature of the substrate is reflected in the following order of declining stability: Al(OH)3 is more stable than Al(OH)3 + CaSO4 or CaCO3 is more stable than SnO2 .

Lakes on pure aluminum hydroxide substrate are the most stable. Ca complexes of colorants are less stable than Al complexes because Ca salts can already compete with the Al cations during precipitation, forming more stable complexes. After precipitation, precipitated Ca salts also reduce the amount of more strongly bound Al complexes, and fillers generally reduce the stability of the lakes. The influence of light depends on its energy; harmful is mainly high-energy UV radiation below 400 nm wavelength. A UV filter can reduce the light hazard. Unfortunately, the lead white frequently used in Old Master paintings does not absorb any UV light and cannot slow down the deterioration of the lakes. In contrast, TiO2 in titanium white absorbs UV light and provides effective light protection. The influence of the painting medium is shown in the following order of declining resistance: Oil or highly pigmented (water) paint application is more stable than egg tempera is more stable than aqueous medium (gum or similar).

A fat medium, or rather the physical film that results from it, and a high pigment density protect against dangerous UV light and too intense irradiation. Lean media such as tempera or water offer no protection. Lakes in aqueous media (watercolor) are the least lightfast because there is hardly any protective medium and a high light reflection through the white paper. The exact nature of the decomposition products is currently unclear. In the case of red lakes, yellow intermediates form during the process. Could today’s lake pigments also lead to similar problems? The damage patterns mentioned before are unlikely because modern lake pigments such as alizarin crimson resemble the earlier ones mostly only in name. The main differences are: – Synthetic lakes contain pure colorants such as alizarin instead of a natural mixture of many similar derivatives. – Modern extraction of plant and animal colorants is more focused on capturing the colorant than the summary extraction of all possible coloring and accompanying substances. – From the fourteenth century to the sixteenth century, it was common to extract vegetable and animal red lakes from dyed rags, i. e., to reuse old dyed textiles as a

7.5 Protein systems (poster paint, gouache paint, glue paint, size paint, distemper paint)

–

� 703

dye source instead of the direct source (plants, insects). In both cases, all kinds of accessory substances were extracted along with the dyes, influencing the pigment quality. Today, production is carried out with defined pure substances. The salts used in precipitation and as substrates influence color and stability. Historically, salts of unknown and varying compositions were used, and diverse substances were added. Today, laking is carried out according to controlled parameters.

7.5 Protein systems (poster paint, gouache paint, glue paint, size paint, distemper paint) Numerous aqueous paint systems, among them some of the earliest we know of, are based on the ability of some naturally occurring proteins to form gels and durable films: – Albumin belongs to a group of similar proteins found in the albumen (egg white) and the blood serum. Because of these readily available materials, it is not surprising that egg white and blood were already used in antiquity to produce paints. While blood did not become the classic binding agent, egg white could hold its own in medieval book illumination and is still used today to fix leaf gold to the ground. – Collagen and its lighter hydrolysates are the proteins that give the animal glue its adhesive power. Again, the raw materials have been known since antiquity and have resulted in various glues: rabbit glue from skin and fur waste, fish glue from skin and bones, and hide and bone glue from skin and bones of various animals. Nowadays, we know glue paints from decorative paints: poster paints and gouache paints are based on an aqueous glue binder. – Casein is the calcium-binding protein of milk, which develops a high binding capacity in casein paints. Therefore, glue made from casein was already available in the earliest times as a binder. Film formation Films are formed by proteins based on the ability of the chain-like protein molecules to become entangled, similar to the threads of a ball of wool. Intermolecular bonds crosslink different protein molecules together, forming ordered structures; the analog of wool is solid felt fabric. Regarding binders for painting, our interest lies in two-dimensional extended films. However, the same film formation also underlies protein foams, such as egg whites made from albumin or milk foam made from casein. Therefore, film formation is also crucial from a technical and economic point of view, and it is not surprising that numerous studies on it are available [289], [286, Chapter 6], [287, Chapter 4.4], [868–871]. They investigate the possibility of producing packaging films for food products or the solidification of foams during baking. In particular, Dickinson and McClements [224] provide numerous details about the driving forces of film formation due to enthalpic and entropic factors.

704 � 7 Paint systems in art As a precursor to the film, the binder forms a gel, in which the protein assembles a more or less loose network in the solvent water. The gel can be characterized as a liquid that prevents the collapse of the protein network or a protein matrix that prevents the water from flowing out. In the painter’s workshop, the gel often materializes during the preparation of the binder. The gel network results from an equilibrium between the intramolecular interactions within a protein molecule and the intermolecular protein-protein and proteinwater interactions. Many proteins we use as binders exhibit a tertiary structure in native form in aqueous solution, essentially characterized by intramolecular interactions. In most cases, only a small number of molecules are involved, or dissolution does not occur. This structure of isolated molecules is retained even when the solution dries up so that a gel or film formation with high tear strength and adhesion does not ensue. For gel and film formation, we must let the intermolecular protein interactions become the dominant force and unfold the proteins so they can contact as many neighbors as possible. The necessary denaturation can be achieved in food technology by heat, solvents, salts, bases, acids, and interfacial exposure (surface denaturation). For the preparation of the binder gel for painting, we employ the following mechanisms: – Albumin is surface-denatured during brush-on by exposure to the air/water interface. In this process, the hydrophobic regions of the protein hidden inside the globular molecule in an aqueous solution are deposited on the hydrophobic air side by unfolding. – In collagen, we open up the structure by boiling with hot water and obtain a viscous solution of the more or less disentangled collagen chains. – Casein per se already has the structure of an open-chain protein without secondary or tertiary structure. However, in its native form, the protein is insoluble. To use it as an aqueous binder, we have to add alkalis to dissolve it in ionized form and obtain the gel. With the onset of denaturation, the protein in the aqueous solution exhibits a more or less association or coagulation in the aqueous solution. This process is reflected in an increase in the viscosity and stickiness of the solution. The transition from gel to film is characterized by the progressive removal of the solvent (water). As the solution dries up due to water evaporation, the molecules are pushed closer together; the loose network forms a film of intertwined protein molecules, which increasingly solidify by the more frequent formation of stronger intermolecular bonds. Typical structural elements of a protein film are: – twisted helices (collagen) – interconnected β-sheet structures, disulfide bridges (ovalbumin) – linear aggregations of hydrophobic sections (casein)

7.5 Protein systems (poster paint, gouache paint, glue paint, size paint, distemper paint)

� 705

Strong gels and films are obtained with high molecular weight proteins and a high content of thiols and hydrophobic amino acids. The binding interactions that lead to the formation of the secondary and tertiary structure and cause the cohesion of the gels and films are composed of several components ([280], the energies are taken from [287, Chapter 2.2.5]). We will, in principle, also find these interactions in the binding of colorants, ▶Section 6.1.4. The contribution of each force to gel and film strength varies greatly depending on the protein: – Van der Waals bonds (1–9 kJ/mol) exist between permanent and induced dipoles in hydrophobic sections (leucine, alanine, phenylalanine, proline, valine, tryptophan). – Hydrophobic effect (4–12 kJ/mol) occurs between aliphatic or aromatic side chains. The order of water molecules forced by hydrophobic moieties is thermodynamically unfavorable and can be eliminated by combining several molecules to form more extensive hydrophobic regions. The effect is strongly solvent-dependent. – Hydrogen bonds (8–40 kJ/mol) reside between hydroxylated amino acids (serine, threonine) or the amine and carbonyl group of the amide bond. They stabilize secondary structural elements (α-helix and β-sheet). – Electrostatic force (40–80 kJ/mol) acts between charged groups (acidic and basic amino acids, serine, threonine, tyrosine, aspartic and glutamic acids, lysine, histidine, arginine). – Covalent bonds (330–380 kJ/mol) are formed via disulfide bridges (for thiols such as cysteine) and various other groups, possibly upon UV irradiation (light). – Protein-typical superstructures emerge, e. g., triple helices in collagen or layered β-sheets. Despite their low binding strength, the cohesion of protein molecules can be very strongly determined by the hydrophobic interaction if the protein, like casein, has extensive hydrophobic regions. As the concentration of the protein increases, even initially small portions increasingly contribute to the overall binding. Surface activity With their hydrophilic and hydrophobic regions, proteins are, in principle, surfaceactive and show surfactant-like effects since hydrophilic regions can be oriented into an aqueous phase. In contrast, hydrophobic regions can be directed into a nonaqueous or air phase [286, Chapter 3]. Proteins with a high proportion of hydrophobic regions, in particular, exhibit high surface activity and lower the surface tension between the solvent water and air significantly: ovalbumin 61 mN/m, κ-casein 54 mN/m [287, p. 116], 12 % egg white solution 49.9 mN/m [280], pure water 72 mN/m. As a result, proteinaceous binders show an improved wetting of the painting ground compared to water, which is advantageous for painting, especially book and miniature painting.

706 � 7 Paint systems in art 7.5.1 Albumin as binder (whole egg, egg white) Albumins are a group of very similar proteins found, among other things, in the blood serum, egg white, egg yolk, and whey. Due to their structure, we can use them as aqueous binders, as our ancestors have done since antiquity to make mural and panel paintings [100]. Then in the early Middle Ages, they were used in egg tempera. Albumin The term albumin refers to a family of proteins with several different representatives, which we find, among other things, in the serum of blood (human albumin or bovine serum albumin BSA), egg white (ovalbumin), egg yolk, and whey [280], [286, p. 289], [287, Chapter 6.2], [291, Chapter 6]. They are essential for maintaining the colloid-osmotic pressure of the serum and serve as carriers for various substances such as fatty acids. Egg white consists of approximately 10 % proteins, of which ovalbumin accounts for 54 % and is particularly capable of film formation due to its denaturing properties. As a globular phosphoglycoprotein, albumin exists in the globular (spherical) form in an aqueous solution. The polar phosphatized saccharide moieties predominantly point outward into the aqueous phase. In contrast, apolar regions and thiol groups are hidden inside the globule. Since there is no significant cross-linking of the globules in the solution, the albumin binder is, in contrast to the casein binder or glue, not a gel but a low-viscosity solution. Egg white Egg white was already used in earlier times and later in medieval book illumination as a binder, the so-called clarea [109, 779]. The wettability of the pigments and the painting grounds could sometimes be improved by adding ox gall as a wetting agent. Fixing leaf gold to the substrate during gilding is still done today with egg white. Thick or pasty layers are brittle and fragile and must not be applied with egg white paint. Today we can buy albumin from hen’s egg (ovalbumin) as a ready-prepared powder; it is obtained from egg whites by removing the saccharide components and spray drying. For a protein-bound paint, only a few ingredients are needed, as shown in the following recipe: Egg white-bound paint [93, p. 84] Egg white, pigment, possibly wetting agent (ox gall, synthetic wetting agent), dilute with water to desired consistency and binding power. Be careful not to inhalate hazardous or toxic pigments and avoid direct body contact!

After stirring pigment into the egg white diluted with water, this paint is similar to waterbased paint, suitable for thin-bodied painting on paper.

7.5 Protein systems (poster paint, gouache paint, glue paint, size paint, distemper paint)

�

707

Clarea As the previous recipe showed, we can dilute the egg white with water and use it without further treatment for painting. However, to prepare the medieval illuminator’s clarea, another approach was followed: clarea, binder for illuminated manuscripts [109, Chapter 6.3.1.1] Egg white is repeatedly beaten to foam and let stand for some time. The clear, thin liquid running of the foam serves as a binder, which must still be diluted with water. Then it is mixed with gum Arabic, vinegar, or honey depending on the intended use.

In this preparation process, ovalbumin is concentrated in the effluent liquid. According to [879, 884], up to 95 % of ovalbumin is recovered in this process, compared to the original egg white. Most of the undesirable other components of the egg white (▶Table 7.8) either build up the resulting egg white foam or are retained in it; after four further whippings of the effluent, a decrease of most proteins except ovalbumin to below one-third of the original value was observed [881–884]. The liquid obtained is thin compared with the egg white and essentially contains ovalbumin, which is suitable as a binder due to its tendency to denature readily. Table 7.8: The major proteins in egg white [280], [286, p. 289], [287, Chapter 6.2], [291, Chapter 6]. Ovalbumin

54 %

Capable of gel formation, readily surface denatured while spreading, therefore foam-forming; globular phosphoglycoprotein with four thiol and two disulfide groups (masked, are exposed and activated by denaturation) and approximately 50 % hydrophobic amino acids, highly water-soluble

Conalbumin (transferrin)

13 %

Thermally denaturable, forms complexes with metals, coagulates on foaming, no thiol but 15 disulfide groups, best foaming properties

Ovomucoid

11 %

Concentrated in viscous egg white, highly hydrated glycoprotein with sialic acid and high content of α-helices, rigid structure due to nine disulfide groups, insoluble in water

Ovomucin

1.5 %

With an insoluble fraction concentrated in the viscous albumen, surface denaturable, foam-forming, very large fibrous glycoprotein, electrostatically stretched and defibered by high sialic acid content, increases the viscosity of an aqueous solution

Lysozyme

3.5 %

Four disulfide groups; antibacterial, basic; binds to ovomucin, ovotransferrin, and ovalbumin

G� , G� globulin

4%

Readily surface denaturable; have hydrophobic regions inside that can easily occupy newly formed air-water interfaces (foaming agents)

Based on the protein properties, it can be assumed that the globulins (G2 , G3 ) are quickly surface denatured by the impact of the air. The hydrophobic regions hidden inside the globulins immediately occupy the newly formed interfaces to the air and rapidly form a foam. We supply the energy for the conformational change in the form of mechanical

708 � 7 Paint systems in art energy through the whisk. Cross-linking the proteins in the lamellae around the air bubbles by ovomucin stabilizes the foam. Ovalbumin, conalbumin, and lysozyme can also form polymers or aggregations as the whisking time increases. Furthermore, ovomucin increases the viscosity of the liquid in the lamellae and slows down the gravitational outflow of the liquid. As a result, the outflowing water carries large amounts of the desired water-soluble ovalbumin, while most other substances remain in the foam or lamellar fluid. In agreement with this, in [291, Chapter 10.1.2.2], it was found that ovalbumin, conalbumin, ovomucoid, lysozyme, and globulins are responsible for foam formation; and in [290, Chapter 8 III.C] that the foaming ability decreases in the series globulins > ovotransferrin > ovomucoid > ovalbumin > lysozyme. Ovalbumin, also due to its quantity fraction, forms the principal mass of the foam. However, it shows a tendency to restore and can then apparently escape from the foam with the draining liquid. Film formation The protein of the egg white (ovalbumin) dries physically to a transparent solid film [289, Chapter 9] and [872–880], in which pigments can be excellently bound. Experiments on the technical use of albumin binders have shown that a slightly alkaline pH value and moderate heating are necessary for optimum film formation. In this process, the protein is denatured, i. e., it is unfolded, and polar and hydrophobic regions are opened up, which subsequently bridge different molecules via intermolecular interactions. These processes are visible in the gelation of albumin solution, among other things, during beating or frying eggs. In these examples, the gelation can easily lead to a solid product, seen as a cohesive three-dimensional film. The processes are similar for painting conditions (room temperature, neutral pH). A surface denaturation takes place at the air/water interface [280], [286, p. 289], [287, Chapter 6.2]. Since proteins are surface-active substances due to their polar and nonpolar fractions, they are readily adsorbed at the interface and change their conformation. Consequently, most polar groups are in contact with the aqueous phase, and most nonpolar regions are in contact with the air, ▶Figure 7.20. In the case of albumin, the adsorbed globules lose their spherical shape under unfolding. In this way, the albumin can reduce interfaces with high tension (air-water) and enlarge those with low tension (air-apolar regions and water-polar regions), and thus reduce the surface energy (surfactant effect). Due to the adsorption tendency of albumin on the water surface, the concentration of the unfolded protein in the near-surface layer is high so that intermolecular reactions and cross-linking of the polar and apolar regions can be initiated via van der Waals, electrostatic, and hydrophobic interactions. Helical structures could be transformed into β-sheet structures, providing large areas for intermolecular interactions [885, p. 31]. The exposed side chains of hydrophobic amino acids can also contribute to strengthening and aggregation through hydrophobic interactions. Since these forces depend significantly on the distance between the binding partners, they rapidly become stronger with

7.5 Protein systems (poster paint, gouache paint, glue paint, size paint, distemper paint)

� 709

Figure 7.20: Film formation of the albumin. (a): Albumin in egg white as a dense hermaphroditic-ionic tangle, dissolved as globules in water (blue). (b): Denaturation of albumin at the surface of the solution. Polar groups protrude into the aqueous solution. Hydrophobic inner regions are oriented toward the air side, globules unfold and build a dense network. (c): Upon drying, additional molecules accumulate over stable hydrophobic and polar crystalline regions. Disulfide bridges stabilize the film.

progressive compaction during drying, and the network solidifies. The coherent forces are so strong that the formed protein film is hard and brittle. With paints such as egg tempera or those for book illumination, we can therefore not apply thick layers nor paint impasto; the chemistry of the proteins strongly determines the impression of the egg-based painting. Since denaturation proceeds preferably on the surface, the film of more native albumin forms in the deeper layers. Significant from the painting point of view is the presence of free thiol groups of the amino acid cysteine in albumin, which become effective in the long term during film formation [879, 885]. After drying, they form disulfide bridges by oxidation and irreversibly stabilize the network. The protein can also polymerize via carbon bridges in the long term because UV radiation forms radicals from phenyl residues, which recombine to form covalent C-C bonds. During film formation, the inherent hydrophilic properties of the protein are retained so that even dried films retain a certain degree of sensitivity to moisture. However, this sensitivity decreases over time due to the slowly progressing polymerization.

7.5.2 Collagen as a binder (poster, gouache, glue, size, distemper paint) As early as ancient Egypt, glues were prepared as binders from the animal protein collagen [100]. The modern poster, gouache, and glue, size or distemper paints are also based on such animal glues, which consist of degradation products of collagen, so-called glutin: Rabbit glue from skin and fur waste, fish glue from skin and bones, skin and bone glue from skin and bones of various animals. Pearl glue is a commercial form of animal glue, not a particular substance. The well-known gelatin is an animal glue that is specifically purified and bright. All glue binders dry physically, forming a stable network. The ingredient list for glue paints is short:

710 � 7 Paint systems in art

Glue paint [74, p. 223] Glue (1 part by volume to 10 parts by volume water), pigment (3 parts by volume to 1 part by volume glue solution), for opaque colors filler like chalk (1 part by volume per part by volume pigment) Allow the glue to pre-swell in cold water for one day, then dissolve into a viscous liquid in a warm water bath. Although the required temperatures depend on the type of glue, the solution must not boil; otherwise, the adhesive properties will be lost. As soon as the glue has reached the required consistency, add and mix pigments and fillers and dilute with water if necessary. Nonpreserved glue paint must not be stored for long since proteins are a good breeding ground for microorganisms. Clove oil or camphor was used in earlier times, later phenol served as preservatives. Today, isothiazolinone derivatives and several other substances can be used. Be careful not to inhale hazardous or toxic pigments and avoid direct body contact!

Collagen The essential component of the glues is collagen, which in higher animals is the building block of all connective tissues and is the most abundant protein, accounting for about 30 % of total protein [886–890], [289, Chapter 11], [286, Chapter 6], [287, Chapter 6.2], [292]. It consists of approximately 600–3000 amino acids (MR ≈ 100 000) with a high proportion of glycine, hydroxyproline, and proline. Due to this composition, the molecule assumes a helical shape: the proline contained in the protein backbone forms “corners,” and small glycine molecules provide flexibility to form tight coils. Stabilization of this spiral structure occurs via the spatial requirements of the proline groups and clusters of equally charged polar amino acids, which occur at certain intervals and exert an electrostatic repulsion on each other. In natural connective tissue fibers, the structural unit is tropocollagen, consisting of triple helices in which three individual collagen strands are twisted. This structure resembles a strong rope twisted from individual thin ropes. Tropocollagen strands are connected via numerous hydrogen bonds (directly and via intercalated “bridge-water”), which emanate from peptide bonds, hydroxyproline, and polar amino acids. The bridges are the first points of action in producing soluble glue. Furthermore, as the living organism ages, covalent cross-links form between the triple helices, which further stiffen the structures and must be dissolved during glue boiling. These cross-links form at various times during life [292, Chapter 4]. The collagen synthesis is first followed by oxidative deamination of (hydroxy)lysine, ▶Figure 7.21. The resulting allysines react in young tissue with further (hydroxy)lysine residues from other collagen strands to aldimines and ketamines, thus linking two strands. With progressive aging, bonds with further collagen strands occur via histidine and (hydroxy)lysine residues, which increasingly stiffen the tissue via aromatic rings. Other possible cross-links include: – ester between β- or γ-carboxyl groups of aspartic acid and glutamic acid, as well as the hydroxyl groups of serine or hydroxylated amino acids – isopeptides between terminal carboxyl groups of acidic amino acids and the ϵ-amino group of lysine

7.5 Protein systems (poster paint, gouache paint, glue paint, size paint, distemper paint)

�

711

Figure 7.21: Frequent cross-links in collagens [292, Chapter 4]. Lysine oxidases convert lysine and hydroxylysine to aldehydes, which in young tissue react with (hydroxy-)lysine residues from other collagen strands to form aldimines or ketamines, cross-linking the involved strands. During aging, histidine and (hydroxy)lysine residues of other strands react with the cross-linking site to form aromatic rings and increase crosslinking.

712 � 7 Paint systems in art

Due to aggregated helices’ cross-links and crystalline regions, collagen has high tensile strength and is insoluble in cold, neutral water. The tendency of collagen to form triple helices is so high that it largely determines the chemistry of the glue. The ease with which different tissues (skin, bones, and others) can be boiled down to glue differs, and the nature of the cross-links explains it. Aldimines formed from allysine can be easily hydrolytically cleaved by dilute acids, whereas ketamines derived from hydroxyallysine, e. g., bone tissue, are more stable to hydrolysis. (Glutin) glue Glues are obtained by extracting the tropocollagen-containing starting materials with hot water and acidic or basic additives (milk of lime). The hot water first partially penetrates the (tropo)collagen structures, replaces hydrogen bonds, and thus separates (multiple) strands from each other. Other cross-linking bonds are also cleaved. Added acids or bases can now ionize exposed hydrophilic amino acids of the collagen in the exposed strands. As a result, the electrostatic repulsion ultimately separates the already separated collagen strands from each other. The collagen strands are now more or less free and are in contact with each other only at specific points, so a wide-meshed network or gel results. The viscous soluble mass forms the glue and consists of a mixture of individual collagen strands, which, compared to tropocollagen, have a lower molecular weight with a broad distribution (MR ≈ 60 000). In the incomplete separation of the helices, double or even triple helices are also found. The glue mixture is also called glutin. When the boiled glue dries, collagen strands are reorganized into triple helices. The viscous gel severely restricts the mobility of the strands and no longer allows the formation of extended crystalline structures: the dry glue has lost stability compared to the tropocollagen and can be easily liquefied again with warm water before use. From about 40 ℃, the moderately ordered triple helices dissolve again, and single helices change into a disordered wide-meshed gel structure.

7.5 Protein systems (poster paint, gouache paint, glue paint, size paint, distemper paint)

� 713

Film formation During gelation, a partial renaturation of the gel to the shape of the triple helix occurs. The gel of selectively interconnected collagen strands in the glue solution can condense and solidify during drying as strands approach each other and attempt to form again ordered helical regions, ▶Figure 7.22. If drying is slow enough and the glue concentration is high enough, triple helices form with the participation of various collagen strands. The strands become entangled and triple helices, in turn, become part of higher-order structures. The tendency for glutins to crystallize is so high that glue films dry with high tension and continue to shrink for a long time with further solidification.

Figure 7.22: Film formation of collagen glue (triple-helical regions shown in white). (a) Dry glue shows irregular, dense collagen triple helices. (b) The dense triple-helices are dissolved in an aqueous solution (blue). Triple-helical regions cross-link individual disordered collagen strands; a viscous, sticky network results. (c) The triple helices reform extensively and solidify the film upon drying.

In the case of rapid cooling or low collagen concentration, no such crystalline areas can develop: Often, only sections of the same strand can be deposited together to form a “loop” so that no uniform felting occurs. As a result, the emerging loosely-connected sections achieve only a low overall strength.

7.5.3 Casein as binder Casein paints are bound with glue formed when quark or curd cheese is digested with alkalis such as borax or calcium hydroxide. This binder is extraordinarily sticky and dries up with high tension. Traditionally, casein was made from milk, but nowadays, we can also buy it ready-made as a powder. Casein or milk as a whole could, according to [111], have been used as early as 49 000 years ago in South Africa to produce a liquid paint from ocher and milk. However, this application does not yet employ the binding power of casein, which is opened up by a reagent such as slaked lime, wood ash, or sodium tetraborate. This process became known only later. In the eighth century, casein was used as a base of fresco secco and in the thirteenth century for book illumination [100]. Before dispersion paints became the dominant color, casein paints were often used to paint interiors because of their high

714 � 7 Paint systems in art breathability and the fact that they do not seal off walls, unlike dispersion paints. When mixed with warm earth pigments, they give interiors a southern flair and are a highquality ecological paint. The following exemplary recipe shows the basic structure of a borax-casein paint: Borax-casein paint [92], [93, p. 56] 12.5 g casein in 80 ml water, 4.13 g sodium tetraborate (borax) in 20 ml hot water, 40 g chalk in 60 ml water, 12.5 g pigment in 10 ml water Allow casein, chalk, and pigment to soak in the water overnight. Dissolve the sodium tetraborate in hot water and add it to the casein the next day. The result consists of a thick, sticky paste. After half an hour, add the chalk paste and the soaked pigment (additional water depending on the desired consistency). Concerning glazes, do not add chalk; the color is sufficient for a larger area. If the substrate is highly absorbent, apply a primer of highly diluted paint or a pure casein-borax solution beforehand. The paint is at risk of microbial attack and should be used fresh. Be careful not to inhale hazardous or toxic pigments and avoid direct body contact! Composition of casein paint [74, p. 223] Add 8 parts by volume distilled water to 2 parts by volume casein, then stir to a thin paste. Mix a bit distilled water with 1 part by volume ammonium carbonate or concentrated ammonia water, and add the mixture to the aqueous casein. Stir and allow to rest for an hour. Stir in 8 parts by volume distilled water, yielding a syrupy solution. Predisperse pigment in water, grind the pigment paste, and the casein syrup. Be careful not to inhale hazardous or toxic pigments and avoid direct body contact!

Casein Casein is a milk protein and represents the main component of the protein fraction with a share of about 80 % or 25 g/l [282, 283, 288], [289, Chapter 7], partly also [873–878]. Its biological function is the complexation and transport of calcium ions from calcium phosphate, which is required during the growth phases of young mammals and their nutrition. Casein can also form complexes with other small ions by this function but has no tertiary structure for enzymatic or other functions. Casein consists of four phosphorus-containing components with molecular weights around 20000–25000 and typical properties listed in ▶Table 7.9. Phosphorus is present as a phosphoric acid ester with serine and confers the ability to bind calcium. The phosphate residues are also responsible for the high charge of the serine-rich regions of the proteins. Caseins, especially β-casein, contain a high proportion of proline, poorly incorporated into secondary structural elements such as α-helices or β-sheets. Therefore, they have no distinct secondary structure and exist as flexible, disordered proteins. With the help of their extended hydrophobic regions, caseins, especially β and κ, form higher molecular aggregations and interact with interfaces. The readily oxidizable cysteine can meanwhile form disulfide bridges to stiffen the aggregates. In milk, caseins are present as larger complexes or micelles. Their exact structure is rather complicated and is still being discussed in detail, so that we will restrict ourselves to an overview.

7.5 Protein systems (poster paint, gouache paint, glue paint, size paint, distemper paint)

� 715

Table 7.9: The components of casein, their contribution to total casein, and specific properties [282, 283, 288], [289, Chapter 7]. αs�

40–45 %

8–9 seryl phosphates per mole, no cysteine, two distinct hydrophobic terminal regions with high proportions of proline and hydrophobic amino acids such as valine, leucine, isoleucine, and phenylalanine. Between the hydrophobic regions is a calcium-binding polar region containing seryl phosphate

αs�

10 %

10–13 seryl phosphates and 2 cysteines per mole, pronounced negative charge at one, positive charge at the other chain end

β

35–40 %

κ

9–15 %

5 seryl phosphates per mole, no cysteine, a typical structure of a surfactant with hydrophobic and highly charged hydrophilic chain end 1–2 seryl phosphates and 2 cysteines per mole, a glycoprotein with hydrophobic, hydrophilic, and polysaccharide moieties. Stabilizes with saccharide chains the micelles from the other units (acts as sterically active protective colloid)

Calcium-binding sites The effective region for the complexation of the αs1 -subunit consists of phosphoserine, and the bound phosphates represent calcium-binding sites:

In addition, many free phosphates form the “colloidal calcium phosphate” (CCP), which acts as a fluid cement between the calcium-binding components and enhances the calcium complexation. Hydrophobia, association tendency Hydrophilic and hydrophobic regions of casein are irregularly bent into random coils by the high proline content so that no ordered structures form [286, Chapter 6], [287, Chapter 6.2]. Since structure-preserving disulfide bridges are absent, casein is largely unfolded (denatured) at room temperature and possesses neither secondary nor tertiary structures. Even when heated, the structure hardly changes, so casein is very heat resistant. Due to the folding up, a protective tertiary structure is missing. At the surface of the complexes, calcium-binding sites and hydrophobic regions are exposed so that casein is insoluble in water. The hydrophobic regions lead through van der Waals and hydrophobic forces to a strong association tendency, to which we owe its suitability as a binder and film former. As a result, micelles form, consisting of a few tens to hundreds of casein

716 � 7 Paint systems in art molecules in milk. The steric repulsion of the saccharide chains of the κ caseins near the surface stabilizes the emulsion. In the coated casein binder, caseins occupy the interface with the hydrophobic air medium and orient themselves with their charged or hydrophobic areas toward the aqueous or gas phase. This process occurs easily and quickly since hardly any ordered structures have to be unfolded. During drying, the orientation of the molecules allows intermolecular interactions and the formation of a stable film. Emulsification, stabilization, precipitation of casein Without further stabilization, the micelles in the milk could aggregate into ever-larger units. Instead, they are emulsified by the κ-components incorporated as amphiphilic proteins in the micelle’s surface layer. The long polysaccharide chains of the κ-caseins protrude 5–10 nm into the solution and give the micelles a “hairy” structure. Steric repulsion of the saccharide chains prevents the progression of the aggregation, supported by electrostatic repulsion of phosphate, carboxylate, and hydroxylate groups. We need pure casein to produce a milk-based binder, which we can precipitate from the milk by adding acid. The natural milk products (curd and others) used to produce casein color are already precipitated due to microbial activity. During acidification, calcium ions are transferred from the micelles to a solution. The complexing phosphate, hydroxylate, and carboxylate groups are protonated and neutralized. This process removes the loose cement of calcium ions, and the micelles disintegrate. Furthermore, κ-casein dissolves, and after electrostatic repulsion, steric repulsion also ceases. Then, through thermal motion and thus entropically driven, the now freely mobile casein components can interact via their hydrophobic regions and finally coagulate. The enzymatic hydrolysis of the κ-casein also abolishes the stabilizing effect and leads to micelle aggregation. After cleaning and drying, the resulting white powder is insoluble in water, which shows no adhesive properties in this form. These only develop when the powder is brought back into an anionic form, and thus into a solution with alkalis (ammonia, sodium tetraborate, calcium hydroxide) into an anionic form, and thus into solution. Borax acts as a base and reacts with the carboxylic acids of casein to release boric acid to form water-soluble sodium caseinate:

While dissolving, electrostatic repulsion between the phosphate, carboxylate, and hydroxylate groups supports the dissociation of the individual casein molecules.

7.6 Tempera

� 717

Film formation The insoluble casein can be dissolved as described above by adding alkaline solvents. Due to the flexibility and aggregation tendency of the dissolved caseins, long chains are present in the solution, which bind much water. The resulting sizeable hydrodynamic volume leads to the high viscosity of the casein solution. The powerful adhesive effect of the viscous solution is caused by the extended hydrophobic regions of casein, which form wide-meshed networks through van der Waals interaction, ▶Figure 7.23. Since casein is one of the most hydrophobic proteins, the drying process creates such dense and stable networks that they are almost impossible to redissolve and form an extremely durable film. In addition, electrostatic interactions and hydrogen bonds also contribute to film stability.

Figure 7.23: Film formation of casein glue. (a) Precipitated pure casein in a dense tangle. (b) The casein clusters are transformed into their anionic phosphate, hydroxylate, and carboxylate forms by the alkaline dissolution and then expand by electrostatic repulsion. The resulting network and the long water-binding casein chains lead to a sticky, viscous, aqueous solution (blue). (c) During drying, the film solidifies with the formation of stable crystalline regions by hydrophobic interactions and hydrogen bonds.

Divalent cations such as Ca2⊕ increase film hardness because they can ionically bridge and fix two peptide strands while aggregation proceeds. Therefore, lime casein binders, prepared with calcium hydroxide as the alkaline component, provide good film-forming properties. Assuming milk is used instead of pure casein, the contained β-lactoglobulin can also form intermolecular disulfide bridges over the long term via its thiol groups, and thus contribute to film hardening [872].

7.6 Tempera The tempera technique refers to a time that created countless masterpieces long before the invention of classical oil painting. Unfortunately, the term is not easy to grasp; many names existed for tempera precisely during the period of its most extensive use. After that, interpretations abounded of what exactly tempera was. The term itself is complicated further because “tempera” denotes different systems in each language. We can get

718 � 7 Paint systems in art a notion of the problem by reading [891], which attempts to trace the essence of tempera in the course of European painting. From a scientific point of view, tempera is not a binder in its own right but designates a paint system created by emulsifying an aqueous and nonaqueous system [74, ch. 11], [75, Chapter 5]. One of the systems dominates, and thus defines the actual binder, whose properties are modified by the second system. Therefore, “tempera” is not unambiguous; we can create numerous variants by combining the possible aqueous and oily base systems. Some important examples may demonstrate this to us [100]: – fatty egg tempera, a particular case of aqueous egg white binder and drying oil, plus the egg yolk as another emulsion system – gum-oil tempera, made of aqueous vegetable gum and drying oils Although the components of tempera can be combined almost arbitrarily, we often think first of the classical egg yolk tempera. However, even from this, there are several variants, as we will see in a moment. Tempera is suitable for panel painting; however, it was also widely used in coating techniques of the pre-plastic period. Therefore, if we would like to devote ourselves besides painting to our workshop’s wooden decoration with love and care, we could learn a lot about this application in [92]. Since tempera combines two opposing phases, its production requires an emulsifier. Some aqueous binders have the advantage of already containing emulsifiers in the form of proteins. If proteins have pronounced hydrophilic and hydrophobic areas, they have a surface-active effect and mediate between water and the nonaqueous phase. Especially hydrophobic proteins such as casein, which are already denatured, show powerful emulsifying abilities [286, Chapter 3], [287, Chapter 4.7.5]. However, globular proteins such as ovalbumin, which can surface denature, also stabilize emulsions to a certain extent. Most of the components of a chicken egg, especially the egg yolk, are particularly valuable in this respect, which is why we want to examine egg tempera in more detail.

7.6.1 Egg yolk tempera, pure egg tempera The simplest and ancient way to produce tempera colors rests primarily on the egg, as pure egg yolk or whole egg. Advantages (and disadvantages) of this means are: – The rapid evaporation of the water leads to quick drying of the paint and complete drying. – Due to the lecithin and oil content, tempera dries without tension (pure distemper films, on the other hand, show high tensions). – Egg yolk tempera is a lean painting and binding agent. – The dried film can no longer be dissolved by essential oils (turpentine oil) and does not turn yellow because fatty oils are missing.

7.6 Tempera

–

– –

� 719

The egg yolk components do not react with lead paints like fatty oils, white surfaces do not become transparent due to the formation of colorless lead soaps, and dark underpaintings do not shine through (no darkening). The film is brittle (the actual binder is egg white). Colors lighten when drying because there is no physical film around the pigments.

According to Doerner [891], egg yolk tempera is the medium initially used in early European panel painting, which the writings of Cennini around 1390 confirm (production of a panel painting tempera with egg yolk, a mural tempera with whole egg). This tempera tends to flake off in thick layers by forming a brittle egg white film. Since the opaque underpaintings (white highlights) of later Old Master paintings often form such layers, there was a need to discover the fatty egg tempera (see below). The actual egg tempera is made in a simple way: Egg tempera (pure egg tempera) [93, p. 88], [74, p. 227], [75, p. 267] Egg yolk (without yolk sac), pigment Pierce an undamaged egg yolk so the contents without the yolk sac can flow into a vessel. Mix the thick liquid with a teaspoon of water and then grind it with the same amount of pigment. Be careful not to inhale hazardous or toxic pigments and avoid direct body contact!

The components of the egg yolk form a complex system, which can be classified as a fatty glue binder: Lecithin (emulsifier) + egg oil (nondrying, 30 %) + egg white (glue binder, 15 %) + water (50 %) Lecithin is a very efficient emulsifier for a system of egg whites and fatty egg oils. It is phosphatidylcholine, which has a surfactant-like structure with a hydrophobic end of fatty acids and a hydrophilic end of a quaternary ammonium salt.

Lipoproteins and other egg yolk phospholipids also play a role as emulsifiers, although not as significant as lecithin. Egg oil consists primarily of nondrying oils, mainly C16:0 and C18:1 triglycerides, and some C18:2 and C18:0 triglycerides (the indication n:m means “n carbon atoms, m double bonds”). It does not contribute to film formation. The sole binder is albumin, a protein of the yolk, forming brittle films. Therefore, the egg oil’s significant contribution makes the albumin film pliable.

720 � 7 Paint systems in art Since most of the paint volume consists of water and evaporates, only a thin, bodiless film remains. Unlike oil paint, the pigments are put into the foreground. Consequently, the typical matte or satiny appearance of the lean tempera surface occurs, depth of light, and saturation of the color are largely missing. We see a similar appearance with acrylic films. The mechanism is discussed in more detail in ▶Section 7.4.10. 7.6.2 Egg tempera To produce more impasto paint, we need more binder. Since albumin is the binder in the egg yolk tempera, it is obvious to add more of it by using the whole egg including the egg white, getting the following paint system: Whole egg = egg yolk + egg white (albumin, 87 % water) = emulsion binder + glue binder We added to the egg yolk (the binder of an egg yolk tempera) albumin, a pure glue binder identical to the one already in the egg yolk (see also ▶Section 7.5 for more information about albumin glue paints). Now that more albumin is available, the tendency to film formation is more significant, and the importance of the plasticizing components (egg oil) increases. 7.6.3 Fatty egg tempera, egg-oil emulsions Due to the efficient emulsifier in the egg, an egg (yolk) tempera can be mixed with either water or fatty oils without breaking the emulsion. In this way, the properties of oil paints can be imparted to the tempera, which is the actual discovery of the van Eycks. These have replaced the former egg yolk tempera with the invention of the water-oil emulsion, building the foundation of the layer and glaze technique of the Old Masters. In this technique, alternately tempera layers (underpainting, whitening) and oil paint layers (glazes) follow each other (Doerner in [891]). After the Renaissance, the fatty egg tempera became the primary technique of the Old Masters. Such a tempera paint is composed according to recipes of various artists: Fatty egg tempera (egg-oil emulsion) [74, p. 230], [75, p. 272] 1 part by volume of egg (whole), 1 part by volume of a mixture of stand oil (or linseed oil), dammar varnish, or turpentine oil (e. g., in the ratio 1:1:1), possibly 1–2 parts by volume of water, pigment. Put the whole egg in a glass and mix the yolk and egg white. Next, add the same volume of the fatty components and remix everything. The proportions of the oils and resins can be varied. Finally, add a maximum of twice the egg volume of water and again mix everything thoroughly. Grind the pigment into this binder. Mayer [75, p. 274] denotes more compositions concerning oily ingredients. Be careful not to inhale hazardous or toxic pigments and avoid direct body contact!

7.7 Watercolors

� 721

The resulting paint dries faster than oil paints but slower than tempera and allows smearing of paint and impasto application. If we added water, the painting medium would be water; otherwise, the unpigmented emulsion. The added fatty oils improve the brittle protein film of pure egg tempera so that it is possible to create impasto and opaque areas without the risk of spalling. Furthermore, by adding a second binder in the form of oil, which in contrast to the egg white glue, has a very long drying time, we can now work on the painting for a longer time. As it dries, the fat egg tempera forms a corporeal transparent oil film, which creates a deep light and gloss compared to the lean tempera. In contrast, the added water serves as a painting agent that evaporates quickly and has no influence on film formation. However, a significant advantage is a precise contour that brush strokes can create with water-based emulsion on fatty paint layers, which cannot be achieved with oil paints. In addition, the delicate, exact drawing of fur, hair, and lace fabrics becomes possible through such a water-on-oil technique.

7.7 Watercolors Watercolors (aquarelles) belong to the oldest paint systems as a particular case of colors on a water basis. Exquisite examples are undoubtedly the elaborately executed works of the medieval book illuminators. However, pigments were mixed with plant gums to form an aqueous color long before. From around 1700, opaque watercolors, the so-called gouaches, enjoyed great popularity. Watercolors consist of very finely ground pigments suspended in water and ideally have a transparent character, although opaque pigments can also be made transparent by suitably fine grinding. In addition, pigments in watercolor must have the highest lightfastness. No other components of the painting layer are involved in light absorption, so they alone must absorb the radiation energy. The actual painting ground for watercolors is a rough-fibered, textured paper into which the pigment suspension can penetrate deeply. Essential is the ability of the paper to keep pigment particles between the fibers and fix them, just like dyes, only by secondary interactions. Together with the transparency of the pigments, we can certainly speak of coloring in the sense of a dye. Watercolor paints, however, still contain vegetable gum as a binder, usually gum Arabic, mixed with lower saccharides (glucose, honey) or glycols as a plasticizer. 7.7.1 Basic composition of watercolors The following recipe shows the basic structure for preparing a watercolor: Watercolors (transparent and opaque) [74, Chapter 10] 4 parts by volume distilled water, 2 parts by volume gum Arabic, 1 part by volume humectant glycerol, pigment. For opaque paints, mix pigment with precipitated chalk. Dissolve gum Arabic in hot water, and stir in the glyerol. Wet the dry

722 � 7 Paint systems in art

pigment with small amounts of ethyl alcohol if necessary. Predisperse pigment or pigment paste in distilled water. Grind the predispersed pigment into the binder until thick paste forms. Be careful not to inhale hazardous or toxic pigments and avoid direct body contact! Watercolors (transparent and opaque) [75, p. 332] 4 fl. oz. boiling distilled water, 2 oz. gum Arabic, 1 41 fl. oz. humectant honey-water, 1 21 fl. oz. humectant glycerol, 2 to 6 drops wetting agent, 41 tsp. preservative or phenol solution, pigment. For opaque paints, mix pigment with precipitated chalk according to [75, p. 341]. Dissolve gum Arabic in hot water, add other ingredients in the order given. Mix pigment with a spatula with the vehicle to a stiff paste. Grind thoroughly with a muller, adding distilled water when necessary to keep paste fluid. Be careful not to inhale hazardous or toxic pigments and avoid direct body contact! Watercolors (transparent and opaque) [105] 1 part by volume water, 1 part gum Arabic, 41 part by volume humectant glycerol, pigment, for opaque watercolors (gouache) about the same amount of chalk as the pigment, possibly a wetting agent (ox bile or synthetic, a few drops), possibly a preservative (clove oil or synthetic, a few drops). First, heat water and add gum Arabic. After 1 or 2 days, the gum should be completely dissolved. Then add glycerol. Stir the pigment into the solution until thick paste forms. Finally, pour this into small tins or boxes for storage and mix with water if necessary. Be careful not to inhale hazardous or toxic pigments and avoid direct body contact! Watercolors (transparent) [91, p. 20] 6 parts by volume boiling distilled water, 1 part by volume gum Arabic, small additions of honey, dissolved in water, sugar syrup, or glycerol, preservative, pigment. Dissolve gum Arabic in hot water and let rest for 2 days. Add other ingredients. Predisperse pigment with distilled water and grind it into the vehicle to a stiff paste. Grind thoroughly with a muller. Be careful not to inhale hazardous or toxic pigments and avoid direct body contact!

It is advisable first to prepare a pre-dispersion from the pigment: the pigment is mixed with a few drops of ethanol as a wetting agent, then with water to form a paste that has the consistency of oil paint. After adding the binder, the actual color can be rubbed with a glass runner. Water evaporates during the grinding, and the paint thickens, which can be compensated, if necessary, by spraying a little water until the color is thoroughly rubbed. Under no circumstances should a binder be used for moistening; otherwise, too much binder will remain in the paint! Binders From a chemical point of view, watercolor paints are simple in structure since the chemistry is limited to pigments and binders (vegetable gums, mostly gum Arabic). The binder in watercolor paints has a purely physical effect, i. e., after evaporation of the water, the unchanged gum remains on the paper as a thin layer and encloses the pigments. The gums are high-molecular polysaccharides; the polymer molecules are linked by hydrogen bonds and form a saccharide film. We can well imagine how hard such a film can be if we think of similar cases in nature. Polysaccharides play a supporting role in the truest sense: In trees (cellulose fibers), bacterial cell walls, shirts

7.7 Watercolors

� 723

starched with starch, and the extraordinarily stable stale hard bread. (The comparisons are not quite adequate since these examples involve cellulose or starch-like polysaccharides. Due to their linear structure, these are predestined to form very stable crystalline structures. However, plant gums are branched, so we should not expect such stable regular structures.) The basic recipe can be further modified by adding other polysaccharides such as dextrin to the gum. This addition allows the professional manufacturer to control the properties of the film more precisely. Unfortunately, however, a sensible selection of such supplementary substances may not be possible for self-production. Humectants Due to the high molecular weight of the gums and the intermolecular interaction, the film cannot be dissolved as easily with water as would be the case with simple saccharides (sugar, honey). We notice this when we prepare the paint ourselves: if we take too high a proportion of gum Arabic, we can hardly dissolve the color even in the pot. Therefore, watercolor paints always contain humectants, which help prevent complete drying so that the color paste can be dissolved again quickly. The humectants are polar, sugar-like substances with moderately high vapor pressure, i. e., low polyalcohols such as glycerol. Simple sugars are also adequate but do not evaporate and remain permanently as a soluble component in the film. As long as the paint remains stored in the pot, glycerol acts as a plasticizer and averts premature interactions by entering between the polysaccharides, thus preventing too close contact. In addition, it does not participate in the adhesion process. Water can penetrate the space around the polysaccharide molecules, separated from the glycerol, to facilitate the dissolving process. After applying the paint, the glycerol evaporates so that no soluble substance remains, and the film becomes more stable to moisture. Preservatives Due to polysaccharides as binders, the storage of watercolors is problematic; some test samples, stored, however, as moist color paste, showed mold growth after 8 months. So far, well-dried batches did not show any adverse effects. The addition of a preservative is recommended if, instead of the classical dry watercolor, a doughy, moist, tube-like mass is to be produced. Traditionally, essential oils such as clove oil were selected for this purpose. Nowadays, we can choose among various synthetic products.

7.7.2 Gum Arabic Gum Arabic is an extrudate from trees of the species acacia senegal. When the bark is cut, it exudes 2–7 cm tears and solidifies more or less clearly [278, 279]. The extrudate comes from Africa, mainly Sudan.

724 � 7 Paint systems in art Chemically, gum Arabic is a branched polysaccharide with a molecular weight between 190 000 and 600 000. It is composed of four significant saccharides with the following approximate molar ratio: 3.5 mols L-arabinose (L-Araf) 2.9 mols β-D-galactose (β-D-Galp) 1.6 mols β-D-glucuronic acid (β-D-GlcpA) 1.1 mols α-L-rhamnose (α-L-Rhap) The main chain is formed of (1→3)-linked galactopyranose; branchings are possible via the 6-position. Branches can consist of another, shorter polygalactose chain, or short tufts of arabinose, glucuronic acid, and rhamnose, ▶Figure 7.24. The molecule is selfsimilar due to the ability to duplicate the main chain as a side chain and creates a spherical shape.

Figure 7.24: Structural detail of gum Arabic showing a galactose base structure and side chains of galactose, arabinose, glucuronic acid, and rhamnose [280, Chapter 4.4.4.5]. Galp: galactopyranose, Araf : arabinofuranose, GlcpA: glucuronic acid, Rhap: rhamnopyranose.

Due to the content of glucuronic acid, gum Arabic forms neutral to slightly acidic salts with calcium, magnesium, or potassium. As a polysaccharide and uronic acid, it is also soluble in water; we can easily prepare 50 % solutions. An increase in viscosity is only noticeable at high concentrations since the gum molecules are spherically shaped and do not form an extended network, which is a prerequisite for gel formation (similar to an acrylic dispersion). Therefore, gum is often used as an emulsifier and stabilizer in the food industry. This behavior makes gum Arabic very suitable as a binder for water-based paints. The paint remains watery-thin. After water evaporation, a thin film of the nonvolatile polysaccharide remains, stabilized by hydrogen bonds and van der Waals forces, ensuring the pigment’s binding. In addition, since the binder does not change chemically during drying, it retains its water solubility. The stability of watercolor is nevertheless high since the pigments deposited in the paper fibers can be adsorbed by the paper via

7.7 Watercolors

� 725

secondary interactions, as the noncovalently bound dyes can achieve a reasonably high adhesion, ▶Chapter 5. 7.7.3 Gum tragacanth Another plant gum is tragacanth, obtained from trees of the species astragalus gummifer, at home in the dry mountains of Turkey and Iran [278, 279]. It is also used as an aqueous binder and as an emulsifier and thickener in food applications, but to a lesser extent since the deposits are noticeably smaller, making it more expensive. Chemically, gum tragacanth is also a polysaccharide, but it consists of two components: an arabinogalactan (approx. 30–40 %), and the “tragacanth acid,” a poly-uronic acid, ▶Figure 7.25. Arabinogalactan is readily soluble due to its spherical form, like gum Arabic; tragacanth acid is rod-shaped and forms colloids easily. Therefore, gum tragacanth does not dissolve completely in water but forms highly viscous solutions and mucilaginous gels.

Figure 7.25: Structural detail of the two components of gum tragacanth [280, Chapter 4.4.4.7]. Galp: galactopyranose, Araf : arabinofuranose, GalpA: galacturonic acid, Xylp: xylopyranose, Fucp: fucopyranose.

726 � 7 Paint systems in art 7.7.4 Ox gall Gall consists of about 80 % water. As functional components, we find inorganic electrolytes, lipids, and, as the main constituent, about 12 % salts of gall acids. These are hydroxylated carboxylic acids with a steroid skeleton. Like all steroids, they originate from tetraterpene metabolism. Cholic acid is the essential gall acid and the starting point of other derivatives (such as deoxycholic acid, chenocholic acid, lithocholic acid). The gall acids do not occur in free form but as amides (“conjugated bile acids”) with taurine (▶ taurocholic acid) and glycine (▶ glycocholic acid):

In terms of their chemical structure, gall acids are surfactants and emulsifiers. The function of gall in the body is to emulsify fats absorbed with the help of gall acids as part of fat digestion. This function makes gall in “gall soap” suitable as a detergent for stubborn cases, but it can also be used as a wetting agent in watercolor preparation. In addition to the gall as such, primarily taurocholic acid is used, which, according to J. Horadam (1893), shows the most favorable application properties (still today, the company Schmincke carries the brand name “Horadam Watercolors”); less suitable are glycocholic acid and the other gall acids.

7.7.5 Paper We have presented the elemental composition, function of the components, and production processes of paper in ▶Section 6.7 and the unique features of artists’ papers in ▶Section 6.7.7. In the following, we look at the characteristics of watercolor paper. First and foremost, it is uncoated and was traditionally made from rags, which results in high resistance to light and aging. Still today, high-quality, durable watercolor papers are made 100 % from rags or cotton [1004]. The hollow cotton fibers provide the high adsorption and buffering capacity for water consequential for a painting technique like watercolor, which depends on the controlled flow of moisture on the paper, even with large amounts of water.

7.8 Alkyd colors

� 727

In addition to these 100 % rag or cotton papers, paper grades are now available made from blends of rag and lignin-free pulp or pure chemical pulp [1004]. During pulp production, woody substances (lignins) susceptible to yellowing and aging are removed. Adding a few percent CaCO3 as a buffer against acid slows down the aging process so that most of the papers offered can be rated as aging-resistant, according to ISO 9706. Some artistic effects depend on the paper’s possible satinage or surface sizing. Watercolor paper is not calendered when a naturally rough surface is desired. The screen structure can introduce matte or fine-ribbed textures during dewatering or embossing, yielding “rough” and “cold-pressed” paper. For brilliant and conspicuous colors, as they are intended for botanical watercolors, a smooth surface is created by calendering, possibly in combination with surface sizing, yielding “hot-pressed” paper. The grammage ranges from 200–600 g/m2 . A surface sizing may protect paper against mechanical stresses typical of watercolors. These stresses include, e. g., rubbing away masking fluid or “rubbing crepe,” removing the tape to fix the paper sheet, and intensive brushing at one spot to make corrections or remove paint. For this application, AKD is a suitable neutral sizing agent. According to [179, p. 98], using chalk and low molecular weight polymers (modified polyethyleneimine, polyamine, polyvinylamine) can achieve high opacity and a uniform surface. Subsequently, any optional sizing is carried out with cationic styrene acrylate and CMC, starch and with or without low molecular weight polyacrylamide or polyvinyl formamide. As a result, these agents control water absorption, oil absorption, and paper stiffness. The necessary abrasion resistance is achieved in this case by surface sizing with starch or a copolymer of polystyrene and polyacrylate.

7.8 Alkyd colors For some time now, artists’ paints based on alkyd resins as a binder are offered commercially [98, 185, 189, 194, 196]. The advantages cited are a much shorter open time than oil-based paints while maintaining the transparency and body of an oil-like film. For the artist, such alkyd paints are a relatively recent development. However, they have been used in coating for almost a 100 years, and alkyd resins are among the most widely used binder systems, accounting for around 45 % of all binder systems. Their popularity is due to their excellent film-forming properties and the cheap chemicals: fatty oils from natural sources and acids such as phthalic acid from large-scale petrochemical products. Alkyd paints, bound in a crystal-clear, oil-like, bodily alkyd resin, are similar to oil paints in their application behavior, opacity, transparency, intensity, and luminosity, in contrast to acrylic paints, which also dry quickly but do not form a physical film. Alkyd resin dries within a day and can be painted over so that numerous glazes can quickly be applied in the Old Master painting technique. Chemically speaking, alkyd resins are synthetic polyester resins. The name alcyd already indicates the components alcohol and acid. To obtain polymers instead of simple esters, we have to use polyfunctional carboxylic acids and polyols, ▶Figure 7.26.

728 � 7 Paint systems in art

Figure 7.26: Synthesis of alkyd resins from dicarboxylic acid and polyol.

The alkyd resin shown is oil-free. The first alkyd resins comprised glycerol (polyol) and phthalic anhydride (dicarboxylic acid). However, these glyphthalic resins had properties that made them unsuitable for application: the trifunctional glycerol either immediately leads to excessive cross-linking or prevents it, and in addition, pure polyesters only dry physically. Furthermore, glyphthalic resins were incompatible with fatty oils, so no resin-oil blends could be prepared that would cross-link chemically. Oil-modified alkyd resins The breakthrough did not occur until around 1930 with the invention of oil-modified alkyd resins by Kienle, who used a monoglyceride as the polyol. Per monomer, a fatty acid molecule is present in the structure, ensuring the oil compatibility and chemical drying in the manner of oil paint. Due to rapid drying, better film hardness, and weather fastness, oil-alkyd resins quickly became a success. Depending on which oil we use, we obtain different types of alkyd resins. For example, in artists’ paints, glycerides from fatty oils are essential, leading to long-oil alkyds. In a first pathway, the required monoglyceride is obtained by transesterification of a normal triglyceride (fat or oil) with glycerol. Thereby a statistical distribution of the acid residues of the higher fatty acids to glycerol molecules takes place:

7.8 Alkyd colors

� 729

In a second pathway, the triglyceride first reacts with the dicarboxylic acid, releasing the fatty acid:

A third pathway directly synthesizes oil-alkyd resins from alcohol, acid, and fatty acid. It is now used on a large scale, as all starting products are industrially produced. The wide range of monomers and the possible combinations allow for the preparation of numerous resins with different properties. The resin should be soluble in common solvents (aromatic and aliphatic hydrocarbons, terpenes hydrocarbons, terpenes, alcohols, and more recently, water). Incorporating hydrophilic areas (polyethers, polyols) increases water compatibility considerably, as does the introduction of free carboxylic acids, whose salts are also water-soluble. However, solubility can also be achieved by adding emulsifiers. Dicarboxylic acid component The dicarboxylic acid used is mainly the cheap and widely available phthalic acid, along with other benzene dicarboxylic acids and some aliphatic dicarboxylic acids:

730 � 7 Paint systems in art

Aliphatic acids form softer resins but are less commonly used. Maleic acid is an additive used to modify the properties (▶ maleinate oils). Trimellic acid is a component of watersoluble alkyd resins since the free carboxyl group can form soluble salts. Polyol component Simple alcohols, mainly glycerol and pentaerythritol, are used as polyol components:

Glycols are mainly employed for short and medium oil alkyds. In oil-modified resins, we predominantly find glycerol esterified with: natural C18 fatty acids, pelargonic acid (C9 ), iso-C8 - to C10 -acids, lauric acid (C12 ) as well as lactic acid. In the past, the fatty triglycerides of natural vegetable oils (linseed oil) were used in a one-pot process. However, because of the yellowing tendency of these oils (▶Section 7.4.3 at p. 669), soybean oil is today more commonly chosen, which shows similar drying properties. Even better (and more expensive) is safflower oil. Drying and film formation Pure alkyd resins consist of polyester dissolved in a solvent. It quickly physically dries to a film after the solvent evaporates. Oil-modified alkyd resins such as, e. g., the artists’ alkyd paints of Winsor&Newton [995] show through their content of unsaturated fatty

7.9 Acrylic paints

� 731

acids in the subsequent time also chemical film formation, which corresponds to the oxidative drying of linseed oil, ▶Section 7.4.3.

7.9 Acrylic paints The middle of the last century was a productive time for artists, as it brought forth, in addition to alkyd resins, another alternative to the classic oil binder: The binder of acrylic paints is a dispersion of copolymers of acrylic acid and methacrylic acid [185, several volumes], [98, 99, 833, 837]. Today, polymerized acrylic acid in the form of plexiglass is well known to us by the work of O. Röhm. Around 1901, he began investigating the valuable properties of acrylic acid and its derivatives: their polymerization properties and the ability of their polymers to form dispersions. The first acrylic dispersions were exclusively solvent-soluble and were used for coating. Experiments with aqueous dispersions started around 1930 but did not continue for a while. It was not until around 1950 that the first artists’ acrylic paints came onto the market. Initially, they also contained solvents, but in the following years, aqueous dispersions (AC-33 as methyl methacrylate/ethyl acrylate copolymer) became available on the market. As research continued for 40 years to improve properties for all applications ([834] shows some milestones of this exciting development), the ease of use of aqueous dispersions has quickly established itself as a standard alongside traditional oil-based paints. Their advantages include: – short open time, already after a quarter of an hour, a new paint layer can be applied to the picture – good adhesion to many different substrates (such as paper, canvas, wall, metal, and wood) – water solubility and, therefore, unproblematic processing – glazed or opaque application, numerous paint media for low or high viscosity – formation of nonyellowing, flexible nontearing films – overpainting with acrylic and oil paints, the establishment of a quasi-old-masterly mixed technique with possible rapid acrylic underpainting, and final elaboration in oil Acrylic paints are also complex paints and pose significant problems for restorers, as is illustrated in [837–839]: Acrylic surfaces are susceptible to solvents and direct contact in the course of cleaning or a conservation process. For consideration of the durability of acrylic paints, see [841]. Acrylic-based dispersions are versatile [185] in exterior and interior paints, coatings, varnishes, adhesives, sealants, and joint compounds. In painting, acrylic and methacrylic acid esters are applied and copolymerized with styrene or several vinyl compounds (vinyl alcohol or vinyl acetate) depending on the intended use. The selection of the monomers and their proportions controls the properties of the polymer to a

732 � 7 Paint systems in art large extent. Acrylic polymers are either dissolved in organic solvents (toluene or others) or dispersed in water. In the case of the first variant, industrial applications such as coating or varnishing are in the foreground. However, aqueous acrylic dispersions are the chosen products for artists and interior painting. The drying of acrylic dispersions takes place by physical means. The water evaporates rapidly. The acrylic polymer particles isolated from one another in the dispersion move closer together and fuse to a no longer dissolvable film even if a solvent is added. The film is not formed like an oil film by polymerizing the components to an extensive network but by secondary interactions between the polymer particles. This mechanism casually explains some significant differences between oil and acrylic paintings: Oil films do not suffer a material loss due to evaporated solvent. The thick transparent oil films have bodies and can develop a high proportion of deep light. The light shines through numerous transparent, colored glazing to opaque layers under certain circumstances; a complex color experience results. On the other hand, the dried acrylic film lacks body since the solvent completely evaporates, and thus much material is lost. Only a relatively small part of the actual binder remains. The color impression is mainly determined by scattered light, similar to a low-oil matte prima oil painting. Due to the low film mass, the high pigment concentration can quickly lead to sharp and glaring color impressions.

7.9.1 Basic composition The basic composition of an acrylic paint is complex [185, Chapters 2, 3], [98, 99, 192, 196, 833–835, 837, 840, 853, 854, 995, 998, 1003]: – residues of polymerization (initiators such as potassium persulfate, “chain control agents” such as dodecyl mercaptan to influence the chain length) – acrylic dispersion as a binder, possibly stabilized by anionic and nonionic surfactants – pH buffer – pigments – dispersion stabilizers – wetting agents (surfactants) as wetting aids and dispersants – film formers – thickeners and rheology modifiers to adjust the viscosity – further additives (preservatives, defoamers, fungicides, modifiers for thawing and freezing behavior) However, we can grind pigments into a commercially available acrylic dispersion specifically available for the production of artists’ paints. In doing so, we must ensure the most delicate possible distribution as with oil paints. In the case of such an in-house production, it is best to disregard other additives since, on the one hand, commercial acrylic

7.9 Acrylic paints

� 733

dispersions already have good properties. On the other hand, we do not know the necessary substances and properties for a suitable paint formulation. Dispersions We have already addressed dispersions in general terms in ▶Section 6.3.2, so we will concentrate on acrylic ones in the following. In the context of artists’ paints, we mean specifically suspensions of polyacrylates in aqueous solvents. The polyacrylates are copolymers of various acrylic acid-based monomers. The aqueous suspensions are also called latex dispersions because natural latex was the first polymer used for this application. They consist of approximately spherical polymer particles in the magnitude of 1 µm downwards, which are finely dispersed in water. Larger particles give the dispersion a milky white appearance, smaller ones below 0.05 µm that of a transparent liquid. In between, the semitransparent dispersion shows a gray or bluish tint. The cause of these color appearances is the scattering of light, ▶Section 1.6.6. Acrylic dispersions The polymers in acrylic dispersions are copolymers based on (chain-building) methacrylic acid esters and (chain-modifying) acrylic acid esters [835]. Different acids and alcohols introduce different properties into the copolymer and allow the optimum to be achieved for a particular application since the monomer composition for the copolymer can be fluidly adjusted. However, the copolymers are hardly ever explicitly developed for artists because of the small market share. Instead, they are derived from weatherresistant house coatings, which, similar to a painting, should also remain unchanged for as long as possible. The essential monomers in artists’ paints are methyl methacrylate (MMA), ethyl acrylate (EA), ethyl methacrylate (EMA), and methyl acrylate (MA); since the 1980s, increasingly n-butyl acrylate (BA) and to a lesser extent 2-ethylhexyl acrylate (EHA) [98, 99, 192, 833, 834, 854, 998, 1003]. Small amounts of the unesterified acrylic acid (AA) or methacrylic acid (MAA) electrostatically stabilize the dispersion. The polymer has the following typical structure:

An important property is the glass temperature. Below this temperature, the polymer chains in the amorphous regions of the polymer are frozen in a glassy state. However, they begin to move and soften above this temperature. The result is a viscous or rubbery

734 � 7 Paint systems in art mass that transitions to a melt. (In contrast, the melting temperature has a fixed value and describes the softening temperature of the crystalline regions in the polymer). The monomers used in artists’ paints form homopolymers with the following glass transition temperatures: Methyl methacrylate at 105 ℃, ethyl methacrylate at 65 ℃, methyl acrylate at 8 ℃, ethyl acrylate at −22 ℃, n-butyl acrylate at −54 ℃, 2-ethylhexyl acrylate at −55 ℃. The data show that methacrylates with high glass transition temperatures lead to hard homopolymers, while acrylates lead to soft homopolymers. We can qualitatively explain the reduced mobility of the methacrylate chains, compared to the acrylate chains, by the additional methyl group sterically hindering free movement. Similarly, we can explain the influence of the increasing branching from the methyl to the ethylhexyl ester. Hard also means brittle and abrasion-resistant, and soft means supple and sticky. To obtain dispersions with ideal properties depending on the application, manufacturers vary the proportions of monomers accordingly. Commercial acrylic binders such as Primal® AC 33/34/234 p(EA-MMA), Primal® AC 35, Paraloid® B-72 p(EMA-MA), Primal® AC 235 p(BA-MMA), Plextol® D 360, D 498, D 528, B 500 p(BA-MMA, BA-MMA, BA-MMA, EA-MMA), or Mowilith® DM 771 essentially contain the dispersed copolymer as well as water and optionally low alcohols as solvents. Typical characteristics of such a dispersion are particle size of around 0.1 µm, minimum film formation temperature of 8 ℃,and glass temperature of 16 ℃. According to [837], many acrylic dispersions available on the market do contain pure acrylates and cheaper polyvinylacetates or polystyrenes. Designated mixed or nonacrylate binders are, e. g., Mowilith® 20 (polyvinylacetate PVOAc), Mowiol® 4-88 (polyvinylalcohol PVOH), Klucel® E (hydroxypropyl cellulose HPC) and Mowilith® DM 5 (polyvinylacetate-n-butyl acrylate p(VAc-BA)) [192, 854, 988].

7.9.2 Irreversible film formation Irreversible film formation is the most important process in acrylic painting [186, 836], [185, Volume 3, p. 139ff], [196, 842–846, 852]. We have already described the fundamental processes involved in the approach of two dispersion particles in ▶Section 6.3.2. Let us consider the energy curve of two approaching acrylic polymer particles in ▶Figure 6.7, which ends when the primary energy minimum is reached (point (1) in the figure). The van der Waals forces, which are very strong over short distances, hold the particles tightly together; the available energy is insufficient to move the system out of the deep potential minimum. Moreover, particles fuse at this point, and their molecules intertwine, so re-dissolution in water is no longer possible. How can this primary minimum be reached, and what are the detailed processes? After applying the aqueous acrylic dispersion paints, the system as a stable dispersion dwells at point (3), the secondary minimum. Subsequently, we observe four phases, ▶Figure 7.27.

7.9 Acrylic paints

� 735

Figure 7.27: Process of film formation of acrylic paints in four phases [185, vol. 3, Chapter 3.2.1.8], [844, 845, 852]. White block: painting support; blue: solvent. (a) Evaporation of the solvent from the dispersion. (b) Compaction of the latex particles. (c) Compression of the particles with loss of their own shape. (d) Film formation by coalescence of the particles.

Evaporation phase Water evaporates, the concentration of the polymer particles increases, and their mutual distance decreases. A possible maximum (2) is surpassed. Compaction phase Water has evaporated largely; the approximately spherical polymer particles form a dense sphere packing. They are fixed in the primary minimum (3) by van der Waals forces, which are strong at close range; still, they retain their identity so that we do not yet speak of cohesion or even of a film. Compression phase During this phase, the polymer particles are deformed and densely compressed, and the viscous spherical packing becomes one of the dodecahedrally deformed particles. This packing has no gaps, no interstices, and no more water. Visually, we recognize this point because the milky white dispersion becomes transparent. The now homogeneous polymer material exhibits a uniform refractive index; scattering no longer occurs due to fluctuations in the refractive index of water and polymer. For the deformation of the particles, the temperature must rise above the polymer’s minimum film-forming temperature (MFT). Below this temperature, the particles are too stiff to be sufficiently deformed, thus impairing film formation. Therefore, the processing temperature is a vital parameter for the film quality of dispersion paints. The driving force for deformation is a reduction in the surface area between the polymer particles and the surrounding environment, and thus the minimization of the surface energy, ▶equation (6.13) at p. 553. The pressures range up to one hundred bar. In the literature, various possibilities are discussed as to how this force can emerge [98, 189, 842–846, 852]. Some of the proposed options (comparative values for interfacial energies can be looked up in ▶Table (8.7) at p. 809):

736 � 7 Paint systems in art –

–

–

In wet sintering, polymer-water surfaces are the dominant factor. The deformation reduces or eliminates surface areas and gains surface energy accordingly. We speak here of surface tensions in the range γpw ≈ 15 mN/m (Vanderhoff [850]). In dry sintering, the surfaces between polymer and air are the decisive factor. They exist in voids between polymer particles from which water has already evaporated. The deformation causes these surfaces to be minimized to air with γpa ≈ 30–40 mN/m and surface energy is also gained (Dillon [847]). Capillary forces occur due to the water-air meniscus of the trapped water (Brown [848]). According to the equation of Young–Laplace, a sphere of water with radius r in air exerts a pressure (“capillary pressure”) p=

2γwa r

(7.1)

on its surroundings since the surface tension γwa wants to compress the water sphere to minimize its surface area. The internal pressure p balances this tension and maintains the sphere in a stable equilibrium of forces, ▶Figure 7.28.

Figure 7.28: Emergence of capillary pressure in droplets. The surface tension γwa tries to compress a sphere of water with radius R. This compression is acted upon by the capillary pressure p, which is directed outward.

As a result of the continuous escape of water during the drying of the dispersion, polymer particles approach each other to such an extent that the water enclosed between them does not form droplets or spheres but a meniscus as we know it from capillary tubes. This concave surface can be described as a sphere with a negative radius R < 0, which has a capillary pressure, pc =

2γwa R

R0

(7.3)

or pc = −

7.9 Acrylic paints

� 737

The concave water surface pulls its surroundings (polymer particles) thus with a pressure pc , ▶Figure 7.29. With the continued evaporation of the water, R decreases, pc continues to increase, i. e., ever more pressure is necessary to produce the increasing curvature of the surface. Once the water has evaporated from the channels between the particles, we can extend the observation to water spheres that remain in the spandrel between the closely packed particles. These are “real spheres” with a convex surface (R > 0).

Figure 7.29: Emergence of capillary forces, leading to compression of polymer particles (white ovals). (a) The aqueous medium (blue) evaporates. (b) Consequently, the polymer-air interface and the surface energy of the polymer system increase. (c) When the polymer particles move closer, the polymer-air interface can decrease again (or the polymer-water area can increase). The meniscus can be described as a sphere with a negative radius R1 and R2 . The internal pressure of this upturned sphere acts not as pressure towards the outside. Instead, it pulls inward and draws the particles closer.

–

Typical values for the surface tensions in question are γwa ≈ 75 mN/m. Other mechanisms, e. g., diffusion (Sheetz [849]).

Film-formation phase Under sustained high pressure, the surfaces of the individual particles crack. The repulsive forces are passed, and coalescence occurs, i. e., polymers of different particles come into contact. Due to the reduction of the particle surface, a state of lower energy is reached. The first film of still low strength forms. Polymers diffuse into the neighboring particles in the following, taking several days. This diffusion occurs in a layer thickness of the order of magnitude 20–150 Å. For the polymers to have sufficient mobility, the temperature must be above the polymer’s glass transition temperature. Below this temperature, the polymer chains are frozen and diffuse insufficiently. Better film formation can be achieved by raising the temperature, lowering the glass transition temperature, or adding plasticizers. The polymer chains become entangled during diffusion or form partially crystalline areas. As a result, the previously soft film hardens considerably and gradually reaches

738 � 7 Paint systems in art its final strength. Furthermore, the attractive van der Waals forces acting in the vicinity between the polymer chains become so strong that the aggregation of the polymer chains is irreversible, and the resulting film cannot be dissolved again with water. 7.9.3 Retarders In the acrylic paint sector, we can choose from a rich selection of auxiliaries. They extend the short open time of acrylic paints, give them more body, a glossy or a matte surface, act as a painting media or modify the rheological properties (in particular, increasing the low viscosity of the aqueous dispersion) [185, Chapter 4], [196, 833, 995, 998]. Retarders extend the low open time of acrylic colors from a few minutes to the range of hours. They thus occupy a position between solvents (highly volatile) and plasticizers (lowly volatile), being similar to both in their structure [995, 998]: Alcohols or alcohol ethers with low molecular weight, such as propanediol (propylene glycol) or dipropylene glycol monobutyl ether. In addition, amino-alcohols such as 2-amino2-methylpropanol-1 also act as a weak base in conjunction with ASE thickeners (see below), which is necessary for the swelling of the thickener:

Alcohols decrease the water’s evaporation rate through the interaction of the polar groups. As a result, they remain for a longer time in the film being formed, thus remaining paintable longer and sensitive to water. To ensure regular filming, the proportion of retarders must not be too high. In this case, the processes of the compression and film-forming phases would be severely delayed or incomplete and exposed to disturbing and changing influences for a long time. 7.9.4 Media, thickeners, gels, acrylic butter These auxiliaries change the consistency of the thin aqueous dispersion. They consist of the same binder as the paint themselves. The media of Lascaux and Winsor & Newton, e. g., comprise the copolymers p(EA-MMA) and p(BA-MMA) [98, 99], [185, Chapter 4], [833, 840, 995, 998]. Thickeners and gels are adjusted with water and a cellulose derivative or a modified polyacrylic acid to the desired consistency. The thickening effect of the gels and media is based on the two types “hydrogeling agent” and “associative thickener,” ▶Section 6.4.

7.9 Acrylic paints

� 739

In addition to viscosity during processing, these gels also control the surface appearance of the finished film. In glossy gels, the gloss is generated by the acrylic film itself. The gel’s viscosity and rheology allow the polymer particles to blend into a film with a smooth surface, which leads to directional reflection, possibly with portions of scattered light, ▶Figure 1.21 and ▶Figure 1.26 left and center. If necessary, film-forming aids (Texanol® ) support a clean film formation. Matte gels contain white silica particles (microcrystalline quartz), which lead to slight irregularities of the film surface. As a result, light is diffusely reflected, and there is a higher proportion of scattered light, which contributes to the matte appearance quality, ▶Section 1.68, specifically ▶Figure 1.26 on the right. Media are gels adjusted to a thinner consistency and applicable as a painting medium. Modeling pastes or structural gels contain additional fillers such as marble powder or quartz dust, which can be structured and form, after filming, a corporeal mass. Cellulose-based hydrogelling agents Cellulose-based hydrogeling agents can be found in products such as Klucel® E or Tylose® . They contain methylhydroxyethyl cellulose (MHEC, Tylose® MH [990]), methylhydroxypropyl cellulose (MHPC, Tylose® MO [990]), hydroxyethyl cellulose (HEC, Tylose® H, HS, HA [990]), hydroxypropyl cellulose (HPC, Klucel® [988]), sodium carboxymethyl cellulose (NaCMC), or sodium carboxymethyl hydroxyethyl cellulose (NaCMHEC), ▶Section 6.4. Polyacrylic acid-based hydrogelling agents Available in artists’ supplies is ASE 60® , a copolymer of methacrylic acid and ethyl acrylate p(MAA-EA), which we discussed in ▶Section 6.4. Its traded form is an acidic, low viscosity dispersion of undissociated polyacrylic acid. It reacts with a sharp increase in viscosity when neutralized. If the medium has enough alkali capacity by itself, it is possible to achieve the thickening effect without prior neutralization. Associative thickeners As associative thickeners, products from the groups ether-urethanes (HEUR, an example is Tafigel® [989]), polyacrylic acids/ASE (HASE, an example is Tafigel® AP [989]), and hydroxyethyl cellulose (HMHEC) are used.

7.9.5 Wetting agents and dispersants The importance of wetting agents and dispersants was highlighted in ▶Section 6.3. In the manufacture of acrylic paints, the unique features of a pigment dispersion must be con-

740 � 7 Paint systems in art sidered and supported by suitable auxiliaries [99, 835, 837–840]. As wetting agents for acrylic paints, suitable nonionic surfactants (alkylphenol ethoxylates), anionic surfactants (alkyl sulfates and alkylaryl sulfonates, e. g., sodium lauryl sulfate and sodium dodecyl benzene sulfonate), sulfosuccinates and polyphosphates are mentioned in ▶Section 6.3.1. The artists’ supplies trade offers compounds such as Triton® X100 (octylphenoxypolyethoxyethanol) or Surfynol® 61 (3,5-dimethyl-1-hexin-3-ol):

In ▶Section 6.3.3, we have examined dispersants for acrylic paints using the example of acrylic dispersion. The compounds mentioned above are effective as dispersants and also as wetting agents. In addition, hydroxyethyl cellulose (HEC), polyvinylalcohol (PVOH), polyvinylpyrrolidone (PVP), or fatty acid polyglycol esters and polyglycol ethers act as dispersants. Furthermore, as ionically and sterically effective stabilizers, polycarboxylates based on methacrylic acid are popular. Examples from the trade are Orotan® 731K and Tamol® 960 as sodium polycarboxylate based on a polymethacrylic acid [1002]; Tamol® 731A and Orotan® 731A, as copolymers of diisobutylene and maleic anhydride [999–1001]:

7.9.6 Film formation aids The film formation described above can only occur without any problems if we maintain the minimum film formation temperature (MFT) during processing so that the polymer particles are sufficiently deformable. In the case of conventional acrylic dispersions for artists, this is approximately at room temperature or slightly below. In order to ensure uniform film formation even at lower temperatures, we can add film formation aids, which will temporarily reduce the MFT. Their function, general structure, and frequently used compounds have already been discussed in ▶Section 6.5. In

7.10 Lithographic printing, lithography

� 741

artists’ paints, we find, in addition to various glycols and glycol ethers, pyrrolidones such as N-methyl-2-pyrrolidone, esters of benzoic acid or dibutyl phthalate. Examples from the artists’ supplies trade since the 1970s are Texanol® (2,2,4-trimethylpentane-1,3-diolmonoisobutyrate or (3-hydroxy-2,2,4-trimethylpentyl)-2-methylpropanoate), Velate® 386 (2-ethylhexyl benzoate) or Velate® 262 (isodecyl benzoate):

7.9.7 Other additives Acrylic paints may contain numerous other ingredients [840]: – defoamers to prevent foaming of the dispersion during processing, e. g., mineral or silicone oils (poly dimethylsiloxane) – adjuvants to control freezing and thawing (freeze-thaw), including ethylene glycol and propylene glycol – preservatives to improve shelf-life like barium metaborate BaB2 O4 ⋅ H2 O or isothiazolinone derivatives

7.10 Lithographic printing, lithography Lithography is a printing process highly dependent on the finely balanced interplay of chemical components [608]. Therefore, it is worth taking a closer look at this technique.

742 � 7 Paint systems in art Only letterpress printing and intaglio were available to reproduce texts and images until the nineteenth century. As a type of letterpress printing, woodcut was the most common technique for producing book illustrations and printed matter but allowed only black and white representations. In addition, cutting the lines into the wood was a laborious task, and the vividness of the lines was often lost. Copperplate engraving, an intaglio technique, allowed a more subtle expression. Nevertheless, even here, the metal’s toughness into which the lines had to be scratched slowed down the liveliness of the lines. The etching technique of delicate engraving lines and the possibility of working in intermediate tones broadened the possibilities of expression more and more. However, in total, the processes were costly, and the possible amount of editions was low. At the beginning of the nineteenth century, A. Senefelder invented planographic printing on stone. As a result, the artwork’s printing and nonprinting areas are no longer separate but lie next to each other in the same plane. However, they differ in their chemical properties, especially their hydrophilicity or hydrophobicity. Senefelder, therefore, called his printing process chemical printing; today, however, the process is known as lithography. Planographic printing became immediately popular everywhere. It was cheaper than copperplate printing and allowed more numerous and faster print runs. The basis of lithography is a flat, fine-grained stone plate into which greasecontaining drawing media (lithographic ink or chalk) can penetrate. Since the beginning of lithographic printing, the Solnhofen limestone has been particularly suitable for lithographic printing since it consists of more than 97 % CaCO3 built from the most delicate foraminifera shells. A quality feature is its uniform grain size, which actively absorbs the drawing media through capillary effects. The following sequence of drawing, etching with a gum-acid solution, and washing off is only one possibility to work with lithographic processes, [608] introduces others. Applying the drawing The lithographic chalk (moistened for use) and the lithographic ink contain sodium stearate C17 H35 COONa (soap). Since the carboxyl group is hydrophilic like the stone surface, it attaches to the stone plate. In contrast, the stone repels the lipophilic alkyl radical. As a result, hydrophobic areas form in the area of the drawn lines or painted surfaces. First preparation and etching After the drawing is available as a hydrophobic pattern, an aqueous solution of gum Arabic and 2–4 % nitric acid is applied, anchoring the hydrophobic areas of the drawing and the hydrophilic residual areas in the stone. The nitric acid releases stearic acid from the soap, which reacts with the lime of the stone slab to form a stone soap (calcium stearate) and thus firmly anchors the alkyl residues of the soap in the stone:

7.10 Lithographic printing, lithography

� 743

While the hydrophobic pattern is attached to the stone, the acid dissolves the carbonate in the unmodified areas of the stone:

The exposed pores absorb the gum Arabic solution well and represent the negative of the drawing. After rinsing off the gum solution and acid, gum Arabic solution is applied again to reinforce the hydrophilic areas. Blow-drying promotes the drying of the gum solution, which dries to a permanent film that does not dissolve in water even when moistened. Thus, the drawing is present as a hydrophobic positive and hydrophilic negative. Reinforcement of the oil-based drawing Any soap residue can be washed off with turpentine oil. Then, hydrophobic lithographic ink was rolled up (applied with a roller) to strengthen the drawing, which adheres only to the hydrophobic areas of the picture. The ink contains stand oil (linseed oil varnish) as a binder. Second etching A second etching with aqueous gum Arabic solution and nitric acid, this time 4–6 %, enhances the differences between hydrophobic and hydrophilic areas. The acid oxidizes the linseed oil at the double bonds, cross-links the fatty acid residues via oxygen bridges, and thus solidifies the hydrophobic film. At the same time, the stone is etched deeper and absorbs more gum. Reinforcement and third etching The excess ink can now be washed off with turpentine oil. Next, the new printing ink is rolled on, dusted with rosin, rubbed with talcum powder, and heated (“burned in”) with a soldering lamp. Talcum and rosin melt, blend with the printing ink, penetrate deep into the stone in the hot thin liquid state, and create an acid-proof film. The etching is continued with a gum-acid solution of 6–8 %. So much of the stone is etched away that the hydrophobic drawing lies slightly above the stone surface.

744 � 7 Paint systems in art Gum Arabic and printing ink are now removed with water and turpentine, and the actual printing process can begin. 7.10.1 Lithographic crayons and inks Lithographic crayon consists of soap, grease, carbon black, and binders. Typical compositions are: Lithographic crayon [608] Wax 10 parts, tallow 5 parts, soap 5 parts, carbon black 2 parts, gum Arabic 2 parts, shellac 40 parts, sodium carbonate 6 parts Lithographic crayon [608] Wax 30 parts, soap 50 parts, carbon black 8 parts, gum Arabic 10 parts Lithographic crayon [608] Wax 48 parts, soap 36 parts, carbon black 20 parts

Carbon black is the colorant in this crayon. Depending on the hardness of the crayon, tallow or wax predominates. Since the transfer of oil-containing outlines to stone is decisive, an artist could also use wax crayons for experiments with stone printing. Gum Arabic and shellac broken down by sodium carbonate serve as light binders to hold the crayon together in pencil form. For use, the pencil is moistened. The liquid lithographic ink also contains carbon black as a colorant, wax, and soap as oily components, and shellac as a binder, as the following recipes show: Lithography ink [608] Wax 12 parts, tallow 4 parts, soap 5 parts, carbon black 1 part, mastic 2 parts, turpentine 1 part, shellac 2 21 part Lithography ink [608] Wax 8 parts, tallow 2 parts, soap 4 parts, carbon black 2 parts, mastic 2 parts, turpentine 21 part Lithography ink [608] Wax 3 parts, tallow 3 parts, soap 4 parts, carbon black

3 4

part

Lithography ink [608] Wax 5 parts, tallow 3 parts, soap 3 parts, carbon black 1 21 part

7.10.2 Reprint and materials for reprint Instead of directly on the stone, the drawing can be more conveniently drawn on transfer paper. This picture can then be reprinted onto the stone. To make this possible, transfer papers have a water-soluble coating that separates the drawing from the paper and prevents the grease from the lithographic ink penetrating it. Exemplary coatings are: Coating for transfer paper [608] Bone glue 2 parts, starch 1 part, chalk 2 parts, alum Coating for transfer paper [608] Bone glue canth 1 part

1 2

1 2

part

part, starch 4 parts, gum Arabic 2 parts, gum traga-

7.11 Silicate paint

� 745

Coating for transfer paper [608] Bone glue 1 part, starch 2 parts, chalk 1 part, gum Arabic gum tragacanth 21 part

1 4

part,

Coating for transfer paper [608] Bone glue 1 part, starch 5 part, glycerol 1 41 parts

Reprinting is done by placing the paper with the drawing side on the stone, moistening it from the reverse side, and pressing it several times with a hand press. The drawing is almost entirely transferred to the stone and can be prepared after one day with a weak etching.

7.10.3 Printing inks for lithography Printing inks for lithography consist of a pigment and linseed oil varnish as a binder, i. e., linseed oil, which is thickened at 300–320 ℃, possibly with the addition of siccatives. The paints may contain additives, such as siccatives (cobalt, manganese, and lead salts) or thickening agents (Bolognese or Champagne chalk). Nondrying oils such as corn or almond oil, added dropwise, slow down drying.

7.11 Silicate paint For the sake of completeness, we should mention silicate paints. Water glass is the binder used for this type of paint [196]. Water glass is the alkali salt of polysilicic acid and is formed by melting quartz sand with alkali carbonate at high temperature:

The resulting glass mass can be dissolved in water under pressure at about 100 ℃ and forms water glass. Water glass hardens chemically by decomposition, which already occurs due to carbon dioxide in the air:

The resulting hydrated SiO2 is (amorphous) silica gel, which tightly encloses pigments and preserves the painting. We do not use silicate-based paints for panel painting. However, they offer advantages for interior house coating and artistic mural paintings. Recipes and application instructions are given in [92].

746 � 7 Paint systems in art

7.12 Low binder systems: chalks and pencils We can assume that the abrasion of a colored piece of material on a rough surface was one of the earliest means of drawing that people used artistically. A naturally occurring piece of white chalk, colored ocher earth, a colored stone, or charcoal may have been used conveniently as an early drawing stick. In addition, red chalk appeared in the fifteenth century, which consisted of red ocher mixed with clay. Since the abrasion of such drawing agents is not fixed, no considerations are necessary regarding a binder. The adhesion of the pigment takes place only by adsorption on the painting surface or by being entrapped in cavities of rough support. The effect of these drawing materials is based solely on the diffuse reflection and scattering of the pigment powder. If we wanted to fix the drawing with a binder, the inevitably developing film would reduce the scattering and replace it with depth light. These effects would destroy the specific character of such a drawing. The development of abrasion-based drawing materials is mainly characterized by the search for colored materials that are soft enough to be rubbed off clearly and fluidly but are also dimensionally stable and not too brittle. Attempts to offer the colored compounds in a handy form led to the invention of sticks (charcoal, chalk) and pencils.

7.12.1 Blackboard chalk This primarily white chalk consists only partly of natural chalk, mineralogically calcium carbonate CaCO3 : Pressed blackboard chalk [203, Keyword “Drawing and Writing Materials”] Natural chalk 67.5 %, talcum 5 %, bentonite 5 %, as binder carboxymethyl cellulose 2 %, wetting agent 2 %, water 10 %

Today’s blackboard chalk is also based on white anhydrous calcium sulfate CaSO4 (alabaster gypsum), which is mixed with fillers (kaolin Al2 O3 ⋅ 2SiO2 ⋅ 2H2 O, talcum, chalk) to improve the writing properties: Cast blackboard chalk [203, Keyword “drawing and writing materials”] Alabaster gypsum 6 parts, water 5 parts, fillers (talcum, kaolin, chalk), binders (cellulose derivatives, gum Arabic)

Depending on the water-gypsum ratio, the mixture filled into a mold solidifies into sticks of different hardness. Optionally, an added light binder such as carboxymethyl cellulose or gum Arabic prevents dust formation. Colored chalks differ from white chalk only by colorants; the addition of pigments and dyes is possible. However, of course, only nontoxic colorants are used, since hazards are caused not only during manufacturing, but also during application of the chalks due to their abrasion dust.

7.12 Low binder systems: chalks and pencils

�

747

7.12.2 Pastel crayons These crayons are, by nature, colored blackboard chalks and are formed from an aqueous paste that contains only minimal amounts of binder to preserve the shape of the sticks and crayons: Pastel crayon [96], [203, keyword “drawing and writing materials”] Inorganic nonhazardous pigments 10–15 % by weight, mineral fillers (gypsum CaSO4 , chalk CaCO3 , talcum, kaolin Al2 O3 ⋅ 2SiO2 ⋅ 2H2 O) 60–70 % by weight, binders (gum tragacanth, gum Arabic, alginates, cellulose derivatives, or polyvinylalcohol) 5–10 % by weight (15–20 % of a solution 5–10 %), bentonite 3–5 % by weight Pastel crayon [74, Chapter 13] Nonhazardous pigments (titanium white, zinc white, or precipitated chalk for white artist-grade crayons), white mineralic fillers for lighter tints (precipitated chalk CaCO3 , kaolin Al2 O3 ⋅ 2SiO2 ⋅ 2H2 O), binders (gum tragacanth, gum Arabic). Dissolve 1 part of gum Arabic in 2 parts hot distilled water, or 1 part of gum tragacanth in 25 parts distilled water. Make a few standardized dilutions of this stock solution, which is too strong for some pigments. Grind pigment and filler with binder, add binder in small charges until a smooth, doughty paste is achieved. Form sticks and let slowly dry. Be careful not to inhale pigment dust! Pastel crayon [75, Chapter 8] Nonhazardous pigments (precipitated chalk CaCO3 for white crayons and for making lighter tints), binders (gum tragacanth, methyl cellulose), preservative. Dissolve 31 oz. gum tragacanth in 8 fl. oz. distilled water. Make a few standardized dilutions of this stock solution, which is too strong for some pigments. Grind pigment and filler with binder, add binder in small charges until a smooth, doughty paste is achieved. Form sticks and let slowly dry. Be careful not to inhale pigment dust!

7.12.3 Pencils The pencil likes to mislead us: it only consisted of the metal lead in antiquity (the Roman scribes used lead rods to write on papyrus) and early modern times and produced rather unspectacular, faint gray lines. Its first manifestation was flat pieces of lead, which the artist used with a narrow side to achieve fine lines. This medium only acquired greater color depth and blackness by discovering rich deposits of pure, soft graphite in Borrowdale in 1564, which started the triumphal procession of the graphite pencils [164, 165]. Graphite quickly replaced lead in the seventeenth century, applied as sticks cut from graphite and wrapped in twine or strings or hold in a special tool (porte-crayon). The shapeless pieces were then placed in wooden sleeves, barrels, or shafts to protect the pigment core from breaking and the user from dirt. Finally, in 1795, the artificial core made of a graphite-clay mixture was invented by N.-J. Conté. Today’s pencils also consist partly of graphite, partly of clay (15–60 % by weight), as well as fillers like kaolin Al2 O3 ⋅ 2SiO2 ⋅ 2H2 O or talcum [203, keyword “drawing and writing materials”]. The proportion of graphite and clay determines texture and depth;

748 � 7 Paint systems in art higher amounts of clay yields harder, more silvery pencils (denoted by letter “H”), higher amounts of graphite softer, blacker pencils (denoted by letter “B”). In addition, aluminosilicates, methyl cellulose, or lignin serve as binders. Carbon black may be added to deepen the tone. All components are mixed according to the required degree of hardness, kneaded together, and fired at 900–1200 ℃. Silicate clay minerals act as binders, as they lose water and sinter at these temperatures. The porous graphite sticks are then immersed in a hot mixture of greases and waxes. These fill the graphite pores, which can amount to approximately 20 % of the volume, and provide a soft and light abrasion of the pencil. In addition to classic pencils, today, we also know very thin cores (0.3–1 mm), which are sold loose. They can be inserted in mechanical or technical drawing pencils and no longer need sharpening. Unfortunately, graphite-clay mixtures do not provide the necessary stability for this purpose. Therefore, a thermoplastic (polystyrene, polyvinylacetate) or lignin is used instead of clay. During firing, the plastic pyrolyzes, leaving a rigid carbon skeleton, which takes over the role of the clay sinter and exhibits high strength. The color determining factor is still the graphite portion, which has not changed its structure during firing and is embedded in the polymer carbon skeleton. These polymer mines are also finally impregnated with a grease-wax mixture. Cause of color When drawing, tiny sheds of graphite are detached from the graphite core by abrasion on the rough surface of the drawing ground. These sheds show the silvery-gray appearance, typical of graphite, as discussed in ▶Section 3.1. While charcoal sticks possess a deeper black appearance due to their microscopic structure of carbonized fibers, cell remnants, and cavities, causing manifold reflections and absorption of light, graphite abrasion possess a flaky structure, causing reflections rather than deep absorption. Erasers Graphite can easily be removed from the drawing using an eraser [164, 166]. Traditionally, pieces of (moist) bread were used. In 1752, it was noted that the condensed latex or “caoutchouc” from rubber trees could be used to remove graphite. According to [166], J. Priestley coined the term “rubber” after this capability of “rubbing out” graphite marks. Caoutchouc or polyisoprene is the polymer of isoprene:

7.12 Low binder systems: chalks and pencils

�

749

As was the case for all early rubber products, also the graphite rubber had the disadvantage of softening upon heating, and embrittling upon cooling. The vulcanization process of C. Goodyear transformed the raw rubber goods into versatile, cured products, ▶Figure 7.30. Upon addition of sulfur to raw latex, heating, and pressing, sulfur crosslinks the individual polymer chains, yielding a stable three-dimensional polymer. Soon, graphite erasers were made of vulcanized rubber.

Figure 7.30: Schematic process of vulcanization of natural rubber. Addition of sulfur and heating yields cross-linked polymer chains.

In the twentieth century, erasers were made from manifold other polymeric materials such as isoprene-isobutylene copolymer (butyl rubber), styrene-butadiene copolymer, and ethylene-propylene copolymer. From the 1950s on, synthetic rubbers including synthetic cis-polyisoprene from the Ziegler–Natta catalytic process replaced natural rubber in erasers. They are made nearly exclusively from synthetic rubber nowadays, especially polyvinylchloride (PVC). To manufacture erasers, the natural or synthetic rubber is heated, mixed with additives (petroleum-based oil, plasticizers, amine or phenol antioxidants, pigments), and vulcanization aids (sulfur), if required. For erasers, abrasives such as silica pumice are added. The mass is extruded into the final form, or placed in a mold, and cured by elevated temperatures. An especially soft eraser is the “putty rubber,” it can remove graphite by light dabbing, yielding soft tonal values, lights and shades.

750 � 7 Paint systems in art 7.12.4 Colored pencils The composition and manufacture of colored pencils are very different from those described above [203, keyword “drawing and writing materials”]. The main reason is the temperature sensitivity of colored pigments in contrast to graphite. In the production of colored pencils, therefore, the firing process is dispensed with; the base compound consists of a binder, pigments, and fillers (kaolin Al2 O3 ⋅ 2SiO2 ⋅ 2H2 O, talcum, chalk CaCO3 ). The binder can either be dissolved in water (methyl cellulose, polystyrene, polyvinylacetate) or an organic solvent (cellulose nitrate, ethyl cellulose), and enclose the pigments homogeneously. After drying at below 100 ℃, the colorants are firmly embedded in the binder matrix. Impregnation with a grease-wax mixture then follows. Crayons that will be applied with water later (watercolor pencils) contain fats and emulsifiers or surfactants from the outset; the final impregnation with the grease/wax mixture is unnecessary. Emulsifiers and surfactants ensure that the lipophilic colored mass can be moistened and dispersed with water. 7.12.5 Paper We have presented the elemental composition, function of the components, and production processes of paper in ▶Section 6.7 and the unique features of artists’ papers in ▶Section 6.7.7. Traditionally, artists used parchment for drawings. Nowadays, wood pulp-free papers made of cellulose are available for drawing and pastel papers, sized in the mass and mostly also surface-sized [182, 183]. There are also cheap wood-containing school drawing papers, but the aging problems of wood pulp make them unsuitable for artistic purposes. A surface sizing, e. g., with AKD, strengthens the paper against shear forces, which can occur when drawing with hard or sharp pencils, especially when erasing. Drawing paper is textured with fine to coarse grains so that the pigments can be easily rubbed off the lead or chalk core. Adding a few percent CaCO3 as a buffer against acid slows down aging processes so that such wood-free papers may usually be called aging resistant, according to ISO 9706.

7.13 Fingerpaint Besides chalk, colors painted directly with the hands, bypassing any brushes or other tools, contribute not only to children’s creative materials. In this application, the nontoxicity of the materials is paramount, and the components of these paints are selected accordingly [704]: – The binders consist of nontoxic natural substances. Proven to be effective are modified cellulose and starch.

7.14 Intarsia art

–

– –

–

–

� 751

Fillers give body and opacity to paints. Since many regular fillers are chemically relatively inert and nontoxic, we find well-known and food-approved compounds such as chalk (calcium carbonate), gypsum, silicates, kaolin, alumina (Al2 O3 ), magnesium oxide, and titanium white. Polyglycols and glycerol are used as humectants. The natural or modified natural binders used in edible paints are susceptible to microbial attack, so preservatives must be added. An efficient combination is 2-methyl-2,3-dihydro-isothiazolin-3-one and 5-chloro-2-methyl-2,3-dihydro-isothiazolin-3-one.

Nonhazardous components are required. However, the paints must also be distinguishable from food in terms of taste, eliminating any risk of swallowing the fingerpaint. Bitter substances effectively signal “nothing to eat.” We know naringin from bitter orange peels; synthetic embittering agents are sucrose octa-acetate or the particularly effective denatonium benzoate. Pigments and colorants as the main components are subject to strict regulations and must be approved colorants for food. In the EU, e. g., the standard “EN 71 Safety of Toys – Part 7: Finger paints – requirements and test methods” (ISO 8124-7) lists requirements for ingredients of finger paints. Possible pigments are PY1, PY3, PY42, PR5, PV23, PB15:1, PG7, and PBk7 [968, Chapters 3.11, 3.12]. Besides pigments, manufacturers offer paints made from natural dyes.

7.14 Intarsia art The creation of patterns by joining different woods (marquetry) belongs to the craftwork but becomes interesting for our topic since pieces of wood are colored. Some materials from around 1570 until the eighteenth century are listed in ▶Table 7.10 [775, 776]. Yellow, red, and blue wood colors are caused by flavonoids (▶Section 4.4.4), neoflavonoids (▶Section 4.4.6), and anthraquinones (▶Section 4.6.3). The dyes can be added to the wood by dyeing or are, as in the case of wood from dyer’s sumach, naturally already contained in the wood. Natural blue-green woods owe their color to an attack by fungi: the micelles of chlorociboria secrete the dye xylindein, a naphtho-

752 � 7 Paint systems in art Table 7.10: Colored woods from the sixteenth to the eighteenth century for intarsia [775, 776]. Color

Origin of colorant

Colorant

Yellow

Dyer’s weed Barberry Dyer’s sumach, smoketree

Luteolin (flavone/flavonoid) Berberine (isoquinoline) Fisetin (flavone/flavonoid)

Red

Brazilwood Cochineal

Brazilin (neoflavone/flavonoid) Carminic acid (anthraquinone)

Blue

Indigo Indigo carmine Bluewood

Indigotine Indigotine-5,5’-disulfonic acid Hematein (neoflavone/flavonoid)

Green

Copper Various mixtures Chlorociboria fungus

Verdigris Various Xylindein (naphthoquinone)

quinone. At that time, the fungi were cultivated on the wood, and the colored wood was collected. Berberine from barberry is one of the few color bases.

8 Inks In this chapter, we dwell on inks, a paint system that, due to its suitability for drawing and writing, has had a tremendous significance for the cultural development of humankind. Introductory sources of information are, in addition to specific references, [155], [203, keyword “drawing and writing materials”], [204, keyword “Tinten und andere Schreibflüssigkeiten”]. Inks are colored liquids that apply visible traces (characters or drawings) to materials such as paper [809]. The components changed over time in typical ways we will use as an organizing principle. ▶Figure 8.1 illustrates the rough chronological course of ink development, and ▶Table 8.1 displays the main compositions.

Figure 8.1: Rough chronological overview of the essential inks [155]. For writing purposes, after the era of Indian ink and carbon black inks, metal inks (iron gall ink) dominated until the discovery of dye inks.

Chinese or Indian ink Already 5400 years ago, black ink made of lampblack and gum Arabic was found on Egyptian papyri. Later, red inks with hematite/cinnabar/read lead as pigment were used [777]. Suspensions of carbon black in a binder were the medium of choice for black inks for a long time. Natural dye inks Since antiquity, colored plant juices and extracts were used, rarely also animal products. Thus, e. g., sepia is supposed to have been used, though it probably appears only from the eighteenth century onward. Especially the scribes of the medieval monastic scriptoria developed brown and black inks from natural materials such as barks, thorns, and metal https://doi.org/10.1515/9783110777123-008

754 � 8 Inks Table 8.1: Essential classes of (historical) inks and their schematic composition [155]. Class Carbon

Metal

Composition (other than water) Chinese ink, Indian ink, carbon black ink

Carbon black, binder

Bister

Fatty wood soot, glue

Iron gall ink

Earlier: gall apples, vitriol, binder New: gallic acid, tannin, iron(II) sulfate, hydrochloric acid, water, binder

Dye

Plants

Bark/thorn ink

Bark/fruit/peel, vitriol, binder

Bluewood ink

Bluewood, alkali, possibly metal salt, binder

Dye ink

Dye, binder

Bark/thorn ink

Bark/fruit/peel, binder

Bluewood ink

Bluewood, binder

Plant-based ink

Colored plant extracts, binder

compounds. From numerous plants, they obtained colored inks, which found their most beautiful application in book illumination. Metal-based inks Almost 2000 years old are iron gall inks, which possess a permanent color and show on papyri as early as 300 BC [777]. They originate from vitriol (iron(II) sulfate) and aqueous extracts of gall apples. Later, old pieces of iron often substituted vitriol. These early iron gall inks contained the finished, insoluble colorant suspended in ink. The suspension was stabilized by the binder (usually gum Arabic). In 1856, A. Leonhardt first produced the modern iron gall ink, which could later also be employed in fountain pens. Initially, only soluble colorless iron(II) gallate is built by adding acid. The insoluble colorant is only achieved over time by the oxidation of FeII to FeIII by atmospheric oxygen as the writing dries; the addition of acid prevents premature oxidation in ink. A dye is included to see the writing instantly appearing on the material, which does not contribute to the later black writing color. Dye inks Today, in addition to classic writing inks, we find colored inks for writing or artistic purposes, ballpoint pens, fountain pens, felt-tip pens, computer printers (inkjet inks), and printing inks for a wide variety of printing processes and industrial sectors. Using synthetic dyes and pigments, a full range of colors is available. In addition, various solvent combinations allow writing on paper, foils, plastics, and metals.

8.1 Carbon inks

� 755

8.1 Carbon inks The history of carbon-based black inks starts in prehistoric times, as an overview depicts: Neolithic

Red/black pigment marks on neolithic pottery [158, p. 237], [157, p. 47]

4th millenium BC

Egyptian inscriptions on pottery black/red (carbon or charcoal, cinnabar) [157, p. 71]

Around 3000 BC

Probable Chinese ink development [158, p. 233f]

2600 BC

Egypt ink (lamp black, gum Arabic, water) [159, p. 271f]

Around 2600 BC

China ink, ascribed to Tien-Tchen (lamp black, specialty ink: soot of pine logs, ivory, lacquer resin) [159, p. 271]

2500 BC

Egypt ink (burned animal bones) [158, p. 236]

2nd millenium BC (Chang dynasty)

Tchao Hi-Kou (author of thirteenth century), Chinese ink of high antiquity: lacquer or varnish [157, p. 44]

1700 BC (Chang dynasty)

Red (cinnabar) and black (carbonaceous material) pigment marks on oracle bones and stone objects [158, p. 237]

1400 BC

Ink samples [158, p. 233f]

1st millenium BC (Chew dynasty)

Tchao Hi-Kou (author of thirteenth century), Chinese ink of antiquity: “inkstone” [157, p. 44]

Greek

Greek ink (dried wine-lees or burnt ivory), washable [158, p. 236]; inks equal to roman inks [157, p. 74]

Roman

Roman ink (similar to greek ink, additional pigments were used, made from half-charred human bones, earth and minerals, resin or pitch soot, sepia, lamp black), washable [158, p. 236]; Vitruvius, Pliny, Dioskurides, Philo of Byzantium: pigment (soot), binder (gum Arabic), sometimes additives [155], [157, p. 77]

400 BC

India ink [158, p. 236]

5th century BC

Silk as writing medium [158, p. 240]

200 BC (Han dynasty)

Chinese ink (pine soot, glue, miscellaneous additives), permanent, nonwashable [158, p. 233]

Around 200 (Han)

Wei Tan (around 192 to 224): pine soot [158, p. 239]

Late Han

Paper as writing medium [158, p. 240]

1st century

Export of India ink [158, p. 236]

3rd century

Recipe for making Chinese ink, attributed to the calligrapher Wei Tan: pigment (soot or pine soot), binder (glue), additives (egg white, cinnabar, musk, tree bark) [158, p. 237, 243], [157, p. 43]

6th century (Liang dynasty)

Pine soot, cloves, musk, lacquer, glue. Additives herb (purplish), bark (bluish) [158, p. 245]

7th century 9th century

Ink wash painting with India ink Arabic carbon ink, gray-black: (pine) soot or vegetable black, fish glue and vegetable gums [156] Arabic carbon ink, black: lamp black from linseed, sesame, olive, or other vegetable oils or petroleum [156] Arabic cheap charcoal ink, silvery gray: vegetable black or charcoal black, gum Arabic [156]

756 � 8 Inks 10th century (Southern Tang dynasty)

Soot, aqueous extract from horn, plants, bark, sappanwood and sandalwood, fish glue, green vitriol, soot [158, p. 245]

10th century (Song dynasty)

Chinese ink (lamp black from animal (fish), vegetable (rapeseed, bean, hemp, sesame, tung) and mineral (petroleum) oils rivals pine soot) [158, p. 233, 241];

11th century (Song Ink attributed to Chang Yü: oil lamp black, musk, camphor; ink attributed to Phan Ku: dynasty) soot, small amounts of glue [158, p. 246] 11th century

Iron-gall ink (oak gall=tannic acid + iron sulfate, water, gum Arabic) [159, p. 272]

12th century

Theophilus iron-gall ink from buckthorn (crushed buckthorn=tannic acid + iron sulfate) [159, p. 272]

Ming

90 % of ink made from pine soot and 10 % from lamp black [158, p. 241]; ink attributed to Wu Shu-Ta: tung oil, glue, musk [158, p. 246]

Around 1330

Ink attributed to Chu Wan-Chhu: only soot from pine trees [158, p. 246]

Around 1400

Ink, attributed to Shen Chi-Sun: lamp black instead of soot, tung oil lamp black, cowhide glue, fish glue, ash bark, sappanwood bark [158, p. 245]

17th century

India ink with lamp black [158, p. 240]

Early 19th century

Ink attributed to Hu Khai-Wen: lamp black, lard, antler glue, several additives [158, p. 246]

The inks, prepared according to these recipes, represent suspensions of carbon black in a binder, often denoted as tusche if nonwashable, ▶Section 8.8. In the Mediterranean area (Egypt, Greece, Rome), polysaccharides such as gum Arabic (▶Section 7.7.2) function as binders. In the Asian-Islamic area, we find proteins such as casein, albumin, or collagen derived from milk, bone, and skin glue (China/India ink, ▶Section 7.5). Polymeric binders are particularly suitable because they colloidally stabilize the carbon black dispersions as a side effect. Formed into sticks and dried, these inks could be stored and redispersed back to the liquid ink as needed. Reddish-colored bister comprises beechwood soot and by-products of incomplete combustion. Suspended in water with glue, it enjoyed great popularity from the seventeenth and eighteenth centuries onward for pen and wash drawings.

8.1.1 Inks in antiquity Inks from antiquity were soot inks [158, pp. 236], [155, 156], or at least carbon inks [157, p. 96], where artefacts, residues, or literary details exist and allow the testimony. 8.1.1.1 Egyptian inks The use of black in Egyptian inks seems to date back to an even earlier time than in China [158, p. 236], [157, p. 71]; Egyptian ink was probably used as early as the invention of papyrus, which was supposed to happen before 2500 BC. The first inscriptions on pottery are dated before the first dynasty, around the fourth millennium BC. There is no

8.1 Carbon inks

� 757

literary testimony on the subject or their composition; the pigment may be produced by burning animal bones. The only information comes from chemical analyses carried out on the remainders of ink found with the material of scribes. From these, it emerges that these inks were manufactured on the base of pigment and a binder. The black pigment is carbon or also charcoal, following linguistic deduction. Furthermore, soot from cooking equipment was collected. The pigment was used for inks, paints, and lacquers. Their binder probably distinguished inks and paints, but no details are known. Probably gum was used, eventually, in addition, to a proteinous material. The ink was produced, stored as a solid, and diluted in water. 8.1.1.2 Greco-Roman inks Greek inks were colored black through pigments of dried wine-lees or burnt ivory [158, p. 236]. Bat-Yehouda [157, p. 77] summarizes Greek inks under Greco-Roman carbon inks; see below. They were kept in solid form and prepared for use by grinding and mixing with water. Roman ink was similar; the basic composition was pigment (soot from resin, vine, wine yeast, pitch, or charcoal from wine lees), binder (gum Arabic, sometimes protein such as from bulls’ necks) [158, p. 236], [157, p. 77]. Dioscorides also gave a recipe, mixing a metal salt such as copper sulfate into the ink. Probably additional pigments were used compared to Greek ink, made from half-charred bones, earth, minerals, resin or pitch soot, sepia, and lamp black. The ink was solid and dissolved before use in water or wine. In contrast to Chinese ink, Greco-Roman inks were washable. Typical ink recipes are: Greco-Roman carbon black ink after Vitruvius [155, p. 28], [157, p. 77] Black of smoke (lamp black from a resin or pine chips, or vegetable black from burnt residues of wine lees or yeast), gum Arabic Greco-Roman carbon black ink after Dioskurides [155, p. 28], [157, p. 77] 3 parts soot (lamp black from pitch or resin), 1 part gum Greco-Roman carbon black ink after Dioskurides [155, p. 28], [157, p. 77] 3 parts soot (lamp black from pitch or resin), 112 parts gum, 112 proteinous binder, 112 “chalcanton” (copper sulfate) Greco-Roman carbon black ink after Pliny [155, p. 29], [157, p. 77] Soot (lamp black from pine, wine yeast, ivory or pitch), gum

8.1.1.3 Arabic inks In the Arabic-Islamic world, inks as means of scholarship had a high value of place [156]. Deepest black and gloss were sought after, and according to the degree of perfection, inks were ranked. Their origin is Chinese ink, known by Greeks and Romans around the turn of time as atramentum indicum. Due to its cost, cheaper surrogates were developed. The

758 � 8 Inks earliest inks were carbon inks containing soot or vegetable black and vegetable gums as a binder. Early Arabic soot ink [156, p. 10] Pine soot, fish glue, gum Arabic

In tradition to Dioskurides, pine soot was employed and bound by fish glue and gum, delivering a grey-black appearance. For the ruling class, luxurious materials such as sandarac were carbonized. Better quality was achieved by using lamp black as a colorant: Arabic soot ink [156, p. 10] Lamp black (from linseed, sesame, olive, or other vegetable oils or petroleum) or soot from stoves and cooking pans, gum Arabic, fragrances (musk, camphor oil, frankincense, roses), color modifiers (beetroot, ultramarine)

Since soot from vegetable oils is fatty when scraped off the collector, an after-treatment followed. To remove oily residues, the soot was baked on a stove and then lightly roasted over a fire, followed by excessive stamping before mixing with gum. Later, a metal funnel was added to separate oil tar and soot. A cheap ink was made from carbonized vegetables and other readily available materials. Also, pulverized charcoal was employed. Binder was gum Arabic. Cheap Arabic charcoal ink [156, p. 11] Carbonized vegetables (e. g., date seeds, pomegranate peels, gall apples, papyrus perennials) or charcoal, gum Arabic

These char inks appeared silvery gray, and no odorants were added. To produce good charcoal, the material was to carbonize under the control of parameters such as the amount of oxygen, degree of heat, and time. Care had to be taken that the starting material did not burn to ash when too high the temperature. On the other hand, too low a temperature increased the amount of tar and reduced soot quality. 8.1.1.4 Indian inks India uses ink from fourth century BC onwards. From first century on, “Indian ink” was exported, probably being Chinese ink, sought after due to its unique properties. 8.1.1.5 Chinese ink Chinese ink [158, pp. 233–251], [157, pp. 43–63] developed a long time ago, perhaps as early as 3000 BC. Archeological, there is evidence of the widespread use of various types of inks or ink-like pigments before this time. Fragments of bamboo tablets, bone fragments, and silk scrapings dating back to ancient times confirmed the presence of lamp black. On remains from the Chang dynasty (1765–1123 BC), covered with red and black marks, carbon and cinnabar were found as pigments, confirming the use of carbon black in very early times. It is unclear if it was burnt sumac resin or if sumac was simply a binder. Chemical analysis alone cannot rule out one or the other of this hypothesis. The

8.1 Carbon inks

� 759

earliest form of a character for ink as a fluid for writing appeared in the Warring States period (476–221 BC). Before the Han time, lacquers might have been used for short inscriptions. Thirteenth century scholars suggested Chinese ink be made from lacquer, minerals, pine soot, and lamp black. One author, Tchao Hi-Kou, explains the colorants of three phases of ink: – high antiquity (Chang dynasty 1765–1123 BC): lacquer or varnish – antiquity (Chew dynasty 1122–256 BC): “inkstone” – “contemporary” (Han dynasty 200 BC–220): soot ink The use of lacquer is disputed among modern scholars. Possibly, lacquer-based inks were necessary to write on hard, nonabsorbent surfaces, in contrast to inks for writing on silk or paper. There is no precise nor detailed literature about lacquer. The lacquer was supposed to be the resin of a species of Chinese Sumas rhus vernicifera L. The fresh resin sap is whitish at first and browns quickly in contact with air, but it is unclear if it was used to write in this stage or if it was mixed with a colorant. Inkstone would have been long in use, but there is no precise or detailed literature about inkstone or lacquer. Authors from the fifth century speculate about “black stones,” charcoal, graphite, or even antimony sulfide. The terms used also denote naphtha (a natural form of mineral oil) and a greasy black stone. The invention of ink in a more strict sense is attributed to the calligrapher Wei Tan in the early third century. At least, we know that from the Han dynasty (200 BC–220) onwards, a permanent, nonwashable ink was made from soot: Chinese ink from Han dynasty [157, pp. 43–63] soot (lamp black from pine), binder (glue), additives (egg white, cinnabar, musk, tree bark)

Due to its functional and aesthetic properties, it was highly valued for centuries and sought-after in Europa. A description from about 1600 by Sung Ying-Hsing, an author from the late Ming period, explained how to prepare ink from pine soot. The recipe declares the complete elimination of resin as mandatory for free-floating ink. The deresinated tree is felled and sawn into pieces. A round bamboo chamber is built with a length of up to 100 feet. All surfaces and joints are pasted with paper, with small holes as smoke outlets. The pine wood is burned for some days, and the chamber is allowed to cool. The soot is scraped out. Pure quality soot for the best ink comes from the last sections, soot for ordinary inks from the middle sections, and cheap soot for printing and lacquer works comes from the first sections. From the Song dynasty (960–1279), lamp black made from animal, vegetable, or mineral oils, rivaled pine soot. The recipe remains otherwise unchanged: Chinese ink from Song dynasty [157, pp. 43–63] soot (lamp black from pine or animal, vegetable, mineral oils such as fish oil, rapeseed oil, bean oil, hemp oil, sesame oil, tung oil, petroleum), binder (glue), additives (egg white, cinnabar, musk, tree bark)

760 � 8 Inks Lamp black was made by burning animal, vegetable, or mineral oils in lamps with a wick. However, in Ming time (1368–1644), still estimated 90 % of ink was made from pine soot and 10 % from lamp black. According to a 1738 report, lamp black was made by placing several wicks in a vessel full of oil and covered by an iron cover shaped like a funnel. The funnel collects the smoke, is regularly taken off, and soot for the best ink is collected utilizing a goose feather. The sticking soot is only used for ordinary ink. In summary, the elemental composition of Chinese ink remained the same, employing pigments, binders, and additives. – Pigments were traditionally pine soot and remained in use late into the Middle Ages. From the Song dynasty onward, lamp black from vegetable sources such as tung oil or other oil types was also used. – Pine soot and lamp black do not adhere well to materials, so an agent was required to bind carbon on surfaces. As binder, animal glue was predominantly employed, such as fish glue, cowhide glue, or aqueous extract from horn (antler glue). In general, numerous types of glues were prepared from animal remains, such as rawhide, leather, muscles, bones, shells, horns, fish skins, and fish scales. The material was boiled in water, and the hot viscous mass was strained through silk or cotton gauzes and then condensed to solid form until use. The glue was then dissolved for use and mixed with pigment. The binder also held solid ink in shape. – Additives changed the most, comprising cloves, musk, camphor, plants and herbs, bark, sappanwood, sandalwood, green vitriol, and many others. Some served as preservatives, others as dispersants or humectants, or simply as odorants. Pine soot and lamp black do not mix well with water, but the proteinous binder also serves as a protective colloid and dispersant. Chinese ink was generally made to form solid sticks, but for making large amounts of black printing inks, also liquid ink was prepared from low-quality soot. This coarse soot came from the near end of a smoke chamber, was mixed with glue and wine, and stored for a few years before use. (Red printing ink was made by boiling cinnabar and red lead with water and certain plant roots, or more straightforward, by boiling the red-stem amaranth amaranthus tricolor. Blueprinting ink employed indigo as the colorant.) In the fifth century BC, silk was used as a writing medium, but also paper was used from the late Han dynasty (200 BC–220) onward.

8.1.2 Modern carbon inks Carbon inks are employed in numerous fields of application, from artistic drawing and writing to technical drawing and industrial printing. Modern, water-resistant drawing inks, the so-called tusches, are similar to Chinese inks, ▶Section 8.8. These and other modern pigmented black inks exclusively employ carbon black as pigment due to its

8.1 Carbon inks

� 761

controllable properties, ▶Section 3.1. Requirements for the different grades of industrial carbon black pigments for printing inks are, among others: high blackness, high tinting strength, good wettability, good dispersibility, high pigment concentration, viscosity, and flow behavior tailored to the application. Influencing parameters are technical restrictions due to printing speed, solvents used, and materials employed, such as rough, coated, or sized paper. One factor to match the application requirements is the particle size. The finer the particle, the deeper the color becomes at the cost of dispersibility. The tinting strength increases with decreasing particle size, as does the bluish tint, favored over brownish undertones in printing products. Due to their higher surface area, printing inks exhibit higher viscosity with decreasing particle size. Offset and letterpress inks employ carbon black pigments with particle sizes of 20–60 nm to achieve high yield value and blackness and to avoid penetration into the rough paper surface. Inks for magazine gravure printing have a low viscosity to match the high printing speeds and employ low-structure carbon blacks with medium-sized particles, simultaneously achieving good hiding power and gloss. Another factor for the applicability of carbon black pigments depends on functional groups on the surface of the carbon particles, i. e., their surface chemistry, in particular, their surface oxygen [16, Chapter 5.2.8, 5.3]. The more oxidized the surface, the better the pigment’s dispersibility, rheology, wettability, flocculation stability, and gloss. The ink’s flow characteristics improve with increasing surface oxygen content, and the hue shifts from a brownish one to a bluish one. The oxygen content of thermal blacks, produced in a reducing atmosphere, is only 0.2–2 %, while gas and channel blacks are produced in the presence of air and contain up to 3.5 % oxygen. Temperatures above 950 °C destroy functional groups containing oxygen. Surface oxygen can be controlled by oxidative after-treatment of carbon black and depends on the oxidizing agent and the reaction conditions. Typical oxidizers employed at elevated temperatures are: – Hot air of 350–700 °C yields limited oxidation. – HNO3 , NO2 /air, O3 , or NaClO yield higher degrees of oxidized graphite. The resulting oxidized carbon black can contain up to 15 % by weight of oxygen and is then hydrophilic, yielding better wettability and dispersibility in polar inks. As shown in ▶Section 3.1, the oxygen occurs as carbonyl, ether, ketone, peroxide, phenol, lactone, or anhydride oxygen.

8.1.3 Chemistry of carbonization, combustion, and sooting The chemistry of carbonaceous colorants is characterized significantly by high-temperature processes such as burning, charring, and sooting, ▶Figure 8.2. Starting materials are:

762 � 8 Inks

Figure 8.2: Colorants obtained from pyrolysis of wood (charcoal), and from combustion of oils (lamp black) or natural gas (carbon black).

8.1 Carbon inks

– – –

� 763

wood for charcoal (PBk8) and bister in a pyrolysis process lamp oil or resins for lamp black (PBk6) in a sooting combustion process natural gas or other combustible hydrocarbons such as acetylene, propene, or ethene for modern carbon black (PBk7) in a controlled combustion process, yielding soot in a defined configuration

Pyrolysis is essentially the thermal decomposition of organic matter under a limited supply of oxidizer (mostly air) [259]. Under these conditions and temperatures of about 200–500 °C, wood decomposes into char (solid carbonaceous material), tar (nonvolatile, liquid organic compounds), and volatiles (gases, volatile organic liquids), ▶Figure 8.2(a) depicts a simple model. The process is endothermic. If an ignition source is available, the combustible pyrolysis products, mainly chars and volatiles, undergo glowing and flaming combustion, releasing energy and raising the temperature to about 1000 °C or more. The primary combustion product is CO, which ultimately burns in an oxygen-rich atmosphere to CO2 , releasing most of the total energy. In a two-step model, the primary tar undergoes further high-temperature decomposition and conversion reactions to char as well as volatiles, ▶Figures 8.2(b) and (c). The major chemical conversions are schematically [263]:

The reaction scheme indicates that char forms by stepwise conversions of the starting material or by radical reactions involving high-molecular structures in larger steps. The latter is facilitated by the relatively low energy barrier of ≈ 200 kJ/mol for the formation of high-molecular radicals, in contrast to 300–400 kJ/mol for low-molecular radicals [263]. During slow pyrolysis, these high-molecular radicals can recombine and form solid char with an aromatic character. In reality, a complex network of linked reactions between all intermediates exists, depending on reaction parameters, environmental conditions, and details of the fuel. The simple models shown here depict only the principal matter flow; see [259] for details and further reference.

764 � 8 Inks 8.1.3.1 Wood pyrolysis and the origin of charcoal The basic processes in pyrolysis are [259]: – heat transfer from the primary heat source increases the temperature in wood – the pyrolysis starts – volatiles are released, char forms – hot volatiles flow toward the ambient, condense to tar and transfer heat to cool fuel – between all fractions, auto-catalytic, secondary pyrolysis reactions occur, transferring matter between char, tar, and volatiles Pyrolysis processes are, in general, endothermic. Afterward, pyrolysis products can undergo reactions such as glowing or flaming combustion to CO and CO2 , releasing the most energy. Intermediates and reaction products from pyrolysis and combustion participate in forming smoke and soot. Smoke is an aerosol of gaseous compounds, liquid droplets, and solids, comprising a few hundred substances: organic acids, alcohols, aldehydes, ketones, esters, furans, lactones, phenols, and hydrocarbons [256, 257]. ▶Table 8.2 depicts the significant pyrolysis phases in wood. Volatiles amount to about 77 % of dry wood’s pyrolysis products, and the remaining carbon (char) to about 19–22 %. The exact composition and relation depend on wood, reaction parameters, and environmental conditions [257, 261, 262]. Since dry wood comprises about 50 % of weight cellulose, 25 % of weight hemicelluloses, and 25 % lignin, and the pyrolytic profile is different for each chemical class, the pyrolysis processes are depicted separately for each of them in the following sections. Gaseous components Gaseous components’ composition varies with chemical class and pyrolysis temperature [260]. It is assumed that gaseous products form by cracking distinct functional groups. In this model, the high yields of CO2 and CO from cellulose and hemicelluloses are the result of cracking carbonyl and aldehyde groups of monosaccharide derivatives. The high yields of CO and methane for lignin pyrolysis are due to a large number of aromatic rings and methoxy residues. H2 is released from C-H bond scission, requiring more energy. Volatiles and tar The composition of volatiles and vaporizable tarry components vary with chemical class and pyrolysis temperature [260]. For cellulose, levoglucosan is the predominant reaction product in the region 400–550 °C. It is quickly degraded at higher temperatures, resulting in small fragments and gaseous components. From 700 °C on, aromatic hydrocarbons and phenols can be detected. Pyrolysis of hemicelluloses at 400 °C yields predominantly acids, especially acetic acid, due to the abundance of carboxyl groups in hemicelluloses.

8.1 Carbon inks

� 765

Table 8.2: Major phases of pyrolysis in wood [258, 259]. A few hundred products and intermediates form ([256, 257, 261, 262] give some details about composition of smoke and pyrolyzed materials). Hemicelluloses are the compounds most sensitive to heat and react first, followed by cellulose and lignin. Temperature [ °C]

Process

160

Dehydration, removal of moisture (H� O) from wood

Pyrolysis 200

Start of pyrolysis, yielding primary tar, monosaccharide anhydrides, and methoxyphenoles

200–280

Decomposition of hemicelluloses into volatile products (CO, CO� ), condensable vapor or tar (organic acids such as formic acid, acetic acid, and furfural derivatives), and soluble fragments and monosaccharides

280–500

Decomposition of cellulose into volatiles, peak reaction at 320 °C < 300 °C: degradation of polymers, reduction of the degree of polymerization > 300 °C: decomposition of fragments into volatile products (CHO, CH� CO, CH� OH, CO, CO� , CH� , C� H, acids, esters, aldehydes, ketones), tar (levoglucosan), and char (by cracking, cross-linking, repolymerization)

280–500

Decomposition of lignin, yielding more volatiles and less char than cellulose > 320 °C: rapid increase in decomposition rate of lignin Increase in the carbon content of solid residuals (char) Formation of gas (CH� , C� H� , CO) and pyroligneous acid, i. e., aqueous components (CH� OH, acetic aid, acetone, H� O) and tar residues (homologous phenolic compounds, e. g., methoxy phenols)

450–500

End of pyrolysis, yielding primary volatiles and char

Breakdown of fuel and residues 600–700 Breakdown to phenols >700 Breakdown to methylated aromatics Formation of polyaromatic hydrocarbons (PAH) Combustion

Glowing combustion of char, flaming combustion of volatiles

Depolymerization and decomposition reactions start at higher temperatures and convert the glycan chains into C2 -C4 fragments, frequently ketones. With increasing temperature, the product variety increases, and phenolic compounds start to form. From 700 °C on, PAH emerge. The main tar component from lignin are phenols such as guaiacol, being also the main constituent of native lignin. With increasing temperature, guaiacol content decrease, and alkylated phenols form, especially phenol, 2-methyl phenol, 4-methyl phenol, and 4-ethyl phenol. Simple phenols are formed due to the cracking of primary phenolic products. In the range of 600–800 °C, aromatics form, such as styrene, indene, or naphthalene. The cause is the polymerization of cyclic monomers by free radical rearrangements or dehydroxylation of phenols. ▶Figure 8.3 illustrates some significant components of wood tar.

766 � 8 Inks

Figure 8.3: Typical intermediates of the pyrolysis of wood [260, 265–269].

Char composition The elemental composition of char varies with chemical class and pyrolysis temperature [260]. The decrease in O and H content is due to cracking (dehydroxylation, decarbonylation, decarboxylation, demethoxylation reactions) and removal of oxygen functions as gaseous compounds. At 600 °C, all signs for functional groups decline. The increase in aryl-C is caused by the formation of aromatic, graphite-like structures by polycondensation reactions. Due to the conversion of aliphatic chains or carbohydrates into short fragments or aromatic structures, the alkyl-C declines. ▶Figure 8.4 illustrates charring for lignin carbonization. 8.1.3.2 Combustion and precursors of soot, lamp black, and carbon blacks Combustion is the exothermic redox reaction between a fuel and an oxidizer, usually atmospheric oxygen, at temperatures of 500 °C and more. For manufacturing lamp black and bister, typical fuels are combustible volatiles and char from wood, vegetable oils, and resins. In the modern production of carbon black, combustible gases such as natural gas (methane) or acetylene are employed. The combustion reactions are represented

8.1 Carbon inks

� 767

Figure 8.4: Formation of lignin char by polymerization and aromatization from oligosaccharides, or by condensation and deposition of PAH and aromatic and phenolic compounds [260, Figure 10]. The reactions of cellulose and hemicelluloses to char are similar.

by a complex network, including hundreds of species, so we only highlight the general pathways and principles and refer to [243–245] and the original literature for details. Glassman and Yetter [243, Chapter 3.E] explain the oxidation of various fuel classes such as CO, hydrocarbons, aromatic compounds, and oxygen-derivatives in detail. The two principal phenomena in our context are glowing and flaming combustion. Glowing combustion is the surface oxidation process of the solid or nonvolatile products of wood pyrolysis, such as charcoal or heavy tar, yielding CO2 . In contrast, flaming combustion is the gas-phase combustion of all volatile gases and low-boiling liquid products of wood pyrolysis (tar, gaseous compounds), vaporized liquid fuels (lamp oil, resin for production of lamp black), or gaseous fuels (natural gas or methane for production of carbon black). The gaseous reaction volume is observable as a flame; its final products are CO2 or carbon as soot depending on reaction conditions. Flaming combustion The gases produced by wood pyrolysis comprise more than 200 substances, predominantly simple molecules such as CO, CO2 , or H2 O. However, also lower organic acids,

768 � 8 Inks alcohols, ketones, aldehydes, esters, furans, lactones, phenols, and (aromatic) hydrocarbons comprising 20 carbon atoms or more form [257, 264]. As these hot gases rise into the air, they mix with cool air and partially condense into smoke or burn as flames. The smoke, an aerosol, comprises gaseous compounds, liquid droplets, and solids. The flames are luminous and yellow-colored in the case of wood or combustion processes producing small carbon particles burning in the flame. In the case of hydrocarbons, the pyrolysis product mixture is less manifold, and flames are only lightly colored bluegreen, becoming yellow only when soot is produced. For industrial heating and technical processes, the yellow flames act as an intense black-body radiation source and significantly increase the heat transfer to, e. g., boilers and melting furnaces. Larger molecules in the gas not igniting condense into micrometer-sized droplets. The smoke, i. e., the mixture of these, soot formed and the uncondensed gas, has a distinct, typical smell of wood smoke and deposits on chimneys and environmental structures. When burning wood, in particular beech wood, it is this tarry mixture that yields bister. The overall combustion reaction of the fuel, e. g., methane, can follow several pathways, depending on the ratio of methane to oxidizer:

In mixtures rich in oxidizer, methane is completely oxidized to CO2 . This pathway releases the most energy and is aimed at in most combustion processes for energy production. In fuel-rich mixtures, the combustion can be directed to carbon-rich compounds, mainly soot, i. e., carbon. This pathway is generally avoided since it produces an unspecific variety of soot, tars, and other carbon-rich compounds. Nevertheless, it is the goal of historical and industrial incomplete combustion of oil, resins, and natural gas in the course of lamp black or carbon black production. The third pathway represents pure thermal decomposition, ideal for the controlled production of carbon black. Combustion reactions are complex and depend on fuel, reaction parameters, and environmental conditions. For our purposes, only the oxidation of methane, hydrocarbons, and ethene is depicted in some detail. For hydrocarbons and other fuel classes such as aromatic, phenolic, or oxygen-rich compounds, the oxidation processes are dis-

8.1 Carbon inks

� 769

cussed in detail in [243, Chapter 3.E]. As a general statement, aromatic and phenolic compounds are broken down to acetylene C2 H2 . Aliphatic, oxygen-rich compounds split off O-rich fragments such as CO, CO2 , HCHO, formic acid, acetic acid, or CH3 OH, while the remaining carbon skeleton is degraded as the pure hydrocarbons. Combustion of methane The simplest hydrocarbon fuel is methane, the main constituent of natural gas. Its combustion is summarized in ▶Figure 8.5. The possible formation of methyl, ethane, and subsequently C2 –C4 fragments opens the pathway to soot formation.

Figure 8.5: High-temperature combustion of methane [243, Chapter 3.G 2.]. The processes are complex, and models can include hundreds of elementary reactions. Here, the general pathway is outlined. Thermal H ? abstraction initiates the chain reaction. Hot H ? radicals feed the H2 /O2 branching and propagation scheme, building up a pool of reactive radicals X ? via H ? + O2 + H2 → 2 HO ? + H ?. Fast attack of X ? and HO2 ? on methane produce more methyl, as do the reaction CH4 + O2 → ?CH3 + HO2 ?. Hydrogen is stepwise removed from methane by reactions such as RH + X ? → R ? + XH, or RH + YO → RO + YH, Y = −, H, O. Due to the slow chain propagation reaction, methyl radicals can recombine and form considerable amounts of ethane, decomposing to soot precursors. The oxidation of carbon monoxide releases significant amounts of energy. X ?: reactive radical (H ?, HO ?) or diradical (O).

Combustion of higher aliphatic hydrocarbons Due to the numerous reactive sites on the more unstable radicals of higher aliphatic hydrocarbons, their oxidation is fairly complex and differs from methane [243, Chapter 3.H 1.]. It can be summarized as follows, ▶Figure 8.6: – The chain reaction is initiated by thermal abstraction of H ? radicals from the hydrocarbon. Higher hydrocarbons can also decay into two alkyl radicals due to their relatively weak C-C bond, compared to a C-H bond. – Reacting with oxygen, H ? builds up a pool of reactive radicals X ? (H ?, O, HO ?). – X ? abstract H ? from the fuel, yielding an instable hydrocarbon radical R ?. – R ? decomposes into olefins, H ?, methyl, and R′ ?. In a multistep process, olefins and hydrocarbon radicals undergo further decomposition, yielding intermediates such as C derivatives, ethene, and propene.

770 � 8 Inks

Figure 8.6: Combustion of paraffine fuel to lower olefins and methyl [243, Chapter 3.H 1.a., 3.H 1.b.]. The fuel is converted into radicals and degraded stepwise to lower olefins (ultimately ethene and propene) and methyl.

–

The intermediates decompose into formaldehyde, formyl, acetylene, and ultimately CO.

The intermediates comprise olefinic compounds, their amount decreasing in the following order: ethene > propene > butene > pentene … Hydrocarbon combustion produces methyl, its decomposition products, and small olefins such as ethene or propene. As with methane, the most energy is released afterward by the oxidation of carbon monoxide. From C2 –C4 intermediates, carbon as soot is built up. Ethene, propene and acetylene oxidation Besides methyl, all higher hydrocarbons or paraffines ultimately form ethene and/or propene, and even methane can form ethane by recombination of methyl radicals, yielding ethene by abstraction of H ? [243, Chapter 3.H 1.c.]. While propene decays to ethyl, formyl, and small fragments such as CO, the decomposition of ethene is depicted in ▶Figure 8.7. 8.1.3.3 Formation of soot and bister The exact mechanisms of soot formation are not yet fully understood, are complex, and depend on fuel, reaction, and environmental parameters. Nevertheless, there is agreement about the general features of the process, ▶Figure 8.8.

8.1 Carbon inks

� 771

Figure 8.7: Combustion of ethene and acetylene at high temperatures [243, Chapter 3.H 1.c]. Abstraction of H ? from acetylene yields ?C2 H, its oxidation cause blue-green chemoluminescence, typical for hydrocarbon flames. Lumophores are the electronically excited species CH∗ and C∗2 . Furthermore, acetylene can dimerize to diacetylene. With ethene, vinyl, and (di)acetylene, all reactants for soot formation occur in the flame gas.

Figure 8.8: Typical phases and zones in the formation of soot from organic fuels [244, p. 4].

Formation of C2 –C4 precursors of soot, ▶Figure 8.8(a) The direct products of combustion of gaseous (methane or natural gas, ethane, propane), liquid (oils), or solid (char) fuels are C–C2 compounds. Under the conditions of flaming combustion, they react to CO, CO2 , and a set of reactive C2 -C4 compounds, ▶Table 8.3. They serve as building blocks for the formation of PAH and soot, i. e., for the build-up phase of sooting when combined to PAH in manifold ways, ▶Figure 8.8(b). The conver-

772 � 8 Inks Table 8.3: Typical small, reactive species in flames as precursors for soot formation [245]. Species

Species

Species

Species

C� H ? ethynyl C� H� ? propargyl

C� H� acetylene

C� H� ? vinyl

C� H� ethene

C� H� ? cyclopentadienyl

n-C� H� ?

C� H� cyclopentadien

C� H� vinylacetylene

n-C� H� ? n-butadienyl

C� H� diacetylene

sion rate of each step in the complex reaction network strongly depends on temperature. Under a low-temperature regime (T < 1500 °C), predominantly C4 H5 ?forms, while at high temperatures (T > 1500 °C), predominantly C4 H3 ? forms. While there is agreement about pathways involving C2 and C4 species, the literature discusses the role of C3 and C5 . Formation of molecular precursors of soot (aromatics or PAH), ▶Figure 8.8(b) The precursors for soot formation are PAH, with masses of about 500–1000. A growth process starts with small molecules such as benzene, progressing to larger PAH by stepwise addition of C2 , C3 , and other small building blocks to PAH radicals. The grown PAH radicals recombine or join to form larger structures. The first reaction steps, yielding the first aromatic ring, are decisive in soot formation. They were thoroughly investigated and depend on fuel, fuel-oxidizer ratio, and reaction parameters [245, 256]. Starting from intermediates of the fuel decomposition (most important: acetylene C2 H2 , n-C4 H3 ?, and n-C4 H3 ?), a first aromatic ring is built up, generally benzene. From there, lower and higher PAH form through a variety of reactions: – HACA pathway (H abstraction and C2 addition): stepwise extension of aromatic carbon skeletons by adding acetylene C2 H2 . – CPDyl pathway (cyclopentadienyl): stepwise extension of aromatic carbon skeletons by adding cyclopentadienyl radical ?C5 H5 . Key intermediates are naphthalene and indene, or indenyl. – Stepwise enlargement of hydrocarbons by the formation of hydrocarbon radicals and recombination. The backbone of mass growth is the hydrogen abstraction and C2 addition (HACA) sequence, ▶Figure 8.9. Its intermediate PAH must be stable enough against a hightemperature fragmentation, so the PAH must belong to the so-called Stein’s stabilomers. Stabilomers essentially are peri-condensed PAH with six-membered aromatic rings, such as benzene, naphthalene, phenanthrene, pyrene, or corone [248, Chapter 3.1]. Parallel to HACA, other mechanisms play a role in the formation of the first and following aromatic rings and weight growth, e. g., resonance-stabilized radicals such as propargyl (C3 ), phenyl (C6 ), or cyclopentadienyl (C5 ) [245, Figure 14, Section 3.2].

8.1 Carbon inks

� 773

Figure 8.9: Formation of pyrene and intermediate, stable PAH according to the hydrogen abstraction and C2 addition pathway (HACA pathway) [244, Chapter 10.4], [245, Figures 3, 18, 20].

When burning aromatics such as benzene, the fuel does not decompose as quickly as hydrocarbons do. Instead, components such as biphenyl form and grow by repeated addition of C2 H2 up to pyrene. In general, the initial PAH can form by recombination of any aromatic radicals, ▶Figure 8.10, yielding relatively high-molecular PAH in only a few steps. Employing the principle of recombination of small PAH radicals, even larger PAH up to coronene can readily form.

Figure 8.10: Formation of higher PAH by initial recombination of aryl radicals, followed by HACA steps. This reaction occurs in the early stages of PAH formation, [244, Chapter 10.4], [245, Figures 3, 18, 20].

The exact reactions depend on fuel. The soot formation capability of fuels rises in the following order: paraffin < olefin, diolefin < benzene < naphthaline This tendency to sooting relies on the breakdown of aromatic fuels into, e. g., acetylene, and aryl radicals. In contrast, the breakdown of aliphatic hydrocarbons and olefins yields methyl predominantly, reacting to CO and only small amounts of acetylene and other C2 –C4 building blocks of soot. Therefore, ring-forming agents are present in lower concentrations than in aromatic flames.

774 � 8 Inks Particle inception or nucleation from heavy PAH, ▶Figure 8.8(c) bottom In this phase, mass is converted from molecules to particles. The nascent soot particles have masses around 2000 and diameters of ≈ 1.5 nm. Some authors regard the dimerization of pyrene C16 H10 , or reactions yielding PAH such as C20 H16 or C20 H10 (BIN 1A and BIN 1B in the so-called BIN model [246, p. 200]) as the onset of soot formation or soot inception [256]. The chemical details are poorly understood since experimental techniques are limited in this size and mass range. The literature discusses three conceptual pathways from PAH to soot [248, Chapter 3.2]: – the stepwise growth of PAH into curved, fullerene-like 3D structures, e. g., by HACA (Cn + C2 H2 → Cn+2 ) – the physical coalescence of moderate-sized PAH into di and oligomeric stacks and clusters (Cn + Cm → Cn ⋅ Cm ) – the chemical coalescence of PAH by cross-linking PAH and 3D structures (Cn + Cm → Cn+m ) Martin, Salamanca, and Kraft [255] review the state of knowledge about the inception and discusses the current mechanisms and theories. Growth of particles by adding gas-phase molecules, ▶Figure 8.8(c) The nascent soot particles increase in mass and size by adding gas-phase species such as C2 H2 , PAH, and PAH radicals. They probably involve radical sites on soot for reactions with C2 H2 and stable PAH, but not necessarily for reactions with PAH radicals. In the BIN model, BIN 5 and higher constitute soot particles, comprising more than 320 carbon atoms, Mr ≈ 4000–500 000, and particle diameter ≈ 2–10 nm [246, p. 200]. Coagulation by reactive particle-particle collisions, ▶Figure 8.8(c) top The size and mass of the soot particles considerably increase by sticking collisions between smaller particles. In addition, gas-phase species are continuously added to all particles. In the BIN model, BIN 13 and higher are regarded as soot aggregates, comprising 80000 and more carbon atoms, Mr >≈ 970 000, and particle diameter > 12.7 nm [246, p. 200]. Carbonization At higher residence time, soot particles undergo the elimination of functional groups, cyclization, ring condensation, ring fusion, and dehydrogenation. These reactions result in the formation of polyaromatic layers of increasing carbon content. The conditions correspond to those of pyrolysis and occur in the post-flame zone. Carbonization reactions convert the loose, amorphous soot into an increasingly graphitic structure.

8.2 Chemistry of phenolic ink constituents

� 775

Oxidation Finally, the soot particles formed are oxidized. The oxidation of PAH and soot competes with their formation, decreasing the mass of soot particles by forming small molecules such as CO or CO2 . The reactive agents are radicals such as HO ? or O, and O2 . HO ? predominates under fuel-rich conditions, O2 under oxygen-rich conditions. 8.1.3.4 Bister Bister is the tarry residue of burning wood, mainly birch wood, scraped from chimneys near the fireplace, ▶Section 3.1.2 at p. 216. It comprises a vast amount of small and intermediate tarry, organic compounds, unreacted or partially reacted starting material, and soot, and it has no fixed composition. The colorant prepared from it has a faint yellowto-brown color. The previous sections have discussed the pyrolysis of wood, combustion of char and volatiles, and smoke and soot formation. Among them, the final carbonaceous and graphite-like structures account for the more or less black color of charcoal, soot, and impure tar. Unfortunately, the manifold research results are focused on details of combustion, cleanness of combustion, or energy exchange and do not discuss color phenomena. The nature of bister colorants except carbon is therefore not clear. However, we can speculate that beyond the PAH formed, some are at least weakly colored yellow-to-red and that numerous undefined intermediates are quinoid or have a quinone methide structure, giving rise to a more or less yellow, red, or brown tint.

8.2 Chemistry of phenolic ink constituents Before discussing natural and iron gall inks in more detail, we need to look at the chemistry of natural phenols, which provide the basis of brown bark inks, thorn inks, and early iron gall inks. We begin with the essential oxidation reactions that natural phenolic compounds undergo. These compounds are representatives of tanning agents, by which quite vaguely polyphenols, i. e., phenolic compounds with numerous hydroxyl groups, are denoted. An overview of three important classes of compounds follows that play a role in the context of extracts used for inking and that are widely distributed in leaves, barks, fruits, seeds, and woods: – Hydrolyzable tannins are colorless but significant as a natural source of gallic acid for obtaining iron gall inks, ▶Section 8.4. – Condensed tannins (nonhydrolyzable tannins, proanthocyanidins, PA) are also colorless and contribute mainly to the flavor and astringency of natural products such as fruits and juices. – Tannin-like tanning agents include heterogeneously composed simple and polymeric compounds, often red, brown, or dark-colored, used to prepare natural brown inks, ▶Section 8.3.2.

776 � 8 Inks They can have low but also high molecular weights; the low-molecular representatives are water-soluble, colorless, yellow, red, or brown, while the higher-molecular or polymeric compounds are insoluble and more intensely colored red to brown. The denotation derives from using the compounds for tanning animal hides during leather preparation in antiquity. Due to many hydroxyl groups, these phenols can build a large number of strong hydrogen bonds with the amide groups of the proteins in the hides, especially with those of the collagens. The proteins are thus forced into a different conformation and denatured or tanned [296, p. 374]. They also exert their tanning effect on saliva proteins in the oral cavity. We notice the denaturation of these proteins as an astringent taste [296, p. 192]. The oral cavity is covered with a mucous membrane constantly moistened by salivary glands secretions. In addition to polysaccharides, this secretion contains almost exclusively salivary proline-rich protein (PRP), consisting of about 40 % proline. Polyphenols can complex PRP through hydrogen bonds, cross-link with each other, and finally precipitate. As a result, salivary secretion loses its “lubricant,” leading to a dry mouth sensation. (When milk is added to black tea, its tannins complex with the milk casein instead of the PRP so that the astringent taste is significantly reduced).

8.2.1 Oxidation and polyphenols To understand the nature of tanning agents, we need to become familiar with some of the properties of the phenols. Phenols are easily oxidized. In fruits and plants, phenols are susceptible to enzymatic oxidation, and in air, to nonenzymatic oxidation with atmospheric oxygen: – The polyphenol oxidase (PPO, also called catechol oxidase, tyrosinase, phenolase, o-diphenol oxidase) is an enzyme of the so-called enzymatic browning. It occurs in fruits and is responsible for browning the fruit flesh after cutting. Freezing and sulfurous acid can inhibit the process (▶ sulfurization of wine). The preferred substrates of PPO are o-diphenols. In intact fruit cells, the enzyme and substrate are stored in separate cell compartments (cytoplasm and vacuole, respectively). The enzymatic reaction does not begin until the cell structure is destroyed by cutting, squeezing, storage damage, or simply aging. – Another enzyme is laccase or p-diphenol oxidase. Its substrates are diverse, such as monophenols, o- and p-diphenols, and triphenols. In contrast to PPO, this enzyme does not respond to sulfite treatment. – The nonenzymatic reaction with oxygen in the air is slower than the enzymatic one but yields similar end products. – Newly formed quinones, which are primary end products of enzymatic oxidation, can, in turn, act as potent oxidizing agents by oxidizing phenols (chemical browning).

8.2 Chemistry of phenolic ink constituents

� 777

We can get a good overview of the thrilling chemistry of phenol oxidation with [726, 727, 741, 748, 750, 767, 772] and [747, 761], which show us mechanisms, reaction classes, and the variety of (natural) end products. The last aspect is highlighted in particular by [749], of particular interest for our topic is [751], which investigates colored oxidation products of phenols for their suitability as possible food colorants. The reactions occurring during phenol oxidation lead from 1,2- or 1,4-diphenols to quinones and C-C- or C-O-linked dimers:

▶Figure 8.11 shows the oxidation mechanism using the example of a simple phenol,

leading primarily to the quinones and C-C linked dimers, but also O-linked products (“depsidones”) are possible. The resulting, possibly slightly colored, quinones are very reactive and can be attacked by phenols in a vinylogenic nucleophilic addition, yielding C-C linked dimers I, ▶Figure 8.12. These rapidly rearrange into the corresponding aromatic compounds II. Under oxidative conditions, the dimers can react to quinoid structures IV, the socalled quinone methides. As quinone analogs, they are intensely colored (yellow to red) and often introduce a chromophore into the oxidation products. Some of the essential quinone methides for our topic we have already encountered in the redwood flavonoids, ▶Section 4.4.7. Further reactions at the quinone methides can lead to polycyclic dicarbonyl compounds, as they are found in insect carapaces. Depending on the starting material, we rapidly obtain high molecular weight structures [749]:

The end product shows quinone methide structure; polycyclic end products are often dark brown to black. Such build-up reactions are common in decaying fungi

778 � 8 Inks

Figure 8.11: Schematic representation of oxidation of polyphenols [726, 741, 748, 750]. An oxygen radical forms from the phenol, which further reacts to o-quinone; or connects C-C and C-O bonds with a second radical to form dimers.

metabolism, so decomposition and mold phenomena are associated with unattractive dark colors. Since products II from ▶Figure 8.12 again have the structure of the starting materials and phenols and quinones are easily oxidatively converted, the dimers can react further in the same way to form higher molecular weight oligomers III. With an increasing degree of polymerization, the products exhibit extended π-electron systems, auxochromic hydroxyl groups, and quinoid structures and are then colored yellow, red, or brown. Such brown pigments are typical for barks, roots, woods, shells, and leaves. Water or amino groups can also act as nucleophiles and attack the quinones. As ▶Figure 8.13(a) and (b) show, the addition of water introduce another hydroxyl group. If the oxygen is part of a hydroxyl group within the quinone, an intramolecular attack

8.2 Chemistry of phenolic ink constituents

� 779

Figure 8.12: Schematic representation of nucleophilic addition of polyphenols to quinones [726, 741, 748, 750]. The initially formed dimeric enone I very readily reconstitutes the aromatic ring II. Since II again shows the o-dihydroxy phenol structure, II can further react to the trimer III and oligomers. By enzymatic oxidation, II can also give rise to extended diphenoquinones or quinone methides IV.

on the quinone can lead to a complex quinoid ring system, and thus a chromophore. By nucleophilic addition of proteinaceous amino groups, phenolic oligomers are linked to proteins to form complex structures, ▶Figure 8.13(c). 8.2.2 Hydrolyzable tannins We have seen how fundamental reactions form chromophoric elements and various colorless compounds from phenols. These include colorless hydrolyzable tannins. They represent esters of saccharides (typically glucose) with phenolic carboxylic acids [729, 730], [296, p. 51], [302, p. 193], [745, 746]. We distinguish two groups: – Gallotannins are esters of gallic acid with glucose or gallic acid. Examples are glucogallin (1-O-galloyl-glucose) and 3-O-galloyl gallic acid. – In ellagitannins, phenolic carboxylic acids such as hexahydroxy diphenic acid, ellagic acid, or valonic acid occur, which form by oxidative linkages and conden-

780 � 8 Inks

Figure 8.13: Schematic representation of nucleophilic addition of water or hydroxyl groups to quinones.

sations from gallic acid, ▶Section 8.2.1 and ▶Figure 8.11. These polybasic acids can form multiple ester bonds simultaneously with saccharides, yielding various (macrocyclic) structures. For example, in eugeniin, a diester of hexahydro diphenoic acid occurs besides typical gallic acid esters. Moreover, different saccharides can be bridged via such diesters, and a single saccharide can carry several diester rings. The substance name “tannin” or “tannic acid” per se represents glucose, penta-Osubstituted with gallic acid and m-galloyl gallic acid. Of the five substituents, one or two usually consist of a galloyl residue and the remaining consist of m-galloyl-galloyl residues. A great variety exists, from which we can only highlight a few examples:

8.2 Chemistry of phenolic ink constituents

� 781

Hydrolyzable tannins are found, e. g., in the sumac plant, ellagitannins in black alders, oaks, spruces, pines, and chestnut shells. These tannins are colorless but play an essential role as a source of gallic acid and polyphenols for preparing iron gall ink, ▶Section 8.4. In the past, catechu and sumac tannins were also used for black dyeing (Natural Brown 3 and 6). 8.2.3 Condensed or nonhydrolyzable tannins, proanthocyanidins Colorless oligomers of catechin, condensed tannins, or proanthocyanidins, are responsible for the astringent taste of tea, wine, fruit, or seed peels and the tanning effect of many plant extracts. In these compounds, up to four flavan-3-ols (catechins) are linked via their A rings by C-C bonds. In contrast to the hydrolyzable tannins, these C-C bonds are not easily hydrolytically cleaved. However, with strong acids, we obtain the underlying flavanols in the form of anthocyanidins, the source of the name for this group of substances. The structure of proanthocyanidins and their position in flavonoid metabolism is well known, ▶Section 4.4 [722–724, 738, 742], [302, pp. 193]. ▶Figure 8.14 shows the major branching pathways, which lead via colorless to yellow/red and finally to brown derivatives.

782 � 8 Inks

Figure 8.14: Overview of phenolic dyes and pigments in the secondary metabolism of plants [722–724, 738]. The flavanoid basic structures are primarily colorless, while the dimers and higher oligomers are often colored yellow, red, or brown due to the oxidative formation of quinone chromophores.

The individual flavanols can link to one another via one or two bonds, and the stereochemistry of each bond can vary. In addition, linkages via oxygen atoms are also possible. As a result, a large variety of known proanthocyanidins exists [722, 724], [296, pp. 23]. However, they are colorless and not of interest to our topic; some examples may therefore suffice for illustration:

8.2 Chemistry of phenolic ink constituents

� 783

8.2.4 Tannin-like tanning agents Simple polyphenols can be oxidatively linked to oligomers. They are similar to tannins in their properties (astringency, tannicity, polyphenolic character) but show a variety of complex structures [723, 725, 728, 732, 734, 735]. In contrast to the colorless tannins, these substances are often yellow, red, or brown and, therefore, interesting for brown inks. These pigments and compounds are based on flavonoids, especially flavan-3-ols (catechins), flavan-4-ols, and anthocyanins. Their position in flavonoid metabolism is shown in ▶Figure 8.14 [738], [722–724]. Flavonoids undergo the oxidation processes described in ▶Section 8.2.1. Enzymatic oxidation with polyphenol oxidase (PPO) and peroxidase (POD) converts o- and p-diphenols into quinones, which react with phenols, thiols, or amines to various colored condensation products. Among others, dark brown to black insoluble products may form. The intermediate quinones can also oxidize flavonoids (chemical or nonenzymatic oxidation) so that the spectrum of colored products widens again. Trace metals often catalyze the reactions. In many plants, browning phenomena occur during harvesting or processing (e. g., squeezing fruits for juice extraction), as the cell compartments deteriorate and the enzymes encounter their substrates (flavanols). Numerous plants also form colored oxidation products in dying or dead tissues (barks). As we see from the vague formulation “colored products,” the reaction products constitute a heterogeneous group, which we subdivide further. Dimer products Oxidative dimerization of flavonoids leads to a number of different reaction products, ▶Figure 8.15 [725, 728, 735, 754, 755]. Common to all products is the linkage of B or A rings of two catechol molecules. Depending on the linkage, we distinguish between several dimer classes, whose chemical structure is known and which show yellow to red colors due to their quinoid structures. They contribute to the hue of many natural products, including tea, white and red wine, and natural inks made from plant material, often barks or woods. Theaflavins These bright yellow, orange, or brown compounds contribute to the copper-red color of black tea, ▶Section 4.4.3. The characteristic element is a color-giving benzotropolone ring formed by the oxidative condensation of the B rings of catechins and gallocatechins and subsequent ring expansion. A similar compound is the orange purpurogallin, a degradation product from brown iron gall ink. Its formation illustrates the mechanism of oxidative ring expansion. For example, we see how 2 C6 -bodies transform into a C11 -body in ▶Section 8.4.3.

784 � 8 Inks

Figure 8.15: Development of some oxidation products of flavones belonging to the condensed or nonhydrolyzable tannins [725, 735, 754, 755]. Many of them are colored (yellow, red, and brown). Theaflavins, theasinensins, and theanaphthoquinones give, e. g., black tea its beautiful copper-red color.

Theasinensins They consist of yellow to brown dimers of flavan-3-ols, whose B rings are linked by biphenyl bonds. Darker-colored oligomers form when biphenyl bonds link many flavonoids continuously via their B rings to create a polyphenyl chain, similar to brown pigments in older red wine [737].

8.2 Chemistry of phenolic ink constituents

� 785

Theanaphthoquinones They are colored dimers in which the B rings of the participating catechins are fused to form an o-naphthoquinone. Theacitrins, oolongtheanins, dicatechins In these yellow or orange dimers, the linkage of B-B and A-A rings results in complicated quinoid ring systems.

Polymers The previous dimeric flavonoids already show yellow to red colors. However, for our topic, “ink,” the polymeric products or those of higher molecular weight, colored red to brown, are even more compelling. Due to their complex composition, their structure is unfortunately still largely unexplored. Phlobaphenes They are reddish-brown or brown components of many barks, woods, and roots. These flavan-4-ols polymerize via the A ring, as with proanthocyanidins [723]. Unfortunately,

786 � 8 Inks these exciting compounds’ structures and formation pathways are unknown. We must assume oxidative and condensation reactions, transforming the colorless poly-flavanols into red-brown compounds. Conceivable is the occurrence of quinoid structures II, similar to those discussed for red wine pigments [733, 741, 742].

Thearubigins These reddish-brown, insufficiently identified complex compounds account for up to 60 % of the phenolic substances in fermented black tea. They are formed from flavan3-ols as products of irregular oxidative polymerization with molecular weights up to about 2500 [725] and contribute to the copper-red color of the black tea. Until recently, the characterization posed a problem because thearubigins consist of thousands of individual substances and the applied analytical methods were insufficient to separate these substances. Haslam [735], [296, p. 335] proposed structures, but only lately a clear picture has been obtained through advanced analytics [754, 755]. According to these studies, thearubigins are catechins converted in four steps, the so-called “oxidative cascade,” into complex and structurally similar oligomers. 1. In the first stage, catechins dimerize to theaflavins, theasinensins, theacitrins, or theanaphthoquinones. Next, the dimers react similarly with other catechin molecules until oligomers consisting of two to seven catechin molecules and two to four gallic acid residues form. 2. These oligomers readily oxidize to o-quinones and then react according to ▶Figure 8.13 with water to form more highly hydroxylated compounds. This reaction continues until all reaction sites are hydroxylated. 3. The vicinal hydroxyl groups present in these polyhydroxy compounds can, in turn, be oxidized to o-quinones. 4. In a final step, the (quinoid) polyhydroxy compounds are further modified, e. g., by methylation or glycosylation.

8.3 Inks based on natural materials, book illumination

� 787

The example of a dimeric theaflavin illustrates some of these modifications (polyhydroxylation, quinone formation):

8.3 Inks based on natural materials, book illumination Inks made from natural materials such as colored plant juices have probably been used since time immemorial. However, due to their low resistance, we know plant-derived extracts primarily from the Middle Ages, employed as writing fluids and watercolors for manuscript and book illumination. At the same time, pigmented inks were applied. 8.3.1 Colored natural inks, book illumination Colored, natural inks have been produced at all times and in every cultural area. We consider, in particular, the period of the medieval manuscript and book illumination between the second century and the fifteenth century since it summarizes the findings since antiquity and represents a climax. On the materials and the procedure of the book illumination focus especially [71, 109, 119, 779], also [23, 67, 70, 156, 496, 721, 802]. (If we want to make inks ourselves, we can find a detailed collection of recipes for colored plant inks in [95].) Writing supports A papyrus served as writing support in antiquity, processed into scrolls. From the second century to the fifteenth century, parchment (tanned animal skins) dominated, which was durable and suitable for valuable editions and, in the form of the codices, resembles today’s books. Moreover, the planar writing surface offered advantages for thicker painting with viscous colors and, at the same time, provided a framework for pictorial elaboration. After the fifteenth century, letterpress printing, woodcut, and intaglio thrived and became the predominant artistic multiplication techniques; consequently, paper replaced parchment.

788 � 8 Inks Binders In ancient Egyptian times and the Mediterranean region, the colorants were bound with gum Arabic (▶Section 7.7.2 at p. 723) or the sap of the papyrus plant, a mixture of galactose, arabinose, and rhamnose [777]. In the Asian-Islamic area, proteins from skins and fish glue, eggs, milk (collagen, fish glue, albumin, and casein), cherry gum, and gum Arabic were used [156]. In the Middle Ages, monks worked with fish glue, egg white (clarea), and gum Arabic [71, 119], [109, Chapter 6.3], [779]. The proteins are surface-active and, as binders, can reduce the aqueous writing fluid’s surface tension and improve the support’s wettability, ▶Section 7.5. Manuscripts consistently mention clarea since antiquity. Since it dries up brittle, as a binder, it is often mixed with other substances, e. g., for paints with gum Arabic and vinegar, and with gum Arabic and honey as a glazing agent. However, as described in ▶Section 7.5.1 at p. 707, the actual binder is the low viscosity liquid made from several times whipped egg whites after some waiting time. Before use, it is diluted with water; with aging, it dries up insoluble in water. Egg yolk is usually an additive to other aqueous binders. For example, it is added dropwise to egg white and used in the rubric because the resulting paint has a thicker consistency and a higher gloss due to its oil content. Exemplary mixtures are: Paint for the rubricated letter [109, p. 577] Clarea, egg yolk, vermilion Paint for the rubricated letter [109, p. 577] Clarea, gum Arabic, egg yolk, red lead Black tusche [109, p. 577] Gum Arabic, egg yolk

The often-used glutin glues are transformation products of collagen and are obtained from animal connective tissue, e. g., parchment, leather, hide, bone, fish bones, and swim bladders, ▶Section 7.5.2. Collagen obtains its adhesive strength by shortening protein chains so for glue production, the material is boiled, i. e., hydrolyzed. The putrefaction of glue also shortens the chain length. On the one hand, cohesion is reduced. On the other hand, adhesion is increased, so old glue was often used for gold primers. The sensitive material was preserved by mistletoe berry juice, which is toxic to cells, or by alum. Plant-based gums have also been common binders since antiquity. However, they exhibit varied compositions and possess different degrees of tack. For example, gum tragacanth or the extract of Aloe hepatica is not very adhesive but remains elastic during drying, ▶Section 7.7.3. Gum arabic, the predominant gum, is more adhesive but brittle, ▶Section 7.7.2. It serves as a stand-alone binder (“gum water”). However, historically it was mixed with clarea, vinegar, or both. Other possible additives were honey, wine, sugar, and vinegar (sadly, the old recipes do not disclose the purpose and effectiveness of these additives, as far as the author knows). Adding gum tragacanth can compensate for the brittleness. A considerable advantage is that gums as heteropolysaccharides are less prone to microbial attacks than proteins when solved in water.

8.3 Inks based on natural materials, book illumination

�

789

Colorants Most of the colorants known since antiquity remained unchanged and formed a part of colored inks until modern times. Only the knowledge of Egyptian blue was gradually lost. ▶Table 8.4 lists the pigments used in illuminations. We have already learned about them in ▶Chapter 3. Oltrogge [109, Chapter 6.2] provides information about them in greater detail. Table 8.4: Common pigments for historical colored inks [71, 109, 119, 156, 496, 522, 523, 777–780]. Source

Pigment

Composition

White Chalk, eggshells Artificial Mineral Bone, deer horn

Eggshell white, shell white Lead white, cerusa Gypsum Bone white, horn white

Calcium carbonate CaCO� Basic lead carbonate �PbCO� ⋅ Pb(OH)� Calcium sulfate CaSO� Apatite

Yellow Mineral Artificial Artificial Ocher earth

Orpiment Massicot (lead yellow), cerusa flava Lead-tin yellow I Yellow ocher, ogra

Arsenic sulfide As� S� Lead oxide PbO Lead-tin oxide Pb� SnO� Iron oxide hydrate α-FeOOH

Red Artificial Mineral Mineral Mineral Mineral

Burnt ocher, rubeum Red ocher, hematite, red chalk, bolus Red lead, minium Cinnabar, cenobrium Realgar

Iron oxide α-Fe� O� Iron oxide α-Fe� O� Lead oxide Pb� O� Mercury sulfide HgS Arsenic sulfide

Green Artificial

Verdigris, viride

Earth mineral Mineral Artificial

Green earth, prasinus Malachite Egyptian green

Basic copper acetate or chloride, hydroxide, carbonate Cu(CH� COO)� ⋅ Cu� (OH)� Iron silicate Basic copper carbonate Cu(OH)� ⋅ CuCO� Copper silicate

Blue, purple Artificial Artificial Woad Mineral Mineral

Egyptian blue Lime blue Indigo, indicum Azurite, lazur Ultramarine, lazur

Copper silicate CaCu[Si� O�� ] Calcium copper acetate or hydroxide, chloride Indigotine Basic copper carbonate Cu(OH)� ⋅ � CuCO� Aluminosilicate Na�.� [Al�.�� Si�.�� O�� ]S�.��

Brown, black, miscellaneous Earths Brown ocher, umber Plants Charcoal black, plant black, atramentum Oil Soot, lamp black, atramentum Artificial Iron gall ink Ore, metals Silver, gold

Iron oxides xFe� O� ⋅ yFeOOH ⋅ zMnO� Carbon Carbon Iron phenol complex Silver, gold

790 � 8 Inks While it is possible to assign some colorants unambiguously to a pigment, this is more difficult with others, especially with the blue and green colors, which are based on a variety of often unspecified copper compounds, ▶Section 3.2 at p. 224. A complicating factor for research in this field is that most given recipes are only poorly or incompletely described. Much is assumed to be known; many reactions are merely based on impurities of the starting materials or mixtures that are not exactly explained. Sometimes they are misinterpreted in an alchemical manner. Thus, e. g., a “silver-blue” is mentioned, although silver yields no blue colorants. Probably, the reference alludes to a copper pigment created from silver-copper alloy. Likewise, no blue colorant can be extracted from mercury; today, the alleged “mercury blue” is interpreted as bluish-black metacinnabarite. Since lightfastness was not a significant criterion for manuscript and book illuminations, which were protected from the light by the book covers, a variety of natural dyes from fruits or parts of plants could be used, ▶Table 8.5. The compounds underlying the dyes (flavonoids, carotenoids, and anthraquinones) are the subject of ▶Chapter 4. Artists can directly paint with some “direct dyes,” ▶Section 5.4. However, most of them must be stabilized by complexation with metal salts and possibly converted into a pigment by laking. Oltrogge [109, Chapter 6.2] gives a detailed description of the colorants used. Nonetheless, the challenge remains that many natural colorants represent complex mixtures of similar compounds, whose composition depends on the plant, the selected parts of it, and the processing. Furthermore, many plants yield similar mixtures of dyes, and some colorants’ compositions or structures are not yet definitively elucidated. Manufacture of plant-based paints The colorants extracted from plant parts, juices, fruits, or flowers are organic by nature and often present as glycosides in their raw state. In order to avoid microbial infestation and obtain the actual colorants, the plant material must be extracted and hydrolyzed. The extraction was carried out with potash K2 CO3 , lime solution, or old urine at an alkaline pH, with water at a neutral pH, vinegar, or wine at an acidic pH. Depending on the nature of the dyes, the chosen pH value often influences the color of the extract, and thus several colors become possible. A good example is anthocyanin dyes, which show a broad color palette: alkaline–blue, neutral–purple, acid–red. Glycosides were hydrolyzed and removed either enzymatically by fermenting or putrefying or chemically through boiling, possibly with acids or alum. Complexation with metal salts could stabilize the color extract. Mordants mainly used until the Middle Ages were alum, copper(II) oxide CuO, eggshell lime CaO, or alum chalk. The dyes’ Al, Cu, Ca, or Al-Ca complexes form, which can also change the hue of the extract, usually toward darker tones or shorter wavelengths, ▶Section 2.6.3. In addition, the products are more lightfast due to the metal-oxygen bonds, and alum lowers the pH, which also decreases the oxidative influence of atmospheric oxygen. Moreover,

8.3 Inks based on natural materials, book illumination

� 791

Table 8.5: Common dyes for historical colored inks as well as colored lakes, produced from the dyes by laking with aluminum (alum), iron salts, and tin salts [23, 67, 70, 71, 109, 119, 156, 496, 721, 802]. Numerous other plant saps and extracts from flowers and fruits are also stated. Source Yellow Dyer’s mulberry Dyer’s weed, reseda Dyer’s broom

Colorant

Proper name of colorant

Barberry Animal gall

Morin (flavonoid) Luteolin (flavonoid) Luteolin (flavonoid), genistein (isoflavonoid) Quercetin (flavonoid) Rhamnetin (flavonoid) Crocetin (carotenoid) Gambolic acid, morellic acid (xanthone) Safflower yellow A and B (quinochalcone) Curcumin Berberine, sanguinarine, chelerythrine, coptisine (basic dyes) Berberine (isoquinoline) Bilirubin, biliverdin

Red Safflower Dyer’s madder, rose madder Kermes Kerria lacca Cochineal Rocella and other lichens Brazilwood, redwood Dragon tree

Carthamine (quinochalcone) Alizarin (anthraquinone) Kermic acid (anthraquinone) Laccaic acid (anthraquinone) Carminic acid (anthraquinone) Orcein (phenoxazine) Brazilin (neoflavone) Dracorubin (quinonmethide)

Green Rhamnus berries (ripe) Irises, leeks, cabbage, parsley, rue, ivy

Rhamnetin (flavonoid), anthocyanin Mangiferin (xanthone), anthocyanin

Sap green Succus

Blue, purple Bilberry, elderberry, privet, viola, barberry, delphinium, belladonna, danewort Bluewood

Delphinidin, cyanidin, malvidin, petunidin (anthocyanidins and their glycosides) Hematein (neoflavone)

Succus

Black oak Rhamnus berries (unripe) Saffron Garcinia tree Safflower Curcuma Celandine sap

Brown, black Squid ink sac Bark extract Wood and bark extract, gall apples with iron salts

Eumelanin (polymer) Complicated mixture of quinoid compounds Iron gallic acid complex

Stil de grain Croceum Gum gut, gamboge

Madder lake Kermes carmine Lac dye Crimson Orseille Dragon’s blood

Sepia

“Iron gall ink lake”

792 � 8 Inks the increase in molecular weight and size by complexation also desirably reduces the dyes’ solubility. Adding alkaline precipitating reagents to the inky alum-containing dye solutions produces bodily color lakes that are paintable like pigments. ▶Section 2.6 describes that this involves substrates such as aluminum hydroxide, chalk, or gypsum, on which the dye is deposited, and thus acquires the body. More often, this happened with flavone dyes (yellow lakes, ▶Section 4.4.4), brazilwood, and anthraquinone dyes (carmine and madder lake, ▶Section 4.6.3), less frequently with anthocyanin dyes (blue lakes, ▶Section 4.4.5). Traditional storage of colors The liquid, colored products could be used directly as colored inks, e. g., for illuminations. Sap colors were stable, preferably in the acid medium; they were thickened for more extended storage. Due to their storage in animal bladders or mussel shells, the colors were also named bladder colors or mussel colors. The so-called “Tüchleinfarben” or “small cloth colors” (German for “fabric” and “flax”) were a widely used method for painters to store colors for a long time. They comprised linen cloth, soaked several times with the dye solution, and the intensively dyed cloth could be kept for years. For using the dye, the colorants were extracted from the cloth with water or an aqueous binder; the resulting colored solution was ready for use immediately. 8.3.2 Brown inks The simplest way to obtain brown writing inks is to extract brown colorants from saps, barks, woods, and seed pods of trees and shrubs, especially oaks, willows, and blackthorns (sloes). Gall apples, growths on oak branches caused by gall wasps, are also an essential source of brown compounds. The brown ink is ready after the brown extracts are mixed with a binder. Especially from the sixth century until the thirteenth century, such bark and thorn inks enjoyed great popularity in scriptoria [155]. Thorn ink Theophilus 1100 [155] Extract from the bark of the blackthorn, water, wine, gum Arabic Add vitriol, red-hot iron, or rust if the color is too weak

The above plant parts contain tanning agents in varying compositions and mainly tannin-like color carriers, ▶Section 8.2.4. Low-molecular representatives are watersoluble, colorless, yellow, red, or brown and yield more or less brown inks. In contrast, higher molecular weight, oligomeric and polymeric compounds are insoluble and more intensively colored from red to brown. They must be dispersed by suitable means. Fuchs [802] provides a good overview of all ink types. Since extracts from tanning agents also contain gallic acid and glycosides, it was possible to obtain a brown-black iron gall ink by adding iron salts as described in Theophilus’ recipe. We will examine this significant type of ink in ▶Section 8.4.

8.4 Durable writing inks (iron gall inks)

� 793

8.4 Durable writing inks (iron gall inks) The earliest black writing fluid is based on soot, and its use as a color body in Egyptian inks is documented from around 3000 BC. Two thousand years ago, another black liquid was developed based on iron salts and gallic acid [155, 156]. We know Roman recipes for iron gall inks from 200 BC and corresponding compounds from Egypt [777]. Iron gall inks spread in Central Europe in the early Middle Ages and eventually became the dominant ink until dye inks took the top spot in modern times. The most prominent characteristic of iron gall inks is their deep black dye, which is so stable that it is considered documentproof. We need two ingredients to prepare iron gall ink: a FeII salt and a tanning agent containing gallic acid [808]. Early iron gall inks The development of iron gall inks comprises two identifiable stages. The first, beginning with recipes from Roman times, is characterized by recipes like this one: Iron gall ink, Tegernsee, about 1500 [155] 5.2 g gall apples, 3.9 g vitriol, 3.5 g gum Arabic, 100 ml water or wine

The raw materials were of natural origin and were neither used in strict ratios nor coordinated with one another. Furthermore, a wide variety of substances were used as tanning agents, which were also required to prepare brown plant-based inks (▶Section 8.3), mainly tree bark and gall apple extracts. Gall apples are more suitable due to their higher content of gallic acid. Iron vitriol, metallic iron, or rust was historically used mainly as iron salt. In addition to iron(II) sulfate, natural vitriol contains iron(III), copper, and other metal sulfates. This basic mixture can be enriched by adding different auxiliaries. The following substances and processes were intended to release gallic acid from hydrolyzable tanning agents: wine, beer, vinegar, boiling, fermenting, and heating. In addition, fragrances (rose, myrrh, frankincense, and camphor) were added in the Arabic region. Since tanning agents also play a role in leather tanning, this reaction was familiar to leatherprocessing craftspeople: to dye leather goods black, tanned hides were treated with iron vitriol solution, so the same processes took place as in ink preparation. The essential point of early recipes is that the insoluble black colorant already forms during preparation or storage. The ink prepared with a FeII salt is initially colorless. Due to oxidation with air and the FeIII salts already present in the vitriol, the insoluble black pigment forms during preparation. The added gum Arabic thus serves not only as a binder but also to stabilize this pigment dispersion. Excursion: early iron gall ink in the laboratory The simplicity of this early iron gall ink tempted the author to prepare it in a laboratory experiment. ▶Figure 8.16 contains the recipes used and shows the main steps as well as the results obtained for two inks made with tannin as a source of gallic acid and old iron screws and FeSO4 , respectively, as FeII sources.

794 � 8 Inks

Ink no. 1 darkens over several days as much as iron is dissolved from the iron rust. In contrast, ink no. 2 with iron(II) sulfate shows, as expected, instantaneous deep blackness due to the immediate availability of the iron cations. The role of a colloid stabilizer such as gum Arabic also became apparent. Ink no. 1 flocculates irregularly. In contrast, ink no. 2 forms a uniform black colloidal solution suitable for writing despite the high colorant concentration due to the gum added. The gum now acts as a colloid stabilizer. The reflectance spectra of both inks show an almost uniform gradient, which explains the deep neutral black color of ink no. 2, ▶Figure 8.17. To what extent the brown color of ink no. 1 is a result of the slow reaction with iron rust and only slight formation of the black colorant or an expression of the phenol oxidation described in ▶Section 8.4.3 and does not represent an actual iron gall ink, cannot be decided with the available means. Ink no. 2 shows, due to the formation of thin colorant membranes during air oxidation on the paper, a metallic-iridescent play of interference colors, ▶Figure 8.16(b).

Modern iron gall inks The second stage of development began in 1856 with the development of alizarin ink by A. Leonhardi. The latter added acid and one of the new synthetic tar dyes. Schematically, the recipe looks as follows: Iron gall ink [155] 3 Parts gall apples, 1 part iron(II) sulfate, 1 part gum Arabic, 40 parts water, 1/4 part hydrochloric acid, colorant (alizarin, indigo sulfonic acid, bluewood) Iron gall ink [809] 230 g Tannin, 77 g gallic acid, in 5000 ml distilled or demineralized water, add 25 g hydrochloric acid conc., 100 ml gum Arabic solution (1:1), then add cold solution of 300 g iron(II) sulfate in 3000 ml distilled or demineralized water German Imperial Recipe for the preparation of an officially approved ink 1888 [801], [203, keyword “drawing and rriting materials”] 30.0 g Iron(II) sulfate, 23.4 g tannin, 7.7 g gallic acid, 10.0 g gum Arabic (stabilizer, binder), dye (e. g., from bluewood and alum or a synthetic dye; primary dye until the iron black has developed), 1.0 g phenol (preservative), 10.0 g hydrochloric acid or acetic acid, per 1000 ml water

Late recipes use pure chemicals in place of natural products, give precise ratios of amounts, and add preservatives. From the eighteenth century onward, the works of Scheele (isolation of gallic acid), Deyeux/Seguin (discovery of tannic acid or tannin), and Lewis (quantification of ink components, discovery of black pigment as an irontannin complex) provide the basis for new developments. The key innovation is an acid additive that prevents the premature formation of the black pigment during preparation and in the vessel. The conversion of FeII into FeIII is blocked by acid and only occurs during writing by atmospheric oxygen. The formed hydroxyl ions neutralize the acid:

8.4 Durable writing inks (iron gall inks)

� 795

Figure 8.16: Simple preparation of iron gall inks. To prepare ink No. 1, tannin was dissolved in hot water, and rusty iron screws were added. The reaction to the dark reddish-brown dye (a) sets in within a few minutes to hours and develops a purple-black flocculent solution and a reddish-brown ink over the next few days. To prepare ink No. 2, 1.4 g of tannin was dissolved in 12 ml of water and mixed with 1.8 g of FeSO4 , also dissolved in 12 ml of water; and 1 ml of saturated gum Arabic solution was added as a protective colloid. The reaction to form the black ink starts instantaneously and yields a black colloidal solution (b, d). The handwriting sample shows how the ink oxidizes to deep black within a few seconds (e).

Figure 8.17: Reflectance spectrum of iron gall inks from the author’s production, normalized to an arbitrary unit: Tannin with iron screws (ink no. 1) and tannin with iron(II) sulfate (ink no. 2). We can see a uniform absorption almost over the entire visual range, which explains the balanced deep black color of ink no. 2. With available means, it cannot be clarified whether the brown color of no. 1 is due to phenol oxidation, ▶Section 8.4.3.

796 � 8 Inks The black color will develop after writing within hours and days. However, a dye additive is necessary to ensure immediate readability precisely at the moment of writing. The significance of this discovery was that ink was now available that was suitable for fluid writing with the newly developed steel nibs and fountain pens. The earlier iron gall inks with the ready-made colorant would have easily clogged the fragile capillary system of the nibs from the beginning or, at the latest, in the course of use when exposed to air. 8.4.1 Chemistry of iron gall inks It is astonishing that despite the considerable age of the iron gall inks, the structure of the coloring compound has been elucidated only in the last two decades, although numerous works have been devoted to this subject throughout the history of chemistry [804, 805]. The colored principle is an insoluble, polynuclear 1:1 complex of FeIII and the gallic acid contained in ink. Six oxygen atoms octahedrally surround iron. According to [799, 800], the complex is structured as follows:

The FeII complex I is colorless, water-soluble, and forms when the reactants are mixed. Crucial to its suitability as ink is forming the insoluble black FeIII complex II, which only starts with oxidation. In the course shown, the authors consider that in inks, deviating from the recipes, small amounts of FeIII ions are present in the vitriol from the very beginning. This fact causes a radical decarboxylation of gallic acid to pyrogallol III. Due to its composition as a 1:1 complex, an ideal iron gall ink contains stoichiometric amounts (1:1) of iron(III) sulfate and gallic acid (or 1:3.6 for tannin). However, this composition is only partially realized in historical inks, as no pure substances were available. For example, vitriol contains iron(III) sulfate and sulfates of copper, zinc, and other

8.4 Durable writing inks (iron gall inks)

� 797

metals; natural tanning agents vary significantly in composition and contain varying amounts of gallic acid. This fact often gives rise to damage patterns in old documents: an excess of FeIII is partly responsible for the browning of old writings (see below). Investigations on the black pigment under laboratory conditions [801, 803] come to a deviating structure for grown crystals, which emerge according to the following reaction:

These crystals also contain a 1:1 complex of gallic acid and FeIII , in which iron is also coordinated octahedrally by six oxygen atoms. In this proposal, however, not two gallic acid molecules share two iron atoms, but two of each of three molecules coordinate two of each of three iron atoms in the manner of formula I. The two remaining coordination sites of the iron are occupied by oxygen atoms of the carboxyl groups of neighboring complexes, formula II. Since, under laboratory conditions, gallic acid does not decarboxylate, the iron complexes are linked via carboxyl oxygen to form a polymeric structure of chains and bands. For a better overview, we see the coordination with hydroxyl or carboxyl groups separately in the formula representations:

The extent to which this structure also occurs in inks is unclear. 8.4.2 Color of iron gall inks In all proposed structures, the intense color is provided by an LMCT transition (ligandmetal charge-transfer) O2⊖ → FeIII , which brings the pigment close to red ocher,

798 � 8 Inks hematite, and other LMCT-based colorants. In these, too, the charge is transferred under the influence of light from ligand orbitals into iron orbitals, ▶Section 2.4.1. The excited state can be formulated as a radical with numerous mesomeric boundary structures, which ultimately spreads over both iron atoms and the entire molecule:

Since the coloring iron complex requires FeIII to form, the ink made with iron(II) sulfate is initially colorless. The actual, durable black color gradually develops by oxidation of FeII to FeIII by atmospheric oxygen. Until the black pigment has developed, the ink must be made visible by an added dye.

8.4.3 Brown iron inks Iron gall inks are not always deep black: sometimes, from the very beginning, often only after a long time, we can observe a distinct brown tone. This color is assumed to come from organic degradation products produced by the oxidation of gallic acid and other polyphenols, which have a more or less intense brown color, with quinones appearing as intermediate or final products. Examples of such products are purpurogallin, ellagic acid, iron oxides, and various phenols [806]:

8.4 Durable writing inks (iron gall inks)

� 799

The oxidant for this reaction is excess FeIII that was not consumed to form the pigment and FeII that has been oxidized by atmospheric oxygen to FeIII . Phenol oxidation is readily possible because, under the conditions of the inks, the redox potentials of the FeII /FeIII and phenol/quinone systems are practically the same. Both reactions can therefore proceed side-by-side. Musso [726] provides a comprehensive overview of the phenol oxidation and also explains the formation of purpurin:

Neevel [807] shows how phenols are attacked by FeII or FeIII cations via the formation of peroxy or hydroxyl radicals. This reaction also happens with the written ink on paper, which is why old manuscripts are often subject to ink corrosion that extends to the paper:

800 � 8 Inks The recent works mentioned above have invalidated the assumption of being able to produce a brown “copper gall ink” by the use of copper vitriol since no colored copper complexes form. Instead, iron(II) sulfate is always sufficient in impure historical copper vitriol to produce an ordinary iron gall ink. 8.4.4 Excursion: the iron-phenol reaction The essential components of the iron gall ink, FeIII salts, and a polyphenol evoke memories of a classical color reaction of metal cations, mostly Fe3⊕ cations, with phenols. It was described as early as 1834 by Runge and was often used as a nonspecific verification for monovalent and polyvalent phenols, naphthols, oximes, hydroxamic acids, and others. This beautiful reaction was also a means of quantitative photometric analysis at a time when specialized reagents were still unknown. Depending on phenol and metals, it offered numerous colors in the entire visual spectrum, from yellow to purple. For a universal color reaction, FeIII salts ultimately proved to be the most suitable [673, 674]. Much was written early on about the constitution of these colored complexes but did not immediately achieve clarity. Today the topic has regained importance: Iron-phenol complexes play a role in intentional and unintentional discolorations in the food, e. g., in the coloring of olives [681] or the absorption of iron when eating while tea is present. Outside the food sector, an excellent example is the corrosion of steel saw blades by phenols contained in wood [682]. From the present point of view, iron forms octahedral complexes with phenols. If coordination number 6 cannot be achieved by phenolic ligands due to geometric or other factors, water plays the role of a secondary ligand. Monovalent phenols form charged complexes [FeL]2⊕ in the ratio 1:1 with iron I, where, in the case of hydroxybenzoic acids, the carboxylic oxygen also forms a chelating ring II [674, 675]. Polyvalent phenols or o-hydroxy carbonyl compounds can generally form five- or six-membered chelate rings III–V [673, 676, 752]:

If applicable, solvent molecules (water) as secondary ligands enter to achieve the octahedral coordination of the iron, indicated by arrows. In this reaction, the pH value regulates the availability of the ligands through the deprotonation of the hydroxyl groups. Depending on the reactant concentrations, complexes with different iron-ligand ratios FeL, FeL2 , and FeL3 arise and transform into each other [672, 677–680]:

8.5 Dye inks (fountain pen, felt-tip pen, ballpoint pen, inkjet printing)

� 801

Hydrolysis reactions are possible, leading to hydroxo complexes, such as [FeL(OH)2 ]⊖ , as are oxidation products of the phenols (quinones), leading to colored precipitates, ▶Section 8.2.4 and [671]. The color appearance LF transitions In the octahedral complexes, LF transitions of FeIII contribute to the color, but also other transition metals with partially filled d orbitals come into question, ▶Section 2.3. Via the ligand field strength, the transition energies depend on the nature of the ligands (phenols). Studies show complete coverage of the visible spectrum [670, 672–674, 677–680, 752]. Due to the pH dependence of the above equilibrium between 1:1, 1:2, and 1:3 complexes, the color of the complexes is pH-dependent. Pale yellow-brown colors indicate that absorption occurs essentially in the UV spectral range. Only an offshoot of the peak protrudes into the visual spectral range and absorbs nonspecifically and with low intensity, especially in the blue region, resulting in a yellowish overall color. LMCT transitions Ligands with ortho and peri-dihydroxy groups forming five and six-membered chelates show high extinction with some metal cations (molar extinction coefficient ϵ > 2000 compared to ϵ ≈ 20–50 for phenols), and hence the intensive color. It is caused by a ligand-metal charge transfer (LMCT transition), as we also observe in chromates, permanganates, and hematite, ▶Section 2.4.1. Charge in the form of π electrons is transferred from the phenol ring via the hydroxyl oxygen into molecular orbitals that are more or less entirely localized at the metal. For example, we can observe this in a classical color pigment, madder lake. The inconspicuous orange-red color of the free alizarin is intensified to a bright red by complexation with aluminum. Electron transfer from the alizarin system into free 3p orbitals, localized at the metal, also induces an intensive CTcomponent, ▶Section 2.6.3, especially ▶p. 202. We also observe a change in color at LMCT transitions with different ligands and metals. Substituents with high electron affinity in the aromatic core, e. g., lower the electron density, which can be transfered to the metal via the oxygen, and thus enter into competition with the CT transition. As a result, the absorption band is hypsochromically shifted. Furthermore, the high stability of the metal-oxygen bond facilitates electron transfer to the metal and leads to a bathochromic shift of the band. The band becomes hypsochromically shifted with increasing ligand coordination: FeL (blue) vs. FeL2 (purple) vs. FeL3 (red). In this series, aquo or hydroxo ligands are increasingly replaced by aromatic ring systems, which cause a stronger LF splitting. Quinone formation Some cations, such as Ba2⊕ , provide a color reaction, although they are neither among the typical colorproviding cations nor those with a tendency to CT transitions. In such cases, oxidation products of the polyphenols (▶Section 8.2) cause the color appearance [673]. Also, due to the high oxidation potential of FeIII ions, some polyphenols can be oxidized to quinones; we observe green to blue and also red colors [677].

8.5 Dye inks (fountain pen, felt-tip pen, ballpoint pen, inkjet printing) With the discovery of the first synthetic dye made from coal tar (mauveine 1856), the era of dye inks began. The possibility of obtaining (almost) any shade of color for writing,

802 � 8 Inks the brilliance and intensity of the coloring, and the smooth, controllable flow quickly led to a large selection of writing inks based on these synthetic dyes. Unfortunately, the most significant disadvantage is their low resistance to light and air. In the early days, dye inks were simple in composition; water, the dye, gum Arabic as a binder, and a preservative (phenol or salicylic acid) initially fulfilled the demands. Mauveine, nigrosine, eosin, fuchsin, methyl violet, and methyl green were available as dyes. If we consider the properties required of ink in countless areas of application today, it becomes clear that modern ink is a highly complex system with numerous components and interactions. The fundamental composition of inks has remained the same: a finely dispersed pigment or a dye in a solvent, often water. Today, numerous auxiliaries and cosolvents are added to this basic mixture: binders, surfactants, pH regulators, wetting agents and dispersants, humectants, biocides, and dispersion stabilizers. The field of application determines the exact composition, as the following exemplary recipes may show [203, keyword “Drawing and Writing materials”], [13, p. 501], [783, 784, 809, 812]. Aqueous writing inks Ordinary writing inks are based on water with anionic triphenylmethane or acid dyes and have a very low viscosity. Writing ink [13, p. 505] Dye (Acid Blue 93) 0.5 %, cosolvent (glycol, glycerol, sorbitol) 1.5 %, H2 SO4 0.2 %, traces of biocides, traces of surfactants, solvent (water) 97.5 % Writing ink Dye (indigo carmine) 0.2 g, binder (gum Arabic) 0.2 g, biocide (salicylic acid) 0.03 g, solvent (water) 10 g Multicolored ink for fountain pens, felt-tip pens, markers [783] (Figures are given in percentage by weight.) Colorant, according to ▶Table 8.6, 0.1–10, binder (gum Arabic) 0.1–2, consistency regulator (beeswax) 0.05–5, humectant (diethylene glycol, glycerol, or propylene glycol) 10–40, emulsifier (glycerol fatty acid ester, PEG fatty acid ester or pentaerythritol fatty acid ester) 1–7, buffer (triethanolamine) 0.1–3, preservative (tert-butyl hydroperoxide or monophenyl glycol ether) 0.1–3, solvent (water) 34–88.5

Felt-tip pens, aqueous and nonaqueous, markers Felt-tip pens contain a thin water-based ink made from acidic or basic dyes held in a sponge-like reservoir of synthetic fibers. The dye is transferred evenly to the paper by a fine tip of a pressed felt-like fiber bundle, transporting the ink by capillary forces. The fiber bundle consists of nylon, acrylonitrile, or polyester. The thinly liquid solvent of water and glycols (viscosity approximately 4 mPa ⋅ s, surface tension approximately 30 mN/m) enables easy writing and drawing of even fine lines. A high proportion of glycols prevents rapid drying when the tip remains uncovered, while small amounts of synthetic resins (polyvinylpyrrolidone) prevent the ink

8.5 Dye inks (fountain pen, felt-tip pen, ballpoint pen, inkjet printing)

� 803

Table 8.6: Options for mixing a wide range of colors for aqueous inks applied in fountain pens, felt-tip pens, and markers employing food colorants [783]. Figures are given in percentage by weight. Black Brown Purple Light blue Light green Dark green Orange Yellow Pink Red

1.9 FB2 0.8 FB2 0.7 FB2 0.7 FB2 0.3 FB2 3.6 FB2

3.7 FR7 3.5 FR7 1.5 FR14 0.2 FR14

0.3 FR7 0.1 FR7 2.8 FR14 0.5 FR14, 2.7 FR7

1.3 FY4 2.2 FY3

2.5 FY4 1.8 FY4 2.5 FY4 2.7 FY4 0.9 FY4

from leakage into the paper fibers. In addition, surfactants improve the flow properties. The concentration of colorants can increase up to 10 % for saturated blacks and fine liners with narrow line widths. Felt-tip pen ink [13, p. 505] Dye (AB9) 3 %, cosolvent (1,2-propylene glycol 25 % and polyvinylpyrrolidone 0.1 %), traces of surfactants, solvent (water) 72 % Felt-tip pen ink Pigment dispersion 5–20 %, cosolvent (diethylene glycol/glycerol) 20–40 %, traces of surfactants, solvent (water) 80–55 % Felt-tip pen ink [203, keyword “Drawing and Writing materials”] Dye 1–10 %, glycols 20–30 %, surfactant 0.1 %, binder 2 %, solvent (water) to 100 % Fiber-tip pen ink [809] Dye 1.5–3.5 parts, cosolvent (diethylene glycol or propylene glycol) 30–20 parts, nonionic wetting agent 0–0.1 parts, resin 0–0.1 parts, solvent (distilled or deionised water) 70–80 parts Pigmented fiber-tip pen ink [809] Pigment preparation 5–20 parts, cosolvent (diethylene glycol or propylene glycol) 20–25 parts, glycerine 0–15 parts, nonionic wetting agent 0–0.2 parts, preservative 0–0.15 parts, solvent (deionised water) 80–55 parts Felt-tip pen ink [809] Dye 2.5–8 parts, cosolvent (diethylene glycol) 30–15 parts, natural or synthetic resin 3–20 parts, solvent (ethanol) 35–25 parts, solvent (isopropanol) 30–25 parts Pigmented felt-tip pen ink [809] Pigment preparation 5–10 parts, cosolvent (diethylene glycol) 30–20 parts, resin 0–5 parts, solvent (ethanol) 40–30 parts, solvent (isopropanol) 30–40 parts

Dry-erasable felt-tip pens, widely replacing blackboard chalks, are formulated similarly to pigmented felt-tip pen inks. Additionally, they contain antistick agents, frequently a polyglycol, or a small amount of silicone oil. This agent prevents the inks from adhering too firmly to the substrate (whiteboard, glass, enameled metal sheets, plastic-coated supports).

804 � 8 Inks Nonaqueous inks are mainly used to write on surfaces such as glass, metal, or plastic (whiteboard or other markers). The formerly popular solvents (toluene, xylene) have been replaced by lower and higher boiling alcohols, allowing faster drying than water. The colorants in these formulations are cationic or solvent dyes. Notable is the high content of film-forming aids (synthetic resin, polyacrylate), which imparts good resistance against water and wiping: Nonaqueous marker ink Basic dye 15 %, cosolvent (butanol 10 % and methoxy propanol 15 %), binder (polyacrylate) 10 %, solvent (ethanol) 55 % Pigmented marker ink Pigment preparation 5–10 %, cosolvent (isopropanol 30–40 % and diethylene glycol 30–20 %), binder (polyurethane or polyvinylbutyrate) 0–5 %, solvent (ethanol) 40–30 % Nonaqueous felt-tip pen ink [203, keyword “Drawing and Writing materials”] Dye 2–15 %, cosolvent (butanol 10 % and methoxy propanol 15 %), binder 10 %, solvent (ethanol) to 100 %

We want to wipe off some marker inks, especially whiteboard markers easily. Polyglycols or silicone oil, which reduce the adhesion of the ink to the substrate, meet this requirement. Ballpoint pen, rollerball pen For the ballpoint pen paste, highly concentrated basic or soluble dyes or pigments are suspended in oleic acid or acidic resins and dissolved in organic solvents (benzyl alcohol, glycols, glycol ethers, polyethylene glycols). High dye concentrations of up to 40 % ensure that writers draw a crisp line with sufficient coloring power despite the fine nib and the associated small amount of ink. Pigments can also be included in the color paste, e. g., phthalocyanines or metal complexes. These are usually more lightfast than dyes, and pastes containing phthalocyanine are even document-proof. Leakage of the color paste from the barrel is prevented by the high viscosity of 10–20 Pa ⋅ s and surface tension of around 40 mN/m. Ballpoint pen ink [13, p. 505] Dye (SB89) 30 %, oleic acid 10 %, solvent (benzyl alcohol) 35 %, binder (polyacrylic acid) 25 % Ballpoint pen ink [203, keyword “Drawing and Writing materials”] Dye 30 %, oleic acid 10 %, solvent (phenyl glycol 30 %, benzyl alcohol 2 %, and 1,2-propanediol 8 %), binder (synthetic resin 10 % and polyvinylpyrrolidone 0.2 %)

The resin (e. g., phthalate or cyclohexanone resin) provides the required flow properties in benzyl alcohol, phenylene glycol or propylene glycol, and polyvinylpyrrolidone. Aqueous ballpoint pen ink (Figures are given in percentage by weight.) Dye (acidic, basic, direct, pigment) 2–15, solvent (water) 20–75, cosolvent 5–35, adhesion-promoting resin (acrylic resins, styrene-maleic acid copolymers, cellulose derivatives, PVP, PVOH, dextrin) 1–30, binder (polyviny-

8.5 Dye inks (fountain pen, felt-tip pen, ballpoint pen, inkjet printing)

� 805

lalcohol), anionic surfactant (sodium 2-ethylhexyl sulfosuccinate 5–20 and cocoylamide propyl dimethylamino acetic betaine (cocamidopropyl betaine) 2–10)

In contrast to the classic ballpoint pens, rollerball pens work with a thin liquid water-like ink stored in a sponge. Instead of oleic acid, polyols serve as lubricants for the writing ball, which also prevent the tip from drying out quickly: Rollerball ink [203, keyword “Drawing and Writing materials”] Dye 2–8 %, polydiol (ethylene glycol, diethylene glycol, other glycols) 20–30 %, surfactant 0.5 %, solvent (water) to 100 % Pigmented rollerball ink [809] Pigment preparation, ultrafine dispersed 15–30 parts, cosolvent (ethylene glycol) 30 parts, polyvinylpyrrolidone 0.2 parts, solvent (deionized water) 55–40 parts

Water-based inkjet inks Inkjet printing places particular demands on ink [151]: the small system dimensions (micrometer-fine nozzles and channels) require a colorant of the highest purity with a fineness below 1 µm. It should be readily soluble in ink but later be resistant to water and abrasion. Ink must not dry out even after a long period of nonuse, but it should dry quickly on the paper and must be immediately removable for cleaning the print head. It should form stable drops, but its surface tension must not prevent it from flowing easily through the fine capillaries. It should form drops readily but must not splash and must not be too viscous or thin. Finally, the colorants must not settle, become encrusted, mold, or be corrosively active. A unique discipline has developed from these demands, so we will look at recipes only in principle [151]. Suitable values for the surface tension of bubble-jet (TIJ) inks are 30–60 mN/m, and for piezo jet (PIJ) inks, 30–35 mN/m; typical viscosity values of TIJ inks are 1–5 mPa ⋅ s, of PIJ inks 10–20 mPa ⋅ s [812]. The pH value is between 6 and 9. Inkjet ink Acid dye or direct dye for good adhesion to paper (cellulose) 4 %, cosolvent (diethylene glycol 10 %, N-methyl-2-pyrrolidone 5 %, and triethanolamine 0.5 %), traces of biocides, solvent (water) 80 % Cyan inkjet ink [812] Dye (DB199), 2-pyrrolidone 8 %, 1,5-pentanediol 9 %, 2-ethyl-2-hydroxymethyl-1,3propanediol 8 %, solvent (water) Pigmented inkjet ink [812] Pigment 2.25 %, N-methyl-N-oleyl taurate 1.4 %, diethylene glycol 8 %, glycerol 12 %, solvent (water) Inkjet ink in general [151] Colorant 2–8 %, solvent (water, alcohols such as methanol and ethanol, methyl ethyl ketone, ethyl acetate, aliphatic hydrocarbons, oil) 35–80 %, surface tension regulator (Tergitol 15-S-5, secondary alcohol ethoxylates) 0.1–2 %, humectant 10–30 %, penetration aid (isopropanol, helps penetrate the paper for faster drying) 1–5 %, viscosity regulator 1–3 %, solubilizer for dye (N-methyl-pyrrolidone, keeps dye soluble even at a high concentration near the printhead) 2–5 %, dispersion aid (specific per pigment) 3–8 %, fixative (water-soluble latex, polyamides, PVP, styrene-acrylate latex) 1–3 %, pH buffer 0.1–1 %, auxiliary to reduce paper cockling 20–50 %

806 � 8 Inks Inkjet inks for industrial printing Industrial inks are often designed to adhere to metal or plastic surfaces. Since water has too high a surface tension for these substrates, they are based on organic solvents such as glycol esters and glycol ether esters or lower ketones. Dyes and binders are adapted to the organic solvent. Solvent-based inkjet ink Soluble dye (SBk27) 5 %, solvent (methyl ethyl ketone) 80 %, cosolvent (propylene glycol) 10 %, binder (polyvinylchloride-polyvinylacetate copolymer) 5 % Solvent-based inkjet ink for continuous product identification (CIJ) [176, ch. 9.6.5] Dye 2 %, solvent (methanol) 42 %, cosolvent (methyl ethyl ketone) 30 %, cosolvent (water) 1.5 %, drying retarder (ethylene glycol methyl ether) 9 %, binder (resin acid methyl ester) 1.4 %, binder (styrene-acrylic acid copolymer) 13 %, wetting agent (nonylphenol polyethoxy ethanol) 4 % Water-based inkjet ink [176, Chapter 9.6.5] Dye 2.8 %, solvent (water) 73.5 %, cosolvent (glycerol) 18 %, biocide 0.2 %, wetting agent 5.5 %

8.5.1 Function of components Using a menu sequence for the composition of an ink (▶Figure 8.18), we can investigate what function the individual components have. For example, polymers perform many tasks: stabilization of pigment dispersions, adhesion to the substrate, film formation, abrasion resistance, print quality, and rheology control. Solvents The choice of the solvent (▶Figure 8.19 shows a selection) depends on the surface for writing. The solvent’s volatility and dissolution capability for the dye play a crucial role in the applicability. Another property directly related to the solvent is its surface tension. The surface tension of ink significantly influences its writing, drawing, and printing quality since the ink in writing instruments is sucked up by capillary forces in the ink conduit and is held in reserve or prevented from flowing out directly. The same applies to the print heads of inkjet printers. For a proper function, the ink must have a low viscosity and the correct surface tension; a good value for writing inks is about 50 mN/m. High surface tension makes it difficult to wet the surface to be written on and to feed the ink to the nib tips and print heads, making writing eventually impossible. Conversely, low surface tension causes the ink to flow out of the fountain pen tip or print head too quickly and in too large a quantity, flows on the medium and forms feathered writing or leaving blurred traces. Therefore, the writing instrument, print head, and print medium must be considered when selecting the solvent. Weibel [172, Chapter 2] sheds light on the physics of capillary ink flow systems using the fountain pen as an example. Water is often the medium of choice for writing and printing inks on paper. For writing on glass, metal, or plastic (whiteboards), highly volatile organic solvents (simple ketones such as ethyl methyl ketone or esters such as ethyl acetate and butyl acetate) are

8.5 Dye inks (fountain pen, felt-tip pen, ballpoint pen, inkjet printing)

� 807

Solvents and cosolvents Alcohols (ethanol, n-propanol, isopropanol, glycerol, n-butanol, 1,2-butanediol, 2-isopropyloxyethanol, benzyl alcohol, (di)ethylene glycol, butyl(di,tri)ethylene glycol, (poly)propylene glycol), esters (ethyl acetate, butyl acetate), ethers (monomethyl, monoethyl, monopropyl, and monobutyl ethers of (di)ethylene glycol, (di)propylene glycol, and ethylene glycol phenyl ether), benzamide, butyrolactone, diisopropanolamine, N,N-butyl-1-ethanolamine, ethanolamine, diethanolamine, triethanolamine, propylene carbonate, ethylene carbonate, 2-pyrrolidone, N-methyl-2-pyrrolidone Colorants Dyes or dispersible pigments Dispersants Animal glues (proteins such as collagen), polyvinylalcohol, methyl cellulose, hydroxyethyl cellulose, carboxymethyl cellulose, and other water-insoluble polymers, polyacrylates, surfactants Humectants Hygroscopic polyhydric lower alcohols (diethylene glycol, triethylene glycol, 1,1,1-trishydroxymethylpropane=trimethylolpropane, 1,2-ethanediol, 1,2-propylene glycol, butyl diglycol, glycerol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, PEG-n), heterocycles (2-pyrrolidone, N-methyl-2-pyrrolidone), thiodiglycol, urea, caprolactam, N,N-dimethylurea Surfactants Dialkyl and dialkenyl sulfosuccinates (C7 −C18 linear and branched), sodium lauryl sulfate, acetylene glycols, alkylphenyl polyethylene glycols (e. g., nonylphenol ethoxylate, nonoxinol-n), polyethylene glycol fatty alcohol ethers (lauryl C12 , myristyl C14 , cetyl C16 , stearyl C18 ), alkyl polyglycosides, alkyl sulfonates, acoylamine-propyl dimethylamino acetic betaine (e. g., cocoyl C7 −C17 ), alkyl dimethylamino acetic betaine (e. g., stearyl C18 ), alkyl phenyl sulfonates, dimeticone propolyols, alkyl pyrrolidones (Surfadone® LP 100), Surfynol® 465, Triton® X Rheology modifier Alginic acid, polyvinylpyrrolidone polymers and copolymers, polyacrylic acid pH regulators Low organic acids (glycolic acid, acetic acid, succinic acid, citric acid), 3-morpholinopropane sulfonic acid, organic amines (triethanolamine, 2-amino-2-hydroxymethylpropane-1,3-diol) Binders Simple sugars, polysaccharides (gum Arabic), vinyl-based (polyvinylalcohol, polyvinylpyrrolidone) or acrylic-based polymers (ethyl acrylate, methyl methacrylate, n-butyl acrylate, 2-ethylhexyl acrylate, poly methacrylic acid, polyacrylic acid), styrene-maleic acid copolymers Emulsifiers Alkylated amines and fatty acid amides (dodecylamine, cocoyl trimethyl ammonium chloride, sodium N-cocoyl-3-aminobutyrate, N-cocoyl diamino propane, N-oleyl-1,3-diamino propane), fatty acid esters (glycerol fatty acid esters, PEG fatty acid esters, pentaerythritol fatty acid esters) Figure 8.18: Menu sequence for inks [13, p. 501], [783, 784, 809, 812].

808 � 8 Inks

Figure 8.19: Solvents and cosolvents used for inks.

employed. With its numerous different requirements, the printing industry uses many of the organic solvents also known in the laboratory. For inkjet inks, aqueous or organic solvents are suitable, depending on the intended application. They must have low viscosity (maximum 20 mPa ⋅ s, better < 10 mPa ⋅ s) and are subdivided as follows [176, Chapter 9.6.5]: – Aqueous inkjet inks. The solvent is water, but cosolvents (alcohols, glycerol, diethylene glycol monobutyl ether) and wetting agents or detergents are necessary. They dry by absorption into the paper and by evaporation. – Solvent-based inkjet inks. The solvent is organic in nature, such as methanol or methyl ethyl ketone (butanone). They dry by absorption into the paper and by evaporation. – Oil-based inkjet inks. The oil is an aliphatic hydrocarbon; drying takes place by absorbing the oil into the paper. – Hot-melt inkjet inks. The hot-melting wax is a mixture of wax and amides with a sharp glass temperature of TG ≈ 70–120 ℃. The wax cools on the paper and solidifies.

8.5 Dye inks (fountain pen, felt-tip pen, ballpoint pen, inkjet printing)

� 809

For the reasons mentioned above, the solvent and oil-based types are suitable for industrial purposes; in the office and home sector, we prefer to find harmless aqueous and hot-melt inks. The move away from solvent-based inks to water-soluble formulations has significantly driven recent developments. However, this initially led to quality problems, particularly in the printing sector: organic solvents’ surface tension γs is 20–30 mN/m, slightly below the tension γp of the surfaces and pigments wetted, ▶Table 8.7. With γs < γp , solvents fulfill an essential condition for successfully wetting and forming a thin uniform film, ▶Figure 8.20(b). When printed, solvent inks show sharp edges and uniform colors [152, 781]. Table 8.7: Important parameters of water, solvents, and writing materials for ink systems [220, p. 23], [781, p. 171].

Water Ethanol 2-Propanol n-Butanol Glycol Ethyl acetate n-Hexane Acetone Benzene Toluene Polyethene film Polypropylene film Polyester film Polyamide 6,6 film

Evaporation rate (ether=1)

Evaporation heat [kJ/kg]

Surface tension [mN/m]

45 8.3 10.5

2236 841 675

2.9

367

75.87 23.3 21.4 26.11 49.34 26.84 21.31 23.3 28.9 30.76 31 29 43 46

Since water has a considerably higher surface tension, wetting of the printing material and pigments is less, ▶Figure 8.20(a). Via ▶equation (6.3) in ▶Section 6.3, the contact angle θ depends on γs : if the term γs cos θ is to remain the same when the solvent changes to water, cos θ must be halved. Thus θ must become larger. With water, we obtain isolated drops; print layers adhere poorly, are uneven, and lack luster. As a measure, e. g., polyacrylates are added to aqueous inks, which form a cohesive acrylic film and provide the necessary uniformity and abrasion resistance. Another problem when switching to aqueous inks is the heat of water evaporation, which is about three times higher than common solvents, ▶Table 8.7. The significant changes in the drying properties (speed, temperatures) had to be compensated by design features.

810 � 8 Inks

Figure 8.20: Wettability of a solvent on a surface. (a): γs > γp . The solvent (blue) with the high surface tension γs cannot wet the surface of the pigment with a surface tension γp . Isolated drops form by the contact angle θ > 0. (b): γs < γp . Due to its low surface tension γs , the solvent form a smooth film, the contact angle θ ≈ 0 °.

Colorants Both pigments and dyes serve as colorants. However, some chemically possible colorants are no longer used in more recent times due to carcinogenic properties [785, 787, 993]. Dyes They can belong to different dye classes: solvent dye, food dye, acid dye, direct dye, sulfur, or reactive dye. The main criterion is the nature of the medium (water, solvent) in which the dye must dissolve optimally. Only a tiny amount of dye remains on the substrate after writing; therefore, high dyeing power is a prerequisite. Since dyes do not scatter light and their chemical structure is optimal for color purity, they often deliver brilliant colors. Despite many other favorable properties (low cost, no sedimentation, or clogging), dyes are problematic because they usually have low lightfastness. Only a few dye structures, such as the water-soluble phthalocyanine direct dyes, show high lightfastness. Spinelli [862] provides a direct comparison of the lightfastness and colorfastness of dye and pigment inks. Most writing inks are based on water-soluble acid and triphenylmethane dyes since low lightfastness can be accepted in favor of brilliant colors. Hofmann’s first dye patented in 1858 was Acid Blue 93, which is still in use today for royal blue inks. Unfortunately, black dyes are problematic, so black inks are mixtures of several colors, e. g., yellow, red, and blue (AY23, AR18/AR52, AB1). Pigments They are more lightfast, optically denser, and more water-resistant than dyes. Pigments are used for opaque inks and have to be finely dispersed. Properties such as opacity and tinting strength improve with decreasing particle size; typical particles have a 10–200 nm diameter [782]. The opacity passes through a maximum for particles whose size is at half the wavelength of light, i. e., 0.2–0.4 µm. Transparency and tinting strength are essential for inks, superimposed in very thin layers (typically about 3 µm) in fourcolor printing. Therefore, a small particle size (10–100 nm) is required. Fine-grained pigments have high surface energies and aggregate to reduce their surface area. This formation already occurs during the synthesis of the pigments and leads to tightly bound

8.5 Dye inks (fountain pen, felt-tip pen, ballpoint pen, inkjet printing)

� 811

aggregates, which aggregate loosely. During the dispersion process, we can essentially only break up these loose aggregates so that the correct synthesis control is essential to achieving the desired particle size. The crucial properties of the pigment are determined not only by its chemical structure but also by vital macroscopic factors such as crystal shape, crystal lattice type, and surface character and are determined microscopically by the molecular shape. The mostly completely flat organic colorant molecules tend to get stacked on top of each other like coins. The stacks are held together by van der Waals forces or hydrogen bonds and form larger structures up to crystals of a platelet, cubic, or rod shape. The molecules can be twisted against the stacking axis at an angle that depends on the chemical structure. In addition, the individual molecules’ electron systems can interact intensively in these structures, as in the diketo-pyrrolo-pyrrole pigments. The alignment of the molecules and structures and the electronic interaction strongly modify the electronic and optical properties in the crystals compared to the isolated molecule. If several crystal modifications exist, they also differ, e. g., the color shades of the individual modifications of copper phthalocyanine. In addition, the crystals are anisotropic in structure, exposing different physicochemical properties along the different crystal axes (stacking axes) and crystal faces. Thus, the crystal geometry positively or negatively influences the ink we pigment with the crystals. For example, an individual crystal shape may expose hydrophilic groups of the molecules at specific crystal faces so that this form is particularly suitable for aqueous polar inks. Conversely, other crystal forms may lead to higher hardness, resulting in increased abrasion in printing machines. We can prefer desired crystal shapes by producing mixed crystals with small amounts of other pigments. This process often allows us to influence the crystal growth of the primary pigment in the desired direction. The correct choice of the synthesis conditions is also crucial and contributes to a good pigment. Dispersants Pigmented inks must be stabilized dispersions. Traditionally, this was achieved with animal glue (proteins) or gum Arabic, but several additives are available nowadays. Some of these are widely applicable polymeric dispersion stabilizers, which we have already seen in ▶Section 6.3.3. In addition, stabilizers specifically intended for use in inkjet inks will be covered in the discussion of inkjet colorants in ▶Section 8.5.4 at p. 823. Cosolvents Cosolvents have numerous functions: They are humectants, they prevent crystallization and improve the solubility of dyes, and they regulate viscosity, surface tension, and pH value. In this way, they ensure that the ink does not dry too quickly, that it flows smoothly from the nib or print head, wets the paper or substrate appropriately quickly, but does not deliquesce or splash. Alcohols and short-chain alkyl ethers of glycols are suitable

812 � 8 Inks for controlling surface tension. Alcohols, glycerol, or ethers such as diethylene glycol monobutyl ether are used for aqueous inkjet inks. Humectants Humectants for inks (▶Figure 8.21 shows a selection) are substances soluble in the solvent. They have such a high vapor pressure that they evaporate under normal conditions of use more slowly than the solvent. Therefore, they prevent the inks from drying out. When applied to porous media, they are distributed capillary and evaporate slowly. This behavior is disadvantageous when writing on metal, glass, and plastic or heavily sized papers. On such nonabsorbent surfaces, the ink remains moist and smudgeable for a long time.

Figure 8.21: Humectants used for inks.

Recent patents propose an admixture of solids that can prevent drying. Hygroscopic low alcohols with multiple hydroxyl groups and some amides are suitable. The alcohols sorbitol, glycerol, or glycols are often used and can be easily prepared, ▶Section 6.3.1.

8.5 Dye inks (fountain pen, felt-tip pen, ballpoint pen, inkjet printing)

� 813

Surfactants Solvents or suitable cosolvents can control the surface tension, as do the added humectants. Precise control of this parameter is achieved with detergents or (anionic) surfactants. Easy-to-produce, inexpensive alkyl sulfonates, glycols, or alcoholates are used, ▶Section 6.3.1. ▶Figure 8.22 displays a selection.

Figure 8.22: Surfactants used in inks.

Rheology modifier In addition to cosolvents, dedicated rheology modifiers, i. e., thickening agents, can also determine viscosity. Examples are alginic acid, polyvinylpyrrolidone polymers and copolymers, alginates, and polyacrylic acids. The correct consistency ensures a smooth ink flow and a pleasant writing experience. In pigmented inks, these substances can also stabilize the dispersion. Emulsifiers Emulsifiers keep the aqueous and apolar phases that may occur in ink (e. g., higher alcohols to prevent drying out) in an emulsion [206]. Surfactants are also used as emulsifiers, ▶Section 6.3.1. Depending on the pH of the ink, besides anionic surfactants regulating surface tension, cationic surfactants derived from alkylated amines or amides may also be necessary. ▶Figure 8.23 shows a selection. The cocoyl residue is not a clearly defined substance but a mixture of fatty acids with 8–18 carbon atoms produced by the saponification of coconut oil. The main components are lauric acid (C12 ), with approx. 50 %, oleic acid (C18:1 ), and linoleic acid (C18:2 ). The tallyl residue refers to the fatty acids obtained by saponification of tall oil, a byproduct of papermaking from wood, which also contains tree resins and other lignin degradation

814 � 8 Inks

Figure 8.23: Emulsifiers used in inks.

products, ▶Section 6.7. The main components here are C18 -fatty acids with one and two double bonds (oleic acid and linoleic acid). In addition, surfactants based on glycol, polyethylene glycol, and other alcohols (e. g., pentaerythritol) esterified with fatty acids are also suitable emulsifiers. pH-Regulators The pH value is regulated by low organic acids such as glycolic acid, acetic acid, succinic acid, citric acid, 3-morpholino propanesulfonic acid, and organic amines such as triethanolamine. They adjust the degree of ionization of acid or cationic dyes to achieve an appropriate solubility, color depth, and color brilliance. Rheological auxiliaries (thickeners) and binders also require specific pH ranges to be effective. Binders Binders may consist of simple sugars, polysaccharides such as gum Arabic, or polymers based on vinyl or acrylics, polyamides, or PVP. Acrylates are often copolymers of styrene, ethyl acrylate, methyl methacrylate, n-butyl acrylate, 2-ethylhexyl acrylate, methacrylic acid, or acrylic acid [188]. They are indispensable for pigmented inks and ensure that the pigments adhere more or less permanently to the medium after the solvent has evaporated. There is hardly any binder with inkjet inks, so fixatives must be present in the paper. Dyes can adhere to the substrate on their own by the mechanisms described in ▶Chapter 5, especially in ▶Sections 5.1 and 5.2. In inks containing self-adhesive dyes (direct or acid dyes), weak binders such as polyvinylpyrrolidone prevent ink bleeding into the paper fibers. Polyvinylpyrrolidone,

8.5 Dye inks (fountain pen, felt-tip pen, ballpoint pen, inkjet printing)

� 815

readily soluble in water, can form firm, transparent, and glossy films on glass, metal, and plastics. At the same time, it can act as a thickener and dispersant for pigments. Preservatives Biocides prevent biological activity (fungus, mold) once the ink is on the shelf or its use has begun. Simple and early biocides include phenol and salicylic acid. Today, a large variety of biocides are designed with heterogeneous chemistry [185, Volume 4]. Special additives Particularly in the field of inkjet inks with special requirements, small proportions of special additives are incorporated [151, 815]. These include, e. g., penetration aids (lower alcohols such as isopropanol) that help the solvent penetrate the paper to accelerate drying. In addition, in some printing processes, solubilizing aids are needed to keep the dye in solution even when the concentration has become high, e. g., near the print head. Furthermore, other additives reduce the cockling of wet paper. For this purpose, lower alcohols (pentanediol) are often employed, reducing the ink’s interaction with the paper fibers. The cockling of the paper also decreases if tension in the fiber mesh is reduced during production.

8.5.2 Colorants for fountain pen ink ▶Table 8.8 lists specific dyes for colored writing inks. Many ink colors are not obtained by a single dye but by mixing several ones, as shown in ▶Table 8.6. Mainly black is mixed

from several color shades, e. g., from AY23, AR18 or AR52, and AB1 [809]. We can test this with simple chromatography, i. e., by dripping black ink on coffee filter paper. We drop water (poorly suited but readily available) as a running medium on the ink stain. The chromatogram obtained roughly shows us different ink components, for black, e. g., yellow, purple, and blue. The chapter on dyes already introduced many of them; nonetheless, some food dyes should be mentioned:

816 � 8 Inks Table 8.8: Dyes for writing inks [13, p. 495], [783, 809]. * denotes colorants commercially available currently [969, p. 10], [968, Chapter 4.4–4.7], [979, 985]. Manufacturer information no longer available are [970, 971, 978], [976, p. 10f]. The selection is exemplary and not complete. Yellow Orange Red Purple Blue Green Brown Black

AY3, AY5, AY9, AY23∗∗ , AY36∗ , AY42, AY73, DY5, DY11∗ , DY86∗ , DY132∗ , RY37∗ , SY62∗ , SY79∗ , SY81, SY82, SY83:1∗ AO4, AO7, AO10, AO56, DO102, SO3∗ , SO41∗ , SO54, SO56, SO62∗ , SO99 AR18, AR51, AR52∗ , AR73, AR87, AR92, DR75, DR239, DR254∗ , RR23, RR24:1∗ , RR120∗ , RR180∗∗ , SR8, SR49∗ , SR89, SR91∗ , SR92, SR118, SR119, SR122, SR124∗ , SR127, SR160 AV17∗ , AV49, AV66∗ , BV3∗ , BV10, DV99∗ , RR141∗ , SV8∗ AB1, AB9∗∗ , AB15, AB90, AB91, AB93∗ , AB104∗ , DB86, DB199∗∗ , SB4∗ , SB44∗ , SB45∗ , SB70 AG1, AG16, SG7∗ ABr4, ABr101, ABr268, SBr42 ABk2∗ , ABk194∗∗ , ABk234∗ , DBk19, DBk154, DBk168∗ , DBk205∗ , RBk8∗ , RBk31∗ , SBk5∗ , SBk7∗ , SBk27, SBk29, SBk45

Edible writing ink dyes Yellow E100=FY3, AY3=E104=FY13, AY23=E102=FY4, E110 Magenta AR18=E124=FR7, AR51=E127=FR14 Cyan AB9=E133=FB2, AB3=E131=FB5 Black E152=FBk2

Inks in four-color printing (CMYK) We can easily determine the typical spectra of dyes that can be used as primary colors of the four-color system (CMYK) based on the numerous writing inks available on the market. ▶Figure 1.8(a) in ▶Section 1.5.4 shows the yellow, magenta, and cyan ink spectra of a CMYK-based color mixing system for writing inks. The colorants used in this system are the classic writing ink dyes AY23, RR180, and AB9. Ink erasers Ink eraser pens (“ink-killer”) contain two tips, one at each end: first, a white felt-tip with a bleaching solution to convert the blue writing ink into an invisible form, and secondly, a blue felt-tip with a dye that, unlike regular writing ink, is resistant to the bleach so the invisible spot can be overwritten.

8.5 Dye inks (fountain pen, felt-tip pen, ballpoint pen, inkjet printing)

� 817

Nucleophiles such as hydrogen sulfite anions are employed as bleaching agents for standard royal blue school inks. The hydrogen sulfite anion is formed from sodium sulfite Na2 SO3 in an aqueous solution via an acid-base reaction:

Its bleaching action is due to the addition of the nucleophilic hydrogen sulfite anion to the central carbon atom of a triarylmethane dye, ▶Figure 8.24.

Figure 8.24: Effect of an ink eraser by destroying the π system of the chromophore.

The addition creates a carbon center with tetrahedral sp3 geometry. As a result, the formerly planar molecule loses its planarity, and the electronic resonance over the entire molecule is blocked. Thus the basis of the chromaticity is lost, and a colorless leuco dye emerges. Dyes stable to adding the hydrogen sulfite anion cannot be neutralized with the ink eraser, which is the case with the usual black inks and the blue ink contained in the overwriting tip of the ink eraser pen.

818 � 8 Inks Another possibility for dye bleaching is employing a reducing agent like sodium dithionite Na2 S2 O4 . This usage reduces the dye to the colorless leuco form. An example of the large-scale application of such a reaction is the treatment of indigo with sodium dithionite to produce a colorless to a slightly yellowish aqueous solution (“vat”) of leucoindigo, which contains the reduced and thus colorless form of indigo, ▶Section 4.7.3 at p. 364. Finally, potent oxidizing agents can also destroy an ink dye, e. g., hypochlorites such as chlorinated lime (calcium hypochlorite Ca(OCl)2 ) or sodium hypochlorite NaOCl. Chemicals of this type, however, are not used in ink pencils. 8.5.3 Colorants for felt-tip, fiber-tip, ballpoint pens For producing color pastes for felt-tip, fiber-tip, and ballpoint pens, the colorants in ▶Table 8.9 are suitable. In principle, all acid or basic colorants, as well as pigment dispersions, can be employed. In particular, fine drawing pens (fineliners) with line widths around 0.1–0.5 mm require ultrafine dispersions with pigment grains below 0.5–1 µm. Again, we find dye mixtures, such as those shown in ▶Table 8.6, specifically for black colors. 8.5.4 Colorants for inkjet inks Inkjet printing inks are realized with both dyes and pigments, ▶Table 8.10. It contains examples of colorants that are specifically suitable for inkjet printing. A good overall representation of the inkjet domain is available with [150, 151, 810, 812, 813], partly also with [6, p. 145]. The table shows the primary colors required for four-color printing; yellow, magenta, cyan blue, and black. Primary colors from the food colorants allow us to print edible images, provided we choose a suitable substrate. The CMYK primary color system dominates the consumer sector exclusively. ▶Section 1.5.4 has already illustrated the spectral characteristics of good CMYK primary colors with the help of writing inks. In the professional sector, e. g., in the printing of flyers and brochures, color palettes are increasingly extended to achieve high-quality color reproduction. Inkjet printers are equipped with six or more inks instead of four, ▶Section 1.5.4. One example was the CMYKOG Hexachrome system, which used orange and green in addition to the CMYK primary colors. Purple can be added as another primary color. The chemistry of inkjet inks is already familiar to us from other areas. However, the selection made must consider the specifics of inkjet printing [151]. For example, in the thin layers of ink, colorants are strongly exposed to light, so inks with simple dyes have had problems with fading for a long time. This problem led to the development of pigmented inks. Nonetheless, their advantages (higher hiding power, high light fastness) and disadvantages (finely dispersed distribution, clogging of the printing unit)

8.5 Dye inks (fountain pen, felt-tip pen, ballpoint pen, inkjet printing)

� 819

Table 8.9: Pigments and dyes for felt-tip or fiber-tip and ballpoint pens, and permanent marker [13, p. 495], [809]. * denotes colorants commercially available currently [969, p. 4, 10], [968, Chapter 4.4], [982]. Manufacturer information no longer available are [970, 971, 978], [976, p. 10f]. The selection is exemplary and not complete. Yellow Orange Red

Blue Green Purple Brown Black White

PY3, PY13∗ , PY16, PY17, PY74, PY83∗ , AY3∗ , AY5, AY9, AY17∗ , AY23∗∗ , AY73, BY2, BY29, BY40∗ , DY86∗ , DY132∗ , DY156, SY56∗ , SY62∗∗ , SY79∗ , SY82∗ , SY83∗ , SY83:1∗ PO13, PO34, PO43, AO7, AO10∗ , BO2∗ , Food Yellow 3∗ , SO3, SO41∗ , SO43∗ , SO45∗ , SO54∗ , SO56∗ , SO58∗ , SO62∗ , SO99∗ PR112∗ , PR122∗ , PR146, PR170, PR179∗ , PR184∗ , PR187, PR188∗ , PR208, PR222, AR18∗∗ , AR52∗∗ , AR53, AR73, AR87∗ , AR92∗ , AR94∗ , AR249∗∗ , AR289∗ , BR1, BR10, BR22, DR23, DR81, RR24:1, RR180∗ , SR1, SR23∗ , SR24∗ , SR89∗ , SR91∗ , SR119∗ , SR122∗ , SR124∗ , SR127∗∗ , SR132∗ , SR160∗ PB15:1, PB15:3∗ , AB1∗ , AB9∗∗ , AB19, AB93∗∗ , AB182∗ , BB7∗ , BB26, DB1∗ , DB86, DB199∗∗ , Food Blue 2∗ , Food Blue 5:2∗ , SB4, SB38∗ , SB44∗ , SB45∗ , SB48∗ , SB70∗ , SB80, SB89 PG7∗ , AG1, AG16, BG4∗ , SG3∗ PV19, PV23, AV17∗ , AV49∗ , AV66∗ , BV16, SV8 PBr25, PBr27, ABr4, ABr101, ABr268 PBk7∗ , ABk2∗ , ABk194, DBk19∗ , DBk22∗ , DBk154∗ , DBk168, DBk171, SBk3∗ , SBk5∗ , SBk7∗ , SBk27∗∗ , SBk29∗ , SBk35, SBk45, Food Black 2 PW6∗

Fluorescent dyes (markers), highlighter dyes Yellow AY254, BY40, SY160:1 Red AR52∗ , AR87, BR1 Blue BB3, DB199 Green SG7 Purple BV10

need to be assessed for any intended application. Furthermore, since printed matter, unlike paintings, is exposed to mechanical stresses, applied colorants must have a high affinity to the printing substrate (paper, overhead, packaging films). Dye ink The first inkjet inks were dye inks [13, Chapter 5.6], [13, Chapter 5.6.3.1], [151, 810]. They contained dyes from the food and textile sectors that were only purified. An early primary color quartet from the acid dye range, ▶Section 5.7, is AY23/AR52/AB9 and Food Black 2, which is still used today in edible inks. The dyes show good solubility in aqueous inks, but at the same time, this results in poor fastnesses regarding water, smear, and marker applicability. Triphenylmethanes and xanthenes are also extremely sensitive to light. From the 1980s onward, demand for solvent-free (aqueous) inks to adhere to standard noncoated writing paper increased. This need was due to improved environmental awareness, the price, and the distinct feel of the thick-coated specialty papers. Unfortunately, the changed, low viscosity of the inks initially resulted in thin, translucent ink areas with frayed edges and blurred printing dots.

820 � 8 Inks Table 8.10: Pigments and dyes for inkjet printing, four-color primary and extended process inks [13, p. 500], [205, 810, 812, 818]. * denotes colorants commercially available currently [968, Chapters 2.6, 2.7, 4.5, 4.6], [980, p. 10, 14, 18], [979, 982, 985]. Manufacturer information no longer available are [970, 971, 978], [976, p. 10f]. The selection is exemplary and not complete. Pigmented four-color primary inks Yellow PY13∗ , PY74∗ , PY77, PY83∗∗ , PY93∗ , PY120∗ , PY126, PY128∗ , PY138∗ , PY139∗∗ , PY150∗ , PY151∗∗ , PY154, PY155∗∗ , PY180∗ , PY181, PY185∗ Magenta PR57:1∗ , PR122∗∗∗ , PR146∗ , PR147, PR168, PR176, PR184∗ , PR185, PR202∗ , PR264∗ , PR293∗ , PV19∗∗ Cyan PB15:�∗∗∗ , PB15:�∗∗ , PB16∗ Black PBk7∗∗ Pigmented extended process inks Orange PO34∗∗ , PO43∗ , PO64∗∗ , PO71∗ , PY95∗ , PY110∗ Red PR48:2∗ , PR144∗ , PR166∗ , PR254∗∗ , PR255∗ Purple PV23∗∗ , PV37∗ Blue PB15:6∗ , PB60∗ Green PG7∗∗∗ , PG36∗ Dye-based four-color primary inks Yellow AY5, AY17∗ , AY23∗∗∗ , AY36∗ , BY40∗ , DY11∗ , DY86∗∗ , DY132∗∗ , RY37∗ , SY62∗ , SY82∗∗ , SY83:1∗ , SY88∗ , SY89∗ , SY162∗ Magenta AR14, AR37, AR52∗∗ , AR149, AR249∗ , DR75, DR254∗ , RR23, RR24:1∗ , RR120∗ , RR141∗ , RR180∗∗ , SR49∗ , SR91∗ , SR118∗ , SR119∗∗ , SR122∗∗ , SR127∗∗∗ Cyan AB1∗ , AB9∗∗∗∗ , AB92, AB93∗ , DB199∗∗∗∗ , RB19, SB4∗ , SB27, SB44∗ , SB45, SB48∗ , SB67∗ , SB70∗∗ Black ABk2∗∗ , ABk194∗ , ABk234∗ , DBk19∗ , DBk22∗ , DBk154∗ , DBk168∗ , DBk205∗ , Food Black 2, RBk8∗ , RBk31∗ , SBk3∗ , SBk5∗ , SBk7∗∗ , SBk27∗∗∗ , SBk28∗ , SBk29∗∗ , SBk45 Dye-based extended process inks Orange AO10∗ , BO2∗ , SO3∗ , SO11∗ , SO43∗ , SO45∗ , SO54∗∗ , SO56∗ , SO58∗ , SO99∗∗ Red AR18∗ , AR87∗ , AR92∗ , AR94∗ , AR289∗ , SR23∗ , SR24∗ , SR89∗ , SR125∗ , SR132∗ , SR160∗∗ Purple AV17∗∗ , AV49∗ , BV3∗ , BV10∗ , DV99∗ , SV8∗ Blue AB104∗ , BB7∗ , DB1∗ , SB38∗ Green BG4∗ , SG3∗ , SG7∗ Edible four-color primary inks Yellow AY23=E102 Magenta E122, AR18=E124 Cyan AB9=E133 Black E151 + E110 + AY3=E104

Subsequently, the industry used more light-resistant structures. Until today, very stable phthalocyanines are employed for the blue range, which must be converted to dyes (DB199). In the yellow and red range, azo dyes are used (RR180, AR37, DY86, DY132, AY17, AY23); in the black range, polykis azo dyes (DBk19, DBk154). We already have described these dyes in the sections on reactive dyes, ▶Section 5.3, direct dyes, ▶Section 5.4, and acid dyes, ▶Section 5.7.

8.5 Dye inks (fountain pen, felt-tip pen, ballpoint pen, inkjet printing)

� 821

Metallization further increases lightfastness (RR23, RBk31). Therefore, in industrial lettering printing, lightfast metalized, solvent-soluble dyes such as SBk27 are employed [819]:

Another step toward high lightfastness is taken with pigments, as we will see in the next section. The problem of solubility in water with simultaneous water resistance can be solved by replacing the strongly ionized sulfonic acids and sulfates with carboxylic acids or weakly alkaline amines with pKa values between 6.5 and 8. These compounds show a strongly pH-dependent ionizability. In alkaline inks (pH between 7.5 and 10), they show sufficiently high dissociation, and thus solubility; on acidic paper (pH between 4 and 6.5), dissociation decreases sharply, and the dyes aggregate, reducing solubility. Carboxylic acids reacted with volatile bases such as ammonia or lower amines can be introduced as soluble ammonium salts into the ink. After printing and evaporating the base, the insoluble carboxylic acid remains. Examples include modified DY86, RR180, DB199, and DBk195 [818, 819]:

822 � 8 Inks

The modified DY86 and DB199 use carboxylic acids instead of sulfonic acids for a pHdependent decrease in the degree of ionization. In the modified RR180, a weakly alkaline diamine mediates the pH dependence. In DBk195, both options are combined. Another way to increase water resistance is to increase the molecular weight. The simplest way to do this is to double the molecule by linking with aliphatic, aromatic, or heterocyclic bridges [818]. In DY86 or DY132, this has already been done with a triazine or urea bridge. In addition, simple red mono azo dyes can be coupled, e. g., with a ditriazinyl-diamine bridge to form a dimer:

Black tris-azo dyes such as DBk168 naturally possess a high molecular weight while serving as a nontoxic substitute for older dyes such as DBk154 containing bridging components that could release benzidine:

8.5 Dye inks (fountain pen, felt-tip pen, ballpoint pen, inkjet printing)

� 823

The solubility can be improved by using the lithium salt and introducing a hydroxyethyl sulfonyl group while maintaining the water resistance [819]:

▶Section 5.2.3 illustrates other modifications that can improve the colorant’s adhesion

to paper.

Pigmented inks In ink formulations, pigments are used as colorants when high optical densities (opacity) and high lightfastness are required. Despite challenging operating conditions (see below), pigmented inks [150, 151] are increasingly gaining acceptance due to their advantages. While phthalocyanines (PB15:3) almost exclusively serve as the cyan component, the selection of magenta pigments is more comprehensive. Lightfast quinacridones (PR122 or the warmer PV19 or PR202) dominate this color range because the likewise red naphthol AS pigments such as PR184 may not exhibit the required fastnesses. The most comprehensive range is available for the yellow component. However, the following pigments show that hardly one of them meets the high demands of modern yellow pigment inks in equal measure. For example, the less lightfast PY74 is frequently used, while the more robust benzimidazolone pigments PY180, PY120, PY154, or PY175 have poor dispersibility. Suitable but weaker in color is PY155. Inexpensive and high-color diaryl yellow pigments (e. g., PY13) can potentially release benzidine during thermal inkjet processes and are therefore unacceptable. Very lightfast but weak in color due to its high molecular weight is the disazo condensation pigment PY128. Known since the 1930s, but only recently available due to a new synthesis, are quinolo-quinolones (structurally related to quinacridones) such as PY220. They are very lightfast and, in a triplet with phthalocyanines (cyan) and quinacridones (magenta), provide a color fit for outdoor use.

824 � 8 Inks

The colorants of pigmented inkjet inks, unlike artists’ pigments, must be present as fine particles. On the one hand, they must not clog the fine feed lines and nozzles of the printing unit, and the pixels must be tiny to allow high image resolution in the printout. The particle size of today’s inkjet inks is 25–150 nm. This size is achieved by grinding coarser particles or by suitable synthesis control so that the pigments are obtained directly as a microcrystalline precipitate or in fine particles in emulsions. We have already concentrated on this problem in “pigments” on ▶p. 810. The pigment particles obtained in this way are used as a dispersion to prevent settling and clogging. However, a technological challenge lies in that dispersed pigments tend to reaggregate and form coarser lumps. These processes and possible stabilization measures (electrostatic or steric stabilization) have already been discussed in ▶Section 6.3. Dispersants Pigmented inks contain dispersants, which are, in particular, polymeric by nature, ▶Section 6.3.3. They can be added to the ink as a separate component but also be firmly anchored to the pigment by chemical modifications [782]: – The additive is a classic sterically active stabilizer added without pigment modification: an AB or ABA block copolymer anchors to the pigment crystal via the B group, the A groups sterically stabilize, ▶Figure 8.25(a).

Figure 8.25: Four ways to stabilize pigment dispersions (drawn after [782]). The circles symbolize polar groups, zigzag lines the sterically or electrostatically acting additive, and curved lines the binder. The gray disc represents the pigment aggregation.

–

The pigment is provided with supplemental polar groups and added as an additive. The base body of this additive is incorporated into pigment aggregates and binds

8.5 Dye inks (fountain pen, felt-tip pen, ballpoint pen, inkjet printing)

� 825

molecules of the binder present in ink at the polar groups. The modified pigment thus forms an anchor for a layer of binder that sterically stabilizes the dispersion, ▶Figure 8.25(b). Exemplary is a phthalocyanine pigment with 2–5 polar anchors:

–

–

The pigment serves as an auxiliary covalently linked to protective groups, acting sterically or electrostatically. The modified pigment is, in turn, adsorbed onto the growing pigment crystals and forms a stabilization layer on them, ▶Figure 8.25(c). In a combination of options one and two, two additives are used: a pigment modified with polar groups is incorporated into the growing pigment crystals. The second, independent additive is a sterically or electrostatically active stabilizer. It is adsorbed on the polar groups and forms the actual protective layer, ▶Figure 8.25(d).

The auxiliary can also be precipitated on top of the pigment and remains loosely adsorbed. Therefore, there is a risk that other system components dissolve the auxiliary, which would lead to coagulation of the dispersion. Consequently, the ink must be formulated with respect to this risk. Type no. 1, separate dispersants Suitable agents for type no. 1 are polyvinylalcohol, methyl cellulose, hydroxyethyl cellulose, carboxymethyl cellulose, surfactants, acrylate-styrene polymers, or acrylatepyrrolidone copolymers, among others. In more detail, the polyacrylates intended specifically for use in inkjet inks are examined in [862]. They act via ionic and steric repulsion and accordingly exhibit two blocks. The first is a hydrophobic block A p(MMA/BMA/EHMA), which ensures adhesion to the pigment and consists of methyl methacrylate (relatively hydrophilic), butyl methacrylate, or ethylhexyl methacrylate (hydrophobic). The second, a hydrophilic part B, extends into the aqueous phase and causes steric and ionic repulsion. It consists of a copolymer between the monomer of block A and methacrylic acid (MMA/MAA, EHMA/MAA, or BMA/MAA). ▶Figures 6.8(b) and 6.9(b) illustrate the achieved effect. Type no. 3, covalently bonded dispersants We can achieve a stable covalent bond to the pigment if we choose styrene-maleic anhydride polymer as an auxiliary and at the same time introduce an anchor group such as the methylamino group into the pigment. Subsequently, anhydride and amine form a covalent amide bond between pigment and polymer:

826 � 8 Inks

The method assumes that a pigment can be suitably modified. In the case of synthetic organic pigments, we can consider the dispersant in the pigment design and introduce voluminous or ionizable groups. Examples are the benzimidazolone group, alkyl chains (sterically demanding), sulfonates, sulfonic acids, or carboxylic acids (ionizable):

Type nos. 2 and 4, introduction of (polar) anchor groups Anchor groups can be subsequently introduced into the pigment in various ways. For example, in the case of carbon black as the most common black pigment, we can create various functional groups through surface oxidation (hydroxyl, carboxyl, oxo groups), ▶Section 3.1 at p. 211. We can attack carbon in organic pigments with diazotized reagents to introduce sulfonic acids, carboxylic acids, or phosphates, ▶Figure 8.26 (reactions at the top). Phosphates are of particular interest in inks for paper printing since they form complexes with calcium traces from the paper, and thus increase the adhesion of the pigment to the substrate. Other possibilities include sulfonation and chloromethylation reactions, ▶Figure 8.26 (reactions at the bottom). 8.5.5 Paper, inkjet support materials In ▶Section 6.7, we already explained the elemental composition, the function of the components, and the manufacture of paper. Looking at the material on which the colorants are ultimately printed helps us understand why an office supply store can now carry more than ten different printer papers. First, (calendered) writing paper, like copying paper or multifunctional office paper, is uncoated, usually wood-free, and can also be smoothed by calendering. Sizing is used

8.5 Dye inks (fountain pen, felt-tip pen, ballpoint pen, inkjet printing)

� 827

Figure 8.26: Various ways to add organic pigments with anchor groups to covalently link dispersants and other functions.

to adapt the surface properties to the requirements of the aqueous writing ink. It must be strong enough to prevent the low-viscosity ink from running on absorbent paper and obtaining a cleanly standing typeface but weak enough not to repel the aqueous ink. The determining factor here is the high surface tension of water.

828 � 8 Inks We have already examined the unique features of artists’ papers in ▶Section 6.7.7. High-quality design and layout paper consists of mass-glued wood-free pulp to increase its strength. Surface sizing prevents ink from running and low-viscosity ink from bleeding through to the reverse side. The sizing must match the solvent in ink. While many inks are water-based, designers, cartoonists, illustrators, and mixed media artists often use markers and fine liners with alcohol-based inks, which have considerably lower surface tensions, and thus run more quickly than aqueous inks. For particularly brilliant colors or pin-sharp drawings, the surface is therefore additionally smoothed by calendering (Bristol). Sizing and calendering work together and increase the open time of the inks, i. e., slowing down the absorption of the solvents so that drawing can continue wet-on-wet for a short time. Furthermore, the sharpness of the drawing is increased by calendering. Designers also work on papers with high basis weights, so-called cardboards. Chromo board is a multilayered board with a top layer of pulp coated with 18–20 g/m2 to present a dense, smooth surface for brilliant colors and precise drawing [182, 183]. The basis weight is 250–500 g/m2 . Bristol board is cardboard glued from three or more layers with a wood-free top layer and a usually wood-containing inlay. It is available in grammages of 250–1000 g/m2 . Wood-containing grades are not recommendable for artistic works with a high life expectancy. The structure of modern substrates for inkjet printing is more complex than those mentioned before. In its early days, printing on simple office paper happened with commercially available dyes from the textile sector. However, problems became quickly apparent. First the ordinary paper was too porous and had too high an absorbency. Second, water-based inks posed further challenges because they caused the ordinary paper to cockle and considerably reduced its strength in the wet state. The statement “it is just paper” is no longer valid for modern inkjet paper [151, 811, 813–817], [176, Chapter 9.5]. Since it was not until around 1992 that ink and dye formulations were developed suitable for standard office papers, the search for better substrates began. For example, the industry developed coatings based on silicate clays to reduce the absorbency and improve the print image. Inventions led to regular paper coatings, water-tear-resistant and dimensionally stable papers, and films. Considerable research efforts are still being made, as the substrate material must match the constantly evolving dyes and pigments. Today’s inkjet printing papers are coated with a color containing silica gel, polyvinylalcohol, CMC, and low molecular weight polymers (modified polyethyleneimine, polyvinylamine, polyacrylamide) [179, p. 98]. The base paper is sized with AKD, ASA, or tree resins. The papers have a complex structure of several layers, ▶Figure 8.27.

8.5 Dye inks (fountain pen, felt-tip pen, ballpoint pen, inkjet printing)

� 829

Figure 8.27: Principle structure of a modern inkjet paper. IRL: ink reception layer. An IRL and a back coating are applied onto a stable support to prepare the surface for printing. The IRL can consist of several thin individual layers, as indicated here.

Substrates Today, we can choose between paper, synthetic papers, and polymer films as the actual mechanical carriers. The primary function is mechanical stability, but the substrate can also serve to absorb the ink solvent (usually water) immediately after printing. We thus obtain a quick finger-dry printout, from which the solvent completely evaporates subsequently. Paper The simplest substrate is regular paper, ▶Section 6.7. Due to its porous structure, it absorbs large amounts of the ink solvent, but its dimensional stability and tear resistance are low in the wet state. Too high absorption or unfavorable porosity leads to washedout and bleeding prints. Therefore, a sharp print image depends mainly on the specific properties of the paper coating. Paper specially designed for inkjet printing contains optical brighteners for brilliant colors and additives to prevent cockling and deformation in the wet state. However, a definitive solution to this problem is not yet in sight. Synthetic papers The poor water and tear resistance and lack of dimensional stability of plain paper make it unsuitable for outdoor applications. Therefore, today synthetic papers made of plastic fibers do not change their properties even when wet and remain dimensionally stable outdoors. These include Teslin® and Tyvek® , made from polyethylene fibers. They are similarly absorbent compared to traditional paper but water and tear-resistant. More like films are Crisper® (polyester), Kimdura® (polypropylene, PP), and Typar® (PP). The polymers are welded as fibers and can be made opaque by microscopic inclusions of air (“microvoided”) while they are transparent in the normal state. Films Plastic films are needed especially for overhead projection and transmitted light printing. They do not consist of polymeric fibers but of rolled or blown films. The most con-

830 � 8 Inks ventional plastics are polyethylene terephthalate (PET) and polyvinylchloride (PVC). The colorless plastics provide transparent films; opaque ones are obtained by white pigmentation or microscopic air inclusions. Films are considerably stiffer than synthetic papers, which is usually desirable. Composites Very uniform and fine-pored papers are coated with polyethene or polypropylene for printing digital photographs. We find many different sandwich-type construction principles; usually, the paper is coated on both sides. The front ply has a smooth, uniform surface and is often pigmented and provided with optical brighteners. Additional front or back layers provide mechanical stability and stiffness even when wet. They are usually transparent and allow watermarks to show through. Ink Reception Layer (IRL) While the substrate is primarily a mechanical carrier, the IRL is a single-layer or multilayer coating that ensures clean and durable printed images in conjunction with the ink formulation. The layers predominantly control the spread of the ink solvent and the deposition of dyes and pigments in the right places. They absorb solvents or transfer them to the substrate to quickly obtain finger-dry prints. From there, slow evaporation then takes place over the next few minutes. ▶Figure 8.28(a) shows the structure of an inkjet paper with a porous IRL; ▶Figure 8.28(b) shows that of a multilayer paper with layers for dye fixation and a liquid barrier, which simultaneously produces an optical brightening. Porous IRLs They consist of fine particles and a binder. Particles used are the following: siliceous clays (montmorillonite, bentonite, synthetic laponite), amorphous silica SiO2 , alumina Al2 O3 , or calcium carbonate CaCO3 (PCC, precipitated calcium carbonate). The size of the particles varies from less than 100 nm to 1 µm. The thickness of IRL on paper is typically 10–20 µm, while on films, it is 30–50 µm to provide sufficient volume for the solvent to be absorbed. The capillary forces acting in the pores ensure rapid absorption of the ink solvent after the printing process. Water-soluble hydrophilic polymers serve as binders. They fix the porous coating, support the capillary processes due to their hydrophilic properties, and slowly absorb more solvent after the liquid’s initial rapid absorption through the pores. As the liquid evaporates from the pores, they subsequently release the solvent. Suitable polymers are the following: polyvinylalcohol (PVOH), polyvinylpyrrolidone (PVP), polyacrylic acid (PAA), styrene-acrylate latices (styrene-AA copolymers), polyacrylamide, styrene-butadiene latices, polyvinylacetate (PVOAc), methyl cellulose (MC), cellulose derivatives, or gelatin. The monomer for PVOH is vinyl acetate, so the

8.5 Dye inks (fountain pen, felt-tip pen, ballpoint pen, inkjet printing)

� 831

Figure 8.28: Schematic representations of inkjet papers [176, Chapter 9.5.6]. (a): Structure of inkjet paper with porous IRL on paper support. The IRL consists of silica or precipitated calcium carbonate (PCC). (b): Multilayer structure of a photo-printing paper with image fixation layer (IFL) on paper or polymer substrate. The substrate is protected against liquids by a polyethylene (PE) film. Such a barrier can be formed with TiO2 as a light base layer. The IRL can be porous or homogeneous.

polymeric products differ in their degree of hydrolysis, e. g., Selvol 540® used in high-end photographic papers, consists of 87–89 % PVOH and 11–13 % PVOAc [1008]. Due to the fine particles, porous IRLs are inherently matte; a multilayer structure of three or more layers results in glossy IRL. Such IRLs consist of a coarsely porous layer responsible for distributing the liquid and thinner layers of fine particles, which provide desired surface properties such as gloss, spreading, and deposition of the dyes and pigments. Nonporous IRLs They consist of homogeneous, polymeric films. Since there are no pores available, the polymers themselves have to absorb the solvent and store it temporarily. For aqueous inks, coatings of natural polymers have been used that swell sufficiently quickly and absorb liquid. Gelatin and polysaccharides (cellulose derivatives) are suitable. Synthetic polymers must be hydrophilic, such as PVOH, PVP, polyethylene oxide (PEO), and poly2-ethyl-2-oxazoline (POx). If necessary, other auxiliaries such as mordants ensure good fixation of the colorants in the IRL. Homogeneous IRLs usually show a multilayered structure, with thick layers responsible for fluid distribution and storage and thin layers for optical properties. The homogeneous film makes it easy to obtain glossy surfaces, especially on smooth substrates such as films. For a matte appearance, matting agents have to be added, e. g., amorphous silica particles of size 1–50 µm.

832 � 8 Inks Image fixation layer (IFL), dye fixation layer Auxiliary substances control the printed image depending on the ink used. Anionic dyes are fixed by cationic mordants consisting of poly-diallyl dimethyl ammonium chloride (polyDADMAC), polyvinylamine (PVAm), polyethyleneimine (PEI), styrenemaleimide resins, polyaluminum chloride (PAC), alum, polyamidoamines, polyvinylbenzyl-ammonium chloride, or cationically modified polyvinylalcohols. The mechanism is similar to that for mordant dyes, ▶Section 5.5. We have already seen many fixatives in the paper coating sector as retention aids and fixatives in ▶Section 6.7.3 at p. 592 and ▶p. 602. 8.5.6 Colorants for stamp pads Many of the colorants presented so far are also suitable for the artistic activity of stamping graphics. Therefore, ▶Table 8.11 gives an overview of colorants for filling stamp pads. Table 8.11: Colorants for stamp pads. * denotes colorants commercially available currently [969, p. 4, 10], [982, 985]. Yellow Orange Red

Purple Green Blue Black

PY1∗ , PY3∗ , PY13∗ , PY42, PY74∗ , PY83∗ , PY155∗ , AY3∗ , AY17∗ , AY23∗∗ , BY40∗ , DY11∗ , DY86∗ , DY132∗ , Food Yellow 3∗ , SY56∗ , SY62∗ , SY82∗ PO34∗ , AO10∗ , BO2∗ , SO43∗ , SO45∗ , SO54∗ , SO56∗ , SO58∗ , SO99∗ PR2∗ , PR5∗ , PR9∗ , PR112∗ , PR146∗ , PR170∗ , PR179∗ , PR188∗ , PR254∗ , AR18∗ , AR52∗ , AR87∗ , AR92∗ , AR94∗ , AR249∗ , AR289∗ , RR24:1∗ , RR120∗ , RR180∗ , SR23∗ , SR24∗ , SR89∗ , SR119∗ , SR122∗ , SR127∗ , SR132∗ , SR160∗ PR122∗ , PR184∗ , PV19∗ , PV23∗ , AV17∗ , AV49∗ PG7∗ , BG4∗ , SG3 PB15∗ , PB15:3∗ , AB1∗ , AB9∗ , AB93∗ , AB182, BB7∗ , DB1∗ , DB199∗∗ , Food Blue 2∗ , Food Blue 5:2∗ , SB38∗ , SB48∗ , SB70∗ , SB80 PBk7∗∗ , ABk2∗ , ABk234∗ , DBk19∗ , DBk22∗ , DBk154∗ , SBk3∗ , SBk5∗∗ , SBk7∗ , SBk27∗ , SBk29∗

8.6 Laser or copier toner With a certain degree of freedom, we can regard copiers and laser printers as artistic media and include them in our foray into the world of color. Whereas the printing processes mentioned so far employ inks that are suitably placed on the paper by the drawing motion of the hand or the print head, the colorant is applied electrographically in the case of laser printing or copying machines. In contrast to inks, toners from laser and copier machines are not only colorants but also participants in this electrographic process and, therefore, have a complex structure.

8.6 Laser or copier toner

� 833

Printing principle A good overview of laser printing is available in [781, 791–795]. Therefore, we only look at the basic principle here to understand the requirements for the colorants. In copier or laser printing, in principle, an invisible image of the original or the print file is created on a (negatively charged) imagesetter drum (▶Figure 8.29(a)) by discharging parts of the drum with a light or laser beam, ▶Figure 8.29(b). Copying machines use bright (white) light for the discharge, reflected from the original onto the imagesetter drum. In contrast, the laser beam is directly controlled by the original in laser printers.

Figure 8.29: Principle of laser printer and copier. ⊖ represents negative charges on the exposure drum or the carrier toner, ∙ is the pigmented toner. (a) The fully charged imagesetter drum. (b) Partial discharge of the imagesetter drum according to the original (or the print image). (c) As a result, negatively charged toner adheres to the exposed, discharged areas. (d) After transferring the toner to paper, the toner is fixed.

Next, the exposed drum comes into contact with the toner of the same electrical charge, whereby the transfer process is controlled electrically and magnetically. The toner is repelled from all areas that have not been exposed and are therefore still electrically charged, and it only adheres to the exposed areas of the drum, ▶Figure 8.29(c). Subsequently, the toner is transferred from the drum to the paper and is fixed to the paper by heating, ▶Figure 8.29(d). The toner resin and the embedded pigment melt onto the paper during this process. As a result of this fusing, we can still feel the residual heat on freshly printed sheets. Toners Systems with one or two components can provide pigments with the necessary electrical properties. The toner consists predominantly of a thermoplastic resin, which may be combined with 0–15 % wax. The resin/wax composition determines the flow properties of the molten toner during hot-setting; the wax improves the rheology of the melt and provides the desired gloss level of the cooled-off toner. To avoid toner deposits on mechanical elements of the device, we often find lubricants such as zinc stearate.

834 � 8 Inks Since the resin mixture represents the subsequent binder for the pigment, it is selected mainly based on its rheology to ensure optimum adhesion to the potential substrates (paper, films, and others). It melts between 50–100 ℃ and can be easily incorporated into the pores of a rough substrate such as paper, but also shows good adhesion to smooth surfaces such as films. Suitable resins are the following: styrene-n-butyl acrylate, styrene-n-butyl methacrylate, styrene-butadiene copolymer, a polyester, or a phenolic epoxy resin with molecular weights between 5000 and 100000. Some devices do not use hot fixation but press the resin onto the carrier. For this, lighter polymers of polyethylene, polypropylene, or an ethylene-vinyl acetate copolymer are used, with molecular weights of up to 15000. In four-color printing, the pigments are colored. The challenge for color toners is to achieve a uniform gloss, adhesion, and film thickness despite very high and irregular coverage (up to 30 % per color). By comparison, black and white toner is applied uniformly to the substrate in a single layer up to 8 % coverage. One-component toners They contain the printing ink and exhibit the necessary electrical properties simultaneously. They consist of thermoplastic resin particles of 5–15 µm size, in which colorant, wax, and 30–50 % magnetic particles are embedded. The magnetic material used is magnetite Fe3 O4 , among others. Since it is dark gray to black, it is suitable for black but not colored toners. Two-component toners They consist of the carrier (95 % by weight) and the colored toner (5 % by weight) that is applied to the carrier. The properties of carrier and toner are carefully selected to achieve triboelectricity. Therefore, they become oppositely charged when mechanically shaken in the copying machine before being applied to the imagesetter drum. This charge is necessary on the one hand to transport the toner with the carrier and, on the other hand, to deposit the toner specifically on the exposed, uncharged parts of the drum. The carrier itself is aligned magnetically. Magnetic materials such as iron, steel, ferrite (ZnO ⋅ NiO ⋅ Fe2 O3 ), and magnetite of 50–500 µm particle size are employed. They are coated with a polymer to charge the carrier triboelectrically [796]. In contrast to the toner resin, this coating is selected for its triboelectric properties. Depending on the desired charge (sign and strength) are used: polyacrylates (n-butyl acrylate, methyl methacrylate), or halogenated ethylenes (polytetrafluoroethene, polytrifluorochloroethene, polyvinylidene fluoride). Carrier particles are about ten times larger than the actual toner. The toner consists of 70–90 % of the thermoplastic resin, contains 0–15 % wax, 1–15 % pigment, and possibly 2 % of a charge control agent. The toner particles are smaller than 10 µm and adhere electrostatically to the carrier, ▶Figure 8.30.

8.6 Laser or copier toner

� 835

Figure 8.30: Structure of the carrier of a two-component toner. Dark gray: magnetic core (magnetite, ferrite, iron). Light gray: triboelectric polymer. Blue: toner particles.

Liquid toners In addition to powdered dry toners, there are liquid toners in which the pigment is dispersed in a hydrocarbon, typically nonane to dodecane [797]. Pigments In ▶Table 8.12, we see colorants used as black or color toners in copiers and laser printers. Primary colors for four-color printing are yellow, magenta, cyan blue, and black. The first three pigments are already known to us and belong to a triplet of very lightfast ones. Among them are the well-established phthalocyanine cyan (PB15:3), quinacridone magenta (PR122), and azo yellow (PY12, PY74, PY155). Carbon is used exclusively as black pigment. Since limitations of the CMYK color space are noticeable, e. g., in modern business printing of flyers and brochures, manufacturers also offer toner pigments for the extended process colors orange, green, and purple, which significantly expand the printable color space. Table 8.12: Pigments for laser printing and copier toners, primary colors for four-color printing, spot colors for spot color printing, and extended process colors orange, green, and purple [13, p. 500, pp. 545]. * denotes colorants commercially available currently [968, Chapter 2.6], [980, pp. 10, 14]. Manufacturer information no longer available are [972, 973]. The selection is exemplary and not complete. Four-color primary colors (CMYK system) Yellow PY12, PY13, PY17∗ , PY74∗ , PY83∗ , PY93∗ , PY95∗ , PY97∗ , PY110∗ , PY128∗ , PY136, PY138∗ , PY139∗∗ , PY150∗ , PY151∗∗ , PY155∗ , PY170, PY174, PY175∗ , PY180∗ , PY185∗∗ Magenta PR48, PR53:1, PR57:1∗ , PR81, PR122∗∗ , PR146∗ , PR184∗ , PR202∗ , PR293∗ , PV1, PV19∗∗ Cyan PB15:3∗ , PB15:4∗ , PB16∗ Black PBk7∗∗ Extended process primary colors and spot colors Orange PO34∗∗ , PO64∗ , PO71∗ Red PR48:2∗ , PR144∗ , PR166∗ , PR254∗ , PR255∗ Purple PV23∗∗ , PV37∗ Blue PB15:6∗ Green PG7∗∗

836 � 8 Inks The selection is small compared to writing inks, proving the high requirements of electrographic printing on colorants. In addition, they must withstand the high thermal stress of fusion fixation and are therefore among the most stable classes of organic colorants. In the electrographic process, other dyes are involved, which we do not perceive as such since they absorb in the near-UV or IR spectral range. They serve for the generation of the electrical charge on the imagesetter drum (charge generation agent, CGA), the charge distribution in the printing system (charge transport agent), and the charge control (charge control agent, CCA). In color printing, the number of auxiliary agents is even higher since each primary color has a different chemistry, and thus other electrical properties, which must be controlled separately. Details can be found in [781].

8.7 Printing inks Printing inks are pigmented liquids, used for artistic as well as commercial printing. Their properties are matched to the printing techniques used, which differ predominantly in the creation of the generative matrix, i. e., the block, stone, plate, screen, stencil that is printed onto paper or another suitable surface. Artists create the generative matrix by drawing, carving, or engraving. Essentially, traditional, artistic printmaking techniques are (▶Figure 8.31) [84]: – stencil process – relief printing – intaglio printing – planographic process Besides these, cross-overs employ elements of different techniques. Artistic printing techniques were employed already in prehistoric times, continuously developed and also inseparable linked to commercial applications, as the following overview depicts: Prehistoric times

Impressions from finger tips, hands, palm in caves in southern Europa and China

“Very early”

China [158, p. 135]

Around 4000 BC

Early examples for relief prints, Sumer [84, p. 75]

1600–1046 BC (Shang dynasty)

Prints of seals in clay, then silk, and paper for inscriptions; seals of any hard, flat material (wood, metal, stone); relief or intaglio technique [158, p. 137]

2nd century BC

Stencils from thick paper with needle holes, pressing ink through perforation [158, p. 146]

1st century

Seal print in black ink on silk [158, p. 137]

105

Invention of paper in China [84, p. 75]

Carvings or incisions in stone, bones, metal [84, p. 105]

8.7 Printing inks

� 837

5th century

Seal print in vermillion ink [158, p. 137]

6th century

Rubbing (making inked squeezes on paper from stone, metal, one, or hard-surfaced materials) [158, p. 143]

6th to 9th century

Chinese woodblock printing [158, p. 146] [84, p. 75]

7th century

Ink wash painting with Chinese ink

8th century

Chinese woodblock printing [160, p. 20]; surviving specimen from woodblock printing [158, p. 147]

868

First complete, printed and illustrated book on paper roll (“diamond sutra”) [158, p. 151, 253]

10th century

Chinese woodblock printing spreads widely [160]

618–907 (Tang dynasty)

Flourishing market of printed books [158, p. 151]

947

Hand-colored multicolor print [158, p. 280]

1100

Chinese moveable characters [160, p. 20]

Around 1450

Gutenberg letterpress ink (carbon black or lamp black, linseed oil as base) [159, p. 272]; letterpress ink (17–22 % soot, linseed oil) [79, Chapter 18.2] Gutenberg’s first ink: lamp black, varnish, egg white [160, p. 57]

15th century (Ming dynasty)

Technical innovations: metal typography, improved multicolor process for woodblock printing (2–5 colors), refined wood art for book illustrations [158, p. 172] Artistic engravings [84, p. 105]

Early 1500s

Employment of acids for etching metal plates [84, p. 106]

16th century

Development of etching process [84, p. 106]

17th century

Application of copper plates [84, p. 106]

1642

Mezzotint [84, p. 106]

1650

Tonal rather linear images (J. van de Velde) [84, p. 106]

Late 1700s

Aquatint [84, p. 106]

End of 18th century

Wood engraving

Stencil process In the stencil process, also called screen printing process, the print is created by forcing ink through a shaped opening onto the paper [84, Chapter 3]. The opening can be a handcut stencil or a screen that is partially blocked by a screen filler. The stencil or filler protect certain areas of the paper from being inked. Stencil printing originates in one of the oldest forms of printing, the stenciled handprint. It was widely used in China and Japan around 500. Intricate paper stencils were used to print a resist made from starch onto fabrics that were subsequently dyed, yielding a colored pattern. The stencil process came to Europa by trade routes. In the late nineteenth and early twentieth century, it was commercially used for book illustra-

838 � 8 Inks

Figure 8.31: Classification of artistic printing techniques according to four basic processes.

tion and textile printing, but the paper stencils used were not durable enough to withstand hundreds of copies. Therefore, paper stencils were fixed to fine-woven silk frames. A first patent for a silk-screen process was issued in 1887 in the USA. The inks were first applied with a stiff brush. Later on, the squeegee was invented. Around 1930, screen printing on paper was recognized as a fine-art technique, called serigraphy. Around 1960, it influenced the pop art movement. The screens were traditionally made of silk, nowadays of polyester meshes. Tight meshes have around 200 threads/inch (83 threads/cm) or higher, commonly used with water-based inks. For high-details stencils made from photo-emulsions, meshes can be as tight as 280 threads/inch (116 threads/cm). For bold imagery, rough surfaces, or textiles, 60–180 threads/inch (43–90 threads/cm) are employed. To create a stencil from light-sensitive emulsions (photo emulsions), the screen is coated with a light-sensitive emulsion, exposed to light, cured by UV light, and rinsed with water. The parts exposed to UV light were cured and hardened, while the parts covered by the drawing can be washed away, creating openings in the mesh to let ink pass. Photo emulsions employ diazo, dual-cure, or presensitized photopolymers. Relief printing Relief printing applies a smooth generative matrix that is cut or carved [84, Chapter 4]. The remainder of the matrix is inked and creates the image, which can be high-contrast. Precursors are carved objects, used to stamp pattern on a surface or to create impres-

8.7 Printing inks

� 839

sions into clay. Early Sumerian examples originate from 4000 BC. The technique became important with the invention of paper in China around 105. Woodblock printing originates in the ninth century in China to reproduce texts with illustrations. It spread throughout Asia and Europe along the trade routes. In Europe, it was used to print textiles and religious materials. For mass prints, relief printing only became relevant when Gutenberg invented the printing press in the fifteenth century, and the paper-making industry grew. Later on, artists like A. Dürer, H. Holbein the Elder, L. Cranach, or L. van Leyden, employed it for artistic printmaking, mostly in black and white. Around the eighteenth and nineteenth century, the Japanese subjects of printmaking influenced the avant-garde artists. The materials used for the generative matrix are manifold: softwood, hardwood, linoleum (cork pressed with resin), MDF (medium-density fiber board), plastic (e. g., PVC or high-impact polystyrene HIPS). The wood used for traditional woodblock printing is side-grain wood, i. e., wood cut parallel to its fibers, like in plank-making. It is therefore not possible to carve fine details, and a strong black and white expression is possible. Wood engraving was developed at the end of the eighteenth century to rationalize printing for mass publications in cases when copper engraving was too expensive. It differs to woodcut in the type of wood used as generative matrix. For wood engraving, the blocks are made from end-grain hardwood, i. e., they are cut perpendicular to the wood fibers, yielding a hard, dense surface suitable for fine details. Traditionally, boxwood, cherry, or lemonwood were used, while maple is common nowadays. Synthetic materials are also explored. The technique of rubbing, developed in China in the sixth century, comprises the making of inked squeezes on paper from stone, metal, one, or hard-surfaced materials. For stone, an intaglio cutting was made, and paper squeezed into the carving, then inked black, and pulled out, yielding white characters on black. For wood, a relief cutting was prepared, inked black, and paper was pressed flat against the relief, yielding black characters on white. Intaglio printing Intaglio printing usually employs metal-plate matrices that are engraved or etched to develop an image [84, Chapter 5]. Basically, two approaches are followed: inscribing lines by engraving or cutting, and employing acids to manipulate the matrix surface in manifold ways. The technique of incision dates back to Paleolithic times, where carvings into stone, bones, and metals were produced. The first artistic engravings are from the fifteenth century, the artists providing drawings and working together with professional gravers. In the beginning, intaglio printing was intended for mass reproduction. Later on, the artistic quality of prints were recognized, and a variety of techniques established. Some direct techniques exist:

840 � 8 Inks – – –

In the drypoint technique, the plate surface is scratched with a sharp tool. For engraving, the plate surface is carved with a lozenge-shaped tool of variable width. For mezzotint, the plate surface is scratched with a row of fine, sharp teeth to create an overall black impression. Lights are produced by scraping and burnishing.

There are also some acid-based etching techniques: – For hard ground/line etching, the plate surface is coated with a hard ground of beeswax, asphaltum, and rosin, nowadays also an acrylic. Lines are scratched into the ground, exposing the plate for etching and yielding a linear expression. – For soft ground, the plate is covered with a mixture similar to hard ground with grease or tallow to achieve a soft quality. The plate can be exposed by anything that removes the ground, e. g., finger tips or brushes. – Aquatint achieves tonality by covering the plate with fine, randomly distributed particles. Traditionally, rosin powder is applied to the plate and fused to it by heating. Nowadays, spray paint or air-brushed acrylics are also employed. Nowadays, also photo-sensitive grounds are manufactured, which can be attached to the metal plate, exposed to light, developed, and partly rinsed away, exposing parts of the plate for etching. If the photo-ground is thicker and also acts as its own plate, it is called a flexographic plate, used for commercial printing. The intaglio process is the opposite to relief printing. For intaglio, the depressions of the generative matrix are inked, and dampened, soft paper is pressed onto the matrix, pulling the ink out. It achieves a sculptural quality of the ink layer, and frequently, platemarks are visible, the marks of the plate embossed into the paper. Two cylinders of a press exerts enough pressure to push the ink into the matrix’ depressions. The plates employed as matrix are zinc and copper, brass and steel are also used. Typical etchants are listed in ▶Table 8.13. Table 8.13: Etchants, acids, and mordants used for etching metal plates to create generative matrices for intaglio techniques [84, pp. 109–112]. Etchant

Metal

HCl FeCl� HNO�

Cu Cu Zn, steel

CuSO� CuSO� /NaCl

Zn Zn

Typical composition ≈ 40 % (45 °Baumé) Typical 7–10 parts water per part acid, for soft ground and aquatint 15–20 parts water 25–200 g/l, the lower concentration for soft ground and aquatint “saline sulfate etch,” 75 g/l CuSO� , 75 g/l NaCl

8.7 Printing inks

� 841

Lithographic process The lithographic process or lithography is a chemical process, ▶Section 7.10 [84, Chapter 7]. The generative matrix is created by drawing with lithographic ink on stone or metal plates, and the process uses the hydrophobic properties of the ink. 8.7.1 Inks Inks for commercial and artistic printing are generally composed like other inks: pigment, binders, solvents, and auxiliaries. However, the printing processes employed (artistic techniques, letterpress, lithography, and intaglio printing) place different demands on printing inks; consequently, many formulations differ in binders, solvents, and auxiliaries. 8.7.1.1 Artistic printing inks The main types of inks for artistic printmaking are depicted in ▶Table 8.14 [84, pp. 22–24]. There are two principal vehicles in use: – Oil-based inks are bound with polymerized linseed oil. Depending on the processing time, the polymerization degree changes, achieving thin-bodied burnt-plate oils and medium-bodied litho varnishes and reducing oils. Addition of MgCO3 can stiffen the oil and shorten the ink. Principal qualities are – stiff, short, more viscous, less oily – loose, long, less viscous, more oily Typical for oil-based inks is that the polymerization continues after printing when drying (curing). – Water-based inks employs water as primary vehicle. Frequently, the binder is acrylic, yielding inks that dries to be water-insoluble. Plant gums such as gum Arabic yield inks that can remain water-soluble after drying. – Water-washable inks are oil- or soy-based inks, but modified so they can be cleaned with water and soap. Differences in inks are due to their field of application. Intaglio inks are wiped into the depressions of the generative matrix. They contain large pigment particles and are loose inks with a short, low-tack, buttery consistency. When oil-based, thin-bodied burnt-plate oil vehicles are used. Relief and lithographic inks are rolled into a thin film and applied to the generative matrix. Relief printing inks employs a heavier burnt-plate oil or a lighter varnish, and applies small pigment particles. They are slightly tacky to prevent the paper from moving but without picking it. Lithographic inks employs a more viscous varnish or vehicle and very fine-ground pigments to achieve good color strength for the thin ink layers. They are short and possess a higher tack for a sharp print.

Short, low-tack, buttery

More viscous, short, higher tack

Intaglio printing

Planographic process

Very fine particles

Large particles

Small particles

More viscous, short, slightly tacky

Relief printing

Pigment Fine particles

Viscosity

Stencil process

Application

Vegetable oil

Traditional linseed oil varnish

Vegetable oil Water

Traditional linseed oil, burnt plate oil Mostly acrylic, also gum Arabic

Water

Vegetable oil

Traditional linseed oil, heavier burnt plate oil, lighter varnish Mostly acrylic, also gum Arabic

Water

Water Toxic organic solvents

Solvent

Mostly acrylic, also gum Arabic

Mostly acrylic, also gum Arabic Early linseed oil

Binder

Table 8.14: Basic types of printing inks for artistic application and their main characteristics [84, pp. 22–24].

Polymerization

Evaporation, coalescence

Polymerization

Evaporation, coalescence

Polymerization

Evaporation, coalescence

Evaporation, coalescence Polymerization

Drying

842 � 8 Inks

8.7 Printing inks

� 843

Screen-printing inks were in the twentieth century oil-based inks, requiring toxic solvents. Nowadays, water-based inks are standard. They are mostly acrylic, but for special applications such as printing on ceramics or glass, specialty inks are used. To squeeze them properly through very dense screens, fine particles are employed. 8.7.1.2 Commercial printing inks Commercially, essentially three types of printing inks are distinguished, ▶Table 8.15 [993]: – pasty inks – liquid printing inks – screen printing inks Pasty inks are used for letterpress, newspaper letterpress, offset, and intaglio printing, including the artistic variants etching and copperplate engraving. Binders are drying oils, alkyd, rosin, or maleic resins. The inks dry by absorption into the paper and by chemical oxidation. Mineral or vegetable oils are added for thinning, and siccatives for specific printing processes due to the long drying time of the oils. In addition to graphic arts, applications include printed matter, newspapers, books, and packaging. Liquid printing inks are used for publication gravure, flexography, and packaging gravure. Standard inks for woodcut and linocut are also liquid. Binders are mainly cellulose derivatives, rosin, synthetic polymers, and, in the artists’ field, also gum Arabic. Drying takes place physically by evaporation of the volatile solvent. Organic solvents are chosen for publication gravure printing; lower alcohols or water for flexography. Other applications besides linocuts and woodcuts include illustrated magazines, catalogs, newspapers, mass-printed products, and packaging. Screen printing inks are special printing inks that are liquid but have a higher viscosity than gravure or flexographic inks. Binders are synthetic polymers, cellulose derivatives, and alkyd resins; solvents are hydrocarbons, alcohols, esters, and ketones. Ek, Gellerstedt, Henriksson [176, Chapter 9.6] and Baumann, Rothardt [168] provide exhaustive information on this subject. The following tables list the most critical materials used. In addition, [785, 787, 993] contains information on printing inks, e. g., their environmental compatibility or colorants excluded because they are carcinogenic. 8.7.2 Pigments ▶Table 8.16 shows pigments used in commercial inks for artistic printing, i. e., copper-

plate engraving, intaglio printing, linocut, and woodcut. In this area, usually, well-known artists’ pigments are employed. Hollenberg and Preissig recommend, e. g., Krems white, opaque white, chrome yellow, yellow lake, carmine, cadmium red, madder lake, cobalt blue, Prussian blue, burnt sienna, sienna, umber, and lamp black [80, 81]. Authenrieth [79] contains an extensive table of classic artists’ pigments also suitable for printing.

Application

Sheet-fed offset, web-fed offset, letterpress, newspaper letterpress. Print materials, newspapers, magazines, labels, forms, packaging, books

Manifold, printing on paper, cardboard, plastic, wood, metal, ceramics, glass. Art printing, poster printing, advertising material, packaging, labeling

Publication gravure, packaging, specialty gravure, flexography magazines, catalogs, periodicals, mass-printed materials, packaging, wallpaper, furniture decor

Type

Offset printing

Screen printing

Intaglio, flexography

Low

“Liquid,” but higher viscosity

High

Viscosity

Publication gravure: rosin acrylics, modified rosin resins, cellulose nitrate, synthetic resins

Polymers, cellulose derivatives, alkyd resins

Plant-based drying oils, alkyd resins, phenol-modified rosin resins, maleic resins, hydrocarbon resins

Binder

Publication gravure: toluene flexography and packaging printing: ethanol, ethyl acetate, methoxy propanol, ethoxy propanol, water

Volatile organic alcohols, esters, ketones, hydrocarbons, water

Mineral oils, plant-based oils

Solvent

Table 8.15: Commercially used basic types of printing inks and their main characteristics [993]. Artistic printing techniques are omitted here.

Physical

Physical, oxidative

Absorption, oxidative, physical

Drying

844 � 8 Inks

8.7 Printing inks

� 845

Table 8.16: Pigments for artistic copperplate engraving and intaglio, linocut, and woodcut [996, 997]. In parentheses: components of former printing inks without a CI system denotation according to Hollenberg and Preissig [80, 81]. White Yellow Orange Red Purple Blue Green Brown Black

PW4, PW6 (Krems white, opaque white) PY3, PY7, PY42, PY74, PY83, PY153, PY154 (chrome yellow, yellow lake) PO13, PO36, PO62 PR3, PR4, PR48:2, PR83, PR101, PR102, PR112, PR122, PR188, PR254, PR255, PR264 (carmine, cadmium red, madder lake) PV19, PV23 PB15:1, PB15:3, PB27, PB29 (cobalt blue, Prussian blue) PG7, PG8 PBr6, PBr7 (sienna burnt, sienna, umber) PBk6, PBk7, PBk9, PBk10, PBk11 (lamp black)

In principle, most of the pigments (and dyes) used elsewhere are also applicable in inks for the printing industry. ▶Table 8.17 lists colorants generally suitable for printing inks and their sources, [168, Chapter 10.2f] provides more details. The table contains the necessary primary colors, yellow, magenta, cyan blue, and black, for four-color printing. A frequent pigment quartet used is PY12 or PY13, PR57, PB15, and PBk7 [993]. Table 8.17: Pigments and dyes for commercial printing inks, primary colors for four-color printing and other hues [149, 168, 985]. * denotes colorants commercially available currently [968, Chapters 1.1–1.7, 3.9, 3.10]. Manufacturer information no longer available are [977], [976, p. 10f]. The selection is exemplary and not complete. Pigmented and dye-based four-color primary inks Yellow PY1∗ , PY3∗ , PY12∗ , PY13∗ , PY74∗ , PY83∗ , PY120∗ , PY126∗ , PY127, PY154∗ , PY155∗ , PY180∗ , PY181∗ , PY185, SY82 Magenta PR57:1∗ , PR122∗ , PR146∗ , PR168∗ , PR176∗ , PR184∗ , PR185∗ , PV19∗ , RR24:1, SR119, SR160 Cyan PB15:3∗ , SB70 Black PBk7∗ , SBk5, SBk27, SBk29 Other colors Yellow Orange Red

Purple Blue Green Brown Black

PY14∗ , PY17∗ , PY55, PY73∗ , PY81∗ , PY97∗ , PY101, PY111, PY138, PY139∗ , PY151, PY174∗ , PY175∗ , PY176∗ , PY191∗ , PY194∗ PO5∗ , PO13∗ , PO34∗ , PO36∗ , PO38∗ , PO43∗ , PO62∗ , PO72∗ PR2∗ , PR3∗ , PR4∗ , PR5∗ , PR9∗ , PR12∗ (Bordeaux), PR14∗ , PR48:2∗ , PR53:1∗ , PR57, PR81:2, PR95 (carmine), PR112∗ lake (signal red), PR144∗ , PR149∗ , PR169, PR170∗ , PR187∗ , PR188∗ , PR208∗ , PR209, PR210∗ , PR214∗ , PR242∗ , PR254∗ , PR257, PR285∗ PV1, PV2, PV3, PV23∗ , PV27, PV32∗ PB1, PB15:1, PB62 PG1, PG7, PG36 PBr25∗ PBk7, PBk31

846 � 8 Inks Comparing an old manual on printing inks [154] to new catalogs of printing inks, we find that some old pigments vanished, as was the case with pigments from the painter’s palette. Examples are Yellow: Red: Blue:

Chrome yellow Vermilion, cadmium red Iron blue pigments

Azo pigments Quinacridone pigments Copper phthalocyanines

Current research projects investigate the suitability of pigments made from environmentally friendly and renewable raw materials that have been known since immemorial. For example, reseda (dyer’s weed) and rhamnus berries provide yellow pigments (luteolin and rhamnetin lake), madder (madder lake) red ones, and dyer’s woad (indigo) blue ones. For some applications, these natural printing inks seem to be suitable [773]. The printer does not resort to four-color printing for all jobs, especially in book printing or business printing (letterheads in the corporate design and color scheme). It is often necessary to use ready-made hues that do not have to be mixed from primary colors, so-called spot colors. Therefore, the table includes possibilities for their pigmentation. The range is naturally extensive since there is no restriction to specific standard color shades. The decisive factors for whether one of the numerous pigments is suitable for printing are lightfastness and migration fastness, solubility, and other properties relevant to the application.

8.7.3 Binders ▶Table 8.18 contains the essential binders for commercial and artistic printing inks; ▶Table 8.14 depicts the point of view of printmaking in arts. Binders primarily serve to

fix the pigments. Together with solvents, they also determine the drying properties of the printing inks, and thus the possible printing speeds of the respective print process, which can vary significantly. Therefore, the choice of the binder has direct process engineering and economic consequences. In the commercial sector, pigments in ink must also be bonded with a higher abrasion resistance than other inks since such printing units are often used intensively. We will see in the following that the three groups of printing inks (pasty, liquid, and screen printing inks) differ in their binders [176, Chapter 9.6], [993]. Pasty printing inks Artists’ inks for gravure printing processes such as copperplate engraving or etching are pasty. They are based on drying oils such as linseed oil and stand oil, today also on acrylic

8.7 Printing inks

� 847

Table 8.18: Important binders for commercial printing inks and artistic engraving techniques such as copperplate engraving. See also ▶Table 8.14 for binders of printmaking in arts. In italics: less used agents. Abbreviations used: PG publication gravure, SG special gravure, F flexographic, OL offset+letterpress, L letterpress, S screen printing, Cu copperplate and intaglio, WLC woodcut, and linocut [176, Chapter 9.6], [168, Chapter 10.5], [79, Chapters 16.5, 18.2], [78, 82, 83, 996]. Natural resin derivatives Maleic resins Alkaline earth and zinc resinates Linseed oil stand oil, oil-resin boiling, linseed oil varnish rosin ester (hard resin), phenolic resins

PG, SG, F, OL, L PG OL, L, Cu, WLC PG, OL, L, F

Cellulose derivatives Nitrocellulose, cellulose butyrate

SG, F

Fats and fatty oils Linseed oil, oils of canola, soybean, sunflower, wood

OL, L, Cu, WLC

Polyester Alkyd resins Polyester resins Urethane alkyds

OL, L, S SG, F S

Polymerisates Polyacrylates, polyvinylbutyrate, polymethacrylates polyurethanes

SG, F, S, Cu, WLC SG, F, S

Ketone resins and others Ketone resins, polyamide resins

SG, F, S

Natural products Gum Arabic, glue, borax shellac

S, Cu, WLC

binders [84, pp. 22–24] [78], [79, Chapter 18.2], [997]. In the last few years, water-washable inks containing oil-based emulsions have appeared on the market [84, pp. 22–24], [997]. For copperplate engraving, line etching, and drypoint techniques, inks may be “stricter” or “shorter,” i. e., with a more viscous consistency. For aquatint and mezzotint, they can be extended with linseed oil or lavender oil. Pasty inks for letterpress, newspaper and engraving printing contain drying oils derived from linseed or alkyd resins, which dry oxidatively-chemically and have a high open time, ▶Section 7.4 and ▶Section 7.8. The alkyd resins contain unsaturated C18 carboxylic and dicarboxylic acids such as phthalic acid and their isomers. Siccatives include cobalt and manganese salts, which accelerate oxidative drying while antioxidants slow it down. Other widely used binders come from resin acid or rosin chemistries, namely hard resins (rosin esters and salts), maleic resins, and phenol-modified rosin resins. They are based on balsamic resins and the resin acids contained therein and dry without crosslinking, i. e., physically, forming a film. The natural drying oils have a low molecular weight of around 900. Therefore, they possess relatively low viscosity and take a long

848 � 8 Inks time to build a high molecular weight, cross-linked, resistant film. Both are often troublesome for technical applications in paints, inks, and lacquers, so techniques were developed early on to increase the molecular size before use and to achieve the optimum viscosity for an application. These processes include: – Heating linseed oil to 260–300 ℃ for several hours to obtain a product with higher viscosity by thermal pre-polymerization. The result is familiar stand oil, well-known to the painter, ▶Section 7.4.2.2 and ▶Section 7.4.4. – Heating linseed oil to 100–150 ℃ while air is blown in. The result is blown oil, crosslinked via oxygen bridges and hardly used by painters, ▶Section 7.4.2.2. – Blending of the oil with solid natural resins. Solvents must reduce the resulting significant increase in viscosity to the required level. An admixture of hard resins leads to rapid physical drying, while drying oils’ admixture slows chemical-oxidative cross-linking. In the painting and coating industry, rosin is a traditional and still widely used natural resin. It is a component of balsam, which exudes from trees (conifers) as a resinous, sticky oil to seal the wound on the trunk. Rosin remains as a residue when turpentine oil is obtained from balsam by steam or fractional distillation, ▶Section 7.4.8. A significant advantage of rosin is its high oil compatibility; disadvantages are its low softening point of ≈ 65 °C and its high content of acidic groups (carboxylic acids). However, we can eliminate both disadvantages by esterifying with polyols or neutralizing or forming a salt, thus obtaining rosin esters and salts. First, rosin reacts with maleic acid and yields tricarboxylic acids. Subsequently, their esterification with polyols leads to high molecular weight maleic resins. Finally, reacting rosin and rosin derivatives with phenolic resins yields phenol-modified rosin resins. The structure and formation of these three rosin derivatives are described in ▶Section 8.7.4 separately. The following recipes give an impression of the composition of pasty printing inks: Recipe for Chinese multicolor-woodblock printing ink, around the 6th–9th century [158, pp. 146, 279] Earth pigments, vermillion, indigo, peach-tree resin or hide glue, water Letterpress ink Gutenberg, about 1440 [79, Chapter 18.2] 17–22 % soot, linseed oil Copper engraving ink, according to Trevelyan [79, Chapter 18.2] 1 part Frankfurt black (i. e., probably vine black, ivory black, or charcoal black), 2 parts of French black (probably French Frankfurt black), linseed oil varnish Offset ink [176, Chapter 9.6] Pigment (0.1–0.5 µm) 15–20 %, hard resin 20–30 %, alkyd resin 8–12 %, triglyceride oil drying 10–20 %, mineral oil or oil ester nondrying 15–25 %, waxes, siccatives, antioxidants 3–5 %

Liquid inks Liquid printing inks for artistic use can be formulated with a wide range of binders; there are inks based on oil, acrylic, or gum [84, pp. 22–24]. Purely water-based inks are available for linoleum and woodblock printing; the binder is gum Arabic [996]. How-

8.7 Printing inks

� 849

ever, in principle, ink, watercolor, and gouache inks, i. e., inks based on borax shellac, gum Arabic, glycerol or glue or both, can be used [82, 83]. Since watercolor and gouache paints are often too thin, they must be thickened, e. g., with gum Arabic or glue. Oil-based binders for woodcut and linocut contain linseed oil, turpentine, or other oils [82, 83]. In addition, the water-washable copperplate inks based on an oil emulsion are applicable for linocut and woodcut printing. Commercial solvent-based, liquid inks for publication gravure and flexographic printing contain cellulose derivatives as binders, especially nitrocellulose. These show good wettability toward pigments and are compatible with other binders like polyacrylates. Synthetic polymers such as polyacrylates are equally applicable in solvent-based and aqueous inks. They are present in aqueous inks either dissolved ionically, i. e., at a high pH value or emulsified as latex. Binders for alcohol-based inks (ethanol, n-propanol) are rosin esters or polyamides. The above binders dry physically by evaporating the solvent to form the film. Some recipes for modern flexographic inks may show the elemental composition: Flexo ink solvent-based for PE films [176, Chapter 9.6] Pigment 12 %, polyamide 22 %, nitrocellulose 4 %, n-propanol 34 %, ethanol 13 %, n-propyl acetate 12 %, PE wax 2 %, amide wax 1 % Flexo ink solvent-based for folding boxes [176, Chapter 9.6] Pigment 14 %, nitrocellulose 11.5 %, maleic resin 8 %, n-propanol 25 %, ethanol 25 %, n-propyl acetate 10 %, plasticizer 5 %, PE wax 3.5 % Flexo ink water-based for nonabsorbent substrates [176, Chapter 9.6] Pigment dispersion 50 %, soluble polyacrylate 10 %, polyacrylate latex 30 %, water 5 %, organic amines 1 %, PE wax 3 %, antifoam agent 0.5 %, wetting agent 0.5 % Flexo ink water-based for absorbent substrates [176, Chapter 9.6] Pigment dispersion 40 %, soluble polyacrylate 30 %, polyacrylate latex 12.5 %, water 13 %, organic amines 1 %, PE wax 3 %, antifoam agent 0.5 %

Screen printing inks For screen printing, binders are nowadays water-based or water-washable, and mostly acrylic. For special applications such as ceramics, textiles, or glass, specialty binders are employed. Screen printing inks are bound with cellulose derivatives, alkyd resins, or synthetic polymers. In solvent-based publication gravure inks, metal resinates serve as binders. Nitrocellulose acts as a binder for those based on water.

8.7.4 Rosin derivatives as binders We will dedicate a section to rosin as a readily available natural product, which has provided essential components for centuries for painters, graphic artists, and printers.

850 � 8 Inks Numerous derivatives are available. We will examine some essential representatives, specifically rosin esters and salts, maleic resins, and phenol-modified rosins. We have discussed the origin and structure of rosin in ▶Section 7.4.8. In general, the products are dissolved or dispersed in linseed oil. Rosin esters (hard resins) are higher molecular esters of a polyalcohol such as pentaerythritol, glycol, diethylene glycol, or glycerol with balsamic resin acids (▶p. 681), which may also be dimerized or polymerized, ▶Figure 8.32. Rosin salts such as alkaline earth or zinc resinates are calcium or zinc salts of balsamic resin acids, ▶Figure 8.32. Since the resin acids used are generally monobasic, we obtain only products of medium molecular weight this way, despite using polyols.

Figure 8.32: Formation of rosin esters and soaps using abietic acid (abbreviated ABA). The balsamic resin acids form salts with bivalent metal cations and rosin esters with polyalcohols such as pentaerythrol, glycol, diethylene glycol, and glycerol [197], [194, p. 39ff].

Rosin converts into valuable lacquer and ink components in other ways. Maleic resins are products of the Diels–Alder reaction between balsamic resin acids and maleic acid or maleic anhydride. Maleic acid represents the dienophile, and the resin acids represent the diene, ▶Figure 8.33. Since the double bonds are only present in levopimaric acid in the necessary 1,3-diene configuration, other resin acids must be transformed by thermal isomerization more or less entirely to levopimaric acid before the Diels–Alder reaction can start. Then the maleic resin forms from maleopimaric acid, which contains three carboxylic acids as anchor points, and a polyol such as glycerol, which again provides

8.7 Printing inks

� 851

Figure 8.33: Formation of maleic resins from resin acids and maleic acid in a Diels–Alder reaction. Most resin acids must first be thermally isomerized to levopimaric acid with the necessary 1,3-diene configuration. The initially formed maleopimaric acid reacts with a polyol, e. g., glycerol, to form the threedimensionally cross-linked maleic resin [197], [194, p. 39ff]. The serpentine lines indicate cross-links to other resin acids or other resin components.

three hydroxyl groups as anchor points for a high-molecular-weight, three-dimensional polyester network. Phenol-modified rosins are condensation products of balsamic resin acids and phenol-formaldehyde prepolymers or novolak. The basis is phenolic resin formation from phenols and under-stoichiometric formaldehyde. In this process, an electrophilic addition of formaldehyde to the phenol occurs. Next, polycondensation to the novolak follows, cross-linked by methylene groups:

852 � 8 Inks This polymeric product brings us into the realm of phenolic resins. For our topic, a variation of this reaction is significant: Increasing the formaldehyde content and the pH value can slow down its rate in such a way that resols, intermediate stages of polycondensation, are obtained, cross-linked by methylene groups and ether linkages:

The phenolic rings can form ether bridges across the various methylol groups to form cross-links. However, resols are reactive resins because they contain free hydroxy methylene groups. This fact and their lack of elasticity prevent their use as film formers in coatings. On top of that, they are insoluble in oil and water and can only be employed as a coating resin when dissolved in methylated spirits. However, resols can be modified with tree resin acids to form phenol-modified rosin resins. 10–20 % resols react with rosin at high temperatures. However, the expected esterification between the hydroxyl groups of the resole and the resin carboxylic acid does not ensue, ▶Figure 8.34. Instead, the addition of hydroxymethyl and phenolic hydroxyl groups to a double bond of the resin acid occurs, forming a chroman [169, Chapter XIII], [170, Chapter 8.3.1]. The resol can also add to the double bond via its methylol group, forming an ether [197]. The acidic reaction product is subsequently esterified with a polyol such as glycerol or pentaerythrole to lower the acid number. Phenol-modified rosin resins are hard, transparent solids with a melting point of 100–125 ℃ for soft resols and 125–160 ℃ for hard resols. The resols can be boiled into the oil or dissolved to produce a base for printing inks and varnishes.

8.7.5 Solvents ▶Table 8.19 contains the essential solvents for commercial and artistic printing inks.

Together with binders, they give the printing ink the necessary drying properties and speeds. The solvents differ among the three groups of printing inks [993]:

8.7 Printing inks

� 853

Figure 8.34: Formation of phenol-modified rosin resins from a resol prepolymer and a resin acid with ether [197] or chroman structure [169, Chapter XIII], [170, Chapter 8.3.1]. The resol structure is only indicated. The alcohol used for the esterification of the resin acid is a polyol (glycerol, pentaerythrole), which can lead to cross-linking like the resol.

– – –

Pasty inks do not contain volatile solvents but mineral or plant-based oils for letterpress, offset, and intaglio printing. Quick-drying liquid inks contain toluene as a volatile organic solvent (publication gravure inks) or alcohols and water (flexographic inks). Screen printing inks contain mineral oils, alcohols, esters, and ketones.

854 � 8 Inks Table 8.19: Important solvents for inks in commercial printing and artistic techniques such as copperplate engraving. In italics: less used agents. Abbreviations: PG publication gravure, SG special gravure, F flexo, OL offset+letter offset, L letterpress, S screen printing, Cu copperplate and intaglio, WLC woodcut and linocut [176, Chapter 9.6], [168, Chapter 10.6], [79, Chapters 16.5, 18.2], [78, 82, 83, 996]. Hydrocarbons (mineral oils) Toluene Gasoline, mixtures of aliphatic/aromatic hydrocarbons

PG OL, L, S, Cu

Alcohols Ethanol, 1-ethoxy-propanol-2, 1-methoxy-propanol-2 Propanol-1

SG, F SG, F, S

Esters Ethyl acetate, iso-propyl acetate, n-propyl acetate

SG, F

Ketones Butanone Cyclohexanone

SG, F S

Other Water Linseed oil Lavender oil

SG, F, S, Cu, WLC S, Cu, WLC Cu

Pasty copperplate and gravure printing inks containing stand oil or oil-based binders can be extended with linseed oil. Linocut and woodcut inks based on water or oil emulsion contain water as a solvent. Inks for artistic engraving techniques, such as copperplate engraving, are traditionally made with linseed oil or other plant-based oils such as lavender oil [176, Chapter 9.6], [79, Chapters 16.5, 18.2]. Water-based inks can be diluted with water. 8.7.6 Auxiliaries Binders and solvents determine the essential properties of the printing ink, which are fine-tuned by auxiliaries. Essential components are [168, Chapter 10.7] slip improvers, wetting agents, and siccatives. Waxes increase the slippage and abrasion resistance of the paint application. Both natural and synthetic waxes are used. These include polyethylene waxes, paraffin waxes, montan waxes, polyethylene glycol waxes, higher fatty acids (e. g., stearic acid), and amides. Surfactants are added to the ink as wetting agents and influence the flow properties. Common substances are aluminum stearates, lithium soaps, fatty acid and fatty alcohol polyglycol esters and ethers, surfactant sulfates and sulfonates, and aluminum alcoholates. Siccatives (drying accelerators) catalytically accelerate the oxidative drying of drying oils, as discussed in oil paints ▶Section 7.4.3. They are salts of CoII , BaII , CaII , CeIII ,

8.8 Tusche

� 855

CuII , MnII , ZnII , AlIII , or FeIII with C6−19 fatty acids. They are added to oleaginous copper and gravure inks.

8.7.7 Paper ▶Section 6.7 already explained the elemental composition, the function of the compo-

nents, and the manufacture of paper. The unique features of artists’ papers were discussed in ▶Section 6.7.7. For printing papers with a grammage of approx. 200–320 g/m2 , high volume and high absorbency are necessary to facilitate the absorption of the solvent. Embossing techniques usually required paper grammages of more than 250 g/m2 . For printing techniques that employ low amounts of solvent, such as woodcut or wood engraving, traditional asian or “rice paper” with a grammage of 40–90 g/m2 can be used. The surface of paper for printmaking is uncoated and unsized to ensure rapid absorption of the solvent. It may be calendered to provide a sharp image in some printing techniques, such as woodcut. Typical types are: – Rough paper is only pressed between felts during initial forming. – Cold-pressed paper is smoother by being pressed between fine weave felts or cold metal cylinders. – Hot-pressed paper is very smooth by being pressed between hot metal cylinders. The paper can be sized in mass to ensure fiber cohesion and strength, and must be pHneutral. Traditionally, pulp made from 100 % rags was used, and even today, high-quality printing papers are made from 100 % cotton or linen fibers, whereby the short fibers of the linter have a volume-increasing effect [1004]. Lower-priced paper grades consist partly (50–70 %) or entirely of wood-free pulp, an additive of a few percent CaCO3 as a buffer against acid slows down aging processes. Many cellulose papers are considered to be aging resistant, according to ISO 9706. Paper for copper printing, etching, or similar embossing techniques should be soft and voluminous. Although they are produced based on a soft pulp without fillers and sizing [180, p. 101], their high volume is achieved by adding short-fiber pulp from hardwoods or deciduous woods. After moistening, the resulting soft, unsized paper can be easily pressed into the printing plate to absorb sufficient ink in a sharply defined manner.

8.8 Tusche Tusches are water-resistant drawing inks of a particular, opaque blackness, similar to Chinese ink. Some early tusches were removable after drying and are therefore better

856 � 8 Inks regarded as inks, such as Roman ink. Modern tusches also form a water-resistant film after drying, which is particularly desirable for technical and artistic drawings. Tusches differ only slightly from other inks in their composition. They are based on pigments and are therefore opaque in appearance [802, 809], [203, keyword “Drawing and Writing materials”], [204]. For the permanent fixation of the pigment on the writing material, tusches have a high proportion of a binder that forms a film when it dries compared to other inks. This film encloses and fixes the pigment as a painting layer of a painting do. The following recipes illustrate the general compositions: Classical tusche, water-resistant [809], [203, keyword “Drawing and Writing materials”] Black or brown pigment (suspension of soot, lamp black, charcoal, bister, or sepia), water-resistant binder (shellac soap), protective colloid and binder (glue) Modern drawing tusche, water-resistant [809], [203, keyword “Drawing and Writing materials”] Pigment dispersion (color pigment or carbon black) 3–5 %, black 10–12, up to 30–36 %, colloidally dissolved in water-resistant binder (shellac soap or synthetic resin, e. g., polyacrylate, rosinmodified maleic resin, water-soluble polyurethane resin), protective colloid (glue), preservative

Early pigments were soot, lamp black or charcoal, and later bister or sepia. Bister (Natural Brown 11) is a red to brown product of incompletely burnt beech wood containing soot and semi-carbonized phenolic oils, resins, and fats, ▶Section 3.1.2 at p. 216. Sepia is a brown melamine pigment produced by drying the ink of specific squids, ▶Section 8.8.1. While a living squid enzymatically disperses the pigment and therefore has a kind of ink at its disposal, sepia itself is insoluble in water. Colored tusches contained the pigments standard at the respective time, such as white lead, massicot, lead-tin yellow, orpiment, realgar, ocher, red lead, cinnabar, indigo, vermilion, indigo/chalk, verdigris, umber, and ultramarine, ▶Section 8.3.1. Today, ultrafine carbon black dispersions are used. To simultaneously achieve high opacity, easy application, and color brilliance of black tusches, the ultra-fine carbon black dispersion is combined with coarser carbon black pigments. High concentrations of pigment achieve the required black covering line. A moisturizer can be added to prevent the tusche from drying rapidly at the point of pens, at the cost of a more slowly drying tusche. The pigment dispersion is stabilized by a protective colloid, in former times glue, gum Arabic, or other plant-based gums, which also acted as binders. In modern tusches, synthetic polymers such as polyvinylalcohol, methyl cellulose, hydroxyethyl cellulose (HEC), or carboxymethyl cellulose (CMC) are employed, ▶Section 6.3.3. They also act as binders. The resistance of the dried film to water was formerly achieved by using shellac as a binder, ▶Section 8.8.2. It was converted for use into a soluble soap, i. e., into the salt of a carboxylic acid, by weak bases such as borax, NH4 OH, morpholine, or NH4 HCO3 . Today, polyacrylates (▶ acrylics, ▶Section 7.9) are increasingly used as film-forming aids instead of shellac to ensure consistent quality.

8.8 Tusche

� 857

8.8.1 Sepia, Natural Brown 9 The pigment of the sepia tusche consists of the dried ink of the cuttlefish sepia officinalis [768]. Though known since the seventeenth century, it has been used only since the nineteenth century on a large scale. As a result, today’s sepia tusches may include true sepia but, more importantly, characterize the hue and include many other colorants. The process of ink extraction is simple: the dark liquid from the squid’s ink sac is mixed with gum Arabic and a little water, after which the ink is ready for use. The basis of this beautiful reddish-brown pigment is melanin, which also lends hair, skin, and numerous other biological systems their color. Melanin forms by oxidative polymerization from o-diphenols (especially tyrosine) and their o-quinones and is variable in composition [305, 764–767, 770, 772]. Mainly dopa, dopamine, and cysteinyl dopa are considered as the base of o-quinones:

Dopa and dopamine are precursors of brown and black eumelanins, cysteinyl dopa builds the yellow, red, and brown pheomelanins. In the case of an eumelanin such as sepia, dopa, and dopamine oxidize to o-quinones. Subsequent intramolecular addition of the amino group to the o-quinones forms the indole skeleton. Sepia melanin is a copolymer of 5,6-dihydroxyindole (approximately 20 %) and 5,6-dihydroxy-1H-indole-2carboxylic acid (approximately 75 %):

A continuous chain establishes via the 4,7’-linkage of the benzene rings, which is crosslinked by a 2,2’-linkage of the indole rings. The 2,4’-linkage of indole to benzene rings leads to the branching of the melanin chain. As a result, all monomers are randomly distributed, and instead of o-diphenols, quinones, and various degradation products may occur.

858 � 8 Inks We can now ask why sepia is brown, but eumelanin is brown-black? In the same way, we can ask how only two pigments (eumelanin and phaeomelanin) induce the different colors of the hair, from blond over red and brown to black? An important, influential factor is concentration. The strongly absorbing melanins quickly cause a dark color impression. Furthermore, melanins are strongly scattering substances that cause Rayleigh scattering on the molecules and Mie scattering on the colored cells. Depending on the concentration, particle shape, and exact composition of the pigment and the cell, various color effects occur, ranging from bluish reflection to reddish-brown transmitted light [305, 767, 771].

8.8.2 Shellac Shellac [960, 961] is a natural resin marketed in the form of brittle, transparent sheets or powder. Highly purified forms are almost colorless, but depending on their origin, shellac has a beautiful warm, golden yellow to garnet-red color. The resin is a secretion of the lac insect kerria lacca, native to the Asian region, especially India, Pakistan, Sri Lanka, and Thailand. The female lac bugs first bite certain trees with their proboscis and suck up their sap. This food is transformed and deposited on twigs, forming a hard coating for the eggs. A large number of insects form crusts several centimeters thick, which are scraped off and cleaned. In the process, many water-soluble components are removed. The resulting flaky shellac is yellow to red but can be bleached in further steps. It is completely soluble in hot alcohol and incompletely soluble in cold water. Shellac can be saponified with borax and then forms aqueous solutions used as film-forming aids in tusches, polishing agents, or sealing waxes. The nontoxic shellac is also used for coating pills and dragées and as a binder for egg colors and nail polishes. Excursion into sealing waxes At this point, the author cannot resist reproducing some recipes for sealing waxes [991], considering a handwritten, sealed letter on heavy paper as an object of art. The elemental composition is always shellac and Venetian turpentine resin, pigment, and turpentine oil. The wax can be perfumed with fragrant resins such as Peru balsam, Tolu balsam, or benzoin (resin). Red sealing wax, fine [991] Shellac 350 parts, Venetian turpentine 240 parts, vermilion 260 parts, magnesium oxide 60 parts, turpentine oil 90 parts Red sealing wax, cheaper [991] Shellac 240 parts, rosin 160 parts, Venetian turpentine 280 parts, cinnabar 180 parts, chalk 60 parts, plaster 60 parts, turpentine oil 20 parts Other colored sealing waxes, traditional [991] The same resin mixture as for the red wax, the pigments used are: lamp black, zinc green, ultramarine, ocher, vermilion, red lead, or smalt. Add fillers such as chalk, gypsum, or magnesium oxide if necessary.

8.8 Tusche

� 859

Composition The main components of shellac [272, drug: Kerria], [103, 962] are partially esterified hydroxycarboxylic acids, especially aleuritic acid (almost 50 % of the substance) and sesquiterpenoid acids such as shellolic acid (nearly 25 % of the mass). However, it is assumed that the sesquiterpenoid acids are ultimately derived from the parent substance, jalaric acid:

The carboxylic acids are responsible for shellac’s ability to dissolve (“saponify”) in an aqueous borax solution. Borax acts as a base and reacts with the carboxylic acids to release boric acid to form water-soluble sodium salts:

At about 5 %, waxes are also present in the crude shellac. These consist of the usual wax alcohols such as myricyl alcohol (triacontanol, melissyl alcohol, C30 ), and cetyl alcohol (hexadecanol, C16 ). The alcohols occur freely as well as fatty acid esters. The fatty acids are also among the waxes: cerotic acid (hexacosanoic acid, C26 ), melissic acid (triacontanoic acid, C30 ), oleic acid (cis-9-octadecenoic acid, C18:1 ), and palmitic acid (cetylic acid, hexadecanoic acid, C16 ).

860 � 8 Inks The hydroxycarboxylic acids of shellac can form polyesters with themselves. The products comprise two to four units and are correspondingly softer or harder (AA = aleuritic acid, BA = butolic acid, JA = jalaric acid):

Exemplified by three units each of butolic acid and jalaric acid:

The aldehyde function of jalaric acid can be oxidized to a carboxylic acid by atmospheric oxygen after coating. Thus, cross-links can form over time by further esterification, which additionally hardens the film. Partners are these carboxylic acids and unesterified hydroxyl groups of the oligomers. However, the exact mechanism is still unclear. Color If we are passionate about carpentry or beautiful woodwork, we are probably familiar with warm-golden or red polishes made with a deep-colored shellac lake. Shellac can display a golden yellow or deep red color, depending on its origin and cleaning. Anthraquinone dyes, which we have already discussed in connection with cochineal and kermes, are responsible for the color: yellow erythrolaccin and the red mixture of the laccaic acids A–D, ▶p. 352. Film formation For film formation, the properties of the linear fatty acids, in particular aleuritic acid, are decisive. A characteristic feature is the structure of hydrophobic chain sections and numerous hydroxyl and carboxyl groups. On the one hand, these allow the alkane chains to assemble. On the other hand, a film post-curing is also possible through intermolecular esterification of the remaining hydroxyl and carboxylic acid groups or the formation of hydrogen bonds.

Bibliography [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29]

Colour Index, several volumes, Society of Dyers and Colourists, Bradford, 1971. H. Zollinger, Color Chemistry, 3rd ed., Wiley-VCH, Weinheim, 2003. ISBN 978-3906390239. P. F. Gordon, P. Gregory, Organic Chemistry in Colour, Springer, 1983. ISBN 3-540-11748-2. J. Griffiths, Colour and Constitution of Organic Molecules, Academic Press Inc., London, 1976. ISBN 0-12-303550-3. J. Fabian, H. Hartmann, Light Absorption of Organic Colorants, Springer, 1980. ISBN 3-540-09914-X. P. Bamfield, Chromic Phenomena, The Royal Society of Chemistry, 2001. ISBN 0-85404-474-4. A. Bartecki, J. Burgess, The Colour of Metal Compounds, Gordon and Breach Science Publishers, Amsterdam, 2000. ISBN 90-5699-250-3. A. T. Peters, H. S. Freeman (eds.), Colour Chemistry, Elsevier Applied Science, London and, New York, 1991. ISBN 1-85166-577-3. R. Johnston-Feller, Color Science in the Examination of Museum Objects, The Getty Conservation Institute, Los Angeles, 2001. ISBN 0-89236-586-2. https://www.getty.edu/conservation/ publications_resources/pdf_publications/pdf/color_science.pdf, accessed on 2022-06-20. S. Bucklow, The Alchemy of Paint: Art, Science and Secrets from the Middle Ages, Marion Boyars Publishers Ltd, 2009. ISBN 978-0714531724. G. Buxbaum, G. Pfaff, Industrial Inorganic Pigments, WILEY-VCH, Weinheim, 2005. ISBN 3-527-30363-4. K. Hunger, M. U. Schmidt, Industrial Organic Pigments, 4th ed., WILEY-VCH, Weinheim, 2018. ISBN 978-3-527-32608-2. K. Hunger (ed.), Industrial Dyes, WILEY-VCH, Weinheim, 2003. ISBN 3-527-30426-6. H. M. Smith, High Performance Pigments, WILEY-VCH, Weinheim, 2002. ISBN 3-527-30204-2. H. Endriß, Aktuelle anorganische Bunt-Pigmente, Curt R. Vincentz-Verlag, Hannover, 1997. ISBN 3-87870-440-2. G. Pfaff, Inorganic Pigments, Walter de Gruyter GmbH, Berlin, 2017. ISBN 978-3-11-048450-2. H. G. Völz, Industrielle Farbprüfung, 2nd ed., Wiley VCH, 2001. ISBN 3-527-30418-5. D. R. Waring, G. Hallas (eds.), The Chemistry and Application of Dyes, Plenum Press, New York, 1990. ISBN 0-306-43278-1. A. D. Broadbent, Basic Principles of Textile Coloration, Society of Dyers and Colourists Thanet, Press Ltd, London, 2001. ISBN 0-901956-76-7. W. Ingamells, Colours for Textiles, Society of Dyers and Colourists, Bradford, 1993. ISBN 0-901956-56-2. A. K. R. Choudhury, Textile Preparation and Dyeing, Science Publishers, 2006. ISBN 978-1578084043. M. Clark (ed.), Handbook of Textile and Industrial Dyeing: Principles, Processes and Types of Dyes, 1st ed., Woodhead Publishing Series in Textiles, vol. 116, Woodhead Publishing, 2011. ISBN 978-1845696955. H. Schweppe, Handbuch der Naturfarbstoffe, Nikol-Verlag, 1993. ISBN 978-3933203465. T. W. Goodwin (ed.), Plant Pigments, Academic Press, 1988. ISBN 0-12-289847-8. T. W. Goodwin (ed.), Chemistry and Biochemistry of Plant Pigments, Academic Press, 1976. ISBN 0-12-289901-6. K. M. Davies (ed.), Plant Pigments and their Manipulation, Annual Plant Reviews, vol. 14, Blackwell Publishing Ltd., 2004. ISBN 1-4051-1737-0. P. Kurzmann, Mittelalterliche Glastechnologie, Peter Lang GmbH, Europäischer Verlag der Wissenschaften, Frankfurt/Main, 2004. ISBN 3-631-52994-5. K. H. Wedepohl, Glas in Antike und Mittelalter, Schweizerbart’sche Verlagsbuchhandlung, Stuttgart, 2003. ISBN 3-510-65207-X. M. T. Wypyski, Compositional Study of Medieval Islamic Enameled Glass from The Metropolitan Museum of Art, in Metropolitan Museum Studies in Art, Science, and Technology, vol. 1/2010, pp. 109–132, The Metropolitan Museum of Art, New York, 2010. ISBN 978-1-58839-365-4.

https://doi.org/10.1515/9783110777123-009

862 � Bibliography

[30] [31] [32] [33] [34] [35] [36] [37] [38] [39] [40] [41] [42] [43] [44] [45] [46] [47] [48] [49] [50] [51] [52] [53] [54] [55] [56]

W. Vogel, Glass Chemistry, 2nd ed., Springer, 1992. ISBN 3-540-57572-3. H. Scholze, Glas. Natur, Struktur und Eigenschaften, 3rd ed., Springer, 1988. ISBN 3-540-18977-7. M. B. Volf, Chemical Approach to Glass, Elsevier, Amsterdam, 1984. ISBN 0-444-99635-4. C. R. Bamford, Colour Generation and Control in Glas, Elsevier Scientific Publishing Company, 1977. ISBN 0-444-41614-5. W. A. Weyl, Coloured Glasses, The Society of Glass Technology, Sheffield, 1951. W. Spiegl, Die Geschichte vom Glasmachen 1550 bis 1700, Version Oktober 2002, http://www.glasforschung.info/pageone/pdf/cristallo.pdf, accessed on 2022-06-10. Webseite des Centre International Du Vitrail, Chartres, https://www.centre-vitrail.org/de/dietechnik-der-glasmalerei-/, accessed on 2022-06-10. A. Y. Spengler, Zur Technik der Glasmalerei, Webseite des Bezirksmuseums Wien, Mariahilf, https: //www.bezirksmuseum.at/de/bezirksmuseum_6/bezirksmuseum/glasmuseum_mariahilf/technik/, accessed on 2022-06-10. P. Sciau, Nanoparticles in Ancient Materials: The Metallic Lustre Decorations of Medieval Ceramics, in A. A. Hashim (ed.) The Delivery of Nanoparticles, InTech, 2012. ISBN 978-953-51-0615-9, https: //cdn.intechopen.com/pdfs/36899.pdf, accessed on 2022-06-20. M. A. Pollard, C. Heron, Archaeological Chemistry, 2nd ed., Royal Society of Chemistry, 2008. ISBN 978-0854042623. CPMA Classification of Chemical Descriptions and Usage of the Complex Inorganic Color Pigments, 4th ed., Color Pigments Manufacturers Association, Inc., Alexandria, 2013. W. Noll, Alte Keramiken und ihre Pigmente, E. Schweizerbartsche Verlagsbuchhandlung, Stuttgart, 1991. ISBN 3-510-65145-6. Colour Pigments and Colouring in Ceramics, Italian Ceramic Society SALA, Modena, 2003. S. Stefanov, S. Batschwarov, Keramik-Glasuren, Bauverlag GmbH, Wiesbaden, 1988. ISBN 3-7652-2600-1. R. Hopper, The Ceramic Spectrum. A Simplified Approach to Glaze and Color Development, Krause publications, 2001. ISBN 0-87341-821-2. K. und, W. Lehnhäuser, Keramische Glasuren und ihre Farben, 4th ed., Ritterbach Verlag GmbH, Frechen, 2000. ISBN 3-89314-659-8. P. Gregory, Metal Complexes as Speciality Dyes and Pigments, in J. A. McCleverty (ed.) Comprehensive Coordination Chemistry II, vol. 9, pp. 549–579, Elsevier, 2004. ISBN 0-08-0437486. M. Doerner, T. Hoppe, Malmaterial und seine Verwendung im Bilde, 18th ed., Verlag Enke, Stuttgart, 1994. ISBN 3-363-00800-7. V. Herausgeber, Artist’s Pigments Vol. 1–4, Oxford University Press, New York, 1997. ISBN 0-89468-256-3. N. Eastaugh, V. Walsh, T. Chaplin, R. Siddall, Pigment Compendium. A Dictionary of Historical Pigments, Elsevier Butterworth-Heinemann, Oxford, 2004. ISBN 0-7506-57499. K. Lutzenberger, Künstlerfarben im Wandel: Synthetische organische Pigmente des 20. Jahrhunders und Möglichkeiten ihrer zerstörungsarmen, analytischen Identifizierung, Herbert Utz-Verlag, 2009. ISBN 9-783-83160903-1. T. C. Patton (ed.), Pigments Handbook, vol. 1, Wiley-InterScience, 1973. ISBN 0-471-67123-1. R. D. Harley, Artistś Pigments c. 1600–1835, Butterworth Scientific, 2nd ed., 1982. ISBN 0-408-70945-6. G. R. Rapp, Archaeomineralogy (Natural Science in Archaeology), Springer, Berlin, 2009. ISBN 978-3540785934. H. Howard, Pigments of English Medieval wall Painting, Archetypes Publications Ltd., London, 2003. ISBN 1-873132-48-4. H. Berke, Chemie im Altertum – Die Erfindung von blauen und purpurnen Farbpigmenten, Universitätsverlag Konstanz, Konstanz, 2006. ISBN 978-3-87940-802-3. G. Kremer, Web-Informationen zum Pigment 10400 Vivianit, https://www.kremer-pigmente.com/ de/shop/pigmente/104000-vivianit.html, accessed on 2022-06-20.

Bibliography

[57] [58] [59] [60] [61]

[62]

[63]

[64]

[65] [66] [67] [68] [69] [70] [71]

[72] [73] [74] [75] [76] [77] [78] [79] [80] [81]

� 863

L. Resenberg, Zinnober – zurück zu den Quellen, Anton Sigl, München, 2005. ISBN 3-935643-30-6. G. Kremer, Web-Informationen zum Pigment 10620 Zinnober, https://www.kremer-pigmente.com/ de/shop/pigmente/10620-zinnober.html, accessed on 2022-01-10. forum member “lemmi”, Synthese von Zinnober – ein alchemistisches Experiment, https://illuminachemie.de/viewtopic.php?t=3336, accessed on 2022-06-20. D. L. Douglass, C. Shing, G. Wang, The light-induced alteration of realgar to pararealgar, Amer. Mineral., 77 (1992), 1266–1274. G. Kremer, website informationen for pigment 10200 Azurit, https://www.kremer-pigmente. com/de/shop/pigmente/10200-azurit-natur-standard.html and https://www.kremer-pigmente. com/elements/resources/products/files/10200-10280.pdf, accessed on 2022-06-20 and personal communication. G. Kremer, website informationen for pigment 46400 Gofun Shirayuki, https://www.kremerpigmente.com/de/shop/pigmente/eigene-und-historische-pigmente/46400-gofun-shirayuki.html, accessed on 2014-06-20. J. Klaas, Studien zu Ägyptisch Grün. Bildungsbedingungen und Identifizierung der chemischen Zusammensetzung, S. Gerzer, Kaadener Grün. Lagerstätte, Gewinnung und Verwendung der böhmischen Grünen Erde, Anton Sigl, München, 2006. ISBN 3-935643-30-6. I. Stösser, Rote Farblacke in der Malerei. Herstellung und Verwendung im deutschsprachigen Raum zwischen ca. 1400 und 1850, Diplomarbeit, Institut für Technologie der Malerei der Staatlichen Akademie der Bildenden Künste, Stuttgart, 1985. R. M. Cornell, U. Schwertmann, The Iron Oxides, Wiley-VCH, 2003. ISBN 3-527-30274-3. U. Schwertmann, R. M. Cornell, Iron Oxides in the Laboratory, Wiley-VCH, 2000. ISBN 3-527-29669-7. C. Blänsdorf, Blaue Farblacke in Staffeleimalerei und Skulpturenfassung, Schriftenreihe Konservierung und Restaurierung, Hochschule der Künste, Bern, 2004. ISBN 3-9522804-3-7. R. Chenciner, Madder Red: a History of Luxury and Trade, Curzon, 2001. ISBN 0-7007-1259-3. H. Kühn, Die Pigmente in den Gemälden der Schack-Galerie, in Gemäldekataloge. Band II: Schack-Galerie, Bayerische Staatsgemäldesammlungen, München, 1969. J. H. Hofenk de Graaff, The Colourful Past: Origins, Chemistry and Identification of Natural Dyestuffs, Riggisberg, Abegg-Stiftung, 2004. ISBN 3-905014-25-4. H. Roosen-Runge, Farbe, Farbmittel der abendländischen mittelalterlichen Buchmalerei, in Reallexikon zur Deutschen Kunstgeschichte, vol. 6, pp. 1463–1492, Verlag C. H. Beck, 1986. ISBN 978-3406140068. Webzugriff über https://www.rdklabor.de/wiki/Farbe,_Farbmittel_der_abendl%C3%A4ndischen_ ma._Buchmalerei, accessed on 2022-06-20. I. Sandner, B. Bünsche, H.-P. Schramm, G. Meier, J. Voss, Konservierung von Gemälden und Holzskulpturen, Deutscher Verlag der Wissenschaften, 1990. ISBN 3-326-00524-5. J. Hill Stoner, R. Rushfield (eds.), Conservation of Easel Paintings, Routledge, 2012. ISBN 978-0-7506-8199-5. M. D. Gottsegen, The Painter’s Handbook, Watson-Guptill Publications, 2006. ISBN 0-8230-3496-3. R. Mayer, The Artist’s Handbook of Materials and Techniques, Faber & Faber, 1991. ISBN 978-0571143313. P. Taggart, The Essential Painting Guide, Thunder Bay Press, 2002. ISBN 978-1571455635. H. Speed, The Practice and Science of Drawing, Dover Publications Inc., 1988. ISBN 978-0486228709. J. Trevelyan, Tiefdruckgraphik heute, O. Maier Verlag, Ravensburg, 1966. W. Authenrieth, Neue und alte Techniken der Radierung und Edeldruckverfahren, Authenriet, 6th ed., 2010. ISBN 978-3000356193. F. Hollenberg, W. Rabe (Bearbeiter), Radierung. Ätzkunst und Kupfertiefdruck, O, Maier Verlag, Ravensburg, 1970. ISBN 3-473-61107-7. V. Preissig, Zur Technik der farbigen Radierung und des farbigen Kupferstichs. I. Teil., Karl W. Hiersemann, Leipzip, 1909.

864 � Bibliography

[82] [83] [84] [85] [86] [87] [88] [89] [90] [91] [92] [93] [94] [95] [96] [97] [98] [99] [100] [101] [102] [103] [104] [105] [106] [107]

[108]

R. Simmons, K. Clemson, DuMont’s Handbuch Holz- und Linolschnitt, Dt. Erstausgabe, DuMont Buchverlag, Köln, 1990. ISBN 3-7701-2468-5. J. Stobart, Einfach drucken, Haupt Verlag, 2003. ISBN 3-258-06608-6. B. Fick, B. Grabowski, Printmaking: A Complete Guide to Materials & Process, Laurence King Publishing, 2015. ISBN 978-1780671949. V. Mooncie, Print Making Book: Projects and Techniques in the Art of Hand-printing, Guild of Master Craftsman, Publications Ltd, 2014. ISBN 978-1861089212. B. Butts, L. Hendrix, Painting on Light: Drawings and Stained Glass in the Age of Durer and Holbein, J. Paul Getty Museum, 2000. ISBN 978-0892365784. https://www.getty.edu/publications/resources/ virtuallibrary/089236579X.pdf, accessed on 2022-06-20. F. Wirtl, Hinterglasmalerei, Otto Maier Verlag, Ravensburg, 1980. ISBN 3-473-43052-8. A. K. Leonhard, Hinterglasmalerei, 5th ed., Frech-Verlag Stuttgart, 1977. ISBN 3-7724-0272-0. S. Bretz, Hinterglasmalerei, Klinkhardt & Biermann Verlag, München, 2013. ISBN 978-3-943616-12-5. K.-P. Schäffel, Book Wasserfarben aus Naturmaterialien selber machen, Basler Papiermühle, 2007. P. Seymour, Watercolour Painting, Lee Press, 2001. ISBN 0-9524727-4-0. G. Ziesemann, M. Krampfer, H. Knieriemen, Natürliche Farben. Anstriche und Verputze selber herstellen, 4th ed., AT Verlag, 2000. ISBN 3-85502-523-1. L. Edwards, J. Lawless, Naturfarben Handbuch, edition antis im Ökobuch Verlag, 1st ed., 2003. ISBN 3-922964-92-3. W. Blanke, in Malen mit Pigmenten und variablen Bindemitteln, Knaur, 2003. ISBN 978-3-426-66956-3. H. Arendt, Werkstatt Pflanzenfarben. Natürliche Malfarben selbst herstellen und anwenden, 2nd ed., AT Verlag, 2010. ISBN 978-3-03800-407-3. P. Seymour, Making Soft Pastels Using Dry Pigments, Lee Press, London, 1999. ISBN 0-9524727-2-4. G. P. A. Turner, Introduction to paint chemistry and principles of paint technology, St Edmundsbury Press Ltd, 1988. ISBN 0-412-29440-0. T. J. S. Learner, Analysis of Modern Paints, Getty Publications, 2005. ISBN 978-0-89236-779-5. T. J. S. Learner, P. Smithen, J. W. Krueger M. R, Schilling (eds.), Modern Paints Uncovered, in Proceedings from the Modern Paints Uncovered Symposium, Tate Modern London, 2006, Getty Publications, 2007. ISBN 978-0-89236-906-5. H.-P. Schramm, B. Hering, Historische Malmaterialien und ihre Identifizierung, Bücher des Restaurators, vol. 1, Hrg. U. Schießl, Verlag Enke, Stuttgart, 1995. ISBN 3-432-27041-0. M. F. Striegel, J. Hill, Thin-Layer Chromatography for Binding Media Analysis, The Getty Conservation Institute, Los Angeles, 1996. ISBN 0-89236-390-8. https://www.getty.edu/conservation/ publications_resources/pdf_publications/pdf/thin_layer.pdf, accessed on 2022-06-20. M. P. Colombini, F. Modugno (eds.), Organic Mass Spectrometry in Art and Archaeology, John Wiley & Sons, Ltd., 2009. ISBN 978-0-470-51703-1. J. S. Mills R, White The Organic Chemistry of Museum Objects, Butterworth-Heinemann Ltd., 2nd ed., 1994. ISBN 0-7506-4693-4. J. Riederer, Archäologie und Chemie, in Ausstellung des Rathgen-Forschungslabors SMPK 1987/1988, Staatliche Museen Preußischer Kulturbesitz, Berlin, 1987. ISBN 3-88609-212-7. C. Ehrenfort, Aquarell- und Gouachefarben. Beiträge zu Materialzusammensetzung, Veränderungen, Schäden, Diplomarbeit 1993, Institut für Technologie der Malerei, Staatliche Akademie der Bildenden Künste, Stuttgart. A. Roy, P. Smith (eds.), Painting Techniques. History, Materials and Studio Practice, The International Institute for Conservation of Historic and Artistic Works, 1998. ISBN 0-9500525-8-2. A. Wallert, M. Rey, Historical painting techniques, materials, and studio practice, preprints of a symposium, University of Leiden, the Netherlands, 26–29 June 1995, The Getty Conservation Institute, Los Angeles, 1995. ISBN 0-89236-322-3. https://www.getty.edu/conservation/ publications_resources/pdf_publications/pdf/historical_paintings.pdf, accessed on 2022-06-20. J. Riederer, Kunstwerke chemisch betrachtet, Springer, 1981. ISBN 3-540-10552-2.

Bibliography

[109]

[110] [111]

[112]

[113]

[114] [115] [116] [117] [118] [119] [120] [121] [122] [123] [124] [125] [126] [127] [128] [129] [130] [131] [132] [133] [134] [135]

� 865

A. Bartl, C. Krekel, M. Lautenschlager, D. Oltrogge, Der Liber illuministarum aus dem Kloster Tegernsee, Veröffentlichung des Instituts für Kunsttechnik und Konservierung im Germanischen Nationalmuseum, vol. 8, Franz Steiner Verlag, 2005. ISBN 3-515-08472-X. D. A. Scott, Copper and Bronze in Art. Corrosion, Colorants, Conservation., Getty Conservation Institute, 2002. ISBN 0-89236-638-9. P. Villa, L. Pollarolo, I. Degano, L. Birolo, M. Pasero, C. Biagioni, K. Douka, R. Vinciguerra, J. J. Lucejko, L. Wadley, A Milk and Ochre Paint Mixture Used 49,000 Years Ago at Sibudu, South Africa, PLoS ONE, 10(6), e0131273 (2015). https://doi.org/10.1371/journal.pone.0131273, accessed on 2022-06-20. Spektrum der Wissenschaft (online editors), Frühe Kunstkenner, news from 2000-05-04, Spektrum der Wissenschaft Verlag, https://www.spektrum.de/news/fruehe-kunstkenner/344026, accessed on 2022-06-20. Spektrum der Wissenschaft (online editors), Die Urväter der Kunst, news from 2000-10-23, Spektrum der Wissenschaft Verlag, https://www.spektrum.de/news/die-urvaeter-der-kunst/344691, accessed on 2022-06-20. G. Bataille, Die vorgeschichtliche Malerei. Lascaux oder Die Geburt der Kunst, Editions dÁrt Albert Skira Genf/Klett-Cotta Stuttgart, 1983. ISBN 3-88447-069-8. G. Rietschel, et al., Lascaux. Höhle der Eiszeit, in Ausstellungskatalog des Roemer- und Pelizaeus-Museum Hildesheim, Verlag Philipp von Zabern, Mainz, 1982. ISBN 3-8053-0593-1. M. Lorblanchet, Höhlenmalerei, Jan Thorbecke Verlag, 2nd ed., 2000. ISBN 3-7995-9025-0. R. H. der künstlerichen Techniken. Band 1, Farbmittel, Buchmalerei, Tafel- und Leinwandmalerei., Philipp Reclam jun, GmbH & Co., Stuttgart, 1988. ISBN 3-15-010322-3. H. Roosen-Runge, Tüchleinmalerei, Tafelmalerei, in [117]. H. Roosen-Runge, Buchmalerei, in [117], pp. 55–124. Reclams Handbuch der künstlerischen Techniken. Band 2: Wandmalerei, Mosaik, Philipp Reclam jun. GmbH & Co. Stuttgart, 1990. ISBN 3-15-010345-2. A. Knoepfli, O. Emmenegger, Wandmalerei bis zum Ende des Mittelalters, in [120]. R. H. der künstlerischen Techniken. Band 3, Glas, Keramik, Porzellan, Möbel, Intarsie und Rahmen, Lackkunst, Leder., Philipp Reclam jun, GmbH & Co., Stuttgart, 1986. ISBN 3-15-030015-0. A. Nunn, Handbuch der Orientalistik, 7. Abteilung, Die Wandmalerei und der glasierte Wandschmuck im alten Orient, J. Brill, Leiden, 1988. ISBN 90-04-08428-2. M. A. Corzo, M. Afshar, Art and Eternity. The Nefertari Wall Paintings Conservation Project 1986–-1992, J. Paul Getty Trust, 1993. ISBN 0-89236-130-1. D. Stulik, E. Porta, A. Palet, Analysis of Pigments, Binding Media and Varnishes, in [124], pp. 55–65. S. Hood, The Arts in Prehistoric Greece, Penguin Books, 1990. ISBN 0-14-0561-42-0. R. Higgins, Minoan and Mycenaen Art (World of Art), Verlag Thames & Hudson Ltd, 1981. ISBN 978-0500201848. I. Scheibler, Griechische Malerei der Antike, Beck, München, 1994. ISBN 3-406-38491-9. T. Spiteris, Griechische und Etruskische Malerei, Weltgeschichte der Malerei, vol. 3, Editions Rencontre, Lausanne, 1966. T. Spiteris, Bunte Götter. Die Farbigkeit antiker Skulptur, 4th ed., Ausstellungskatalog des Museums für Kunst und Gewerbe Hamburg, 2007. I. Baldassarre, A. Pontrandolfo, A. Rouveret, M. Salvadori, Römische Malerei, DuMont, 2003. ISBN 3-8321-7210-6. H. Mielsch, Römische Wandmalerei, Wissenschaftliche Buchgesellschaft, Darmstadt, 2001. S. Augusti, I colori pompeiani, De Luca Editore, Istituto Grafico Tiberino, Roma, 1967. M. Exner (ed.), Wandmalerei des frühen Mittelalters. Bestand, Maltechnik, Konservierung, ICOMOS Hefte des Deutschen Nationalkomitees XXIII, 1998. ISBN 3-87490-663-9. J. und, K. Hecht, Die frühmittelalterliche Wandmalerei des Bodenseegebietes, 2 Bände, J. Thorbecke Verlag, Sigmaringen, 1979. ISBN 3-7995-7008-X.

866 � Bibliography

[136] [137] [138] [139] [140] [141] [142] [143] [144] [145] [146] [147] [148] [149] [150] [151] [152] [153] [154] [155] [156] [157] [158] [159] [160] [161] [162]

H. Stampfer, T. Steppan, Die romanische Wandmalerei in Tirol, 1st ed., Schnell & Steiner, 2007. ISBN 978-3795415747. D. Jakobs, Sankt Georg in Reichenau-Oberzell, 2, Band, Konrad-Theiss-Verlag, Stuttgart, 1999. ISBN 978-3806214628. A. Burmester, C. Heilmann, M. F. Zimmermann, Barbizon. Malerei der Natur – Natur der Malerei, Klinghardt & Biermann Verlagsbuchhandlung GmbH, München, 1999. ISBN 3-7814-ß424-2. C. Syre, Tintoretto. Der Gonzaga-Zyklus, Ausstellungskatalog, Bayerische Staatsgemäldesammlungen, Hatje Cantz Verlag, München, 2000. ISBN 3-7757-0887-1. D. Bomford, J. Kirby, A. Roy, A. Rüger, R. White, Art in the Making. Rembrandt, National Gallery Company, London, 2006. ISBN 978-1-85709-356-8. C. Brown, Making & Meaning. Rubens’s Landscapes, National Gallery Publications, London, 1996. ISBN 1-85709-155-8. R. Spronk, More than Meets the Eye: An Introduction to Technical Examination of Early Netherlandish Paintings at the Fogg Art Museum, Harv. Univ. Art Mus. Bull., 5(1) (1996), 1–3, 6–64. I. Gaskell, M. Jonker, Vermeer Studies, in Studies in the History of Art 55, National Gallery of Art, Washington, 1998. ISBN 0-300-07521-9. J. Townsend, Turner’s Painting Techniques, 2nd ed., Tate Gallery London, 1996. ISBN 1-85437-202-5. C. Peres, M. Hoyle, L. van Tilborgh (eds.), A Closer Look: Technical and art-historical Studies on Works by Van Gogh and Gauguin, Rijksmuseum Vincent van Gogh, Cahier Vincent 3, Waanders Publishers, Zwolle, 1991. D. Bomford, J. Dunkerton, D. Gordon, A. Roy, J. Kirby, Art in the Making: Italian Painting before 1400, National Gallery, London, 1989, 1992. ISBN 0-947645-67-5. D. Bomford, J. Kirby, J. Leighton, A. Roy, Art in the Making. Impressionism, National Gallery Company, London, 1990. ISBN 0-300-05035-6. G. Goldberg, B. Heimberg, M. Schawe, Albrecht Dürer. Die Gemälde der Alten Pinakothek, Bayerische Staatsgemäldesammlungen, Edition Braus, München, 1998. ISBN 3-89466-216-6. J. D. Sanders, Pigments for Inkmakers, SITA Technology, London, 1989. ISBN 0-947798-07-02. S. Magdassi, The Chemistry of Inkjet Inks, World Scientific Publishing Co. Pte. Ltd., Singapur, 2010. ISBN 978-981-281-921-8. S. F. Pond, Inkjet Technology and Product Development Strategies, Torrey Pines Research, 2000. ISBN 0-9700860-0-8. J. T. Kunjappu, Essays in Ink Chemistry, Nova Science Publishers Inc., 2001. ISBN 1-59033-111-7. J. A. G. Drake (ed.), Chemical Technology in Printing and Imaging Systems, The Royal Chemical Society of Chemistry, 1993. ISBN 0-85186-655-7. N. Underwood, T. V. Sullivan, The Chemistry and Technology of Printing Inks, D. Van Nostrand Company, New York, 1915. ISBN 978-1103807611, Reprint von BiblioBazaar 2009. C. Maywald, Schreibtinten. Einführung und Übersicht, Robert Wiegner Verlag, 2010. ISBN 978-3-931775-15-5. A. Schopen, Tinten und Tuschen des arabisch-islamischen Mittelalters, Vandenhoeck & Ruprecht GmbH & Co. KG, Göttingen, 2006. ISBN 978-3-525-82541-9. M. Zerdoun Bat-Yehouda, Les Encres Noires au Moyen Âge, Centre National de la recherche Scientifique, Paris, 1983. ISBN 2-222-02972-4. J. Needham, Science and Civilization in China: Volume 5, Chemistry and Chemical Technology, Part 1, Paper and Printing, 6th ed., Cambridge University Press, Cambridge, 2020. ISBN 978-0-521-08690-5. K. S. Clair, The secret Lives of Colour, John Murray (Publishers), 2016. ISBN 978-1-473-63082-6. M. Lyons, Books: A Living History, Thames & Hudson Ltd, 2013. ISBN 978-0500291153. D. N. Carvalho, Forty Centuries of Ink, Project Gutenberg Release #1483, https://onlinebooks.library. upenn.edu/webbin/gutbook/lookup?num=1483, accessed on 2022-06-13. M. B. Woods, M. Woods, Ancient Communication: From Grunts to Graffiti, Lerner Publishing Group, 2001. ISBN 978-0822529965.

Bibliography

[163] [164] [165] [166] [167] [168] [169] [170] [171] [172] [173] [174] [175] [176] [177] [178] [179] [180] [181] [182] [183] [184] [185] [186]

� 867

J. A. Smith, Pen & Ink Book: Materials and Techniques for Today’s Artist, Watson-Guptill Publications Inc., U. S., 1999. ISBN 978-0823039869. Website about materials and techniques of drawings and prints, The Met, New York. https: //www.metmuseum.org/about-the-met/collection-areas/drawings-and-prints/materials-andtechniques/drawing/graphite, accessed on 2022-06-20. S. Ritter, What’s that stuff? Pencils & Pencil lead, Science & Technology, 79(42) (2001), 35. CENEAR 79, 42 p., ISSN 0009-2347, http://pubsapp.acs.org/cen/whatstuff/stuff/7942sci4.html?, accessed on 2022-06-20. S. Ritter, What’s that stuff? Erasers, Science & Technology, 80(50) (2002), 33. CENEAR 80, 50 p. 33, ISSN 0009-2347, https://pubsapp.acs.org/cen/whatstuff/stuff/8050erasers.html, accessed on 2022-06-20. R. H. Leach (ed.), The Printing Ink Manual, 4th ed. (Softcover reprint of 1st ed. 1988), Springer, 2012. ISBN 978-9401170994. W. Baumann, T. Rothardt, Druckereichemikalien: Daten und Fakten zum Umweltschutz, 2nd ed., Springer, 1999. ISBN 3-540-66046-1. V. C. Malshe, M. A. Sikchi, Basics of Paint Technology Part I, 2nd ed., Antar Prakash Centre for Yoga, 2004. ISBN 978-8190329859. Oil and Colour Chemists’ Association, Surface Coatings: Volume 1 Raw Materials and their Usage, 3rd ed., Springer Science & Business Media, 1993. ISBN 0-412-55210-8. N. Equiamicus, Kleines Rezeptbuch der historischen Tinten, Bohmeier Verlag, 2011. ISBN 978-3-89094-593-4. G. Weibel, Miniaturisiertes Dosiersystem zur geregelten Dosierung von Tinte in einem Schreibgerät, Dissertation, Universität Stuttgart, 2008, http://dx.doi.org/10.18419/opus-4142, accessed on 2015-02-09. M. Ek, G. Gellerstedt, G. Henriksson (eds.), Pulp and Paper Chemistry and Technologie. 1. Wood Chemistry and Wood Biotechnology, Walter de Gruyter GmbH, 2009. ISBN 978-3-11-048195-2. M. Ek, G. Gellerstedt, G. Henriksson (eds.), Pulp and Paper Chemistry and Technologie. 2. Pulping Chemistry and Technology, Walter de Gruyter GmbH, 2009. ISBN 978-3-11-048342-0. M. Ek, G. Gellerstedt, G. Henriksson (eds.), Pulp and Paper Chemistry and Technologie. 3. Paper Chemistry and Technology, Walter de Gruyter GmbH, 2009. ISBN 978-3-11-048343-7. M. Ek, G. Gellerstedt, G. Henriksson (eds.), Pulp and Paper Chemistry and Technologie. 4. Papier Products Physics and Technology, Walter de Gruyter GmbH, 2009. ISBN 978-3-11-048346-8. D. Zerler, H.-U. Kästner, Werkstoffkunde für den Papiermacher, 1st ed., Schriftenreihe der Papiermacherschule, vol. 19, Papiermacherschule Gernsbach, 1997. J. C. Roberts (ed.), Paper Chemistry, Blackie & Son Ltd., Glasgow, 1991. ISBN 0-216-92909-1. H. Holik (ed.), Handbook of Paper and Board, WILEY-VCH Verlag Chemie, 2006. ISBN 978-3-527-30997-9. W. Baumann, B. Herberg-Liedtke, Papierchemikalien: Daten und Fakten zum Umweltschutz, Springer, 1994. ISBN 3-540-57593-6. J. P. Casey (ed.), Pulp and Paper, Chemistry and Chemical Technology, Vol. III, 3rd ed., John Wiley & Sons, 1981. ISBN 0-471-03177-1. J. Blechschmidt, Papierverarbeitungstechnik, Carl Hanser Fachbuchverlag, 2013. ISBN 978-3-446-43071-6. J. Blechschmidt (ed.), Taschenbuch der Papiertechnik, 2nd ed., Carl Hanser Fachbuchverlag, 2013. ISBN 978-3-446-43802-6. M. Lewin (ed.), Handbook of Fiber Chemistry, 3.rd ed., CRC Press, 2006. ISBN 0-8247-2565-4. H. Kittel, Lehrbuch der Lacke und Beschichtungen, mehrere Bände S, Hirzel Verlag, Stuttgart, 2007. ISBN 978-3-7776-1014-6. J. L. Keddie, A. F. Routh, Fundamentals of Latex Film Formation, Springer, 2010. ISBN 978-90-481-2844-0.

868 � Bibliography

[187] [188] [189] [190] [191] [192] [193] [194] [195] [196] [197] [198] [199] [200] [201] [202]

[203] [204] [205] [206] [207] [208] [209] [210] [211] [212] [213] [214]

D. Stoye, W. Freitag (eds.), Paints, Coatings and Solvents, 2nd ed., WILEY-VCH, 1998. ISBN 3-527-28863-5. D. Urban, K. Takamura, Polymer Dispersions and Their Industrial Applications, WILEY-VCH, 2002. ISBN 3-527-30286-7. Z. W. Wicks, F. N. Jones, S. P. Pappas, Organic Coatings – Science and Technology, vol. 1: Film Formation, Components and Appearance, John Wiley & Sons, Inc., New York, 1992. ISBN 0-471-61406-8. J. Bieleman, Lackadditive, Wiley-VCH, 1998. ISBN 3-527-28819-9. B. Müller, Lackadditive kompakt erklärt, 2nd ed., Vincentz Network Hannover, Farbe und Lack Bibliothek, 2018. ISBN 978-3-74830-011-4. C. V. Horie, Materials for Conservation, Butterworth & Co. Ltd., 1987. ISBN 0-408-01531-4. H. Briehl, Chemie der Werkstoffe, 2nd ed., Vieweg und Teubner Verlag, 2007. ISBN 978-3835102231. A. Goldschmidt, H.-J. Streitberger, BASF Handbook Basics of Coating Technology, 2nd revised ed., BASF Coatings AG, Münster/Germany, Vincentz Network, 2007. ISBN 978-3-86630-903-6. T. Brock, Lehrbuch der Lacktechnologie, 2nd ed., Vincentz-Verlag Hannover, 1998. ISBN 3-87870-569-7. D. Stoye, W. Freitag, Lackharze: Chemie, Eigenschaften und Anwendungen, Hanser-Verlag, 1996. ISBN 3-446-17475-3. M. El-Sayed Abdel-Raouf, A.-R. Mahmoud Abdul-Raheim, Rosin: Chemistry, Derivatives, and Applications: a review, BAOJ Chem., 4, 039 (2018). PERIODIC TABLE OF ELEMENTS, official website of IUPAC regarding the periodic table of elements, https://iupac.org/what-we-do/periodic-table-of-elements/, accessed on 2022-01-20. A. F. Hollemann, N. Wiberg, Lehrbuch der Anorganischen Chemie, 103rd ed., Walter de Gruyter, Berlin, 2017, vol. 1: ISBN 978-3-11-026932-1, vol. 2: ISBN 978-3-11-049573-7. N. N. Greenwood, A. Earnshaw, Chemie der Elemente, 1. Nachdr. d. 1. ed., VCH, 1990. ISBN 3-527-26169-9. R. Steudel, Chemie der Nichtmetalle, 3rd ed., Walter de Gruyter, Berlin, 2008. ISBN 978-3-11-019448-7. M. Winter, WebElements: the periodic table on the WWW. Binary compounds and complexes, University of Sheffield and WebElements Ltd, UK, http://www.webelements.com/compounds.html, accessed on 2014-06-06. F. Ullmann, Ullmanns Encyclopedia of industrial chemistry, Wiley-VCH Verlag GmbH, Weinheim, 2005, electronic ed. F. Ullmann, Ullmanns Encyklopädie der technischen Chemie, 4th ed., Verlag Chemie, Weinheim, 1972–1984. R. Dittmeyer, W. Keim, G. Kreysa, A. Oberholz, Chemische Technik, Band 7: Industrieprodukte, 5th ed., Wiley-VCH, 2004. ISBN 9783527307722. J. I. Kroschwitz (ed.), Encyclopedia of Chemical Technology, 5th ed., Wiley & Sons, New York, 2004. ISBN 0-471-48494-6. A. Wollrab, Organische Chemie, 2nd ed., Springer, 2002. ISBN 3-540-43998-6. H. P. Latscha, U. Kazmaier, A. Klein, Organische Chemie: Chemie-Basiswissen II, 6th ed., Springer, 2008. ISBN 978-3-540-77106-7. E. Breitmaier, G. Jung, Organische Chemie, 5th ed., Georg Thieme Verlag, 2005. ISBN 3-13-541505-8. W. Kutzelnigg, Einführung in die Theoretische Chemie, WILEY-VCH, 2002. ISBN 3-527-30609-9. M. J. S. Dewar, R. C. Dougherty, The PMO Theory of Organic Chemistry, Plenum Publishing Company, New York, 1975. ISBN 0-306-20010-4. E. Heilbronner, H. Bock, Das HMO-Modell und seine Anwendung. Grundlagen und Handhabung, Verlag Chemie, Weinheim, 1968, Verlagsnummer 68 109. A. Rauk, Orbital Interaction Theory of Organic Chemistry, 2nd ed., Wiley-Interscience, 2001. ISBN 0-471-35833-9. T. A. Albright, J. K. Burdett, M.-H. Whangbo, Orbital Interactions in Chemistry, 2nd ed., John Wiley & Sons, Inc., 2013. ISBN 978-0-471-08039-8.

Bibliography

[215] [216] [217] [218] [219] [220] [221] [222] [223] [224] [225] [226] [227] [228] [229]

[230] [231] [232] [233] [234] [235] [236] [237] [238] [239] [240] [241] [242] [243] [244]

� 869

M. Hesse, H. Meier, B. Zeeh, Spektroskopische Methoden in der organischen Chemie, 7th ed., Thieme Verlag Stuttgart, 2005. ISBN 978-3135761077. G. Lagaly, O. Schulz, R. Zimehl, Dispersionen und Emulsionen, eine Einführung in die Kolloidik feinverteilter Stoffe, Dr. Dietrich Steinkopff Verlag, Darmstadt, 1997. ISBN 3-7985-1087-3. R. M. Pashley, M. E. Karaman, Applied Colloid and Surface Chemistry, John Wiley & Sons Ltd., 2004. ISBN 0-470-86883-X. R. J. Hunter, Introduction to Modern Colloid Science, Oxford University Press Inc., 1993. ISBN 0-19-855387-0. E. J. W. Verwey, J. Th. G. Overbeek, Theory of the Stability of Lyophobic Colloids, Elsevier Publishing Company Inc., 1948. K. S. Birdi, Surface Tension and Interfacial Tension of Liquids, in Handbook of Colloid and Surface Chemistry, CRC Press, 2009. ISBN 978-1-4200-0720-6. T. F. Tadros, Colloids in Paints, Wiley-VCH, 2010. ISBN 978-3-527-31466-9. T. F. Tadros, An Introduction to Surfactants, Walter de Gruyter, 2014. ISBN 978-3-11-031212-6. D. Distler (ed.), Wäßrige Polymerdispersionen, Wiley-VCH, 1999. ISBN 3-527-29587-9. E. Dickinson, D. J. McClements, Advances in Food Colloids, 1st ed., Blackie Academic & Professsional, 1995. ISBN 0-7514-0203-6. M. Ladd, Symmetry and Group Theory in Chemistry, Horwood Chemical Science Series, Horwood Publishing Ltd., 1998. ISBN 1-898563-39-X. M. S. Dresselhaus, G. Dresselhaus, A. Jorio, Group theory: application to the physics of condensed matter, Springer, 2008. ISBN 978-3-540-32897-1. G. Steffen, Farbe und Lumineszenz von Mineralien, Enke Verlag, Stuttgart, 2000. ISBN 3-13-118341-1. R. G. Burns, Mineralogical applications of crystal field theory, Cambridge University Press, 1993. ISBN 0-521-430771. D. Reinen, Kationenverteilung zweiwertiger 3d n -Ionen in oxidischen Spinell-, Granat- und anderen Strukturen, in P. Hemmerich (ed.) Structure and Bonding, Band 7/1970, pp. 114–154, Springer, 1970. ISBN 978-3-540-05022-3. A. B. P. Lever, Inorganic Elektronic Spectroscopy, Elsevier, 1984. ISBN 0-444-42389-3. B. N. Figgis, M. A. Hitchman, Ligand field theory and its applications, Wiley-VCH, 2000. ISBN 0-471-31776-4. F. Kober, Grundlagen der Komplexchemie, Otto Salle Verlag, 1979. ISBN 3-7935-5482-1. J. R. Gispert, Coordination Chemistry, 1st ed., Wiley-VCH, 2008. ISBN 978-3-527-31802-5. L. H. Gade, Koordinationschemie, 1st ed., Wiley-VCH, 1998. ISBN 3-527-29503-8. E. König, S. Kremer, Ligand field energy diagrams, Plenum Press, New York, 1977. ISBN 0-306-30946-7. B. Di Bartolo, Magnetic Ions in Solids, in B. Di (ed.) Bartolo, Optical Properties of Ions in Solids, pp. 15–61, Plenum Press, New York, 1975. ISBN 0-306-35708-9. A. S. Marfunin, Physics of Minerals and Inorganic Materials, Springer, 1979. ISBN 3-540-08982-9. I. B. Bersuker, Electronic Structure and Properties of Transition Metal Compounds, 2nd ed., John Wiley & Sons, 2010. ISBN 978-0-470-18023-5. R. S. Mulliken, W. B. Person, Molecular complexes: a Lecture and Reprint Volume, Wiley, New York, 1969. C. Janiak, T. M. Klapötke, E. Riedel, Moderne anorganische Chemie, Walter de Gruyter, 2003. ISBN 3-11-015672-5. U. Müller, Anorganische Strukturchemie, 6th ed., Vieweg+Teubner Verlag, 2008. ISBN 978-3834806260. A. Krüger, Neue Kohlenstoffmaterialien: Eine Einführung, Vieweg+Teubner Verlag, 2007. ISBN 978-3519005100. I. Glassman, R. A. Yetter, Combustion, 4th ed., Academic Press, 2008. ISBN 978-0-12-088573-2. H. Bockhorn (ed.), Soot Formation in Combustion, Springer, 1994. ISBN 3-540-58398-X.

870 � Bibliography

[245] [246]

[247] [248] [249] [250] [251] [252] [253] [254]

[255]

[256]

[257]

[258]

[259]

[260]

[261]

[262] [263]

H. Richter, J. B. Howard, Formation of polycyclic aromatic hydrocarbons and their growth to soot—a review of chemical reaction pathways, Prog. Energy Combust. Sci., 26 (2000), 565–608. W. Pejpichestakula, A. Frassoldatia, A. Parenteb, T. Faravelli, Kinetic modeling of soot formation in premixed burner-stabilized stagnation ethylene flames at heavily sooting condition, Fuel, 234 (2018), 199–206. J. D. Herdman, J. Houston Miller, Intermolecular Potential Calculations for Polynuclear Aromatic Hydrocarbon Clusters, J. Phys. Chem. A, 112 (2008), 6249–6256. H. Wang, Formation of nascent soot and other condensed-phase materials in flames, Proceedings of the Combustion Institute, 33 (2011), 41–67. R. A. Dobbins, R. A. Fletcher, W. Lu, Laser Microprobe Analysis of Soot Precursor Particles and Carbonaceous Soot, Combustion and Flame, 100 (1995), 301–309. T. Totton, Investigations into the Formation of Soot Particles, PhD Thesis, Department of Chemical Engineering and Biotechnology, University of Cambridge, 2009 T. Totton, A. J. Misquitta, M. Kraft, A quantitative study of the clustering of polycyclic aromatic hydrocarbons at high temperatures, Phys. Chem. Chem. Phys., 14 (2012), 4081–4094. M. Sirignano, A. Ciajolo, A. D’Anna, C. Russo, Particle formation in premixed ethylene-benzene flames: An experimental and modeling study, Combustion and Flame, 200 (2019), 23–31. R. A. Dobbins, R. A. Fletcher, H.-C. Chang, The Evolution of Soot Precursor Particles in a Diffusion Flame, Combustion and Flame, 115 (1998), 285–298. S. Iavarone, L. Pascazio, M. Sirignano, A. De Candia, A. Fierro, L. de Arcangelis, A. D’Anna, Molecular dynamics simulations of incipient carbonaceous nanoparticle formation at flame conditions, Combustion Theory and Modelling, https://10.1080/13647830.2016.1242156 J. W. Martin, M. Salamanca and M. Kraft, Soot inception: Carbonaceous nanoparticle formation in flames, Progress in Energy and Combustion Science, 88, 100956 (2022). https://doi.org/10.1016/j.pecs.2021.100956. F. A. Atiku, A. R. Lea-Langton, K. D. Bartle, J. M. Jones, A. Williams, I. Burns, G. Humphries, Some Aspects of the Mechanism of Formation of Smoke from the Combustion of Wood, Energy Fuels, 31 (2017), 1935–1944. https://doi.org/10.1021/acs.energyfuels.6b02639. R. T. Toledo, Wood Smoke Components and Functional Properties, in D. E. Kramer, L. Brown (eds.) International Smoked Seafood Conference Proceedings, pp. 55–61, Alaska Sea Grant College Program, Fairbanks, 20008. https://doi.org/10.4027/isscp.2008.12. A. C. Eriksson, E. Z. Nordin, R. Nyström, E. Pettersson, E. Swietlicki, C. Bergvall, R. Westerholm, C. Boman, J. H. Pagels, Particulate PAH Emissions from Residential Biomass Combustion: Time-Resolved Analysis with Aerosol Mass Spectrometry, Environ. Sci. Technol., 48 (2014), 7143–7150. https://doi.org/10.1021/es500486j. S. Sinja, A. Jhalani, M. R. Ravi, A. Ray, Modelling of Pyrolysis in Wood: A Review, Indian Institute of Technology Delhi, 2004, https://web.iitd.ac.in/~ravimr/Publications/IndianJournals/sesi-sanjiv.pdf, accessed on 2022-06-20. D. Chen, K. Cen, X. Zhuang, Z. Gan, J. Zhou, Y. Zhang, H. Zhang, Insight into biomass pyrolysis mechanism based on cellulose, hemicellulose, and lignin: Evolution of volatiles and kinetics, elucidation of reaction pathways, and characterization of gas, biochar and bio-oil, Combust. Flame, 242, 112142 (2022). S. D. Stefanidis, K. G. Kalogiannis, E. F. Iliopoulou, C. M. Michailof, P. A. Pilavachi, A. A. Lappas, A study of lignocellulosic biomass pyrolysis via the pyrolysis of cellulose, hemicellulose and lignin, J. Anal. Appl. Pyrolysis, 105 (2014), 143–150. https://doi.org/10.1016/j.jaap.2013.10.013. ISSN 0165-2370. P. Kazimierski, D. Kardas, Influence of Temperature on Composition of Wood Pyrolysis Products, Drvna Industrija, 2017. https://doi.org/10.5552/drind.2017.1714. E. Khalimov, et al., Some peculiarities of burnt birch wood pyrolysis, IOP Conf. Ser. Earth Environ. Sci., 316 (2019), 012019.

Bibliography

[264] [265] [266] [267] [268] [269] [270] [271] [272] [273] [274] [275] [276] [277] [278] [279] [280] [281] [282] [283] [284] [285] [286] [287] [288] [289] [290] [291] [292] [293] [294] [295]

� 871

H. W. Emmons, A. Atreya, The science of wood combustion, Proc. Indian Acad. Sci., Eng. Sci., 5(4) (1982), 259–268. F. Shafizadeh, Introduction to pyrolysis of biomass, J. Anal. Appl. Pyrolysis, 3 (1982), 283–305. N. S. Hassan, A. A. Jalil, C. N. C. Hitam, D. V. N. Vo, W. Nabgan, Biofuels and renewable chemicals production by catalytic pyrolysis of cellulose: a review, Environ. Chem. Lett., 18 (2020), 1625–1648. https://doi.org/10.1007/s10311-020-01040-7. H. Kawamoto, Lignin pyrolysis reactions, J. Wood Sci., 63 (2017), 117–132. https://doi.org/10.1007/s10086-016-1606-z. V. Gargiulo, A. I. Ferreiro, P. Giudicianni, S. Tomaselli, M. Costa, R. Ragucci, M. Alfe, Insights about the effect of composition, branching and molecular weight on the slow pyrolysis of xylose-based polysaccharides, J. Anal. Appl. Pyrolysis, 161, 105369 (2022). W. Zhou, W. Li, R. Mabon, L. J. Broadbelt, A Critical Review on Hemicellulose Pyrolysis, Energy Technol. 5 (2017), 52–79. https://doi.org/10.1002/ente.201600327. J. Reinhold, Quantentheorie der Moleküle, 3rd ed., Teubner Verlag, 2006. ISBN 978-3-8351-0037-4. O. Krätz, 7000 Jahre Chemie, Nikol Verlag, 2000. ISBN 978-3933203205. H. Hager, W. Blaschek, K. Keller, Hagers Handbuch der Pharmazeutischen Praxis, vol. 3, Springer, 1998. ISBN 978-3540616191. E. Breitmaier, Terpene: Aromen, Düfte, Pharmaka, Pheromone, Teubner Studienbücher Chemie, 1999. ISBN 3-519-03548-0. G. Michal (ed.), Biochemical Pathways, Spektrum, Akademischer Verlag, 1999. ISBN 3-86025-239-9. G. Richter, Stoffwechselphysiologie der Pflanzen, 4th ed., Georg Thieme Verlag, Stuttgart, 1981. ISBN 3-13-442004-X. H.-W. Heldt, B. Piechulla, Plant Biochemistry, 4th ed., Academic Press, 2011. ISBN 978-0-12-384986-1. G. Tegge (ed.), Stärke und Stärkederivate, 3rd ed., Behr’s Verlag, 2004. ISBN 3-89947-075-3. J. N. BeMiller, Carbohydrate Chemistry for Food Scientists. AACC International Inc., St. Paul, 2007, ISBN 978-1-891127-53-3. A. M. Stephen, G. O. Philips, P. A. Williams, Food Polysaccharides and Their Applications, Marcel Dekker Inc., 2006. ISBN 978-0824759223. H.-D. Belitz, W. Grosch, Lehrbuch der Lebensmittelchemie, vol. 4, Springer, 1992. ISBN 3-540-55449-1. W. Ternes, Lebensmittel-Lexikon, Behrs Verlag, 2005. ISBN 978-3899471656. N. P. Wong, R. Jenness, Fundamentals of Dairy Chemistry, Aspen Publishers, 1999. ISBN 0-8342-1360-5. P. Walstra, R. Jenness, Dairy Chemistry and Physics, J. Wiley & Sons, 1984. ISBN 0-471-09779-9. P. Nuhn, Naturstoffchemie, 3rd ed., S. Hirzel Verlag Stuttgart, 1997. ISBN 3-7776-0613-8. Y. Margalit, Concepts in Wine Chemistry, The Wine Appreciation Guild, San Francisco, 2004. ISBN 1-891267-74-4. J. F. Zayas, Functionality of Proteins in Food, Springer, 1997. ISBN 3-540-60252-6. J. C. Cheftel, J. L. Cuq, D. Lorient, Lebensmittelproteine, Behr’s Verlag, 1992. ISBN 3-86022-071-3. S. Damodaran, A. Paraf (eds.), Food Proteins and their Applications, Marcel Dekker Inc., New York, 1997. ISBN 0-8247-9820-1. A. Gennadios (ed.), Protein-based Films and Coatings, CRC Press, 2002. ISBN 978-1587161070. T. Yamamoto, L. R. Juneja, H. Hatta, M. Kim (eds.), Hen Eggs. Their Basic and Applied Science, CRC Press, 1997. ISBN 0-8493-4005-5. W. Ternes, L. Acker, S. Scholtyssek, Ei und Eiprodukte, Verlag Paul Parey, 1994. ISBN 3-489-63114-5. P. Fratzl (ed.), Collagen. Structure and Mechanics, Springer, 2008. ISBN 978-0-387-73905-2. E. N. Frankel, Lipid Oxidation, The Oily Press, Dundee, 1998. ISBN 0-9514171-9-3. H. W.-S. Chan (ed.), Autoxidation of Unsatured Lipids, Academic Press Inc., London, 1987. ISBN 0-12-167630-7. A. Brossi (Hrg.), The Alkaloids: Chemistry and Pharmacology: 39, Academic Press Inc., 1990. ISBN 978-0124695399.

872 � Bibliography

[296] [297] [298] [299] [300] [301] [302] [303] [304] [305] [306] [307] [308] [309] [310] [311] [312]

[313] [314] [315] [316] [317] [318] [319] [320] [321] [322] [323] [324] [325] [326] [327] [328]

E. Haslam, Practical Polyphenolics. From Structure to Molecular Recognition and Physiological Action, Cambridge University Press, 1998. ISBN 0-521-46513-3. M. Luckner, Secondary Metabolism in Microorganisms, Plants, and Animals, 3rd ed., Springer, 1990. ISBN 3-540-50287-4. B. A. Bohm, Introduction to Flavonoids, Harwood Academic Publishers, 1998. ISBN 90-5702-353-9. W. I. Taylor, A. R. Battersby, Oxidative Coupling of Phenols, Marcel Dekker Inc., New York, 1967. H. A. Stafford, Flavonoid Metabolism, CRC Press Inc, 1990. ISBN 978-0849360855. F. Constabel, I. K. Vasil (eds.), Cell Culture and Somatic Cell Genetics of Plants, vol. 5: Phytochemicals in Plant Cell Cultures, Academic Press Inc., 1988. ISBN 0-12-715005-6. D. S. Seigler, Plant Secondary Metabolism, Kluwer Academic Publishers, 1998. ISBN 0-412-01981-7. F. A. Tomás-Barberán, R. J. Robins, Phytochemistry of Fruit and Vegetables, Proceedings of the Phytochemical Society of Europe, Clarendon Press, Oxford, 1997. ISBN 0-19-857790-7. K. R. Markham, Techniques of Flavonoid Identification, Academic Press, 1982. ISBN 0-12-472680-1. G. Prota, Melanins and Melanogenesis, Academic Press Inc., 1992. ISBN 0-12-565970-9. D. W. Lee, Nature’s Palette: the Science of Plant Color, The University of Chicago Press, 2007. ISBN 0-226-47052-0. A. S. Diamond, D. S. Weiss, Handbook of Imaging Materials, 2nd. ed., Marcel Dekker Inc., 2002. ISBN 0-8247-8903-2. M. Zander, Polycyclische Aromaten, Teubner Studienbücher, Stuttgart, 1995. ISBN 3-519-03537-5. I. Gutman, S. J. Cyvin, Introduction to the Theory of Benzenoid Hydrocarbons, Springer, 1989. ISBN 3-540-51139-3. E. Clar, Polycyclic Hydrocarbons, 2 volumes, Academic Press Inc., London, 1964. D. Dolphin, The Porphyrins. Vol. III. Physical Chemistry, Part A, Academic Press Inc., 1978. ISBN 0-12-220103-5. C. C. Leznoff, A. B. P. Lever (eds.), Phthalocyanines. Properties and Applications, volume I, VCH, 1989. ISBN 3-527-26955-X. Volume II: VCH, 1993. ISBN 3-527-89544-2. volume III: VCH, 1993. ISBN 3-527-89638-4. J. Jiang (ed.), Functional phthalocyanine molecular materials, Springer, 2010. ISBN 978-3-642-04751-0. K. Nassau, The physics and chemistry of color, 2nd ed., Wiley serie in pure and applied optics, 2001. ISBN 0-471-39106-9. M. Fox, Optical Properties of Solids, Oxford University Press, New York, 2008. ISBN 978-0-19-850612-6. E. Hecht, Optik, 7th ed., Walter de Gruyter GmbH, 2018. ISBN 978-3-11-052664-6. J. G. Solé, L. E. Bausá, D. Jaque, An Introduction to the Optical Spectroscopy of Inorganic Solids, John Wiley & Sons, Ltd, 2005. ISBN 0-470-86886-4. D. L. Greenaway, G. Harbeke, Optical Properties and Band Structure of Semiconductors, 1st ed., Pergamon Press, Oxford, 1968. M. Aven, J. S. Prener (eds.), Physics and Chemistry of II-VI Compounds, North-Holland Publishing Company, Amsterdam, 1967. S. Adachi, Properties of Semiconductor Alloys, Wiley, 2009. ISBN 978-0-470-74369-0. R. Sauer, Halbleiterphysik, Oldenbourg Wissenschaftsverlag, München, 2009. ISBN 978-3-486-58863-7. M. Grundmann, The Physics of Semiconductors, 2nd ed., Springer, 2010. ISBN 978-3-642-13883-6. B. Sapoval, C. Hermann, Physics of Semiconductors, Springer, New York, 2003. ISBN 0-387-40630-1. S. Hunklinger, Festkörperphysik, Oldenbourg, 2007. ISBN 978-3-486-57562-0. C. Kittel, Einführung in die Festkörperphysik, Oldenbourg, München, 1999. ISBN 3-486-23843-4. A.-B. Chen, A. Sher, Semiconductor Alloys, Plenum Press, New York, 1995. ISBN 0-306-45052-6. M. L. Cohen, J. R. Chelikowsky, Electronic Structure and Optical Properties of Semiconductors, 2nd ed., Springer, 1989. ISBN 3-540-51391-4. L. I. Berger, Semiconductor materials, CRC Press Inc., 2000. ISBN 0-8493-8912-7.

Bibliography

[329] [330] [331] [332] [333] [334] [335] [336] [337] [338] [339] [340] [341] [342] [343] [344] [345] [346] [347] [348] [349] [350] [351] [352] [353] [354] [355]

� 873

P. K. Basu, Theory of Optical Processes in Semiconductors, Oxford University Press Inc., 1997. ISBN 0-19-851788-2. M. Kerker, The Scattering of Light and Other Electromagnetic Radiation, Academic Press, 1969. U. Kreibig, M. Vollmer, Optical Properties of Metal Clusters, Springer, 1995. ISBN 3-540-57836-6. M. Quinten, Optical Properties of Nanoparticle Systems, Wiley-VCH, 2011. ISBN 978-3-527-41043-9. H. Raether, Surface Plasmons on Smooth and Rough Surfaces and on Gratings, Springer Tracts in Modern Physics, vol. 111, Springer, 1988. ISBN 3-540-17363-3. H. Raether, Excitation of Plasmons and Interband Transitions by Electrons, Springer Tracts in Modern Physics, vol. 88, Springer, 1980. ISBN 3-540-09677-9. W. Kleber, Einführung in die Kristallographie, 16th ed., VEB Verlag Technik, Berlin, 1985. P. Laven, computer programm Mieplot, http://www.philiplaven.com/mieplot.htm, accessed on 2022-06-20. M. Thompson, computer programm ArgusLab, http://www.arguslab.com/arguslab.com/Welcome. html, accessed on 2022-09-10. D. Greig, Elektronen in Metallen und Halbleitern, R. Oldenbourg Verlag, 1972. ISBN 3-486-33551-0. G. Mie, Beiträge zur Optik trüber Medien, speziell kolloidaler Metallösungen, Ann. Phys., 25(3) (1908), 377–445. P. Walstra, Approximation formulae for the light scattering coefficient of dielectric spheres, Br. J. Appl. Phys., 15 (1964), 1545–1552. M. Halik, 2,2-Difluor-1,3,2-(2H)-dioxaborine als Bausteine zur Darstellung von langwellig absorbierenden Methinfarbstoffen, Dissertation der Universität Halle-Wittenberg, http://sundoc.bibliothek.unihalle.de/diss-online/98/99H017/prom.pdf, accessed on 2022-06-20. J. S. Dewar, Modern Theories of Colour, Spec. Publ., Geochem. Soc., 4 (1956), 64–82. Y. Tanabe, S. Sugano, On the absorption spectra of complex ions I–III, J. Phys. Soc. Jpn., 9(5) (1954), 753. K.-A. Kovar, W. Mayer, Elektronen-Donator-Acceptor-Komplexe, Pharmazie Unserer Zeit, 8(2) (1979), 46–53. G. Briegleb, “Charge-Transfer”-Spektren von Komplexen zwischen Neutralmolekülen, Pure Appl. Chem., 4(1) (1962), 105–120. W. Kaim, S. Ernst, S. Kohlmann, Farbige Komplexe: das Charge-Transfer-Phänomen, Chem. Unserer Zeit, 21(2) (1987), 50–58. G. Briegleb, J. Czekalla, Elektronenüberführung durch Lichtabsorption und -emission in Elektronen-Donator-Acceptor-Komplexen, Angew. Chem., 72 (1960), 401–413. R. G. Burns, Intervalence Transitions in Mixed-Valence Minerals of Iron and Titanium, Annu. Rev. Earth Planet. Sci., 9 (1981), 345–383. A. G. Marinopoulos, L. Reining, A. Rubio, V. Olevano, Ab initio study of the optical absorption and wave-vector-dependent dielectric response of graphite, Phys. Rev. B, 69, 245419 (2004). https://doi.org/10.1103/PhysRevB.69.245419. A. K. Solanki, A. Kashyap, T. Nautiyal, S. Auluck, M. A. Khan, Band structure and optical properties of graphite, Solid State Commun., 100(9) (1996), 645–649. R. J. Papoular, R. Papoular, Some optical properties of graphite from IR to millimetric wavelengths, Mon. Not. R. Astron. Soc., 443(4) (2014), 2974–2982. https://doi.org/10.1093/mnras/stu1348. L. P. Bouckaert, R. Smoluchowski, E. Wigner, Theory of Brillouin Zones and Symmetry Properties of Wave Functions in Crystals, Phys. Rev., 50 (1936), 58–67. A. P. Alivisatos, Semiconductor Clusters, Nanocrystals, and Quantum Dots, Science, 271(5251) (1996), 933–937. A. P. Alivisatos, Perspectives on the Physical Chemistry of Semiconductor Nanocrystals, J. Phys. Chem., 100 (1996), 13226–13239. C. B. Murray, D. J. Norris, M. G. Bawendi, Synthesis and Characterization of Nearly Monodisperse CdE (E = S, Se, Te) Semiconductor Nanocrystallites, J. Am. Chem. Soc., 115 (1993), 8706–8715.

874 � Bibliography

[356] [357] [358] [359] [360] [361] [362] [363] [364] [365] [366] [367] [368] [369] [370]

[371] [372] [373] [374] [375] [376] [377] [378] [379]

H. Weller, Kolloidale Halbleiter-Q-Teilchen: Chemie im Übergangsbereich zwischen Festkörper und Molekül, Angew. Chem., 105 (1993), 43–55. A. Riss, O. Diwald, Gestalten in der Nanowelt. Partikel-Morphologien und Festkörper-Eigenschaften, Chem. Unserer Zeit, 43 (2009), 84–92. R. A. Abram, G. J. Rees, B. L. H. Wilson, Heavily doped semiconductors and devices, Adv. Phys., 27(6) (1978), 799–892. A. L. Efros, Density of states and interband absorption of light in strongly doped semiconductors, Sov. Phys., Usp., 16(6) (1974), 789–805. H. C. Casey, F. Stern, Concentration-dependent absorption and spontaneous emission of heavily doped GaAs, J. Appl. Phys., 47 (1976), 631–643. H. C. Casey, D. D. Sell, K. W. Wecht, Concentration dependence of the absorption coefficient for n- and p-type GaAs between 1.3 and 1.6 eV, J. Appl. Phys., 46 (1975), 250–257. F. Bassani, D. Brust, Effect of Alloying and Pressure on the Band Structure of Germanium and Silicon, Phys. Rev., 131(4) (1963), 1524–1529. R. Braunstein, A. R. Moore, F. Herman, Intrinsic Optical Absorption in Germanium-Silicon Alloys, Phys. Rev., 109(3) (1958), 695–710. N. R. J. Poolton, K. B. Ozanyan, J. Wallinga, A. S. Murray, L. Botter-Jensen, Electrons in feldspar II: a consideration of the influence of conduction band-tail states on luminescence processes, Phys. Chem. Miner., 29 (2002), 217–225. C. X. Wu, Y. M. Lu, D. Z. Shen, X. W. Fan, Effect of Mg content on the structural and optical properties of Mgx Zn1−x O alloys, Chin. Sci. Bull., 55(1) (2010), 90–93. J. Dong, D. A. Drabold, Band-tail states and the localized-to-extended transition in amorphous diamond, Phys. Rev. B, 54 (1996), 10284–10287. N. A. Noor, N. Ikram, S. Ali, S. Nazir, S. M. Alay-e-Abbas, A. Shaukat, First-principles calculations of structural, electronic and optical properties of Cdx Zn1−x S alloys, J. Alloys Compd., 507 (2010), 356–363. S. Mecabih, N. Amrane, B. Belgoumène, H. Aourag, Opto-electronic properties of the ternary alloy Hg1−x Cdx Te, Phys. A, 276 (2000), 495–507. S.-H. Weia, S. B. Zhang, A. Zunger, First-principles calculation of band offsets, optical bowings, and defects in CdS, CdSe, CdTe, and their alloys, J. Appl. Phys., 87(3) (2000), 1304–1311. L. Visscher, K. G. Dyall, Dirac-Fock atomic electronic structure calculations using different nuclear charge distributions, At. Data Nucl. Data Tables, 67 (1997), 207–224, see also link given in text or http://www.chem.vu.nl/~visscher/FiniteNuclei/FiniteNuclei.htm, particularly orbital energies: http://www.chem.vu.nl/~visscher/FiniteNuclei/Table3.html (all no longer accessible). M. Klessinger, Konstitution und Lichtabsorption organischer Farbstoffe, Chem. Unserer Zeit, 12(1) (1978), 1–11. J. W. Armit, R. Robinson, Polynuclear Heterocyclic Aromatic Types. Part II. Some Anhydronium Bases, J. Chem. Soc. (1925), 1604. E. Clar, M. Zander, 1:12-2:3-10:11-Tribenzoperylen, J. Chem. Soc. (1958), 1861. H. B. Klevens, J. R. Platt, Spectral Resemblances of Cata-Condensed Hydrocarbons, J. Chem. Phys., 17 (1949), 470. J. R. Platt, Classification of Spectra of Cata-Condensed Hydrocarbons, J. Chem. Phys., 17 (1949), 484. E. Clar, Ein einfaches Prinzip des Aufbaues der aromatischen Kohlenwassenstoffe und ihrer Absorptionsspektren (Aromatische Kohlenwasserstoffe, 20. Mitteil.), Ber. Dtsch. Chem. Ges., 69 (1936), 607. K. Toyota, J-y. Hasegawa, H. Nakatsuji, SAC-CI study of the excited states of free base tetrazaporphin, Chem. Phys. Lett., 250 (1996), 437–442. K. Toyota, J-y. Hasegawa, H. Nakatsuji, Excited States of Free Base Phthalocyanine Studied by the SAC-CI Method, J. Phys. Chem. A, 101 (1997), 446–451. M. Gouterman, Optical Spectra and Electronic Structure of Porphyrins and Related Rings, in [311], pp. 1–166.

Bibliography

[380] [381] [382] [383] [384] [385] [386] [387] [388] [389]

[390] [391] [392] [393] [394] [395] [396] [397] [398] [399] [400] [401] [402] [403] [404]

� 875

M. J. Stillman, T. Nyokong, Absorption and Magnetic Circular Dichroism Spectral Properties of Phthalocyanines Part 1: Complexes of the Dianion PC(-2), in [312], vol. I, pp. 133–290. N. Kobayashi, Synthesis and Spectroscopic Properties of Phthalocyanine Analogues, in [312], vol. II, pp. 97–162. M. J. Stillman, Absorption and Magnetic Circular Dichroism Spectral Properties of Phthalocyanines Part 2: Ring-Oxidized and Ring-Reduced Complexes, in [312], vol. III, pp. 227–296. T. Nyokong, Electronic Spectral and Electrochemical Behavior of Near Infrared Absorbing Metallophthalocyanines, in [313], pp. 45–88. M. Gouterman, Study of the Effects of Substitution on the Absorption Spectra of Porphin, J. Chem. Phys., 30 (1959), 1139. M. Gouterman, Spectra of Porphyrins, J. Mol. Spectrosc., 6 (1961), 138–163. M. Gouterman, G. H. Wagnière, L. C. Snyder, Spectra of Porphyrins. Part II. For Orbital Model, J. Mol. Spectrosc., 11 (1963), 108–127. M. Kasha, H. R. Rawls, M. A. El-Bayoumi, The Exciton Model in Molecular Spectroscopy, Pure Appl. Chem., 11(3–4) (1965), 371–392. M. B. Robin, P. Day, Mixed Valence Chemistry – a Survey and Classification, Adv. Inorg. Chem. Radiochem., 10 (1967), 247–433. C. Miliani, L. Monico, M. J. Melo, S. Fantacci, E. M. Angelin, A. Romani and K. Janssens, Photochemistry of ArtistsQ Dyes and Pigments: Towards Better Understanding and Prevention of Colour Change in Works of Art, Angew. Chem. Int. Ed., 57 (2018), 7324–7334. https://doi.org/10.1002/anie.201802801. U. Karl, Gemälde verwelken wie Blumen, Chem. Unserer Zeit (2022). https://doi.org/10.1002/ciuz.202100021. R. J. Gettens, H. Kühn, W. T. Chase, Lead White, Stud. Conserv., 12(4) (1967), 125–139. R. zum Hagen, Darstellung des Saftgrüns, Pharmazeutisches Zentralblatt für 1831, 2. Band (Nr. 26 bis Nr. 55), Nr. 32, pp. 513–514. F. Hund, Abhängigkeit der Farbe roter Eisen(III)-oxide von Teilchengrösse und Teilchengrössenverteilung, Chem. Ing. Tech., 38(4) (1966), 423–428. L. Resenberg, Das Pigment Zinnober, Restauro, 5 (2005), 362–372. M. Spring, R. Grout, The Blackening of Vermilion: An Analytical Study of the Process in Paintings, Natl. Gallery Tech. Bull., 23 (2002), 50–61. ISBN 978-1-85709-941-9. C. Hogan and F. Da Pieve, Colour degradation of artworks: an ab initio approach to X-ray, electronic and optical spectroscopy analyses of vermilion photodarkening, J. Anal. At. Spectrom., 30 (2015), 588. J. Kirby, M. Spring, C. Higgitt, The Technology of Eighteenth- and Nineteenth-Century Red Lake Pigments, Natl. Gallery Tech. Bull., 28 (2007), 69–95. ISBN 978-1-85709-357-5. J. Kirby, M. Spring, C. Higgitt, The Technology of Red Lake Pigment Manufacture: Study of the Dyestuff Substrate, Natl. Gallery Tech. Bull., 26 (2005), 71–87. ISBN 1-85709-341-0. J. Kirby, R. White, The Identification of Red Lake Pigment Dyestuffs and a Discussion of their Use, Natl. Gallery Tech. Bull., 17 (1996), 56–79. ISBN 1-85709-113-2. J. Kirby, A Spectrophotometric Method for the Identification of Lake Pigment Dyestuffs, Natl. Gallery Tech. Bull., 1 (1977), 35–48. D. Saunders, J. Kirby, Light-induced Colour Changes in Red and Yellow Lake Pigments, Natl. Gallery Tech. Bull., 15 (1994), 79–97. ISBN 1-85709-049-7. J. Kirby, D. Saunders, Sixteenth- to Eighteenth-Century Green Colours in Landscape And Flower Paintings: Composition and Deterioration, in [106], pp. 155–159. A. Burnstock, The Fading of the Virgin’s Robe in Lorenzo Monaco’s ‘Coronation of the Virgin’, Natl. Gallery Tech. Bull., 12 (1988), 58–65. ISBN 0-947645-63-2. R. White, Brown and Black Organic Glazes, Pigments and Paints, Natl. Gallery Tech. Bull., 10 (1986), 58–71. ISBN 0-947645-10-1.

876 � Bibliography

[405] [406] [407] [408] [409] [410] [411] [412] [413] [414] [415] [416] [417] [418] [419] [420] [421] [422] [423] [424] [425] [426] [427]

[428] [429] [430] [431] [432]

J. WinterReviewed, The Characterization of Pigments Based on Carbon, Stud. Conserv., 28(2) (1983), 49–66. J. Lehmann, Seit wann wird mit Zinkweiß gemalt? Restauro, 5 (2000), 356–361. C. Rötter, Auripigment, Restauro, 6 (2003), 408–413. A. Wallert, Orpiment and Realgar, Restauro, 4 (1984), 45–57. H. Kühn, Lead-Tin-Yellow, Stud. Conserv., 13(1) (1968), 7–33. E. Martin, A. Duval, Le deux varietes de jaune de plomb et détain. Etude chronologique, Stud. Conserv., 35(3) (1990), 117–136. M.-C. Corbeil, J.-P. Charland, E. A. Moffatt, The Characterization of Cobalt Violet Pigments, Stud. Conserv., 47(4) (2002), 237–249. H. Pfleiderer, Cadmiumzinnober, eine neue Klasse von Pigmenten, Fette Seifen Anstrichm., 58 (1956), 12. R. J. Gettens, E. W. Fitzhugh, Malachite and Green Verditer, Stud. Conserv., 19(1) (1974), 2–23. A. Burmester, L. Resenberg, Von Berggrün, Schiefergrün und Steingrün aus Ungarn, Restauro, 3 (2003), 180–187. H. Kühn, Verdigris and Copper Resinat, Stud. Conserv., 15(1) (1970), 12–36. R. Woudhuysen-Keller, Aspects of Painting Technique in the Use of Verdigris and Copper Resinate, in [107], pp. 65–69. H. Andreas, Schweinfurter Grün – das brillante Gift, Chem. Unserer Zeit, 1 (1996), 23–31. C. Marrder, Schweinfurter Grün. Teil 1: Geschichte eines künstlichen Farbpigments, Restauro, 5 (2004), 326–331. C. Marrder, Schweinfurter Grün. Teil 2: Eigenschaften, naturwissenschaftliche Untersuchungen, Restauro, 8 (2004), 543–547. W. Feitknecht, K. Maget, Zur Chemie und Morphologie der basischen Salze zweiwertiger Metalle. XIV. Die Hydroxychloride des Kupfers, Helv. Chim. Acta, 32 (1949), 1639–1653. G. Banik, Untersuchungen über den Abbau von Papier durch Kupferpigmente. Ein Beitrag zur Aufklärung des Kupferfraßes in graphischen Kunstwerken, Habilitationsschrift, Institut für Farbenlehre und Farbenchemie an der Akademie der Bildenden Künste, Wien, 1982. F. Schweizer, B. Mühlethaler, Einige grüne und blaue Kupferpigmente, Farbe und Lack, 74(12) (1968), 1159–1173. F. Ellwanger-Eckel, Herstellung und Verwendung künstlicher grüner und blauer Kupferpigmente in der Malerei, Diplomarbeit (1979), Institut für Technologie der Malerei, Staatliche Akademie der Bildenden Künste, Stuttgart. B. Mühlethaler, J. Thissen, Smalt, Stud. Conserv., 14(2) (1969), 47–61. J. A. Darrah, Connections and Coincidences: Three Pigments, in [107], pp. 70–77. M. Spring, C. Higgitt, D. Saunders, Investigation of Pigment-Medium Interaction Processes in Oil Paint containing Degraded Smalt, Natl. Gallery Tech. Bull., 26 (2005), 50–61. ISBN 1-85709-341-0. J. J. Boon, K. Keune, J. van der Weerd, M. Geldof, J. R. J. van Asperen de Boer, Imaging Microspectroscopic, Secondary Ion Mass Spectrometric and Electron Microscopic Studies on Discoloured and Partially Discoloured Smalt in Cross-Sections of 16th Century Paintints, Chimia, 55(11) (2001), 952–960. L. Robinet, M. Spring, S. Pages, D. Vantelon, N. Trcera, Investigation of the Discoloration of Smalt Pigment in Historic Paintings by Micro-X-ray Absorption Spectroscopy at the Co K-Edge, Anal. Chem., 83(13) (2011), 5145–5152. A. Ludi, Berliner Blau, Chem. Unserer Zeit, 22(4) (1988), 123–127. H. J. Buser, D. Schwarzenbach, W. Petter, A. Ludi, The Crystal Structure of Prussian Blue: Fe4 [Fe(CN)6 ]3 ⋅ xH2 O, Inorg. Chem., 16(11) (1977), 2704–2710. A. Börner, Berlinerblau für die Retusche? Restauro, 5 (2002), 327–331. J. Kirby, D. Saunders, Fading and Colour Change of Prussian Blue: Methods of Manufacture and the Influence of Extenders, Natl. Gallery Tech. Bull., 25 (2004), 73–99. ISBN 1-85709-320-8.

Bibliography

[433] [434] [435] [436] [437] [438] [439]

[440] [441] [442] [443] [444] [445]

[446]

[447]

[448] [449] [450] [451] [452] [453] [454] [455] [456] [457] [458] [459]

� 877

J. Kirby, Fading and Colour Change of Prussian Blue: Occurrences and Early Reports, Natl. Gallery Tech. Bull., 14 (1993), 62–71. ISBN 1-85709-033-0. M. Prinzmeier, Dem Bronzieren auf der Spur, Farbe und Lack, 118(2) (2012), 24–27. H. Berke, Chemie im Altertum: die Erfindung von blauen und purpurnen Farbpigmenten, Angew. Chem., 114(14) (2002), 2595–2600. L. A. Polette-Niewold, et al., Organic/inorganic complex pigments: Ancient colors Maya Blue, J. Inorg. Biochem., 101 (2007), 1958–1973. G. Chiari, et al., Pre-columbian nanotechnology: reconciling the mysteries of the maya blue pigment, Appl. Phys. A, 90 (2008), 3–7. P. Kempter, G. Wilker, Blaues bleibt Blau. Dioxazin-Blau: ein neues lösemittelechtes Pigment, Farbe und Lack, 107(11) (2001), 29–31. N. Tozzi, Glaze and Body Pigments and Stains in the Ceramic Tile Industry, http://digitalfire.com/4sight/ education/glaze_and_body_pigments_and_stains_in_the_ceramic_tile_industry_342.html, accessed on 2011-07-05. J. Riederer, Keramik, in [104], pp. 175–201. P. Kleinschmit, Ein Kapitel angewandte Festkörperchemie: Zirkonsilicat-Farbkörper, Chem. Unserer Zeit, 20(6) (1986), 182–190. R. W. Batchelor, Modern Inorganic Pigments, Trans. Br. Ceram. Soc., 73 (1974), 297–301. M. Trojan, Synthesis of a Blue Zircon Pigment, Dyes Pigments, 9 (1988), 221–232. M. Dondi, M. Ardit, G. Cruciani, Turquoise zirkon: new diffraction and optical data — an overview of V4⊕ crystal chemistry and optical spectroscopy in minerals, Period. Mineral. (2015), 65–66. M. Ardit, G. Cruciani, F. Di Benedetto, L. Sorace, M. Dondi, New spectroscopic and diffraction data to solve the vanadium-doped zircon pigment conundrum, J. Eur. Ceram. Soc., 38 (2018), 5234–5245. https://doi.org/10.1016/j.jeurceramsoc.2018.07.033. A. Niesert, M. Hanrath, S. Siggel, M. Jansen, K. Langer, Theoretical study of the polarized electronic absorption spectra of vanadium-doped zircon, J. Solid State Chem., 169(1) (2002). https://doi.org/10.1016/S0022-4596(02)00010-5. A. Siggel, M. Jansen, Röntgenographische Untersuchungen zur Bestimmung der Einbauposition von Seltenen Erden (Pr, Tb) und Vanadium in Zirkonpigmenten, ZAAC, 583(1) (1990), 67–77. https://doi.org/10.1002/zaac.19905830109. M. Trojan, Synthesis of a Yellow Zircon Pigment, Dyes Pigments, 9 (1988), 261–273. M. Trojan, Synthesis of a Pink Zircon Pigment, Dyes Pigments, 9 (1988), 329–342. P. Šulcova, M. Trojan, Synthesis of a Pink Zircon Pigment, Dyes Pigments, 40 (1999), 83–86. R. Aguiar, D. Logvinovich, A. Weidenkaff, A. Rachel, A. Reller, S. G. Ebbinghaus, The vast colour spectrum of ternary metal oxynitride pigments, Dyes Pigments, 76 (2008), 70–75. M. Janson, H. P. Letschert, Inorganic yellow-red pigments without toxic metals, Nature, 404 (2000), 980–982. E. Günther, R. Hagenmayer, M. Jansen, Strukturuntersuchungen an den Oxidnitriden SrTaO2 N, CaTaO2 N und LaTaON2 mittels Neutronen- und Röntgenbeugung, Z. Anorg. Allg. Chem., 626 (2000), 1519–1525. E. Günther, R. Hagenmayer, M. Jansen, Nitrides and Oxynitrides: Chemistry and Properties Preparation, Crystal, J. Eur. Ceram. Soc., 8 (1991), 197–213. R. A. Eppler, Inverse Spinel Pigments, J. Amer. Ceram. Soc., 66(11) (1983), 794–801. W. Lüttke, M. Klessinger, Infrarot- und Lichtabsorptionsspektren einfacher Indigofarbstoffe, Chem. Ber., 97 (1964), 2342–2357. M. Klessinger, W. Lüttke, Das chromophore System der Indigo-Farbstoffe, Tetrahedron, 19(2) (1963), 315–335. M. Klessinger, PPP-Rechnungen am Indigo-Chromophor, Tetrahedron, 22 (1966), 3355–3365. M. Klessinger, W. Lüttke, Der Einfluss zwischenmolekularer Wasserstoffbrücken auf die Spektren von Indigo im festen Zustand, Chem. Ber., 99 (1966), 2136–2145.

878 � Bibliography

[460] [461] [462] [463] [464] [465] [466] [467] [468] [469] [470] [471] [472] [473] [474] [475] [476] [477] [478] [479] [480] [481] [482] [483] [484] [485]

Z. C. Koren, A New HPLC-PDA Method for the Analysis of Tyrian Purple Components, Dyes Hist. Archaeol., 21 (2008), 26–35. C. J. Cooksey, Tyrian Purple: 6, 6’-Dibromoindigo and Related Compounds, Molecules, 6 (2001), 736–769. H. Fouquet, H.-J. Bielig, Biological precursors and genesis of Tyrian purple, Angew. Chem. Int. Ed., 19(11) (1971), 816–817. R. R. Melzer, P. Brandhuber, T. Zimmermann, U. Smola, Der Purpur. Farben aus dem Meer, Biol. Unserer Zeit, 31(1) (2001), 30–39. D. Reinen, G.-G. Lindner, The nature of the chalcogen colour centres in ultramarine-type solids, Chem. Soc. Rev., 28 (1999), 75–84. F. Seel, G. Schäfer, H.-J. Güttler, G. Simon, Des Geheimnis des Lapis lazuli, Chem. Unserer Zeit, 8 (1974), 65–71. F. A. Cotton, J. B. Harmon, R. M. Hedges, Calculation of the Ground State Electronic Structures and Electronic Spectra of Di- and Trisulfide Radical Anions by the Scattered Wave-SCF-Xα Method, J. Am. Chem. Soc., 98 (1976), 1417–1424. W. Koch, J. Natterer, C. Heinemann, Quantum chemical study on the equilibrium geometries of S3 and 2− S2− 3 , The electron affinity of S3 and the low lying electronic states of S3 , J. Chem. Phys., 102(15) (1995), 6159–6167. G.-G. Lindner, D. Reinen, Synthese, Farbe und Struktur eines roten Selen-Ultramarins, Z. Anorg. Allg. Chem., 620 (1994), 1321–1328. H. Schlaich, G.-G. Lindner, J. Feldmann, E. O. Göbel, D. Reinen, Optical Properties of Se⊖2 and Se2 Color Centers in the Red Selenium Ultramarine with the Sodalite Structure, Inorg. Chem., 39 (2000), 2740–2746. C. Heinemann, W. Koch, G.-G. Lindner, D. Reinen, P.-O. Widmark, Ground- and excited-state properties of neutral and anionic selenium dimers and trimers, Phys. Rev. A, 54(3) (1996), 1979–1993. G.-G. Lindner, K. Witke, H. Schlaich, D. Reinen, Blue-green ultramarine-type zeolites with dimeric tellurium colour centres, Inorg. Chim. Acta, 252 (1996), 39–45. S. E. Tarling, P. Barnes, J. Klinowski, The Structure and Si, Al Distribution of the Ultramarines, Acta Crystallogr., Sect. B, 44 (1988), 128–135. F. Hund, P. Köhl, J. Kemper, D. Reinen, Der Chromat-Nosean Na8 [Al6 Si6 O24 ](CrO4 ) – ein zeolithisches Gelbpigment? Z. Anorg. Allg. Chem., 628 (2002), 1457–1458. S. Brühlmann, Kobaltblau. Werkstoffgeschichte und Werkstofftechnologie, Schriftenreihe Konservierung und Restaurierung, Hochschule der Künste Bern, pp. 11–34. D. Reinen, T. C. Brunold, H. U. Güdel, N. D. Yordanov, Die Natur des Farbzentrums im Manganblau, Z. Anorg. Allg. Chem., 624 (1998), 438–442. F. Hund, Anorganische Pigmente durch iso-, homöo- und heterotype Mischphasenbildung, Farbe und Lack, 73(2) (1967), 111–120. E. Korinth, Neuere anorganische Pigmente, Angew. Chem., 64 (1952), 265–269. F. Hund, Mischphasenpigmente mit Rutil-Struktur, Angew. Chem., 74 (1962), 23–27. G. Pfaff, Perlglanzpigmente, Chem. Unserer Zeit, 31(1) (1997), 6–16. P. Sayer, General Aspects of Organic Pigment Technology, in [153], pp. 92–106. Brandenburgisches Landesamt für Denkmalpflege und Archäologisches Landesmuseum, Umweltbedingte Pigmentveränderungen an mittelalterlichen Wandmalereien, Arbeitshefte Nr. 24, Wernersche Verlagsgesellschaft mbH 2009. ISBN 978-3-88462-290-2. Website of the caves of Lascaux, https://www.lascaux.fr/en, accessed on 2022-06-20. Heilbrunn Timeline of Art History, vom Metropolitan Museum of Art, https://www.metmuseum.org/ toah, accessed on 2022-06-20. J. Clottes, M. Menu, Ph. Walter, La préparation des peintures magdaléniennes des cavernes ariégeoises, Bull. Soc. Préhist. Fr., 87(6) (1990), 170–192. D. Garate, É. Laval, M. Menu, Étude de la matière colorante de la grotte d’Arenaza (Galdames, Pays Basques, Espagne), L’anthropologie, 108 (2004), 251–289.

Bibliography

[486] [487] [488] [489] [490] [491]

[492] [493] [494] [495] [496] [497] [498] [499] [500] [501] [502] [503] [504] [505] [506] [507] [508] [509] [510]

� 879

A. El Goresy, Ancient Pigments in Wall Paintings of Egyptian Tombs and Temples. An Archaeometric Project, Max-Planck-Institut für Kernphysik, Heidelberg, 1986, MPI H-1986-V 12. H. Jaksch, Farbpigmente aus Wandmalereien altägyptischer Gräber und Tempel: Technologien der Herstellung und mögliche Herkunftsbeziehungen, Dissertation Universität Heidelberg, 1985. A. Brysbaert, The Power of Technology in the Bronze Age Eastern Mediterranean, Equinox Publishing Ltd., 2008. ISBN 978-1-84553-433-2. P. Westlake, P. Siozos, A. Philippidis, et al., Studying pigments on painted plaster in Minoan, Roman and Early Byzantine Crete. A multi-analytical technique approach, Anal. Bioanal. Chem., 402 (2012), 1413. https://doi.org/10.1007/s00216-011-5281-z. R. E. Jones, E. Photos-Jones, Technical studies of Aegean Bronze Age wall painting: methods, results and future prospects, in British School at Athens Studies Vol. 13, Aegean Wall Painting: A Tribute to Mark Cameron, pp. 199–228, 2005. F. Blakolmer, Minoan Wall-Painting: The Transformation of a Craft into an Art Form, in R. Laffineur, P. P. Betancourt (eds.) TEXNH Craftsmen, Craftswomen and Craftmanship in the Aegean Bronze Age, Proceedings of the 6th International Aegean Conference, Philadelphia, Temple University, 18–21 April 1996, pp. 95–113, 1996. A. Vlachopoulos, S. Sotiropoulou, The Blue Colour on the Akrotiri Wall-Paintings: From the Palette of the Theran Painter to the Laboratory Analysis, in A. Papadopoulos (ed.) TALANTA. Proceedings of the Dutch Archaeological and Historical Society, vol. XLIV, pp. 245–272, 2012. J. Riederer, Pigmente in der Antike, Prax. Nat.wiss., Chem., 7(37) (1988), 3–10. W. Noll, Chemie vor unserer Zeit: Antike Pigmente, Chem. Unserer Zeit, 14(2) (1980), 37–43. W. Noll, Anorganische Pigmente in Vorgeschichte und Antike, Fortschr. Mineral., 57 (1979), 203–263. R. Fuchs, Farbmittel in der mittelalterlichen Buchmalerei – Untersuchungen zur Konservierung geschädigter Handschriften, Prax. Nat.wiss., Chem., 37 (1988), 20–29. J. Riederer, Die Pigmente der antiken Malerei, Naturwiss., 69 (1982), 82–86. W. Noll, R. Holm, L. Born, Chemie und Technik altkretischer Vasenmalerei vom Kamares-Typ, Naturwiss., 58 (1971), 615–618. W. Noll, L. Born, R. Holm, Chemie und Technik altkretischer Vasenmalerei vom Kamares-Typ, II, Naturwiss., 61 (1974), 361–362. W. Noll, R. Holm, L. Born, Bemalung antiker Keramik, Angew. Chem., 87(18) (1975), 639–682. J. Riederer, Die Pigmente der antiken Malerei, in [104], pp. 202–213. R. Germer, Die Textilfärberei und die Verwendung gefärbter Textilien im alten Ägypten, Otto Harrassowitz, Wiesbaden, 1992. ISBN 3-447-03183-2. N. Welter, Untersuchungen von Pigmenten in römischer Wandmalerei und antiken Gläsern, Dissertation Universität Würzburg, 2008. N. Riedel, Provinzialrömische Wandmalerei in Deutschland, Dissertation Universität Bamberg, 2007. B. Mazzei, Preservation and Use of the Religious Sites Case Study of the Roman Catacombs, Eur. J. Sci. Theol., 11(2) (2015), 33–43, http://www.ejst.tuiasi.ro/Files/51/5_Mazzei.pdf, accessed on 2022-06-17. D. Tapete, F. Fratini, B. Mazzei, E. Cantisani, E. Pecchioni, Petrographic study of lime-based mortars and carbonate incrustation processes of mural paintings in Roman catacombs, Period. Mineral., 82(3) (2013), 503–527. A. Zucchiatti, P. Prati, A. Bouquillon, L. Giuntini, M. Massi, A. Migliori, A. Cagnana, S. Roascio, Characterisation of early medieval frescoes by p-PIXE, SEM and Raman spectroscopy, Nucl. Instrum. Methods Phys. Res., Sect. B, 219–220 (2004), 20–25. A. Cagnana, S. Roascio, A. Zucchiatti, A. d’Allesandro, P. Prati, Gli affreschi altomedievali del Tempietto di Cividale: Nuovi dati da recenti analisi di laboratorio, in Forum Iulii. Annuario Del Museo Archeologico Nazionale Di Cividale Del Friuli, Archivi e Biblioteca, XXVII, pp. 143–153, 2003. S. Demailly, P. Hugon, M. Stefanaggi, W. Nowik, The Technique of the Mural Paintingsin the Choir of Angers Cathedral, in [106], pp. 10–15. M. R. Katz, The Mediaeval Polychromy of the Majestic West Portal of Toro, Spain, in [106], pp. 27–34.

880 � Bibliography

[511] [512] [513] [514] [515] [516] [517] [518] [519] [520]

[521]

[522] [523] [524] [525] [526] [527] [528] [529] [530] [531] [532] [533] [534]

H. Howard, T. Manning, S. Stewart, Late Mediaeval Wall Painting Techniques at Farleigh Hungerford Castle and Their Context, in [106], pp. 59–64. G. Horta, I. Ribeiro, L. Afonso, The ‘Calvary’ of S. Francisco’s Church in Leiria: Workshop Practice in a Portuguese Late Gothic Wall Painting, in [106], pp. 65–69. H. C. Howard, Techniques of the Romanesque and Gothic Wall Paintings in the Holy Sepulche Chapel, Winchester Cathedral, in [107], pp. 91–104. S. Cather, D. Park, P. Williamson (eds.), Early Medieval Wall Painting and Painted Sculpture in England, B. A. R. Oxford, BAR, vol. 216, 1990. ISBN 0-86054-719-1. H. Howard, ‘Blue’ in the Lewes Group, in [514], pp. 195–203. R. R. Milner-Gulland, Clayton Church and the Anglo-Saxon heritage, in [514], pp. 205–219. R. J. Cramp, J. Cronyn, Anglo-Saxon polychrome plaster and other materials from the excavations of Monkwearmouth and Jarrow: an interim report, in [514], pp. 17–30. S. Rickerby, Kempley: a technical examination of the Romanesque wall paintints, in [514], pp. 249–262. H. Howard, Pigments of English Medieval Wall Painting, Archetype Publications Ltd., 2003. ISBN 978-1873132487. J. Rollier-Hanselmann, D’Auxerre à Cluny: technique de la peinture murale entre le VIIIe et le XIIe siècle en Bourgogne, in Cahiers de civilisation médiévale, 40e année (n°157), Janvier–mars 1997, pp. 57–90, 1997, https://www.persee.fr/doc/ccmed_0007-9731_1997_num_40_157_2674, accessed on 2016-04-10. O. Emmenegger, Karolingische und romanische Wandmalereien in der Klosterkirche. Technik, Restaurierungsprobleme, Massnahmen, in A. Wyss, H. Rutishauser, M. A. Nay (eds.) Die mittelalterlichen Wandmalereien im Kloster Müstair, pp. 77–140, vdf, Hochschul-Verlag an der ETH, Zürich, 2002. ISBN 3-7281-2803-1. N. Turner, The Recipe Collection of Johannes Alcherius and the Painting Materials Used in Manuscript Illumination in France and Northern Italy, c. 1380–1420, in [106], pp. 45–50. B. Guineau, I. Villela-Petit, R. Akrich, J. Vezin, Painting Technique in the Boucicaut Hours and in Jacques Coene’s Colour Recipes as Found in Jean Lebegue’s Libri Colorum, in [106], pp. 51–54. B. New, H. Howard, R. Billinge, H. Tomlinson, D. Peggie, D. Gordon, Niccolò di Pietro Gerini’s Baptism Altarpiece: Technique, Conservation and Original Design, vol. 33, National Gallery, London, 2012. ISBN 978-1-85709-549-4. J. Dunkerton, A. Roy, The materials of a Group of Late Fifteenth-century Florentine Panel Paintings, Natl. Gallery Tech. Bull., 17 (1996), 20–31. ISBN 1-85709-113-2. J. Dunkerton, A. Roy, Uccello’s Saint George and the Dragon: Technical Evidence Re-evaluated, Natl. Gallery Tech. Bull., 19 (1998), 26–30. ISBN 1-85709-220-1. A. Roy, D. Gordon, Uccello’s Battle of San Romano, Natl. Gallery Tech. Bull., 22 (2001), 4–17. ISBN 978-1-85709-926-5. J. Dunkerton, C. Plazzotta, Vincenzo Foppa’s Adoration of the Kings, Natl. Gallery Tech. Bull., 22 (2001), 18–28. ISBN 978-1-85709-926-5. J. Dunkerton, S. Foister, M. Spring, The Virgin and Child Enthroned with Angels and Saints attributed to Michael Pacher, Natl. Gallery Tech. Bull., 21 (2000), 4–19. ISBN 978-1-85709-251-1. J. Dunkerton, L. Syson, In Search of Verrocchio the Painter: The Cleaning and Examination of The Virgin and Child with Two Angels, Natl. Gallery Tech. Bull., 31 (2010), 4–41. ISBN 978-1-85709-495-4. J. Dunkerton, N. Penny, A. Roy, Two Paintings by Lorenzo Lotto in the National Gallery, Natl. Gallery Tech. Bull., 19 (1998), 52–63. ISBN 1-85709-220-1. Methods and materials of Northern European painting in the National Gallery, 1400–-1550, Natl. Gallery Tech. Bull., 18 (1997), 6–55. ISBN 1-85709-178-7. The materials and technique of five paintings by Rogier van der Weyden and his workshop, Natl. Gallery Tech. Bull., 18 (1997), 68–86. ISBN 1-85709-178-7. A double-sided panel by Stephan Lochner, Natl. Gallery Tech. Bull., 18 (1997), 56–67. ISBN 1-85709-178-7.

Bibliography

[535] [536] [537] [538] [539] [540] [541] [542] [543] [544] [545] [546] [547] [548] [549] [550] [551] [552] [553] [554] [555] [556] [557] [558] [559] [560] [561]

� 881

A. Smith, M. Wyld, Altdorfer’s ‘Christ taking Leave of His Mother’, Natl. Gallery Tech. Bull., 7 (1983), 51–64. ISBN 0-901791-88-1. W. Huber’s, Christ taking Leave of His Mother, Natl. Gallery Tech. Bull., 18 (1997), 98–112. ISBN 1-85709-178-7. J. Dunkerton, N. Penny, M. Spring, The Technique of Garofalo’s Paintings at the National Gallery, Natl. Gallery Tech. Bull., 23 (2002), 20–41. ISBN 978-1-85709-941-9. A. Roy, M. Spring, C. Plazzotta, Raphael’s Early Work in the National Gallery: Paintings before Rome, Natl. Gallery Tech. Bull., 25 (2004), 4–35. ISBN 1-85709-320-8. K. H. Weber, Die Sixtinische Madonna, Restauro, 90(4) (1984), 9–28. L. Keith, A. Roy, R. Morrison, P. Schade, Leonardo da Vinci’s “Virgin of the Rocks”: Treatment, Technique and Display, vol. 32, National Gallery, London, 2011. ISBN 978-1-85709-530-2. M. Spring, A. Mazzotta, A. Roy, R. Billinge, D. Peggie, Painting Practice in Milan in the 1490s: The Influence of Leonardo, vol. 32, National Gallery, London, 2011. ISBN 978-1-85709-530-2. J. Dunkerton, S. Foister, D. Gordon, N. Penny, Giotto to Dürer. Early Renaissance Painting in The National Gallery, National Gallery Publications, Limited, 1991. ISBN 978-0-300-05082-0. P. Joannides, J. Dunkerton, A Boy with a Bird in the National Gallery: Two Responses to a Titian Question, Natl. Gallery Tech. Bull., 28 (2007), 36–57. ISBN 978-1-85709-357-5. A. Lucas, J. Plesters, Titian’s “Bacchus and Ariadne”, Natl. Gallery Tech. Bull., 2 (1978), 25–47. J. Dunkerton, M. Spring, The Technique and Materials of Titian’s Early Paintings in the National Gallery, London, in Titian, Jacopo Pesaro being presented by Pope Alexander VI to Saint Peter, Restoration Vol. 3 No. 1, Journal of Koninklijk Museum, 2003, pp. 9–21. H. Dubois, A. Wallert, Titian’s Painting Technique in Jacopo Pesaro being presented by Pope Alexander VI to Saint Peter, in Titian, Jacopo Pesaro being presented by Pope Alexander VI to Saint Peter, Restoration Vol. 3 No. 1, Journal of Koninklijk Museum, 2003, pp. 22–37. U. Birkmaier, A. Wallert, A. Rothe, Technical Examinations of Titian’s Venus and Adonis: A Note on Early Italian Oil Painting Technique, in [107], pp. 117–126. J. Grenberg, W. Kireewa, S. Pisarewa, Das Gemälde “Die Heilige Familie” von Bronzino, Restauro, 3 (1992), 180–184. A. Burmester, C. Krekel, “Azurri oltramarini, lacche et altri colori fini”: Auf der Suche nach der verlorenen Farbe, in [139], pp. 193–211. J. Plesters, Tintoretto’s Paintings in the National Gallery, Natl. Gallery Tech. Bull., 3 (1979), 3–24. J. Plesters, Tintoretto’s Paintings in the National Gallery, Natl. Gallery Tech. Bull., 4 (1980), 32–48. ISBN 0-901791-72-5. N. Penny, M. Spring, Veronese’s Paintings in the National Gallery Technique and Materials, Part I, Natl. Gallery Tech. Bull., 16 (1995), 4–29. ISBN 1-85709-071-3. N. Penny, A. Roy, M. Spring, Veronese’s Paintings in the National Gallery Technique and Materials, Part II, Natl. Gallery Tech. Bull., 17 (1996), 32–55. ISBN 1-85709-113-2. J. Dunkerton, The Technique and Restoration of Bramantino’s Adoration of the Kings, Natl. Gallery Tech. Bull., 14 (1993), 42–61. ISBN 1-85709-033-0. B. Heimberg, Zur Maltechnik Albrecht Dürers, in [148], pp. 32–54. A. Burmester, C. Krekel, Von Dürers Farben, in [148], pp. 55–101. P. Ackroyd, S. Foister, M. Spring, R. White, R. Billinge, A Virgin and Child from the Workshop of Albrecht Dürer? Natl. Gallery Tech. Bull., 21 (2000), 29–42. ISBN 978-1-85709-251-1. S. Foister, M. Wyld, A. Roy, Hans Holbein’s A Lady with a Squirrel and a Starling, Natl. Gallery Tech. Bull., 15 (1994), 6–19. ISBN 1-85709-049-7. M. Spring, N. Penny, R. White, M. Wyld, Colour Change in The Conversion of the Magdalen attributed to Pedro Campana, Natl. Gallery Tech. Bull., 22 (2001), 54–63. ISBN 978-1-85709-926-5. L. Keith, Three Paintings by Caravaggio, Natl. Gallery Tech. Bull., 19 (1998), 37–51. ISBN 1-85709-220-1. J. Plesters, ‘Samson and Delilah’: Rubens and the Art and Craft of Painting on Panel, Natl. Gallery Tech. Bull., 7 (1983), 30–50. ISBN 0-901791-88-1.

882 � Bibliography

[562] [563]

[564] [565] [566] [567] [568] [569] [570] [571] [572] [573] [574] [575] [576] [577] [578] [579] [580] [581] [582] [583] [584] [585] [586] [587]

A. Roy, Rubens’s ‘Peace and War’, Natl. Gallery Tech. Bull., 20 (1999), 89–95. ISBN 1-85709-251-1. C. Christensen, M. Palmer, M. Swicklik, Van Dyck’s Painting Technique, His Writings, and Three Paintings in the National Gallery of Art, in A. K. Wheelock, S. J. Barnes, J. S. Held (eds.) Anthony van Dyck, National Gallery of Art Washington, pp. 45–52, Harry N. Abrams, Inc., Publishers, New York, 1990. ISBN 0-89468-155-9. A. Roy, The National Gallery Van Dycks: Technique and Development, Natl. Gallery Tech. Bull., 20 (1999), 50–83. ISBN 1-85709-251-1. M. Spring, Pigments and Colour Change in the Paintings of Aelbert Cuyp, in A. Wheelock (ed.) Aelbert Cuyp, pp. 65–73, National Gallery of Art Washington, 2001. ISBN 0-89468-286-5. N. Costaras, A Study of the Materials and Techniques of Johannes Vermeer, in [143], pp. 145–165. K. N. Groen, I. D. Van der Werf, K. J. Van den Berg, J. J. Boon, Scientific Examination of Vermeer’s Girl with a Pearl Earring, in [143], pp. 169–183. A. Burmester, Die Palette Giovanni Battista und Giovanni Domenico Tiepolo, in P. O. Krückmann (ed.) Der Himmel auf Erden. Tiepolo in der Residenz Würzburg. II: Aufsätze, Ausstellungskatalog 1996, pp. 160–162, Prestel-Verlag, München, 1996. ISBN 3-7913-1660-5. I. Stümmer, Die Maltechnik von Giovanni Battista Tiepolo, Restauro, 3 (1996), 194–199. W. Helmberger, A. Staschull, Tiepolos Welt. Das Deckenfresko im Treppenhaus des Residenz Würzburg, Bayerische Verwaltung der staatlichen Schlösser, Gärten und Seen, 2006. ISBN 978-3-932982-73-6. D. Bomford, A. Roy, Canaletto’s ‘Stonemason’s Yard’ and ‘San Simeone Piccolo’, Natl. Gallery Tech. Bull., 14 (1993), 35–41. ISBN 1-85709-033-0. D. Bomford, A. Roy, Canaletto’s Venice: The Feastday of S. Roch, Natl. Gallery Tech. Bull., 6 (1982), 40–44. ISBN 0-901791-84-9. S. L. Fischer, The Examination and Treatment of Watteau’ Italian Comedians, in M. Morgan Grasselli, P. Rosenberg (eds.) Watteau, Ausstellungskatalog, pp. 465–467, National Gallery of Art Washington, 1984. ISBN 0-89468-074-9. P. Ackroyd, A. Roy, H. Wine, Nicolas Lancret’s The Four Times of Day, Natl. Gallery Tech. Bull., 25 (2004), 48–61. ISBN 1-85709-320-8. M. Kirby Talley, “Tous les bons tableaux se craquellent”. La technique picturale et látelier de Sir Joshua Reynolds, in N. Penny, P. Rosenberg (eds.) Sir Joshua Reynolds, Ausstellungskatalog, pp. 83–114, Éditions de la Réunion des musées nationaux, Paris, 1985. ISBN 2-7118-2 017-3. G. ‘Dr, R. Schomberg’, D. Bomford, A. Roy, D. Saunders, Natl. Gallery Tech. Bull., 12 (1988), 44–57. ISBN 0-947645-63-2. M. Leonard, A. Roy, S. Schaefer, Two Versions of The Fountain of Love by Jean-Honoré Fragonard: A Comparative Study, Natl. Gallery Tech. Bull., 29 (2008), 31–45. ISBN 978-1-85709-419-0. D. Bomford, A. Roy, Hogarth’s ‘Marriaga á la Mode’, Natl. Gallery Tech. Bull., 6 (1982), 45–67. ISBN 0-901791-84-9. J. Leighton, A. Reeve, A. Burnstock, A ‘Winter Landscape’ by Caspar David Friedrich, Natl. Gallery Tech. Bull., 13 (1989), 44–60. ISBN 0-947645-70-5. J. H. Townsend, The Materials of J. M. W. Turner: Pigments, Stud. Conserv., 38(4) (1993), 231–254. N. W. Hanson, Some Painting Materials of J. M. W. Turner, Stud. Conserv., 1(4) (1954), 162–173. J. H. Townsend, A Re-Assessment of Hanson’s Paper on the Painting Materials of J. M. W. Turner, Stud. Conserv., 50(4) (2005), 316–318. J. H. Townsend, Painting Technique and Materials of Turner and Other British Artists 1775–1875, in [107], pp. 176–185. S. Cove, Mixing and Mingling: John Constable’s Oil Paint Mediums c. 1802–-1837, Including the Analysis of the ‘Manton’ Paint Box, in [106], pp. 211–216. J. Kirby, A. Roy, Paul Delaroche: A Case Study of Academic Painting, in [107], pp. 166–175. L. Sheldon, Methods and Materials of the Pre-Raphaelite Circle in The 1850s, in [106], pp. 229–234. S. Herring, Six Paintings by Corot: Methods, Materials and Sources, Natl. Gallery Tech. Bull., 30 (2009), 86–111. ISBN 978-1-85709-420-6.

Bibliography

[588] [589] [590] [591] [592] [593] [594] [595] [596] [597] [598] [599] [600] [601] [602] [603] [604] [605]

[606] [607] [608] [609] [610] [611] [612] [613] [614]

� 883

R. und, P. Woudhuysen-Keller, J. Cuttle, C. Hurst, Zwei Gemälde von Camille Corot, Restauro, 5 (1993), 306–315. E. Klein, An Investigation into the Painting Technique of the Two Versions of Corot’s St. Sebastian Succoured by Holy Women, in [138], pp. 178–191. R. Woudhuysen-Keller, Observations Concerning Corot’s Late Painting Technique, in [138], pp. 192–200. A. Burmester, C. Denk, Comment ils inventaient ces verts chatoyants? Blau, Gelb, Grün und die Landschaftsmalerei von Barbizon, in [138], pp. 295–329. A. Burmester, C. Denk, Blue, Yellow and Green on the Barbizon Palette, Kunsttechnologie und Konservierung, pp. 79–87, Wernersche Verlagsgesellschaft, Worms, 1999. S. Hackney, Art for Art’s Sake: The Materials and Technique of James McNeill Whistler, in [107], pp. 186–190. J. Leighton, A. Reeve, A. Roy, R. White, Vincent Van Gogh’s ‘A Cornfield with Cypresses’, Natl. Gallery Tech. Bull., 11 (1987), 42–59. ISBN 0-947645-35-7. D. Bomford, A. Roy, Manet’s ‘The Waitress’: An Investigation into its Origin and Development, Natl. Gallery Tech. Bull., 7 (1983), 3–20. ISBN 0-901791-88-1. A. Roy, The Palettes of Three Impressionist Paintings, Natl. Gallery Tech. Bull., 9 (1985), 12–20. ISBN 0-901791-97-0. E. Reissner, Ways of Making: Practive and Innovation in Cézanne’s Paintings in the National Gallery, Natl. Gallery Tech. Bull., 29 (2008), 4–30. ISBN 978-1-85709-419-0. A. Roy, Monet’s Palette in the Twentieth Century: Water-Lilies and Irises, Natl. Gallery Tech. Bull., 28 (2007), 58–68. ISBN 978-1-85709-357-5. A. Roy, Monet and the Nineteenth Century Palette, Natl. Gallery Tech. Bull., 5 (1981), 22–26. ISBN 0-901791-76-8. J. Kirby, K. Stonor, A. Roy, A. Burnstock, R. Grout, R. White, Seurat’s Painting Practice: Theory, Development and Technology, Natl. Gallery Tech. Bull., 24 (2003), 4–37. ISBN 1-85709-997-4. K. Stonor, R. Morrison, Adolphe Monticelli: The Materials and Techniques of an Unfashionable Artist, vol. 33, National Gallery, London, 2012. ISBN 978-1-85709-549-4. T. Froysaker, M. Liu, Four (or eleven) unvarnished oil paintings on canvas by Edvard Much in the Aula of Oslo University, Restauro, 1 (2009), 44–61. A. Robbins, K. Stonor, Past, Present, Memories: Analysing Edouard Vuillard’s La Terrasse at Vasouy, vol. 33, National Gallery, London, 2012. ISBN 978-1-85709-549-4. R. D. Harley, Oil colour containers: Development work by artists and colourmen in the nineteenth century, Ann. of Sci., 27(1) (1971), 1–12. https://doi.org/10.1080/00033797100203607, accessed on 2022-05-06. Fa. Winsor&Newton, From the Archives: The History of the Metal Paint Tube, online article, published 06-JAN-2015, https://www.winsornewton.com/row/articles/art-history/history-metal-paint-tube/, accessed on 2022-05-09. A. Roy, Barbizon Painters: Tradition and Innovation in Artist’s Materials, in [138], pp. 330–342. A. Callen, The Work of Art: Plein Air Painting and Artistic Identity in Nineteenth-century France, Reaktion Books, 2015. ISBN 978-1780233550. J. Zeidler, Lithographie und Steindruck, Christophorus Verlag, 2008. ISBN 978-3-419-53486-1. G. Baars, Das Färben von Naturfasern mit Naturfarbstoffen, Prax. Nat.wiss., Chem., 47 (1998), 24–32. H. M. Koch, P. Pfeifer, Naturfarbstoffe im Unterricht, Anthrachinonfarbstoffe der Krappwurzel, Nat.wiss. Unterr., Chem., 10(52) (1999), 25–29. G. H. Frischat, Glas – Struktur und Eigenschaften, Chem. Unserer Zeit, 11(3) (1977), 65–74. C. Rüssel, D. Ehrt, Neue Entwicklungen in der Glaschemie, Chem. Unserer Zeit, 32(3) (1998), 126–135. R. J. Schwankner, M. Eigenstetter, R. Laubinger, M. Schmidt, Strahlende Kostbarkeiten. Uran als Farbkörper in Gläsern und Glasuren, Phys. Unserer Zeit, 36(4) (2005), 160–167. E. W. J. L. Oomen, A. M. A. van Dongen, EUROPIUM (III) IN OXIDE GLASSES. Dependence of the emission spectrum upon glass composition, J. Non-Cryst. Solids, 111 (1989), 205–213.

884 � Bibliography

[615] [616] [617] [618] [619] [620] [621] [622] [623] [624] [625] [626] [627] [628] [629] [630] [631] [632] [633] [634] [635] [636] [637]

R. Reisfeld, N. Lieblich, OPTICAL SPECTRA AND RELAXATION OF Eu +3 IN GERMANATE GLASSES, J. Phys. Chem. Solids, 34 (1973), 1467–1476. R. W. Douglas, M. S. Zaman, The chromophore in iron-sulphur amber glasses, Phys. Chem. Glasses, 10 (1969), 125. H. D. Schreiber, S. J. Kozak, C. W. Schreiber, D. G. Wetmore, M. W. Riethmiller, Sulfur chemistry in a borosilicate melt. Part 3. Iron-sulfur interactions and the amber chromophore, Glastech. Ber., 63(3) (1990), 49–60. D. C. W. Sanderson, J. B. Hutchings, The origins and measurement of colour in archaeological glasses, Glass Technol., 28(2) (1987), 99–105. L. R. Green, F. A. Hart, Colour and Chemical Composition in Ancient Glass: An Examination of some Roman and Wealden Glass by means of Ultraviolet-Visible-Infra-red Spectrometry and Electron Microprobe Analysis, J. Archaeol. Sci., 14 (1987), 271–282. P. Colomban, The Use of Metal Nanoparticles to Produce Yellow, Red and Iridescent Colour, from Bronze Age to Present Times in Lustre Pottery and Glass: Solid State Chemistry, Spectroscopy and Nanostructure, J. Nanopart. Res., 8 (2009), 109–132. M. Garcia-Vallès, M. Vendrell-Saz, The glasses of the transept’s rosette of the cathedral of Tarragona: characterisation, classification and decay, Bol. Soc. Esp. Cerám. Vidrio, 41(2) (2002), 217–224. E. Thilo, J. Jander, H. Seemann, Die Farbe des Rubins und der (Al, Cr)2 O3 -Mischkristalle, Z. Anorg. Allg. Chem., 279(1–2) (1955), 2–17. C. J. Ballhausen, Intensities in Inorganic Complexes Part II. Tetrahedral Complexes, J. Mol. Spectrosc., 2 (1958), 342–360. L. H. Hillier, V. R. Saunders, Ab Initio Calculations of the Excited States of the Permanganate and Chromate Ions, Chem. Phys. Lett., 9 (1971), 219–221. K. H. Johnson, F. C. Smith Jr., Scattered-wave Model for the Electronic Structure and Optical Properties of the Permanganate Ion, Chem. Phys. Lett., 10 (1971), 219–223. H. Johansen, SCF LCAO MO calculation for MnO⊖4 , Chem. Phys. Lett., 17 (1972), 569–573. A. P. Mortola, H. Basch, J. W. Moskowitz, An Ab Initio Study of the Permanganate Ion, Int. J. Quantum Chem., 7 (1973), 725–737. H.-L. Hsu, C. Peterson, R. M. Pitzer, Calculations on the permanganate ion in the ground and excited states, J. Chem. Phys., 64(2) (1976), 791–795. J. A. Connor, I. H. Hillier, V. R. Saunders, M. H. Wood, M. Barber, Ab initio molecular orbital calculations of transition metal complexes, Mol. Phys., 24 (1972), 497–509. H. Johansen, S. Rettrup, Limited Configuration Interaction Calculation of the Optical Spectrum for the Permanganate Ion, Chem. Phys., 74 (1983), 77–81. S. Jitsuhiro, H. Nakai, M. Hada, H. Nakatsuji, Theoretical study on the ground and excited states of the chromate anion CrO2⊖ 4 , J. Chem. Phys., 101(2) (1994), 1029–1036. M. Wolfsberg, L. Helmholz, The Spectra and Electronic Structure of the Tetrahedral Ions MnO⊖4 , CrO2⊖ 4 , and CIO⊖4 , J. Chem. Phys., 20(5) (1952), 837–843. R. M. Millier, D. S. Tinti, D. A. Case, Comparison of the Electronic Structures of Chromate, Halochromates, and Chromyl Halides by the Xα Method, Inorg. Chem., 28 (1989), 2738–2743. O. Schmitz-DuMont, H. Brokopf, K. Burkhardt, Farbe und Konstitution bei anorganischen Feststoffen. I. Die Lichtabsorptions des zweiwertigen Kobalts in oxydischen Koordinationsgittern, Z. Anorg. Allg. Chem., 295 (1958), 7–35. D. Reinen, O. Schmitz-DuMont, Farbe und Konstitution bei anorganischen Feststoffen. IV. Die Lichtabsorptions als Mittel zur Auffindung struktureller Feinheiten von Kristallgittern, Z. Anorg. Allg. Chem., 312 (1961), 121–134. D. M. Sherman, The Electronic Structures of Fe3⊕ Coordination Sites in Iron Oxides, Applications to Spectra, Bonding, and Magnetism, Phys. Chem. Miner., 12 (1985), 161–175. D. M. Sherman, T. D. Waite, Electronic spectra of Fe3⊕ oxides and oxide hydroxides in the near IR to near UV, Amer. Mineral., 70 (1985), 1262–1269.

Bibliography

[638]

[639] [640] [641]

[642] [643] [644] [645]

[646] [647] [648] [649] [650] [651] [652] [653] [654] [655] [656] [657]

[658] [659] [660] [661]

� 885

D. M. Sherman, Crystal Chemistry, Electronic Structures, and Spectra of Fe Sites in Clay Minerals, in L. Coyne et al. (eds.) Spectroscopic Characterization of Minerals and Their Surfaces, ACS Symposium Series, American Chemical Society, Washington, DC, 1990. Correlation between structural features and vis-NIR spectra of α-Fe2 O3 hematite and AFe2 O4 spinel oxides (A=Mg, Zn), J. Solid State Chem., 181 (2008), 1040–1047. G. Kämpf, Korngrössenverteilung und optische Eigenschaften von Pigmenten, Farbe und Lack, 71(5) (1965), 353–365. M. Dondi, G. Cruciani, G. Guarini, F. Matteucci, M. Raimondo, The role of counterions (Mo, Nb, Sb, W) in Cr-, Mn-, Ni- and V-doped rutile ceramic pigments. Part 2. Colour and technological properties, Ceram. Int., 32 (2006), 393–405. K. M. Glassford, J. R. Chelikowsky, Structural and electronic properties of titanium dioxide, Phys. Rev. B, 46 (1992), 1284–1298. F. Thomazi, L. Stolz Roman, A. Ferreira da Silva, C. Persson, Optical absorption of rutile SnO2 and TiO2 , Phys. Status Solidi C, 6(12) (2009), 2740–2742. M. C. Comstock, Complex Inorganic Colored Pigments: Comparison of options and relative properties when faced with elemental restrictions, J. Surf. Coat. Aust. (2016), 10–30. C. L. Dong, C. Persson, L. Vayssieres, A. Augustsson, T. Schmitt, M. Mattesini, R. Ahuja, C. L. Chang, J.-H. Guo, Electronic structure of nanostructured ZnO from x-ray absorption and emission spectroscopy and the local density approximation, Phys. Rev. B, 70, 195325 (2004). A. Kobayashi, O. F. Sankey, J. D. Dow, Deep energy levels of defects in the wurtzite semiconductors AlN, CdS, CdSe, ZnS and ZnO, Phys. Rev. B, 28 (1983), 946–956. M. Rohlfing, P. Krüger, J. Pollmann, Quasiparticle Band Structure of CdS, Phys. Rev. Lett., 75(19) (1995), 3489–3492. V. E. Henrich, The surfaces of metal oxides, Rep. Progr. Phys., 48 (1985), 1481–1541. S.-R. Sun, Y.-H. Dong, The optical properties of HgS under high pressures, Solid State Commun., 138 (2006), 476–479. E. Doni, L. Resca, S. Rodriguez, W. M. Becker, Electronic energy levels of cinnabar (α-HgS), Phys. Rev. B, 20 (1979), 1663–1668. X. Zhong, Y. Feng, W. Knoll, M. Han, Alloyed Znx Cd1−x S Nanocrystals with Highly Narrow Luminescence Spectral Width, J. Am. Chem. Soc., 125 (2003), 13559–13563. X. Zhong, M. Han, Z. Dong, T. J. White, W. Knoll, Composition-Tunable Znx Cd1−x Se Nanocrystals with High Luminescence and Stability, J. Am. Chem. Soc., 125 (2003), 8589–8594. X. Zhong, Y. Feng, New strategy for band-gap tuning in semiconductor nanocrystals, Res. Chem. Intermed., 34(2–3) (2008), 287–298. K. Glassford, J. Chelikowsky, Electronic Structure of TiO2 :Ru, Phys. Rev. B, 47(19) (1993), 12550–12553. J. C. Phillips, The fundamental optical spectra of solids, Solid State Phys., 18 (1966), 55–164. R. A. Evarestov, V. A. Veryazov, The Electronic Structure of Crystalline Lead Oxides. I. Crystal Structure and LUC-CNDO Calculations, Phys. Status Solidi (b), 165 (1991), 401–410. D. L. Perry, T. J. Wilkinson, Synthesis of High-Purity alpha-and beta-PbO and Possible Applications to Synthesis and Processing of Other Lead Oxide Materials, Lawrence Berkeley National Laboratory, 2011, https://escholarship.org/uc/item/5xb2g68h, accessed on 2022-06-20. G. Trinquier, R. Hoffmann, Lead Monoxide. Electronic Structure and Bonding, J. Phys. Chem., 88 (1984), 6696–6711. G. A. Bordovskii, M. L. Gordeev, A. N. Ermoshkin, V. A. Izvozchikov, R. A. Evarestov, Energy Band Structure of Rhombic Lead Monoxide, Phys. Status Solidi (b), 111 (1982), K123–K127. G. A. Bordovskii, M. L. Gordeev, A. N. Ermoshkin, V. A. Izvozchikov, R. A. Evarestov, Energy Band Structure of Rhombic Lead Monoxide, Phys. Status Solidi (b), 115 (1983), K15–K19. J. Robertson, Electronic structure of SnO2 , GeO2 , PbO2 , TeO2 and MgF, J. Phys. C, Solid State Phys., 12 (1979), 4767.

886 � Bibliography

[662]

[663]

[664]

[665] [666] [667] [668] [669] [670] [671] [672] [673] [674] [675] [676] [677] [678]

[679]

[680] [681] [682] [683]

G. Pfaff und andere, Rare earth metal sulfide pigments useful in paint, lacquer, printing ink, plastics and cosmetics, Deutsches Patent DE19810317A1, https://patents.google.com/patent/DE19810317A1/en? oq=DE19810317A1, accessed on 2022-04-25. G. Pfaff und andere, SULPHIDE AND OXYSULPHIDE PIGMENTS, WIPO Patent Application WO/1999/046336, https://www.sumobrain.com/patents/wipo/Sulphide-oxysulphide-pigments/ WO1999046336.html, accessed on 2022-04-25. T. Corbiere, Preparation and Methods of Characterisation of the Barium Copper Silicates BaCuSi4 O10 (Chinese Blue), BaCuSi2 O6 (Chinese Purple), Ba2 CuSi2 O7 , BaCu2 Si2 O7 Used as Blue and Purple Pigments, Dissertation, Universität Zürich, 2009. C. Burda, X. Chen, R. Narayanan, M. A. El-Sayed, Chemistry and Properties of Nanocrystals of Different Shapes, Chem. Rev., 105 (2005), 1025–1102. E. N. Economou, K. L. Ngai, Surface plasma oscillations and related surface effects in solids, Adv. Chem. Phys., 27 (1974), 265–354. S. Link, M. A. El-Sayeda, Shape and size dependence of radiative, non-radiative and photothermal properties of gold nanocrystals, Int. Rev. Phys. Chem., 19(3) (2000), 409–453. S. Link, M. A. El-Sayeda, Optical Properties and Ultrafast Dynamics of Metallic Nanocrystals, Annu. Rev. Phys. Chem., 54 (2003), 331–366. M. A. Garcia, Surface plasmons in metallic nanoparticles: fundamentals and applications, J. Phys. D, Appl. Phys., 44(28) 283001 (2011), ISSN 0022-3727. E. F. Wesp, W. R. Brode, The Absorption Spectra of Ferric Compounds. I. The Ferric Chloride-Phenol Reaction, J. Am. Chem. Soc., 56 (1934), 1037–1042. W. Mayer, E. H. Hoffmann, N. Losch, H. Wow, B. Wolter, G. Schilling, Dehydrierungsreaktionen mit Gallussäureestern, Justus Liebigs Ann. Chem. (1984), 929–938. L. Sommer, Über die genaue spektrophotometrische Eisen(III)- und Titan(IV)-Bestimmung mit Polyphenolen und verwandten Verbindungen, Acta Chim. Acad. Sci. Hung., 33 (1962), 23–30. L. Sommer, Über einige analytische Reaktionen der Polyphenole, Fresenius’, Z. Anal. Chem., 187(1) (1962), 7–16. P. H. Gore, P. J. Newman, Quantitative aspects of the colour reaction between iron(III) and phenols, Anal. Chim. Acta, 31 (1964), 111–120. G. Ackermann, D. Hesse, Über Eisen(III)-Komplexe mit Phenolen. I. Reaktion zwischen Eisen(III)-lonen und Monophenolderivaten, Z. Anorg. Allg. Chem., 367 (1969), 243–248. G. Ackermann, D. Hesse, Über Eisen(III)-Komplexe mit Phenolen. II. Chelate mit Salicylsäure- und Brenzkatechinderivaten, Z. Anorg. Allg. Chem., 368 (1969), 25–30. H. Kipton, J. Powell, M. C. Taylor, Interactions of Iron(II) and Iron(III) with Gallic Acid and its Homologues: a Potentiormetric and Spectrophotometric Study, Aust. J. Chem., 35 (1982), 739–756. R. C. Hider, A. Rahim Mohd-Nor, J. Silver, I. E. G. Morrison, L. V. C. Rees, Model Compounds for Microbial Iron-transport Compounds. Part I. Solution Chemistry and Mössbauer Study of Iron(II) and Iron(III) Complexes from Phenolic and Catecholic Systems, J. Chem. Soc., Dalton (1981), 609–622. R. C. Hider, B. Howlin, J. R. Miller, A. Rahim Mohd-Nor, J. Silver, Model Compounds for Microbial Iron-transport Compounds. Part IV. Further Solution Chemistry and Mössbauer Studies on Iron(II) and Iron(III) Catechol Complexes, Inorg. Chim. Acta, 80 (1983), 51–56. B. Howlin, A. Rahim Mohd-Nor, J. Silver, Model Compounds for Microbial Iron-transport Compounds. Part V. Substituent Effects in the Catechol/FeCl3 System, Inorg. Chim. Acta, 91 (1984), 153–160. M. Brenes, C. Romero, P. Garcia, A. Garrido, Effect of pH on the Colour Formed by Fe-Phenolic Complexes in Ripe Olives, J. Sci. Food Agric., 67 (1995), 35–41. A. Krilov, R. Gref, Mechanism of sawblade corrosion by polyphenolic compounds, Wood Sci. Technol., 20 (1986), 369–375. L. J. Porter, K. R. Markham, The Aluminium(III) Complexes of Hydroxy-flavones in Absolute Methanol. Part I. Ligands containing Only One Chelating Site, J. Chem. Soc. (C) (1970), 344–349.

Bibliography

[684] [685] [686] [687] [688] [689] [690] [691] [692] [693] [694] [695] [696] [697] [698] [699] [700] [701] [702] [703]

[704] [705] [706] [707]

[708]

� 887

L. J. Porter, K. R. Markham, The Aluminium(III) Complexes of Hydroxy-flavones in Absolute Methanol. Part II. higands containing More than One Chelating Site, J. Chem. Soc. (C) (1970), 1309–1313. M. Katyal, Flavones as Analytical Reagents – a Review, Talanta, 15 (1968), 95–106. M. Katyal, S. Prakash, Analytical Reactions of Hydroxyflavones, Talanta, 24 (1977), 367–375. L. Jurd, T. A. Geissman, Absorption Spectra of Metal Complexes of Flavonoid Compounds, J. Org. Chem., 21(12) (1956), 1395–1401. H. Deng, G. J. van Berkel, Electrospray Mass Spectrometry and UV/Visible Spectrophotometry Studies of Complexes, J. Mass Spectrom., 33 (1998), 1080–1087. S. H. Etaiw, M. M. Abou Sekkina, G. B. El-Hefnawey, S. S. Assar, Spectral behaviour and solvent effects of some aminoanthraquinone dyes, Can. J. Chem., 60 (1982), 304–307. R. H. Peters, H. H. Sumner, Spectra of Anthraquinone Derivatives, J. Chem. Soc. (1953), 2101–2110. H. Musso, Die Trennung des Orceins in seine Komponenten (II. Mitteilung über Orceinfarbstoffe), Chem. Ber., 89 (1956), 1659–1673. H. Musso, C. Rajtjen, Über Orceinfarbstoffe, X. Lichtabsorption und Chromophor des Lackmus, Chem. Ber., 92 (1959), 751–753. H. Musso, Zur Konstitution der Orceinfarbstoffe, Angew. Chem., 69(5) (1957), 178. H. Musso, H. Beecken, Über Orceinfarbstoffe, VI. Die Konstitution von α-, β- und γ-Amino-Orcein, Chem. Ber., 90 (1957), 2190–2196. H. Musso, Orcein- und Lackmusfarbstoffe. Konstitutionsermittlung und Konstitutionsbeweis durch die Synthese, Planta Med., 8 (1960), 432–446. H. Beecken, E.-M. Gottschalk, U. v. Gizycki, H. Krämer, D. Maassen, H.-G. Matthies, H. Musso, C. Rathjen, U. I. Zahorszky, Orcein und Lackmus, Angew. Chem., 73(20) (1961), 665–673. H. Auterhoff, H. Frauendorf, W. Liesenklas, C. Schwandt, Der Hauptbestandteil des Guttiharzes. 1. Mitt.: Chemie des Gummigutts, Arch. Pharm., 295(11) (1962), 833–846. H. Auterhoff, W. Liesenklas, Der Hauptbestandteil des Guttiharzes. 3. Mitt.: Chemie des Gummigutts, Arch. Pharm., 299(1) (1966), 91–96. H. Auterhoff, W. Liesenklas, Die Konstitution der Gambogasäure und ihre Isomerisierung. 4. Mitt.: Chemie des Gummigutts, Arch. Pharm., 299(9) (1966), 797–798. W. D. Ollis, M. V. J. Ramsay, I. O. Sutherland, The Constitution of Gambogic Acid, Tetrahedron, 21 (1965), 1453–1470. L.-J. Lin, L.-Z. Lin, J. M. Pezzuto, G. A. Cordell, N. Ruangrungsi, Isogambogic acid and Isomorellinol from Garcinia Hanbuyi, Magn. Reson. Chem., 31(4) (1993), 340–347. J.-Z. Song, Y.-K. Yip, Q.-B. Han, C.-F. Qiao, H.-X. Xu, Rapid determination of polyprenylated xanthones in gamboges resin of Garcinia hanburyi by HPLC, J. Sep. Sci., 30 (2007), 304–309. A. López-Montes, R. Blanc García, T. Espejo, J. F. Huertas-Perez, A. Navalón, J. L. Vílchez, Simultaneous identification of natural dyes in the collection of drawings and maps from The Royal Chancellery Archives in Granada (Spain) by CE, Electrophoresis, 28 (2007), 1243–1251. K. Roth, Fingerfarben, Ideal für kleine Künstler, Chem. Unserer Zeit, 40 (2006), 260–267. H. Baumann, H. R. Hensel, Neue Metallkomplexfarbstoffe, Struktur und färberische Eigenschaften, Fortschr. Chem. Forsch., 7(4) (1967), 643–783. F. Beffa, G. Back, Metal-complex Dyes for Wool and Nylon – 1930 to date, Rev. Prog. Color., 14 (1984), 33–42. H. S. Freeman, L. C. Edwards, Iron-Complexed Dyes: Colorants in Green Chemistry, in P. Anastas et al. (eds.) Green Chemical Syntheses and Processes, ACS Symposium Series, pp. 18–32, American Chemical Society, Washington, DC, 2000. J. Sokolowska-Gajda, H. S. Freeman, A. Reife, Synthetic Dyes Based on Environmental Considerations: Part I: Iron Complexes for Protein and Polyamide Fibers, Tex. Res. J., 64(7) (1994), 388–396.

888 � Bibliography

[709]

[710] [711] [712] [713] [714] [715] [716] [717] [718] [719] [720] [721] [722] [723] [724] [725] [726] [727] [728] [729]

Y. Yagi, Studies of the Absorption Spectra of Azo Dyes and their Metal-Complexes. I. The Absorption Spectra of Phenylazo-acetoacetanilide and its Related Compounds, Bull. Chem. Soc. Jpn., 36 (1963), 487–492, II. The Absorption Spectra of Phenolazoacetoacetamide and its Derivatives, Bull. Chem. Soc. Jpn., 36 (1963), 492–500, III. The Electronic Absorption Spectra of Metal-Complexes Derived from Phenolazoacetoacetanilides, Bull. Chem. Soc. Jpn., 36 (1963), 500–506, IV. The Effects of Substituents on the Absorption Spectra of Chromium(III)- and Cobalt(III)-Complexes of Phenolazoacetoacetanilides, Bull. Chem. Soc. Jpn., 36 (1963), 506–512, V. The Electronic Absorption Spectra of Metal-complexed Phenolazoacetoacetbenzylamides, Bull. Chem. Soc. Jpn., 36 (1963), 512–517, VII. The Absorption Spectra of Metal-complexes Derived from Phenolazo-α- and Phenolazo-β-naphthols, Bull. Chem. Soc. Jpn., 37 (1964), 1878–1880, VIII. The Absorption Spectra of Azo Compounds Derived from 5, 8-Dichloro-1-naphthol and Their Metal-complexes, Bull. Chem. Soc. Jpn., 37 (1964), 1881–1883. J. Griffiths, D. Rhodes, The Prediction of Colour Change in Dye Equilibria. II – Metal Complexes of Ortho-Hydroxyazobenzenes, J. Soc. Dyers Colour., 88 (1972), 400. P. S. Vankar, Chemistry of Natural Dyes, Resonance (2000), 73–80. E. G. Kiel, P. M. Heertjes, Metal Complexes of Alizarin. I. The Structure of the Calcium-Aluminium-Lake of Alizarin, J. Soc. Dyers Colour., 79 (1963), 21–27. E. G. Kiel, P. M. Heertjes, Metal Complexes of Alizarin. II. The Structure of some Metal Complexes of Alizarin other than Turkey Red, J. Soc. Dyers Colour., 79 (1963), 61–64. E. G. Kiel, P. M. Heertjes, Metal Complexes of Alizarin. IV. The Structure of the Potassium and Calcium Salts of Alizarin and of 3-Nitroalizarin, J. Soc. Dyers Colour., 79 (1963), 363–367. E. G. Kiel, P. M. Heertjes, Metal Complexes of Alizarin. V. Investigations of Alizarin-dyed Cotton Fabrics, J. Soc. Dyers Colour., 81 (1965), 98–102. C.-H. Wunderlich, G. Bergerhoff, Konstitution und Farbe von Alizarin- und Purpurin-Farblacken, Chem. Ber., 127 (1994), 1185–1190. C.-H. Wunderlich, Krapplack und Türkischrot, Dissertation, Universität Bonn, 1993. P. Soubayrol, G. Dana, Aluminium-27 Solid-state NMR Study of Aluminium Coordination Complexes of Alizarin, Magn. Reson. Chem., 34 (1996), 638–645. H. Harms, Zur Chemie des Karmins und seiner in der Mikroskopie verwendeten Lösungen, Naturwiss., 44(11) (1957), 327. S. N. Meloan, L. S. Valentine, H. Puchtler, On the Structure of Carminic Acid and Carmine, Histochem., 27 (1971), 87–95. C. Clementi, et al., Vibrational and electronic properties of painting lakes, Appl. Phys. A, 92 (2008), 25–33. R. A. Dixon, D.-Y. Xie, S. B. Sharma, Proanthocyanidins – a final frontier in flavonoid research? New Phytol., 165 (2005), 9–28. L. Pourcel, J.-M. Routaboul, V. Cheynier, L. Lepiniec, I. Debeaujon, Flavonoid oxidation in plants: from biochemical properties to physiological functions, Trends Plant Sci., 12(1) (2007), 29–36. F. He, Q.-H. Pan, Y. Shi, C.-Q. Duan, Biosynthesis and Genetic Regulation of Proanthocyanidins in Plants, Molecules, 13 (2008), 2674–2703. C. Santos-Buelga, A. Scalbert, Proanthocyanidins and tannin-like compounds – nature, occurrence, dietary intake and effects on nutrition and health, J. Sci. Food Agric., 80 (2000), 1094–1117. H. Musso, Phenol Oxidation Reactions, Angew. Chem., Int. Ed. Engl., 2(12) (1963), 723–735. R. M. Desentis-Mendoza, H. Hernández-Sánchez, A. Moreno, E. R. del C, L. Chel-Guerrero, J. Tamariz, M. E. Jaramillo-Flores, Enzymatic Polymerization of Phenolic Compounds Using Laccase and Tyrosinase from Ustilago maydis, Biomacromolecules, 7 (2006), 1845–1854. S. Guyot, J. Vercauteren, V. Cheynier, Structural Determination of Colourless and Yellow Dimers resulting from (+)-Catechin Coupling Catalysed by Grape Polyphenoxidase, Phytochemistry, 42(5) (1996), 1279–1288. R. Niemetz, G. G. Gross, Enzymology of gallotannin and ellagitannin biosynthesis, Phytochemistry, 66 (2005), 2001–2011.

Bibliography

[730] [731] [732] [733] [734] [735] [736] [737] [738] [739] [740] [741] [742] [743] [744] [745] [746] [747] [748] [749] [750] [751] [752] [753] [754] [755] [756] [757]

� 889

P. Grundhöfer, R. Niemetz, G. Schilling, G. G. Gross, Biosynthesis and subcellular distribution of hydrolyzable tannins, Phytochemistry, 57 (2001), 915–927. V. de Freitas, N. Mateus, Chemical transformations of anthocyanins yielding a variety of colours (Review), Environ. Chem. Lett., 4 (2006), 175–183. T. Shoji, Polyphenols as Natural Food Pigments: Changes during Food Processing, Amer. J. Food Technol., 2(7) (2007), 570–581. T. C. Somers, The Polymeric Nature of Wine Pigments, Phytochemistry, 10 (1971), 2175–2186. G.-I. Nonaka, O. Kawahara, I. Nishioka, Tannins and Related Compounds. XV. A New Class of Dimeric Flavan-3-ol Gallates, Theasinensins A and B, and Proanthocyanidin Gallates from Green Tea Leaf. (1), Chem. Pharm. Bull., 31(11) (1983), 3906–3914. E. Haslam, Thoughts on thearubigins, Phytochemistry, 64 (2003), 61–73. Y. Wang, C.-T. Ho, Functional Contribution of Polyphenols in Black Tea, in N. Da Costa et al. (eds.) Flavors in Noncarbonated Beverages, ACS Symposium Series, American Chemical Society, Washington, DC, 2010. H. Li, A. Guo, H. Wang, Mechanisms of oxidative browning of wine, Food Chem., 108 (2008), 1–13. B. Winkel-Shirley, Flavonoid biosynthesis. A colorful model for genetics, biochemistry, cell biology, and biotechnology, Plant Physiol., 126 (2001), 485–493. C.-T. Ho, S. Sang, J. W. Jhoo, Chemistry of Theaflavins: The Astringent Taste Compounds of Black Tea, in T. Hofmann (ed.) Challenges in Taste Chemistry and Biology, ACS Symposium Series, American Chemical Society, Washington, DC, 2003. C. F. Timberlake, Anthocyanins – Occurence, Extraction and Chemistry, Food Chem., 5 (1980), 69–80. K. Robards, P. D. Prenzler, G. Tucker, P. Swatsitang, W. Glover, Phenolic compounds and their role in oxidative processes in fruits, Food Chem., 66 (1999), 401–436. N. G. Irani, J. M. Hernandez, E. Grotewold, Regulation of Anthocyanin Pigmentation, Recent Adv. Phytochem., 38 (2003), 59–78. G. Forkmann, Flavonoids as Flower Pigments: The Formation of the Natural Spectrum and its Extension by Genetic Engineering, Plant Breed., 106 (1991), 1–26. R. W. Hemingway, P. E. Laks, Plant Polyphenols, Plenum Press, New York, 1992. G. G. Gross, Enzymes in the Biosynthesis of Hydrolyzable Tannins, in [744], pp. 43–60. E. Haslam, Gallic Acid and its Metabolites, in [744], pp. 169–194. J. Nicolas, V. Cheynier, A. Fleuriet, M.-A. Rouet-Mayer, Polyphenols and Enzymatic Browning, in A. Scalbert (ed.) Polyphenolic Phenomena, pp. 165–175, Institut Nationale de la Recherche Agronomique, Paris, 1993. H. Musso, Phenol Coupling, in [299], chapter 1, pp. 1–94. A. I. Scott, Some Natural Products Derived by Phenol Oxidation, in [299], chapter 2, pp. 95–117. B. R. Brown, Biochemical Aspects of Oxidative Couplings of Phenols, in [299], chapter 4, pp. 167–201. A. J. Taylor, F. M. Clydesdale, Potential of Oxidised Phenolics as Food Colourants, Food Chem., 24 (1987), 301–313. G. Jungbluth, Wechselwirkung von Polyphenolen mit Cu2⊕ , Fe3⊕ , Fe2⊕ und A13⊕ . Analyse der Reaktionsprodukte mittels HPLC, Dissertation, Universität Hannover, 2000. P. S. Vankar, D. Shukla, Spectrum of colors from reseda luteola and other natural yellow dyes, J. Text. Eng. Fash. Technol., 4(2) (2018), 106–119. N. Kuhnert, Unraveling the structure of the black tea thearubigins, Arch. Biochem. Biophys., 501 (2010), 37–51. N. Kuhnert, J. W. Drynan, J. Obuchowicz, M. N. Clifford, M. Witt, Mass spectrometric characterization of black tea thearubigins leading to an oxidative cascade hypothesis for thearubigin formation, Rapid Commun. Mass Spectrom., 24 (2010), 3387–3404. H. Obara, J. Onodera, Structure of Carthamin, Chem. Lett. (1979), 201–204. Y. Takahashi, et al., Constitution of two coloring matters in the flower petals of C. tinctorius, Tetrahedron, 23 (1982), 5163–5166.

890 � Bibliography

[758] [759] [760] [761] [762] [763] [764]

[765] [766] [767] [768] [769] [770] [771] [772] [773] [774] [775] [776] [777] [778]

[779] [780] [781] [782] [783]

K. Kazuma, et al., Quinochalcones and Flavonoids from Fresh Florets in Different Cultivars of C. tinctorius L, Biosci. Biotechnol. Biochem., 64(8) (2000), 1588–1599. G. Britton, D. Hornero-Méndez, Carotenoids and colour in fruit and vegetables, in [303], chapter 2, pp. 11–27. R. Brouillard, P. Figueiredo, M. Elhabiri, O. Dangles, Molecular interactions of phenolic compounds in relation to the colour of fruit and vegetables, in [303], chapter 3, pp. 29–49. M. J. Amiot, A. Fleuriet, V. Cheynier, J. Nicolas, Phenolic compounds and oxidative mechanism in fruit and vegetables, in [303], chapter 4, pp. 51–85. J. M. Harkin, Lignin – A Natural Polymeric Product of Phenol Oxidation, in [299], chapter 6, pp. 243–321. K. Weinges, R. Spänig, Lignans and Cyclolignans, in [299], chapter 7, pp. 323–355. B. B. Adhyaru, N. G. Akhmedov, A. R. Katritzky, C. R. Bowers, Solid-state cross-polarization magic angle spinning 13C and 15N NMR characterization of Sepia melanin, Sepia melanin free acid and Human hair melanin in comparison with several model compounds, Magn. Reson. Chem., 41 (2003), 466–474. A. Lopez-Montes, R. Blanc, T. Espejo, A. Navalon, J. L. Vilchez, Characterization of sepia ink in ancient graphic documents by capillary electrophoresis, Microchem. J., 93 (2009), 121–126. M. G. Peter, Chemische Modifikation von Biopolymeren durch Chinone und Chinonmethide, Angew. Chem., 101 (1989), 572–587. R. H. Thomson, Pigmente aus rötlichen Haaren und Federn, Angew. Chem., 86 (1974), 355–386. M. Piattelli, R. A. Nicolaus, The structure of melanins and melanogenesis – I. The structure of melanin in Sepia, Tetrahedron, 15 (1961), 66–75. J. J. Nordlund (ed.), The Pigmentary System: Physiology and Pathphysiology, 2nd ed., Oxford University Press, 2006. ISBN 978-1-4051-2034-0. S. Ito, K. Wakamatsu, Chemistry of Melanins, in [769], pp. 282–310. M. L. Wolbarsht, A. W. Walsh, G. George, Melanin, a unique biological absorber, Appl. Opt., 20(13/1) (1981), 2184–2186. M. G. Peter, Chemische Modifikation von Biopolymeren durch Chinone und Chinonmethide, Angew. Chem., 101 (1989), 572–587. A. Vetter, Bereitstellung von Naturfarbstoffen aus Färberpflanzen, 2004, Abschlussbericht des Projektes Projekt/Förderkennzeichen (FKZ): 00NR185 der Thüringer Landesanstalt für Landwirtschaft. U. Kynast, Vorlesungsscript Optische Eigenschaften der Materialien II: Farbe und Pigmente, FH Münster, https://www.fh-muenster.de/fb1/downloads/personal/VLGMWFarbPig1.pdf. H. Michaelsen, A. Unger, C.-H. Fischer, Blaugrüne Färbung an Intarsienhölzern des 16. bis 18. Jahrhunderts, Restauro, 1 (1992), 17–25. P. Kopp, H. Piening, Wiederentdeckte Farbigkeit der Renaissance, Restauro, 2 (2009), 107–111. G. Buschle-Diller, A. Unger, Schreib- und Malmittel auf Papyrus, Restauro, 2 (1996), 108–114. C. Jenkins, Analyzing Pigments in the Book of the Dead Using XRF Spectroscopy, Blog-Beitrag vom 26.01.2011, Brooklyn Museum https://www.brooklynmuseum.org/community/blogosphere/ 2011/01/26/analyzing-pigments-in-the-book-of-the-dead-using-xrf-spectroscopy/, accessed on 2022-06-20. O. Wächter, Die Restaurierung und Konservierung von Miniaturen, Beitrag zum IADA-Kongress 1975 Kopenhagen, https://cool.culturalheritage.org/iada/ta75_047.pdf. A. von Bohlen, P. Vandenabeerle, M. De Reu, L. Moens, R. Klockenkämper, B. Dekeyzer, B. Cardon, Pigmente und Tinten in mittelalterlichen Handschriften, Restauro, 2 (2003), 118–122. P. Gregory, Colorants for Electronic Printers, in [153], pp. 150–167. R. B. McKay, Control of the Physical Character and Performance of Organic Pigments for Inks, in [153], pp. 107–126. R. Fraas, Aqueous ink with prolonged cap-off time and method of making it, Europäisches Patent EP0897961, https://patents.google.com/patent/EP0897961B1/en?oq=EP0897961, accessed on 2022-06-10.

Bibliography

[784] [785]

[786]

[787]

[788] [789]

[790] [791] [792] [793]

[794] [795] [796] [797] [798]

[799] [800] [801] [802] [803] [804] [805] [806]

� 891

H. Asami, A. Owariasahi-shi, Wässerige Kugelschreiber-Tintezusammensetzung, Deutsches Patent DE69910670T2, https://patents.google.com/patent/DE69910670T2/de, accessed on 2022-06-10. EuPIA, European Printing Ink Association, https://www.eupia.org/, Onlineinformationen zu Druckfarben, EXCLUSION LIST FOR PRINTING INKS AND RELATED PRODUCTS, 4th ed., 2021, https: //www.eupia.org/fileadmin/Documents/Our_commitment/20210310_-Exclusion_Policy_for_ Printing_Inks_and_Related_Products_final_March_2021.pdf, accessed on 2022-06-20. T. Brüning, H. U. Käfferlein, A. Slowicki, Azofarbmittel und deren Hautgängigkeit beim Menschen, BGFA-Report 2, Februar 2009, BGFA – Forschungsinstitut für Arbeitsmedizin der Deutschen Gesetzlichen Unfallversicherung, Institut der Ruhr-Universität Bochum, ISSN 1867-9358, https: //dguv.de/medien/ipa/publikationen/ipa-reporte/09-02-27_bgfa-report2_azofarbstoffe.pdf, accessed on 2022-06-20. M. Beth-Hübner, B. Brandt, R. Rupp, V. Neumann, U. Eickmann, G. Lindner, et al., Aromatische Amine – Eine Arbeitshilfe in Berufskrankheiten-Ermittlungsverfahren (BK-Report 1/2019), Deutsche Gesetzliche Unfallversicherung e. V., Berlin, 2019. ISBN 978-3-948657-03-1, https://publikationen.dguv.de/ widgets/pdf/download/article/3520, accessed on 2022-05-29. H. S. Freeman, Aromatic amines: use in azo dye chemistry, Front. Biosci., 18 (2013), 145–164. P. Sagelsdorff, R. Haenggi, B. Heuberger, R. Joppich-Kuhn, R. Jung, H. J. Weideli, M. Joppich, Lack of bioavability of dichlorobenzidine from diarylide azo pigments: molecular dosimetry for hemoglobin and DNA adducts, Carcinogenesis, 17(3) (1996), 507–514. K. Golka, S. Kopps, Z. W. Myslak, Carcinogenicity of azo colorants: influence of solubility and bioavailability, Toxicol. Lett., 15(1) (2004), 203–210. F. Nakaya, Y. Fukase, Laser Printer, in N. Ohta, M. Rosen (eds.) Color Desktop Printer Technology, pp. 157–192, CRC Taylor and Francis, 2006. ISBN 0-8247-5364-X. H.-T. Macholdt, Organische Pigmente für Photokopierer und Laserdrucker, Chem. Unserer Zeit, 24(4) (1990), 176–181. P. Gregory, Colorants for High Technology, in A. T. Peters, H. S. Freeman (eds.) Colour Chemistry: The Design and Synthesis of Organic Dyes and Pigments, pp. 193–223, Elsevier, New York, 1991. ISBN 978-1851665778. B. E. Springett, A Brief Introduction to Electrophotography, in [307], chapter 4, pp. 145–171. P. C. Julien, R. J. Gruber, Dry Toner Technology, in [307], chapter 5, pp. 173–208. L. O. Jones, Carrier Materials for Imaging, in [307], chapter 6, pp. 209–238. J. R. Larson, G. A. Gibson, S. P. Schmidt, Liquid Toner Materials, in [307], chapter 7, pp. 239–264. G. Banik, H. Weber, Tintenfrassschäden und ihre Behandlung, Werkhefte der staatlichen Archivverwaltung Baden-Württemberg, Heft 10, Verlag W. Kohlhammer, Stuttgart, 1999. ISBN 3-17-015377-3. C. Krekel, Chemische Struktur historischer Eisengallustinten, in [798], pp. 25–36. C. Krekel, Chemische Untersuchungen an Eisengallustinten und Anwendung der Ergebnisse bei der Begutachtung einer mittelalterlichen Handschrift, Diplomarbeit Universität Göttingen, 1990. C.-H. Wunderlich, Geschichte und Chemie der Eisengallustinte, Restauro, 100 (1994), 414–421. R. Fuchs, Der Tintenfrass historischer Tinten und Tuschen – ein komplexes, nie enden wollendes Problem, in [798], pp. 37–75. C.-H. Wunderlich, R. Weber, G. Bergerhoff, Über Eisengallustinte, Z. Anorg. Allg. Chem., 598/599 (1991), 371–376. F. Zetsche, G. Vieli, G. Lilljeqvist, A. Loosli, Bildung und Altern der Schriftzüge, die primären Tintensalze der Eisentinten, Justus Liebigs Ann. Chem., 435 (1924), 233–264. F. Zetsche, A. Loosli, Bildung und Altern der Schriftzüge, 2. Mitteilung, Liebigs Ann. Chem., 445 (1926), 283–296. A. S. Lee, P. J. Mahon, D. C. Creagh, Raman analysis of iron gall inks on parchment, Vib. Spectrosc., 41 (2006), 170–175.

892 � Bibliography

[807] [808] [809] [810] [811] [812] [813]

[814] [815] [816] [817] [818] [819] [820] [821] [822] [823] [824] [825] [826] [827]

[828] [829]

J. G. Neevel, Phytate: a Potential Conservation Agent for the Treatment of Ink Corrosion Caused by Irongall Inks, Restaurator, 16 (1995), 143–160. G. Farusi, Monastic Inks: Linking Chemistry and History, Sci. School, 6 (2007), 36–40. G. Pfingstag, Colorants in Inks for Writing, Drawing and Marking, JSDC, 109 (1993), 188–192. W. J. Wnek, M. A. Andreottola, P. F. Doll, S. M. Kelly, Ink Jet Ink Technology, in [307], chapter 14, pp. 531–602. D. E. Bugner, Papers and Films for Ink Jet Printing, in [307], chapter 15, pp. 603–627. E. Kuckert, Tinten für Ink-Jet Verfahren, presentation Bayer AG 1999, http://www.f07.fh-koeln. de/einrichtungen/imp/forschung_und_gestaltung/showroom/00161/index.html, accessed on 2011-06-10, no longer available. M. C. Jürgens, Preservation of Ink Jet Hardcopies. An Investigation by Martin C. Jürgens for the Capstone Project, Cross-Disciplinary Studies, at Rochester Institute of Technology, Rochester, NY, August 27, 1999, https://www.ica.org/sites/default/files/WG_1999_PAAG-preservation-of-ink-jet-hardcopies_EN.pdf, accessed on 2022-06-20. W. Li, J. Danio, Fundamentals in the Development of High Performance Inkjet Receptive Coatings, TAPPI Conference Paper, 2008 Advanced Coating Fundamentals Symposium, Product Code 08ADV16. E. Svanholm, Printability and Ink-Coating Interactions in Inkjet Printing, Dissertation, Karlstad University Studies 2007:2, http://www.diva-portal.org/smash/get/diva2:6256/FULLTEXT01.pdf, accessed on 2022-06-20. T. Lamminmäki, J. Kettle, P. Puukko, J. Ketoja, P. Gane, The role of binder type in determining inkjet print quality, Nord. Pulp Pap. Res. J., 25(3) (2010), 380–390. D. L. Briley, Polyester Media Development for Inkjet Printers, Hewlett-Packard J. (1994), 28–34. R. W. Kenyon, Beyond the Black – Novel High Waterfast Dyes for Colour Ink Jet Printing, in I. Rezanka, R. Eschbach (eds.) Recent Progress in Ink Jet Technologies, pp. 278–280, Society for Imaging Science and Technology, 1996. ISBN 0-89208-192-9. W. Bauer, J. Ritter, Novel Dyes for Ink Jet Applications, in I. Rezanka, R. Eschbach (eds.) Recent Progress in Ink Jet Technologies, pp. 295–296, Society for Imaging Science and Technology, 1996. ISBN 0-89208-192-9. K. Lussky, P. Engel, S. Pentzien, J. Krüger, Analyse häufig verwendeter europäischer Papiere, Restauro, 7 (2007), 472–478. Application information of the Competence Center Specialities (Lanxess), Anorganische Pigmente für die Papierindustrie, Januar 2006, and Buntpigmente zur Einfärbung von Dekorpapier, edition 09/2008. A. Pingel Keuth, Von Zellstoff zu Filtertüte, Schreibpapier,…Papierproduktion, Chem. Unserer Zeit, 39 (2005), 402–409. K. Roth, Chemie kontra Papierzerfall, Chem. Unserer Zeit, 40 (2006), 54–62. W. Griebenow, Alterserscheinungen bei Papier – vorwiegend aus chemischer Sicht, Teil 1, Restauro, 5 (1991), 329–335. W. Griebenow, Alterserscheinungen bei Papier – vorwiegend aus chemischer Sicht, Teil 2, Restauro, 6 (1991), 409–415. E. Gruber, Papier-Chemie, Scriptum der Dualen Hochschule Karlsruhe für den Lehrgang Papiertechnologie, http://www.gruberscript.net/papierchemie.html, accessed on 2022-06-10. E. Bojaxhiu, Untersuchung der Abbauprodukte der chemischen und elektrochemischen Reduktion ausgewählter Dispersionsfarbstoffe mittels LC- und GCxGC-(TOF)MS, Dissertation, Wuppertal 2008, http://elpub.bib.uni-wuppertal.de/servlets/DerivateServlet/Derivate-896/dc0811.pdf, accessed on 2022-06-10. Water Soluble Polymers, Brochure by SNF, https://www.snf.com/wp-content/uploads/2019/12/WaterSoluble-Polymers-EN.pdf, accessed on 2022-06-10. B. Kießler, Verbesserung der Papierfestigkeiten und der Prozesssicherheit bei der Oberflächenbehandlung von Wellpappenrohpapieren durch spezifischen Stärkeaufschluss, research report to project InnoWatt-Projekt IW 050275 by Papiertechnischen Stiftung PTS, http://www.ptspaper.de/fileadmin/ PTS/Dokumente/Forschung/Forschungsprojekte/IW_050275.pdf, no longer accessible.

Bibliography

[830]

[831] [832] [833] [834] [835] [836] [837] [838] [839] [840]

[841] [842] [843] [844] [845] [846] [847] [848] [849] [850] [851] [852]

� 893

M. Suhr, G. Klein, I. Kourti, M. R. Gonzalo, G. G. Santonja, S. Roudier, L. D. Sancho, Best Available Techniques (BAT) Reference Document for the Production of Pulp, Paper and Board, European Union, 2015, https://op.europa.eu/en/publication-detail/-/publication/fcaab1a5-d287-40af-b21c115e529685fc/language-en/format-PDF, accessed on 2022-06-02. MedienStandard Druck 2018. Technische Richtlinien für Daten, Prüfdruck und Auflagendruck, Bundesverband Druck und Medien e. V. (bvdm), 2016–2018, Art.-Nr. 86035, https://www.bvdmonline.de/fileadmin/Themen/T_F/R_H/MedienStandard_Druck_2018.pdf, accessed on 2022-06-18. Paper Categorization Meeting ICC/ISO TC130, meeting minutes, ICC DevCon ’06, Leeds, 15.06.2006, https://www.color.org/papermeetingminutes.pdf, accessed on 2022-06-18. D. Simmert, Acrylharzkünstlerfarben, Kunsttechnologie und Konservierung, pp. 78–105, Wernersche Verlagsgesellschaft, Worms, 1995. R. E. Harren, History and Development of Acrylic Latex Coatings, in R. B. Seymour, H. F. Mark (eds.) Organic Coatings: Their Origin and Development, Proceedings of the International Symposium on the History of Organic Coatings, pp. 297–314, Elsevier, 1990. R. N. Hildred, Acrylate als Additive für wäßrige Beschichtungsstoffe, farbe+lack, 96 (1990), 857–859. E. S. Daniels, A. Klein, Development of cohesive strength in polymer films from latices: effect of polymer chain interdiffusion and crosslinking, Prog. Org. Coat., 19 (1991), 359–378. P.-B. Eipper, Problemfall Acrylfarbenoberflächen. Teil 1: Hintergründe zur Beständigkeit von Acrylfarben, Restauro, 1 (2009), 30–35. P.-B. Eipper, Problemfall Acrylfarbenoberflächen. Teil 2: Schäden und Restaurierungsmöglichkeiten, Restauro, 2 (2009), 112–122. P.-B. Eipper, Problemfall Acrylfarbenoberflächen. Teil 3: Untersuchungen von Veränderungen nach einer Oberflächenreinigung und Entwicklung eines Reinigungsablaufs, Restauro, 4 (2009), 244–251. E. Jablonski, T. Learner, J. Hayes, M. Golden, Conservation Concerns for Acrylic Emulsion Paints: A Literature Review, Tate Papers, TATE’S ONLINE RESEARCH JOURNAL, ISSN 1753-9854, Autumn 2004, https://www.tate.org.uk/research/tate-papers/02/conservation-concerns-for-acrylic-emulsionpaints-literature-review, accessed on 2022-06-10. M. Golden, Mural Paints: Current and Future Formulations, The Getty Conservation Institute, Los Angeles, 2004, https://www.getty.edu/conservation/publications_resources/pdf_publications/ pdf/golden.pdf, accessed on 2022-06-20. A. van Tent, K. te Nijenhuis, The Film Formation of Polymer Particles in Drying Thin Films of Aqueous Acrylic Latices. II. Coalescence, Studied with Transmission Spectrophotometry, J. Colloid Interface Sci., 232 (2000), 350–363. F. Dobler, Y. Holl, Mechanisms of Particle Deformation During Latex Film Formation, in T. Provder et al., Film Formation in Waterborne Coatings, ACS Symposium Series, American Chemical Society, Washington, DC, 1996. J. L. Keddie, Film formation of latex, Mater. Sci. Eng., 21 (1997), 101–170. M. A. Winnik, Latex film formation, Curr. Opin. Colloid Interface Sci., 2(2) (1997), 192–199. P. A. Steward, J. Hearn, M. C. Wilkinson, An overview of polymer latex film formation and properties, Adv. Colloid Interface Sci., 86(3) (2000), 195–267. R. E. Dillon, L. A. Matheson, E. B. Bradford, Sintering of Synthetic Latex Particles, Colloid Sci., 6 (1951), 108. G. L. Brown, Formation of Films from Polymer Dispersions, J. Polym. Sci., 22 (1956), 423–434. D. P. Sheetz, E. C. Britton, Formation of Films by Drying of Latex, J. Appl. Polym. Sci., 9(11) (1965), 3759–3773. F. Dobler, T. Pith, M. Lambla, Y. Holl, Coalescence Mechanisms of Polymer Colloids. I. Coalescence under the Influence of Particle-Water Interfacial Tension, J. Colloid Interface Sci., 152(1) (1992). P. A. Lovell, M. S. El-Aasser, Emulsion Polymerization and Emulsion Polymers, John Wiley & Sons, 1997. ISBN 0-471-96746-7. M. A. Winnik, The Formation and Properties of Latex Films, in [851], pp. 467–518.

894 � Bibliography

[853] [854] [855] [856]

[857] [858] [859] [860]

[861] [862] [863] [864] [865] [866] [867] [868] [869] [870] [871] [872] [873] [874] [875] [876] [877]

[878]

P. M. Lesko, P. R. Sperry, Acrylic and Styrene-Acrylic Polymers, in [851], pp. 619–655. C. Heyn, Mikrobieller Angriff auf synthetische Polymere, Dissertation 2002, Universität Oldenburg, https://oops.uni-oldenburg.de/181/, accessed on 2022-06-20. R. D. Hester and D. R. Squire, Rheology of Waterborne Coatings, Journal of Coatings Technology, 69(864) (1997), 109–114. M. Laschet, Rheo-mechanische und rheo-optische Charakterisierung wäßriger Lösungen von Hydroxyethylcellulosen und deren hydrophob modifizierter Derivate im Hinblick auf supramolekulare Strukturen, Dissertation 2002, Universität Hamburg. J. E. Glass (ed.), Associative Polymers in Aqueous Media, ACS Symposium Series, vol. 765, Washington D. C., 2000. ISBN 0-8412-3659-3. V. Tirtaatmadja, K. C. Tam, R. D. Jenkins, Rheological Properties of Model Alkali-Soluble Associative (HASE) Polymers: Effect of Varying Hydrophobe Chain Length, Macromolecules, 30 (1997), 3271–3282. A. A. Abdala, K. Olesen, S. A. Khan, Solution rheology of hydrophobically modified associative polymers: Solvent quality and hydrophobic interactions, J. Rheol., 47(2) (2003), 497–511. R. J. English, S. R. Raghavan, R. D. Jenkins, S. A. Khan, Associative polymers bearing n-alkyl hydrophobes: Rheological evidence for microgel-like behavior, J. Rheol., 43(5) (September/October 1999), 1175–1194. R. J. English, H. S. Gulati, R. D. Jenkins, S. A. Khan, Solution rheology of a hydrophobically modified soluble associative polymer, J. Rheol., 41(2) (March/April 1997), 427–444. H. J. Spinelli, Polymeric Dispersants in Ink Jet Technology, Adv. Mater., 10(15) (1998), 1215–1218. C. M. Miller, K. R. Olesen, G. D. Shay, Determination of the Thickening Mechanism of a Hydrophobically Modified Alkali Soluble Emulsion Using Dynamic Viscosity Measurements, in [857], pp. 338–350. W. P. Seng, K. C. Tam, R. D. Jenkins, D. R. Bassett, The Network Strength and Junction Density of a Model HASE Polymer in Non-Ionic Surfactant Solutions, in [857], pp. 351–368. R. J. English, R. D. Jenkins, D. R. Bassett, S. A. Khan, Rheology of a HASE Associative Polymer and Its Interaction with Non-Ionic Surfactants, in [857], pp. 369–380. P. T. Elliott, L. Xing, W. H. Wetzel, J. E. Glass, Behavior of Branched-Terminal, Hydrophobe-Modified, Ethoxylated Urethanes, in [857], pp. 163–178. H. N. Naé, W. W. Reichert, Rheological properties of lightly crosslinked carboxy copolymers in aqueous solutions, Rheol. Acta, 31 (1992), 351–360. L. A. De Graaf, Denaturation of proteins from a non-food perspective, J. Biotechnol., 79 (2000), 299–306. D. E. Graham, M. C. Phillips, Proteins at Liquid Interfaces III. Molecular Structures of Adsorbed Films, J. Colloid Interface Sci., 70(3) (1979), 427–439. P. Walstra, A. L. de Roos, Proteins at air-water and oil-water interfaces: static and dynamic aspects, Food Rev. Int., 9(4) (1993), 503–525. S. Damodaran, Interfaces, Protein Films, and Foams, Adv. Food Nutr. Res., 34 (1990), 1–79. E. Dickinson, Y. Matsumura, Time-dependent polymerization of β-lactoglobulin through disulphide bonds at the oil-water interface in emulsions, Int. J. Biol. Macromol., 13 (1991), 26–30. E. A. Hauser, L. E. Swearingen, The Aging of Surfaces of Aqueous Solutions of Egg Albumin, J. Phys. Chem., 45(4) (1941), 644–659. A. E. Mirsky, Sulfhydryl Groups in Films of Egg Albumin, J. Gen. Physiol., 24(6) (1941), 725–733. H. B. Bull, Studies on Surface Denaturation of Egg Albumin, J. Biol. Chem., 123 (1938), 17–30. A. Gennadios, C. L. Weller, M. A. Hanna, G. W. Froning, Mechanical and Barrier Properties of Egg Albumen Films, J. Food Sci., 61(3) (1996), 585–589. K. Broersen, A. M. M. van Teffelen, A. Vries, A. G. J. Voragen, R. J. Hamer, H. H. J. de Jongh, Do Sulfhydryl Groups Affect Aggregation and Gelation Properties of Ovalbumin? J. Agric. Food Chem., 54 (2006), 5166–5174. D. Fukushima, J. van Buren, Mechanism od Protein Insolubilization during the Drying of Soy Milk. Role of Disulfide and Hydrophobic Bonds, Am. Assoc. Cereal Chem., 47 (1970), 687–696.

Bibliography

[879]

� 895

N. Kitabatake, E. Doi, Conformational Change of Hen Egg Ovalbumin during Foam Formation Detected by 5, 5’-Dithiobis(2-nitrobenzoic acid), J. Agric. Food Chem., 35 (1987), 953–957. [880] A. C. C. Alleoni, Albumen Protein and Functional Properties of Gelation and Foaming, Sci. Agric., 63(3) (2006), 291–298. [881] H. J. T. Beveridge, A Study of some Physical and Chemical Properties of Egg White, Thesis, University of British Columbia, 1973, https://open.library.ubc.ca/soa/cIRcle/collections/ubctheses/831/items/1. 0100825, accessed on 2022-06-20. [882] A. Deuerling, Die Physik und Chemie der “Mousse au Chocolat”, Schriftliche Hausarbeit für die erste Staatsexamensprüfung, Julius-Maximilian-Universität Würzburg, 2010, http://www.thomaswilhelm.net/arbeiten/Mousse_au_Chocolat.pdf, accessed on 2022-06-20. [883] F. E. Cunningham, Properties of Egg White Foam Drainage, Poultry Sci., 55 (1976), 738–743. [884] S. Poole, S. I. West, C. L. Walters, Protein-Protein Interactions: Their Importance in the Foaming of Heterogeneous Protein Systems, J. Sci. Food Agric., 35 (1984), 701–711. [885] A. Hoppe, Examination of egg white proteins and effects of high pressure on select physical and functional properties, PhD Thesis, University of Nebraska, Lincoln, 2010, https://digitalcommons. unl.edu/foodscidiss/7/, accessed on 2022-06-20. [886] B. Brodsky, J. A. Werkmeister, J. A. M. Ramshaw, Collagens and Gelatins, Band 8, in S. R. Fahnestock, A. Steinbüchel (eds.) Biopolymers, WILEY-VCH, 2003. ISBN 3-527-30223-9. [887] M. Djabourov, J. Leblond, Thermally Reversible Gelation of the Gelatin-Water System, in Reversible Polymeric Gels and Related Systems, ACS Symposium Series, vol. 350, pp. 211–223, American Chemical Society, 1987. ISBN 9-780841214156. [888] A. G. Ward, The physical properties of gelatin solutions and gels, Br. J. Appl. Phys., 5 (1954), 85–90. [889] M. Bansal, C. Ramakrishnan, G. N. Ramachandran, Stabilization of the collagen structure by hydroxyproline residues, Proc. Indian Acad. Sci. A, 82(4) (1975), 152–164. [890] S. Zumbühl, Proteinische Leime – Ein vertrauter Werkstoff? Z. Kunsttechnol. Konserv. (2003), 95–104. Wernersche Verlagsgesellschaft Worms. [891] E. Reinkowski-Häfner, Tempera, Z. Kunsttechnol. Konserv., 2 (1994), 297–317. Wernersche Verlagsgesellschaft Worms. [892] M. Lazzari, O. Chiantore, Drying and oxidative degradation of linseed oil, Polym. Degrad. Stab., 65 (1999), 303–313. [893] J. Mallégol, J.-L. Gardette, J. Lemaire, Long-Term Behavior of Oil-Based Varnishes and Paints I. Spectroscopic Analysis of Curing Drying Oils, J. Am. Oil Chem. Soc., 76(8) (1999), 967–976. [894] J. Mallégol, J.-L. Gardette, J. Lemaire, Long-Term Behavior of Oil-Based Varnishes and Paints. Fate of Hydroperoxides in Drying Oils, J. Am. Oil Chem. Soc., 77(3) (2000), 249–255. [895] H. P. Kaufmann, Oxydation und Verfilmung trocknender Öle, Fette Seifen Anstrichm., 59(3) (1957), 153–162. [896] J. Mallégol, J. Lemaire, J.-L. Gardette, Drier influence on the curing of linseed oil, Prog. Org. Coat., 39 (2000), 107–113. [897] E. Ioakimoglou, S. Boyatzis, P. Argitis, A. Fostiridou, K. Papapanagiotou, N. Yannovits, Thin-Film Study on the Oxidation of Linseed Oil in the Presence of Selected Copper Pigments, Chem. Mater., 11 (1999), 2013–2022. [898] E. N. Frankel, Lipid Oxidation: Mechanisms, Products and Biological Significance, J. Am. Oil Chem. Soc., 61(12) (1984), 1908–1917. [899] E. N. Frankel, Lipid Oxidation, Prog. Lipid Res., 19 (1980), 1–22. [900] E. N. Frankel, Recent Advances in Lipid Oxidation, J. Sci. Food Agric., 54 (1991), 495–511. [901] E. N. Frankel, Secondary Products of Lipid Oxidation, Chem. Phys. Lipids, 44 (1987), 73–85. [902] N. A. Porter, Mechanisms for the Autoxidation of Polyunsaturated Lipids, Ace. Chem. Res., 19 (1986), 262–268. [903] W. J. Muizebelt, M. W. F. Nielen, Oxidative Crosslinking of Unsaturated Fatty Acids Studied with Mass Spectrometry, J. Mass Spectrom., 31 (1996), 545–554.

896 � Bibliography

[904] [905] [906] [907] [908] [909] [910] [911] [912] [913] [914] [915] [916]

[917]

[918]

[919] [920] [921] [922] [923] [924] [925] [926]

W. J. Muizebelt, J. C. Hubert, R. A. M. Venderbosch, Mechanistic study of drying of alkyd resins using ethyl linoleate as a model substance, Prog. Org. Coat., 24 (1994), 263–279. D. Erhardt, C. S. Tumosa, M. F. Mecklenburg, Long-Term Chemical and Physical Processes in Oil Paint Films, Stud. Conserv., 50(2) (2005), 143–150. J. Mallégol, J.-L. Gardette, J. Lemaire, Long-Term Behavior of Oil-Based Varnishes and Paints. Photo- and Thermooxidation of Cured Linseed-Oil, J. Am. Oil Chem. Soc., 77(3) (2000), 257–263. C. Boelhouwer, Th. Knegtel, M. Tels, On the Mechanism of the Thermal Polymerization of Linseed Oil, Fette, Seifen, Anstrichmittel, 69(6) (1967), 432–436. H. Wexler, Polymerization of Drying Oil, Chem. Rev., 64(6) (1964), 591–611. G. Dobson, W. W. Christie, J. L. Sebedio, Saturated Bicyclic Fatty Acids formed in Heated Sunflower Oils, Chem. Phys. Lipids, 87 (1997), 137–147. G. Dobson, J. L. Sebedio, Monocyclic dienoic fatty acids formed from γ-linolenic acid in heated evening primrose oil, Chem. Phys. Lipids, 97 (1999), 105–118. H. P. Kaufmann, Diels-Alder-Reaktionen auf dem Fettgebiet, Fette, Seifen, Anstrichmittel, 64(12) (1962), 1115–1126. G. Billek, Die Veränderungen von Nahrungsfetten bei höheren Temperaturen, Fat. Sci. Technol., 94(5) (1992), 161–172. W. C. Ault, J. C. Cowan, J. P. Kass, J. E. Jackson, Polymerization of Drying Oils, Ind. Eng. Chem., 34 (1942), 1120–1123. J. Koller, U. Baumer, Die Benzinlösemittel (des Restaurators), Restauro, 7 (2001), 528–536. F. Casadio, K. Keune, P. Noble, A. van Loon, E. Hendriks, S. A. Centeno, G. Osmond (eds.), Metal Soaps in Art: Conservation and Research, Cultural Heritage Science, Crown, 2019. ISBN 978-3-319-90616-4. P. Noble, A. van Loon, J. J. Boon, Selective darkening of ground layers associated with the wood grain in 17th-century panel paintings, in J. Townsend, T. Doherty, G. Heydenreich (eds.) Preparation for painting: the Artist’s choice and its consequences, pp. 68–78, Archetype, London, 2008, https: //www.researchgate.net/publication/290997156, accessed on 2022-06-12. P. Noble, A. van Loon, J. J. Boon, Chemical changes in old master paintings II: darkening due to increased transparency as a result of metal soap formation, in I. Verger James, James (eds.) Preprints of the ICOM committee for conservation 14th triennial meeting, The Hague, 12–16 Sep 2005, pp. 496–503, 2005, https://www.researchgate.net/publication/305687557, accessed on 2022-06-12. J. J. Boon, K. Keune, J. van der Weerd, P. Noble, Mechanical and chemical changes in Old Master Paintings: dissolution, metal soap formation and remineralization processes in lead pigmented ground/intermediate paint layers of 17th century paintings, in R. Vontobel (ed.) Icom-cc 13th Triennial Meeting Preprints, Rio de Janeiro, 22–27 Sep 2002, pp. 401–406, 2002. https://www.researchgate.net/ publication/283363916, accessed on 2022-06-12. A. van Loon, Color changes and chemical reactivity in seventeenth-century oil paintings, PhD thesis, University of Amsterdam, 2008, https://hdl.handle.net/11245/1.292118, accessed on 2022-06-12. J. J. Hermans, Metal soaps in oil paint: Structure, mechanisms and dynamics, PhD thesis, University of Amsterdam, 2017, https://hdl.handle.net/11245.1/53663926-183c-40aa-b7b3-e6027979cb7d, accessed on 2022-06-12. J. Koller, U. Baumer, Die Bindemittel der Schule von Barbizon, in [138], pp. 343–369. J. Koller, I. Fiedler, U. Baumer, Die Bindemittel auf Dürers Tafelgemälden, in [148], pp. 102–119. J. Koller, U. Baumer, Jacopo Tintoretto: Die Bindemittel seiner Schnellmalerei, in [139], pp. 213–225. R. White, J. Kirby, Rembrandt and his Circle: Seventeenth Century Dutch Paint Media Re-examined, Natl. Gallery Tech. Bull., 15 (1994), 64–78. ISBN 1-85709-049-7. R. White, The Characterization of Proteinaceous Binders in Art Objects, Natl. Gallery Tech. Bull., 8 (1984), 5–14. ISBN 0-901791-94-6. A. Spyros, D. Anglos, Studies of organic paint binders by NMR spectroscopy, Appl. Phys. A, 83 (2006), 705–708.

Bibliography

[927]

[928] [929] [930] [931] [932] [933] [934] [935] [936] [937] [938] [939] [940] [941] [942] [943] [944] [945] [946] [947] [948] [949] [950] [951] [952]

� 897

A. Lluveras-Tenorio, J. Mazurek, A. Restivo, M. P. Colombini, I. Bonaduce, The Development of a New Analytical Model for the Identification of Saccharide Binders in Paint Samples, PLoS ONE, 7(11) (2012), e49383. https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0049383, accessed on 2022-06-20. I. Bonaduce, L. Carlyle, M. P. Colombini, C. Duce, C. Ferrari, et al., New Insights into the Ageing of Linseed Oil Paint Binder: A Qualitative and Quantitative Analytical Study, PLoS ONE, 7(11), e49333 (2012). https://doi.org/10.1371/journal.pone.0049333, accessed on 2022-06-20. R. White, J. Pilc, Analyses of paint media, Natl. Gallery Tech. Bull., 16 (1995), 85–95. ISBN 1-85709-071-3. R. White, J. Pilc, Analyses of paint media, Natl. Gallery Tech. Bull., 17 (1996), 91–103. ISBN 1-85709-113-2. C. Higgitt, R. White, Analyses of Paint Media: New Studies of Italian Paintings of the Fifteenth and Sixteenth Centuries, Natl. Gallery Tech. Bull., 26 (2005), 88–97. ISBN 1-85709-341-0. J. Mills, R. White, Paint Media Analyses, Natl. Gallery Tech. Bull., 13 (1989), 69–71. ISBN 0-947645-705. R. White, J. Pilc, J. Kirby, Analyses of Paint Media, Natl. Gallery Tech. Bull., 19 (1998), 74–95. ISBN 1-85709-220-1. J. H. Townsend, L. Carlyle, A. Burnstock, M. Odlyha, J. J. Boon, Nineteenth-Century Paint Media: the Formulation and Properties of Megilps, in [106], pp. 205–210. J. Mills, R. White, Analyses of Paint Media, Natl. Gallery Tech. Bull., 5 (1981), 66–67. ISBN 0-901791-76-8. J. Mills, R. White, Analyses of Paint Media, Natl. Gallery Tech. Bull., 1 (1977), 57–59. J. Mills, R. White, Analyses of Paint Media, Natl. Gallery Tech. Bull., 3 (1979), 66–67. J. Mills, R. White, Analyses of Paint Media, Natl. Gallery Tech. Bull., 4 (1980), 65–67. ISBN 0-901791-72-5. J. Mills, R. White, Analyses of Paint Media, Natl. Gallery Tech. Bull., 7 (1983), 65–67. ISBN 0-901791-88-1. J. Mills, R. White, Analyses of Paint Media, Natl. Gallery Tech. Bull., 9 (1985), 70–71. ISBN 0-901791-97-0. J. Mills, R. White, Analyses of Paint Media, Natl. Gallery Tech. Bull., 11 (1987), 92–95. ISBN 0-947645-35-7. J. Mills, R. White, Analyses of Paint Media, Natl. Gallery Tech. Bull., 12 (1988), 78–79. ISBN 0-947645-63-2. R. White, Van Dyck’s Paint Medium, Natl. Gallery Tech. Bull., 20 (1999), 84–88. ISBN 1-85709-251-1. R. G. Weissenhorn, Chemie in der Freskomalerei, Chem. Unserer Zeit, 36(5) (2002), 310–316. J. Koller, U. Baumer, Kunstharzfirnisse. Teil I: Geschichte und Verwendung von Naturharzen und Kunstharzen in Firnissen, Restauro, 7 (2000), 534–537. J. Koller, U. Baumer, Kunstharzfirnisse. Teil II: Mechanische Eigenschaften, Alterungsverhalten und Löslichkeit von polymeren Kunstharzfirnissen, Restauro, 8 (2000), 616–625. J. Koller, U. Baumer, Kunstharzfirnisse. Teil III: Die niedermolekularen (nichtpolymeren) Kunstharzfirnisse, Restauro, 1 (2001), 26–38. S. Zumbühl, R. D. Knochenmuss, S. Wülfert, Rissig und blind werden in relativ kurzer Zeit alle Harzessenzfirnisse, Z. Kunsttechnol. Konserv., 12 (1998), 205–219. E. Wenders, Dammar als Gemäldefirnis, Z. Kunsttechnol. Konserv., 15 (2001), 133–162. P. Dietemann, C. Higgitt, M. Kälin, M. J. Edelmann, R. Knochenmuss, R. Zenobi, Aging and yellowing of triterpenoid resin varnishes – Influence of aging conditions and resin composition, J. Cult. Herit., 10 (2009), 30–40. G. A. van der Doelen, Molecular studies of fresh and aged triterpenoid varnishes, Dissertation Universiteit van Amsterdam, 1999. G. A. van der Doelen, K. J. van den Berg, J. J. Boon, Comparative Chromatographic and Mass-Spectrometric Studies of Triterpenoid Varnishes: Fresh Material and Aged Samples from Paintings, Stud. Conserv., 43 (1998), 249–264.

898 � Bibliography

[953] [954] [955] [956] [957] [958] [959] [960] [961] [962] [963] [964] [965] [966] [967] [968] [969] [970] [971] [972] [973] [974] [975]

S. Zumbühl, R. Knochenmuss, S. Wülfert, F. Dubois, M. J. Dale, R. Zenobi, A Graphite-Assisted Laser Desorption/Ionization Study of Light-Induced Aging in Triterpene Dammar and Mastic Varnishes, Anal. Chem., 70 (1998), 707–715. E. R. de la Rie, A. M. Shedrinsky, The Chemistry of Ketone Resins and the Synthesis of a Derivative with Increased Stability and Flexibility, Stud. Conserv., 34 (1989), 9–19. K. J. van den Berg, J. van der Horst, J. J. Boon, O. O. Sudmeijer, cis-l, 4-poly-β-myrcene, the Structure of the Polymeric Fraction of Mastic Resin (Pistacia lentiscus L.) Elucidated, Tetrahedron Lett., 39 (1998), 2645–2648. E. R. de la Rie, Photochemical and Thermal Degradation of Films of Dammar Resin, Stud. Conserv., 33 (1988), 53–70. P. Dietemann, M. Kälin, S. Zumbühl, R. Knochenmuss, S. Wülfert, R. Zenobi, A Mass Spectrometry and Electron Paramagnetic Resonance Study of Photochemical and Thermal Aging of Triterpenoid Varnishes, Anal. Chem., 73 (2001), 2087–2096. E. R. de la Rie, Old Master Paintings: A Study of the Varnish Problem, Anal. Chem., 61(21) (1989), 1228A–1240A. R. White, J. Kirby, A Survey of Nineteenth and early Twentieth Century Varnish Compositions found on a Selection of Paintings in the National Gallery Collection, Natl. Gallery Tech. Bull., 22 (2001), 64–84. ISBN 978-1-85709-926-5. M. Stappel, Schellack, Restauro, 8 (2001), 596–603. S. K. Sharma, S. K. Shukla, D. N. Vaid, Shellac – Structure, Characteristics & Modifications, Def. Sci. J., 33 (1983), 261–271, https://publications.drdo.gov.in/ojs/index.php/dsj/article/view/6181/0, accessed on 2022-06-20. K. S. Brown, The Chemistry of Aphids and Scale Insects, Chem. Soc. Rev., 4 (1975), 263–288. J. S. Mills, R. White, Natural Resins of Art and Archaeology. Their Sources, Chemistry, and Identification, Stud. Conserv., 22(1) (1977), 12–31. R. M. Carman, D. E. Cowley, R. A. Marty, Diterpenoids XXV. Dundathic acid and Polycommunic Acid, Aust. J. Chem., 23 (1970), 1656–1665. G.-F. Chen, Developments in the field of rosin chemistry and its implications in coatings, Prog. Org. Coat., 20 (1992), 139–167. Brochure Handbuch Cognis Kit für Haushalt & Kosmetik, Cognis Deutschland GmbH & Co. KG, CFM Technical Training, 2002. Pigments Finder by Clariant, https://www.clariant.com/de/Business-Units/Pigments/PigmentsFinder, accessed on 2022-06-20. Sales range for the printing, paints, plastics industries and special applications, Clariant, Dokument DP5525, https://www.clariant.com/-/media/Files/Business-Units/Pigments/Clariant-BrochureSales-Range-202003-EN.pdf, accessed on 2022-06-26. Clariant Creative Colors – Solutions for Coloring your Work Environment, https://www.clariant.com/de/ Business-Units/Pigments/Special-Applications/Stationery, accessed on 2022-06-26. Ultrafine pigment dispersions for fine and durable sketching and drawing, Clariant, document DP6209, no longer available, last accessed on 2013-06-20. Ultrafine pigment dispersions for design and creative materials, Clariant, document DP6208, no longer available, last accessed on 2013-06-20. Products for NON IMPACT PRINTING, Clariant, Dokument DP8530, no longer available, last accessed on 2019-05-20. Products for DIGITAL INKS, Edition 2019, Clariant, Dokument PI 6000 E, no longer available, last accessed on 2019-05-20. Organic pigments (general purpose pigments) by DIC Corporation, https://www.dic-global.com/en/ products/general_pigments/ (pigments for printing inks), accessed on 2022-06-24. Pigments for digital printing by DIC Corporation, https://www.dic-global.com/en/products/nip_ pigments/, accessed on 2022-06-24.

Bibliography

[976] [977] [978] [979]

[980] [981] [982] [983] [984] [985] [986] [987] [988]

[989]

[990]

[991] [992] [993]

� 899

Neozapon Farbstoffe, BASF, Dokument EVP 001905d, no longer available. Pigmente für Flexo-, Tiefdruck- und Offset-Verpackungsdruckfarben, BASF, Dokument EVP 000204d, no longer available. Basacid Farbstoffe, no longer available. Lanxess Farbstoffe für Schreib- und Inkjettinten: Bayscript, Pyranin, Spezial, https://lanxess.com/en/ Products-and-Solutions/Industries/Colorants/Spezialfarbstoffe (index page), https://lanxess.com//media/Project/Lanxess/Corporate-Internet/Products-and-Solutions/Industries/Colorants/10249_ BRO_PLA_ColorantsInk_EN_web.pdf (inkjet and stationary inks), accessed on 2022-06-20. Pigments for digital printing, https://www.basf.com/us/documents/en/general-business-topics/ pigments/industries/printing/BASF-Colors-and-Effects_Brochure_Pigments-for-digital-printing.pdf, accessed on 2022-06-24. Pulp & Paper Dyes and Pigments, ORCO Organic Dyes and Pigments, https://www.organicdye.com/ industries/paper-dyes/, no longer accessible. Ink and Toner Dyes & Pigments, ORCO Organic Dyes and Pigments, https://www.organicdye.com/ industries/inks-toner-dyes/, accessed on 2022-06-21. Paper And Pulp, Dayglo Color Corporation, http://www.dayglo.in/paper_industry.html, accessed on 2022-06-21. DYCROPULP Dyes for Paper, DYCRO Jagson Colorchem Limited, https://www.jagson.com/dycropulpdyes-for-paper.php, accessed on 2022-06-21. Dyes for Ink, DYCRO Jagson Colorchem Limited, https://www.jagson.com/ink-dyes.php, accessed on 2022-06-21. Information about Carbopol™, Lubrizol, https://www.lubrizol.com/Home-Care/Products/CarbopolPolymers, accessed on 2022-06-20. Datasheets for ACRYSOL™ASE-60, Dow, https://www.dow.com/en-us/pdp.acrysol-ase-60errheology-modifier.136370z.html?productCatalogFlag=1#overview, accessed on 2022-06-20. Information about Klucel™products, Ashland, https://www.ashland.com/industries/paints-andcoatings/specialty-and-industrial-coatings/klucel-hydroxypropylcellulose (index page for HPC for coatings), https://www.ashland.com/file_source/Ashland/Product/Documents/Pharmaceutical/PC_ 11229_Klucel_HPC.pdf (HPC structure and properties for pharmaceutics), accessed on 2022-06-20. Information about Tafigel™and similar products, Münzing, https://www.munzing. com/de/downloads/product-brochures/, https://www.munzing.com/static/ bed874f982e762433cc2e88af82651db/Dispersing_Technologies_MUNZING_2016_fffffe5d54.pdf (information about dispersing technologies), all accessed on 2022-06-24. Information about Tylose™products, SE Tylose GmbH & Co. KG, https://www.setylose.com/ en/knowledge-base/industrial/technical-data-sheets (index page), https://www.setylose.com/ fileadmin/user_upload/Tylose%20Paints_EN.pdf (brochure about cellulose ethers), accessed on 2022-06-20. SIEGEL UND SIEGELLACK, website information by Scriptorium, http://www.kalligraphie.com/443-0Siegel.html, accessed on 2022-06-21. ASTM International Standards Worldwide https://www.astm.org/. Verband der deutschen Lack- und Druckfarbenindustrie, Fachgruppe Druckfarben, https: //www.wirsindfarbe.de/service-publikationen/informationsmaterial-druckfarben/allgemeineinformationen-ueber-druckfarben/, https://www.wirsindfarbe.de/fileadmin/user_upload/ Dokumente/Druckfarben/37_2110_Fluessige_Druckfarben.pdf (liquid printing inks), https: //www.wirsindfarbe.de/fileadmin/user_upload/Dokumente/Druckfarben/38_2110_Pastoese_ bis_dickfluessige_Druckfarben.pdf (viscous printing inks), https://www.wirsindfarbe.de/ fileadmin/user_upload/Dokumente/Druckfarben/39_2110_Siebdruckfarben.pdf (screen printing inks), https://www.wirsindfarbe.de/service-publikationen/informationsmaterial-druckfarben/ allgemeine-informationen-ueber-druckfarben/die-auswirkungen-von-druckfarben-auf-die-umwelt (environmental consequences of printing inks), all accessed on 2022-06-21.

900 � Bibliography

[994] [995]

[996]

[997]

[998]

[999]

[1000] [1001] [1002] [1003] [1004]

Various information by D. Smith: https://danielsmith.com/product/original-oils/ (composition artists’ paint “Original Oils”), https://danielsmith.com/product/daniel-smith-extra-fine-watercolors/ (composition artists’ paint “Extra Fine™ Watercolors”), accessed on 2022-06-10. Various information by Winsor&Newton: https://www.winsornewton.com/row/paint/oil/artistsoil/#product-info-colours and https://www.winsornewton.com/na/education/compositionpermanence/artists-oil-colour/ (composition artists’ oil color), https://www.winsornewton. com/row/paint/watercolour/professional-watercolour/#product-info-colours and https:// www.winsornewton.com/na/education/composition-permanence/professional-water-colour/ (composition professional water colors), https://www.winsornewton.com/row/paint/acrylic/ professional-acrylic/#product-info-colours and https://www.winsornewton.com/na/education/ composition-permanence/professional-acrylic-colour/ (composition professional acrylics), all accessed on 2022-06-21, article “The Science behind Artists’ Acrylic and its benefits” (resource center, http://www.winsornewton.com/resource-centre/product-articles/) accessed on 2019-05-15, http://www.winsornewton.com/assets/HealthandSafetyDataSheets/OIL%20COLOUR/Griffin% 20Alkyd/04912258.pdf (MSDS artists’ alkyd colors), accessed on 2013-06-10. Various information by Schmincke: https://www.schmincke.de/fileadmin/downloads/pdf/ Broschueren_2016/MUSSINI_D_EN.pdf (composition MUSSINI oil colors), https://www.schmincke. de/fileadmin/downloads/pdf/Broschueren_2016/HORADAM_AQUARELL_D_EN.pdf (composition HORADAM water colors), https://www.schmincke.de/fileadmin/downloads/pdf/Broschueren_2016/ PRIMAcryl_DE_EN.pdf (composition PRIMAcryl acrylics), https://www.schmincke.de/fileadmin/ downloads/pdf/Broschueren_2016/aqua_Linoldruck_DE_EN.pdf (composition aqua LINOPRINT inks), all accessed on 2022-06-21. Various information by Charbonnel, https://www.charbonnelshop.fr/row/product-category/metalplate-printing-intaglio/intaglio-ink/ (intaglio/engraving- and gravure printing inks), https: //www.charbonnelshop.fr/row/product-category/metal-plate-printing-intaglio/aquawash-intaglio/ (engraving- and gravure printing inks solvent-free (Aqua Wash)), accessed on 2022-06-21. Various information by Lascaux: https://lascaux.ch/dbFile/4392/u-1749/Lascaux_Polysaccharides_ Cellulose_Starch.pdf, https://lascaux.ch/dbFile/4606/u-20a2/Lascaux%20Synthetic%20Resins% 20and%20Dispersions.pdf, https://lascaux.ch/en/products/painting-mediums, accessed on 2022-06-21. Various information by Dow: http://www.dow.com/, https://www.dow.com/documents/enus/mark-prod-info/884/884-02316-01-tamol-dispersants-product-solutions-guide.pdf (dispersants), https://www.dow.com/en-us/pdp.orotan-731a-er-dispersant.242161z.html?productCatalogFlag= 1#overview (Orotan 731A), https://www.dow.com/en-us/pdp.tamol-731a-dispersant.242189z# overview (Tamol 731A), all accessed on 2022-06-21. J. B. Clarke, C. R. Walker, Dispersants for emulsion paints, Europäisches Patent EP 0802951 A1, 1997, http://www.google.com/patents/EP0802951A1?cl=en, accessed on 2022-06-24. J. B. Blackburn, A. W. Field, Pigment preparation, US-Patent 4089699, 1978, https://patents.justia. com/patent/4089699, accessed on 2022-06-24. Fa. Kremer Pigmente, Orotan 731 K Material Safety Data Sheet, https://www.kremer-pigmente.com/ elements/resources/products/files/78032_SHD.pdf, accessed on 2022-06-21. Various information by Celanese: http://www.celanese.com/msds/pdf/993-64529418.pdf (MSDS für Mowilith LDM 7412), accessed on 2011-07-20. Hahnemühle FineArt GmbH, Künstlerpapiere, website to product portfolio, https://www. hahnemuehle.com/de/kuenstlerpapiere.html, in particular https://www.hahnemuehle.com/ de/kuenstlerpapiere/haeufig-gestellte-fragen.html (FAQ), https://www.hahnemuehle.com/ de/kuenstlerpapiere/aquarell.html (watercolor paper), https://www.hahnemuehle.com/de/ kuenstlerpapiere/klassische-druckverfahren.html (printing paper), https://www.hahnemuehle. com/de/kuenstlerpapiere/skizze-zeichnen/skizzenpapiere.html (drawing paper), https://www. hahnemuehle.com/de/kuenstlerpapiere/grafik-illustration.html (layout- and design paper), https://www.hahnemuehle.com/de/kuenstlerpapiere/pastell.html (pastell drawing paper), all accessed on 2022-06-21.

Bibliography

� 901

[1005] Kemira pigments and coatings for paper: https://www.kemira.com/products/?industry=none&oil_ gas=none&pulp_paper=436&water=none and https://www.kemira.com/products/?industry=none& oil_gas=none&pulp_paper=432&water=none, acccessed on 2022-06-21, http://www.kemira.com/ regions/germany/SiteCollectionDocuments/Broschüren_PulpPaper/Farbmittel.pdf (colorants, no longer accessible). [1006] Various websites of federal and state archives to the topic, a selection: https://afz.lvr.de/ de/bestandserhaltung_2/bestandserhaltung_1/vergleich_din_6738_und_din_en_iso_9706/ vergleich_der_papiernormen_din_6738_und_din_en_iso_9706.html, https://www.bundesarchiv. de/DE/Content/Downloads/KLA/positionspapier-alterungsbestaendiges-papier.pdf?__blob= publicationFile, https://www.klug-conservation.com/medien/Wissen/Wissens_Folder/wissen8_ blauer_engel_de.pdf, accessed on 2022-06-21. [1007] Various information by Ferro, https://www.ferro.com/-/media/files/news-and-events/events/ ceramitec-2022/structural/fk02_e_overview_ceramic_pigments.pdf (Technical Information FK02 Ceramic Stains), accessed on 2022-06-21. [1008] Sekisui Specialty Chemicals, Informationen zu Selvol™Polyvinylalkohol, http://www.sekisui-sc.com/ products/selvol/productline.html, accessed on 2022-06-21.

Index Page numbers in blue refer to vol. 1. 4C 34 AB3 372, 527 AB9 372, 527 AB40 526 AB45 526 AB74 363 AB87 385 AB90 372 AB92 415 AB93 372 Abienol 683 Abietane 682 Abietic acid 682 Absorption 28, 29, 41, 44 Absorption coefficient 42 Acetoacetic arylide pigments 395 Acetylene black 211, 214 Acridine 365, 523 Acrylate copolymer 607 Acrylic see binder, acrylic paint Acrylic paint 731, 741 – binder 733 – components 732 – dispersing agent 739 – film formation aids 740 – medium 738 – retarder 738 – thickener 738 – wetting agent 739 Adipic acid 729 After-chroming 521 AG27 527 AG50 372 Aggregation 502 AKD 607 Albumin see binder, albumin Alcohols – acrylate 738 – ink 806, 812 Aleuritic acid 859 Alizarin 191, 348 – aluminum lake 194 – turkish red 196 https://doi.org/10.1515/9783110777123-010

Alizarin crimson 12, 22, 347, 349 Alizarin crimson hue [SC] 421 Alizarin crimson [WN][DS] 354 Alizarin madder lake [KR] 407 Alizarin purple 354 Alkali blue pigment 367, 374 Alkannin 345 Alkenyl succinic anhydride 607 Alkyd color 727, 731 – binder 727 Alkyl ketene dimer 607 Alkyl sulfates 739, 813 Alkyl sulfonates 739, 813 Allura red AC 405 Aluminum chrome pink 263 Aluminum chrome red 259 Aluminum manganese pink 263 Amaranth 405 Amino compound 813 Ammonium cobalt phosphate 22, 293 Amphiboles (blue) 9, 23 – color formation 142 Anatto 318 Anhydrite 631 Annulene 174 Anthraquinoid red [DS] 354 Anthraquinoid scarlet [DS] 355 Anthraquinone – color formation 156 Antimony yellow 234 AO7 527 AO10 527 Apigenin 323 AR1 415 AR18 405, 527 AR27 405 AR37 526 AR51 367 AR52 372, 527 AR57 526 AR73 405 AR87 367 AR92 367 Arachic acid 657

904 � Index

Arsenic 6, 233, 236 Arylamide red pigments 406 ASA 607 ASE 60 739 Associative thickener 557, 562 Atacamite 6, 25, 218, 631 Atrament black [SC] 421 Aureolin 20, 129, 291 – color formation 129 Aureolin hue [SC] 409 Auxochrome 44, 147 AV7 415 AV41 527 AV49 372 AY3 527 AY17 526 AY23 342, 392, 527 AY29 526 AY36 527 AY42 527 AY63 342 AY73 367 Azo colorant 161 – metallization 164 Azo rubine 815 Azo yellow deep [WN] 396 Azo yellow [DS] 409 Azo yellow medium [WN] 396 Azo yellow [WN] 409 Azomethine pigments 425 Azurite 6, 23, 125, 221 – color formation 125 BAK 587 Ballpoint pen ink see ink, ballpoint pen Balsam resin 681 Band see semiconductor Band edge 33 Band gap 84, 86 – chromophore 86 Band structure diagram 85 Barberry 752 Barite 19, 290 Barium yellow 20, 283 Barytes 19 Bathochromic shift 44, 147, 150, 186 – anthraquinone 354 – color lake 199 – lake 199

BB3 379 BB7 371 BB9 379 BB140 507 BB26 523 BBr1 524 BCTMP (SW,HW) 580 BEK, BEKP 587 Benzaurine 367 Benzimidazolone hydrazone pigments 408 Benzimidazolone pigments 408 Benzyl violet 372 Berberine 752 Berlin blue 23, 285 BG1 371 BG4 371 BHK 587 BHS 589 Binder 530, 755 – acrylic see binder, acrylic paint – acrylic paint 733 – dispersing agent 739 – film formation 734 – film formation aids 740 – retarder 738 – wetting agents 739 – albumin 703, 706 – film formation 708 – alkyd resin 727 – film formation 730 – blackboard chalk 746 – book illumination 788 – casein 650, 703, 713, 714 – film formation 717 – chemical drying 536 – clarea 707, 788 – coating (paper) 613 – collagen 703, 709 – distemper paint 709 – egg 707 – egg clarea 788 – egg white 706 – fingerpaint 750 – fresco 650 – glue 650, 709, 788 – film formation 713 – glue paint 709 – glutin glue 788 – gouache 709

Index �

– gum 788 – gum Arabic 722, 723, 746, 747, 755, 788, 814 – gum tragacanth 725 – ink 755, 814 – lime 650 – linseed oil 650 – medium 738 – oil paint 656 – blown oil 659 – boiled oil 660 – degradation 669 – drying 660 – film formation 661 – heavy metals 675 – oil 658 – refined oil 658 – saponification 671 – siccative 675 – stand oil 659, 672 – yellowing 671 – pastel crayon 746 – physical drying 537 – poster paint 709 – printing ink 846 – protein 703 – albumin 706 – casein 713 – clarea 707 – collagen 709, 710 – egg 707 – film formation 703, 713, 717 – glue 712 – rubber 650 – shellac 858 – color formation 352 – film formation 860 – silicate paint 745 – size paint 709 – tempera 650, 717 – thickener 738 – tusche 858 – watercolors 722 Biotite 136 Bisacetoacetic arylide pigments 400 Bismarckbraun 524 Bismuth 278 Bismuth molybdate 20, 279 Bismuth vanadate 20, 279 Bismuth vanadium yellow 278, 279

905

Bister 216, 755, 855 Bixin 318 BKP 583, 587 Black oak 325 Black pigments 18, 87, 136, 207 – ceramic 629 Blackboard chalk 746 Bladder color 792, see color, bladder color Blanc fixe 19, 290 Bleached linseed oil 660 Blown oil 659 Blue bice 221 Blue iron earth 292 Blue ocher 292 Blue pigments 22 Blue verditer 23, 221 Blueberry 332 Bluewood 335 BO2 524 Boiled oil 660 Bone black 19, 215 Bone white 288 Book illumination 191, 223, 238, 318, 324, 326, 332, 335, 342, 787, 792 – binder 706, 788 – color lake 790 – colorants 10, 789, 790 Bordeaux [DS] 410 Bordeaux [SC] 407 Borneol 681 Boviquinone 345 BR1 371, 524 BR22 523 BR111 506 Brazilin 191, 335 Brazilwood 10, 191 Bright red [WN] 423 Brillant Blue FCF 372 Brillant Blue G 372 Brilliant Acid Green BS 372 Brilliant Benzo Fast Green 415 Brilliant Black BN 815 Brilliant green 371 Brilliant red B 405 Brilliant scarlet [SC] 403 Brilliant yellow [SC] 402, 409 Bristol board 828 Bronzing 65 Brown ink see ink, brown

906 � Index

Brown madder [WN] 421 Brown pigments 19, 136 BSK 587 BSS 589 Buckthorn berry 325 Bulk Yellow 191 Burnt Sienna 22, 240, 244 Burnt umber 22, 240, 246 Burnt vitriol 246 Butanol 806 Butolic acid 859 Butter yellow 388 Butyl glycol 806 BV1 371 BV10 371 BV16 523 BY2 523 BY29 523 BY40 523 Cadinene 684 Cadmium 12, 274, 278 Cadmium cinnabar 20, 22, 278 Cadmium green 25, 278 Cadmium mercury sulfide 278 Cadmium orange 20, 277 Cadmium red 22, 99, 277 – color formation 99 Cadmium yellow 12, 20, 89, 99, 275 – color formation 89, 99 Cadmium yellow light 275 Caesar purple [SC] 418 Calcite 630, 631 Calcium carbonate 288 Calcium tantalum yellow 263 Calendering see Paper, calendering Capri blue 379 Caput Mortuum 240, 246 Carbazole violet [DS] 381 Carbon 207, 629, 755 – char 207 – coal 207 – coke 207 – graphite 207 Carbon black 19, 211, 631 – color formation 212 – hydrophilic 213 Carbon black ink see ink, carbon black ink Carene 681

Carmine 9, 22, 191, 350, 352, 815 – color formation 191, 352 Carmine [DS] 410 Carmine Naccarate 347, 352 Carmine Naccarate [KR] 350 Carminic acid 191, 350 Carob flour 595 Carotenoids 317 Carthamin 191, 338 Casein see binder, casein Casein paint – binder 714 – components 714 Catechin 321 Cativic acid 683 CcMmYK 34 Cellobiosis 569 Cellulose 569, 738 Cellulose derivatives 739, 746, 747, 750 Ceramics 627, 640 – cold painting 630 – enamel color 630 – flow 631 – frit 631 – glaze 632 – glaze pigments 632 – Hot painting 627 – iron-reduction technique 627 – iron-reoxidation technique 628 – manganese-black technique 629 – onglaze color 632 – pigments – cold painting 630 – hot painting 629 – polychrome firing 628 – underglaze color 632 Cerium sulfide orange 20, 267 – color formation 267 Cerium sulfide red 22, 267 – color formation 267 Cerroic acid 859 Cerulean blue 12, 23, 263, 265, 266 – color formation 266 Chalk 19, 746 Channel black 211, 214 Char 207 Charcoal black 19, 215 – color formation 216 Charge transfer 77, 131, 143

Index

Chay root 348 Chinese blue 220 Chinese ink 19, 210 Chinese purple 220 Chinese white 273 Chloranil Fast Green 416 Cholic acid 726 Chromate – color formation 138 Chrome aluminum pink 259 Chrome green 25, 284, 287 Chrome iron brown 20, 263, 264 Chrome iron manganese brown 259 Chrome iron nickel black 259 Chrome manganese zinc brown 259 Chrome nickel ferrite black 259 Chrome niob titanium yellow 262 Chrome orange 20, 282 Chrome red 22, 282 Chrome tin pink 264 Chrome titanium yellow 20, 262 Chrome tungsten titanium brown 262 Chrome yellow 12, 20, 138, 280 – color formation 138 Chrome yellow lemon 280 Chrome yellow light 280 Chromium 12, 280 Chromium hematite 263 Chromium orange hue [SC] 409 Chromium oxide green 12, 25, 122, 268 – color formation 122 Chromium oxide hydrate green 269 Chromium yellow hue deep [SC] 396 Chromium yellow hue lemon [SC] 409 Chromo board 828 Chromophore 44 – band gap 86 – bathochromic shift 150 – charge transfer 131 – donor-acceptor 150 – diazo 161 – indigoid 154 – quinone 156 – simple with carbonyl acceptor 153 – d orbital 102 – enlargement 150 – IVCT 138, 139 – ligand field 102 – LMCT 134

� 907

– metal complex 199 – bathochromic shift 199 – MMCT 138 – MO 143 – molecular orbital 143 – OMCT 134 – polyene 164 – annulene 174 – donor-acceptor substituted 168 – linear 166 – phthalocyanine 175 – polycyclic 169 – polycyclic quinone 172 – porphine 175 – polymethine 183 – bathochromic shift 186 – semiconductor 86 – sulfide 189 Chrysocolla 6, 25, 218 Chrysoidin 524 CI 14 CICP 254, 635 Cinnabar 6, 89, 236, 631 – color formation 89 Cinnabar green 25 Clarea see binder, egg clarea CMC 593, 595, 739, 746 CMHEC 739 CMP 577, 579 CMYK – black 36, 211, 512, 516, 527, 818, 835, 845 – color theory 34 – cyan 36, 372, 383, 517, 527, 818, 835, 845 – inkjet inks 818 – laser toner 835 – magenta 36, 355, 372, 405–407, 410, 419, 512, 526, 527, 818, 835, 845 – printing inks 843 – spectra of primary colors 816 – writing inks 815 – yellow 36, 392, 396, 401, 409, 516, 526, 527, 818, 823, 835, 845 CMYKOG 34 Coal 207 Coalescing agents 565 Coating see paper, coating Coating color see paper, coating color Cobalt 12, 291, 292 Cobalt aluminum blue 259

908 � Index

Cobalt ammonium violet 293 Cobalt blue 12, 23, 128, 259, 260, 631 – color formation 128, 260 Cobalt blue dark 264 Cobalt blue turquoise 23 Cobalt chrome blue-green 259 Cobalt chromite blue-green 259 Cobalt chromite green 259, 261 – color formation 261 Cobalt ferrite black 259 Cobalt green 25, 259, 261, 266 – color formation 261 Cobalt silicate blue 264 Cobalt stannate 263 Cobalt tin aluminum blue 259 Cobalt titanium green 259 Cobalt turquoise 25, 259 Cobalt turquoise [WN] 385 Cobalt violet 12, 22 Cobalt violet dark 292 Cobalt yellow 20, 291 Cobalt zinc aluminate blue 259 Cobalt zinc silicate blue 264 Cochineal 350 – color formation 191 Cochineal red 405 Cocoyl compound 813 Coke 207 Colcothar 240, 246 Cold painting see ceramics, cold painting Cold-pressed linseed oil 658 Collagen see binder, collagen Colloid 48, 633, 641 Colophony 681 Color – alkyd see alkyd color – casein see casein color – ceramics see ceramics – chemical cause 2, 77, 203 – coating color (paper) see paper, coating color – color lake see color lake – copier toner 833 – crystal structure dependence 72 – fingerpaint 750 – fresco see fresco – glass 644 – laser toner 833 – pencil 747, 748, 750 – physical cause 2

– reverse glass painting see stained glass, reverse glass – sap color 326, 332, 792 – shape dependence 48, 72 – size dependence 48, 60 – stained glass see stained glass – Tempera see Tempera – toner 833 – tusche 855 – Tüchleinfarben 792 Color black 10 Color etching – reverse glass painting 643 Color formation (chemical) 77 Color formation (physical) 45 Color intensity 112 Color lake 9, 190, 203, 792 – bathochromic shift 199 – blue 332 – book illumination 790 – flavonoid 326 – green 325 – metal 192 – purple 332 – red 347, 352 – sap green 325 – Stil de Grain 324 – structure 194 – substrate 192 – yellow 324 Color matching function 34 Colorant 27 – artists’ paint 205, 427 – ballpoint pen 818 – felt-tip 818 – fiber-tip 818 – fountain pen ink 815 – inkjet ink 818 – laser toner 833 – printing ink 843 – solubility 499 – stamp pads 832 Colored pencil 750 Colour Index 14 Complementary color 29 Complex inorganic color pigments 254 Complex oxides 254 Conduction band 84 Congo red 515

Index �

Copal resin 683 Copalic acid 683 Copier 833 Copier toner see toner Copigmentation 331 Copper 6, 217, 641 – patina 223 Copper chromite black 259 Copper engraving 846 Copper green 222, 224 – transparent 225 Copper oleat 225 Copper phthalocyanine 383 Copper pigments 217, 224 – color formation 125 Copper printing paper 855 Copper resinate 225 Corundum base 263 Cosolvent – ink 811 Cotton 575, 621 CPT red [KR] 403 CPT scarlet [KR] 403 Crabbage 191 Cremnitz white 289 CRMP 577 Crocetin 318 Crossberry 191, 325 Crystal orbital 83 Crystal structure – color 92 Crystal violet 371, 372 CT 77 CT (designation of color mechanism) 131 CTMP 577 Cuprorivait see Egyptian blue Curcumin 337 Cyanidin 327 Cyclohexanone resin 691 Cyprian blue 221 Dactylopius coccus 351 DADMAC 593, 603 Dammar 681, 684 – aging 686 Dammarane 684 DB67 415 DB71 507 DB86 385, 517

DB199 385, 517 DB218 509, 517 DB273 509 DBk19 415, 516 DBk154 516 DBk168 516 DBk195 821 Debye interaction 540 Decay – paper 622 Deep light 69 Deep scarlet [DS] 410 Defoamers 566 Degradation – oil paint 669 – pigments (mural painting) 652 – pigments (panel painting) 691 Delftblau [SC] 355 Delphinidin 327 Dewar rules 186 Dewrocholic acid 726 DG13 415 DG26 416 Diaryl yellow pigments 400 Diazine 365, 379 Diazo chromophore 161 – bathochromic shift 412 – metallization 164 Dicarbonyl system 344 Diethylene glycol 812 Dihydrazone condensation pigments 397 Dihydrazone pigments 397 Dinitraniline orange 388, 392 DIP 568 Dipole-dipole interaction 502, 540 DIR (paper) 616 Disazo condensation pigments 397 Disazo pigments 397 Disazopyrazolone pigments 400 Dispersant 550 – ink 811 – inkjet ink 824 Dispersants 542 Disperse Blue 79 413 Disperse Blue 148 413 Disperse Green 9 413 Dispersing agent 739 Dispersion 54, 733 – acrylic 733

909

910 � Index

Dispersion interaction 502, 540 Dispersol blue B-G 354 Dispersol red A-2B 354 Distemper paint see glue paint, 709 DO102 507, 517 Dodecylamine 813 Dolomite 631 Donor-acceptor chromophore 150, 310, 313 – diazo 161 – indigoid 154 – quinone 156 – simple with carbonyl acceptor 153 Doped rutile see DR pigment DR pigment 101, 262 – color formation 101 DR28 515 DR75 516 DR81 415, 506 DR239 507, 517 DR253 506 Dracorubin 337 Dragon blood 337 Drude–Lorentz model 62 Drying (oil paint) 660 Drying oils 656 DV51 507 DY28 508 DY86 516 DY132 516 DY137 508 DY147 508 Dye 28 – acid dye 524 – aggregation 502 – anionic 524 – basic 505 – cationic 505, 522 – direct 506, 513 – metal complex 518 – modification for paper 506 – mordant dye 518 – paper 503, 514, 517, 522, 524 – reactive dye 510 – substantive 513 – xanthophylls 318 Dyer’s broom 191, 325 Dyer’s dew 191 Dyer’s mulberry 191, 325 Dyer’s ranges 191

Dyer’s reseda 325 Dyer’s saw-wort 325 Dyer’s sumach – wood 752 Dyer’s woad 359 E100 337 E102 392 E104 527 E110 405, 815 E122 815 E123 405 E124 405, 527 E127 367 E129 405 E131 372, 527 E132 363 E133 372 E142 372 E151 815 E152 815 E160 b 318 E163 333 ECF 587, 589 Egg clarea see binder, egg clarea Egg tempera 719, 720 Egg white – binder 706 Egg yolk tempera 718 Egg-oil emulsion 720 Eggshell white 288 Eglomisé 644 Egyptian blue 23, 125, 219, 631 – color formation 125 Egyptian green 25, 219, 631 Elderberry 332 Electrostatic stabilization 553 Ellagic acid 779 Emerald green 12, 25, 226 – color formation 226 Emeraude green 269 Emission 29 Emulsifier – ink 813 – Tempera 718, 719 Enamel color – ceramic see ceramic, enamel color Enamel paint – glass see stained glass, enamel paint

Index

English red 245 Eosin 367 Eperuic acid 683 Epicatechin 321 Epimanool 683 Eraser 748 Eryodictiol 321 Erythrolaccin 352 Erythrosine 367 Ester – ink 806 Ethanol 806 Ethyl acetate 806 Eugeniin 779 Euphane 685 Euxanthic acid 341 Evening red 58 Excited state 108 Extinction 42 False Blue 59 Fast chrome green 25, 284 Felt-tip pen ink see ink, felt-tip pen, ink Film formation – acrylic 734 – albumin 708 – casein 717 – glue 713 – oil paint 661 – protein 703, 708 Film formation aids – acrylic 740 Film-forming aids 565 Fines 577 Fingerpaint 750 Fisetin 323 Fisetwood 325 Fixative see paper, fixative Flake white 289 Flavoceric acid 350 Flavonoid – lake 326 – oxidation 783 – polymerization 783 Florentine red [SC] 421 Fluorescein 367 Flux (ceramic) 631 Flux (glass) 649 Fog 59

Food Black 1 815 Food Black 2 527, 815 Food Blue 2 372 Food Blue 5 372 Food Orange 4 527 Food Red 7 527 Food Red 9 405 Food Red 14 367 Food Red 17 405 Food Red 104 367 Food Red 106 367, 372 Food Yellow 3 337, 405, 815 Food Yellow 4 392 Food Yellow 13 527 Forest glass 296 Fountain pen ink see ink, pen Four-color printing see CMYK – color theory 34 French ultramarine 23, 228 Fresco 649, 655 – binder 650 Fresco buono 650 Fresco secco 650 Frit – ceramic 631 Fuchsin 371 Fungus 345 Furnace black 214 Galium 348 Gall apples 321 Gallic acid 779 Gallocatechin 321 Gamboge 342 Gamboge acid 342 Garnet base 264 Gas black 211, 214 GCC 590 Gelatin 709 Genistein 191 Geranium red [SC] 403 Glass 293 – antique coloring 303 – bottles 300 – colloid staining 301 – coloring 298 – float glass 300 – high refractive index 52 – opaque glass 307

� 911

912 � Index

– silver yellow 301 Glass frit 305 Glass painting – sanguine 648 Glass temperature 733 Glass window 644 – colorant 644 Glauconite 9 Glaucophane 9, 23, 140 – color formation 140, 142 Glaze 632, 642 – iridescence 642 – luster 642 – pigments see ceramic, glaze pigments Glaze frit 631 Glucogallin 779 Glue see binder, glue Glue paint 709, 713 – binder 709 – ingredients 709 Glue-size see color, glue-size Glutin glue 709 Glycerol 723, 730, 812 Glycol ether – acrylate 740 – acrylic 740 – ink 812 Glycols – acrylate 738, 740 – alkyd resin 730 – ink 806, 812 Goethite 9, 118, 631 Gofun Shirayuki 289 Gold 49, 62, 65, 301, 633 Gold ruby glass 49, 301, 634 Gouache see glue paint Gouache paint see glue paint Graphite 207, 209, 747 – color formation 210 Green bice 222 Green earth 9, 25, 140, 142, 248 – color formation 140, 142, 248 Green forest glass 296 Green frit 219 Green pigments 24 Green verditer 25, 222 Grevillin 345 Grinding – oil 656

Ground calcium carbonate 590 Ground state 108 Guaran 593, 595 Gubbio red [KR] 403 Guignet’s green 269 Guimet’s blue 228 Gum Arabic 649, see binder, gum Arabic Gum tragacanth see binder, gum tragacanth Gumdrop 342 – color formation 342 GW, GWD 577 Gypsum 19, 631 Hamaker force 550 Han blue 23, 220 Han purple 22, 220 Hansa yellow deep [DS] 396 Hansa yellow light [DS] 396 Hansa yellow medium [DS] 396 Hansagelb 396 Hard resin 850 HASE 739 Heavy metals (oil paint) 675 HEC 739, 740 Helianthin 388 Helio cerulean [SC] 383 Helio green deep [SC] 385 Helio green light [SC] 385 Helio green [SC] 385 Helio turquoise [SC] 383 Heliogen blue 385 Helvetia Blue 372 Hematein 335 Hematite 9, 118, 245, 263, 630, 631 Hematite base 263 Hemicellulose 571 Henna 345 Hesperitin 321 HEUR 739 Hexahydro-diphenic acid 779 HMHEC 739 Hooker’s green 287 Hopane 684 Horn white 288 Hornblende 136 Hot painting see ceramics, hot painting Hot-pressed linseed oil 658 HPC 739 Huckleberry 332

Index �

Humectants – acrylic 738 – ink 812 – watercolors 723 Huntite 19 HWC (paper) 616 Hydrogel 556 Hydrogelling agents 557 Hydrogen bonds 705 Hydrophobic interaction 502, 541 Hydrophobic modification 609 Inclusion pigment 633 Indamine 378 Indanthrene blue [WN] 355 Indanthron 355 Indanthrone blue [DS] 355 Indian red 245 Indian red deep [WN] 410 Indian yellow 20, 341 – color formation 341 Indian yellow [DS] 401 Indigo 23, 154, 358 – color formation 154, 358 Indigocarmine 363 Indoaniline 378 Indophenol 378 Ink 836 – ballpoint pen 804 – binder 801 – colorant 818 – binder 801, 814 – black 516, 755, 790, 793, 794, 818, 833, 843 – blue 332, 371, 372, 385, 527, 789, 790, 818, 843 – brown 783, 790, 792, 798, 818, 843 – carbon black ink 753 – carbon ink 755 – colorant 810, 843 – components 806 – cosolvent 811 – cyan 372, 385, 527, 818, 833, 843 – dispersant 811 – dye 754, 810 – dye (nature) 754 – emulsifier 813 – felt-tip pen – aqueous 803 – ink 802 – nonaqueous 804

913

– solvent 803, 804 – felt-tip pen ink – colorants 818 – from barks and woods 775 – green 325, 371, 789, 790, 818, 833, 843 – historic colorants 789 – humectants 812 – inkjet ink 801, 805 – aqueous 805 – binder 801, 814 – black 512, 516, 527, 818 – colorant 818 – cyan 517, 527, 818 – dispersant 824 – dye ink 819 – magenta 512, 526, 527, 818 – nonaqueous 806 – pigment ink 823 – solvent 806 – water-based 805 – yellow 516, 526, 527, 818 – iron gall ink 793 – magenta 372, 405, 407, 410, 419, 527, 818, 833, 843 – metal ink 754 – natural ink 787 – orange 318, 402, 818, 833, 843 – pen 801 – pigment 843 – plant ink 787 – purple 332, 372, 789, 790, 818, 833, 843 – red 337, 338, 367, 371, 516, 527, 789, 790, 818, 833, 843 – rheology modifier 813 – rollerball pen 804 – solvent 806, 852 – surface tension 813 – thickener 813 – thorn ink 775, 792 – white 789, 818, 843 – writing ink 793 – binder 788, 801 – black 755, 793, 815 – blue 517, 815 – brown 783, 792, 798, 815 – carbon ink 755 – colorants 815 – cyan 815 – dyes 790

914 � Index

– green 815 – Iron gall ink 793 – magenta 815 – natural ink 787 – orange 815 – pigments 789 – purple 815 – red 512, 526, 815 – yellow 526, 815 – yellow 318, 325, 337, 338, 341, 342, 367, 393, 396, 401, 402, 409, 527, 789, 790, 818, 833, 843 Inkjet ink see ink, inkjet ink Inkjet paper 826 INP (paper) 616 Intarsia – coloring 751 Ionic bonding 501, 539 Iron 9, 50, 285, 627, 628 Iron blue 285, 292 Iron brown 263 Iron chromite black 259 Iron chromite brown 259 Iron cobalt black 259 Iron cobalt chromite black 259 Iron gall ink see ink, iron gall ink Iron hydroxide 238 Iron manganese oxide 263 Iron oxide 238 Iron oxide black 19, 247, 629 Iron oxide red 22, 245 Iron oxide yellow 20, 243 Iron oxides 118, 136, 631 – color formation 50, 118, 136, 241 – transparent 253 Iron phosphate blue 292 Iron red 648 Iron titanium brown 259 Iron-phenol reaction 800 Iron-reduction technique 627 Iso-ozic acid 683 Isogamboge acid 342 Isoindole orange [KR] 425 Isoindoline pigments 425 Isoindolinone pigments 425 Isoindolinone yellow [KR] 411 Isomorellinol 342 Isophthalic acid 729 Isopimaric acid 682 Isopropanol 806

IVCT transition 138, 139 Ivory black 19, 215 Jahn-Teller distortion 125 Jahn-Teller effect 125 Jalaric acid 859 Jarosite 9, 631 Kaempferol 323 Kaolinite 630 Kassler yellow 234 Keesom interaction 540 Kermes 350 – color formation 191 Kermes lake 22 Kermes vermilio 351 Kermic acid 191, 350 Kerria lacca 351, 352, 858 King’s yellow 20, 233 Klucel 734, 739 Kremser white 289 Labdane 683 Lac dye 191, 352 – color formation 191, 352 Laccaic acid 191, 350, 352 Lacsholic acid 859 Laking see color lake Lamp black 19, 210, 214 – color formation 211 Langit 218 Lanthantalum red 263 Lapis Lazuli 189 Laporte rule 112 Larixyl acetate 683 Laser printing 833 Laser toner see toner Latex dispersion 59 Lattice width – color 92 Laux process 253 Lawson 345 Lead 6, 232, 234, 235, 238, 289 Lead antimonate yellow 234 Lead antimony yellow 234 Lead crystal 52 Lead glass 52, 296, 649 Lead red 6 Lead soap 671

Index �

Lead titanate 263 Lead white 6, 19, 289 Lead yellow 6, 20, 94 – color formation 94 Lead-tin yellow 20, 235, 263, 265, 266 – color formation 266 Lecithin 719 Lemon yellow 20, 283 Lemon yellow [DS] 409 Lemon yellow [SC][WN] 396 Leuco base 344 Levopimaric acid 682 LF (designation of color mechanism) 102 Ligand field 77, 102, 131 Lignin 573 – yellowing 624 Lime blue 224 Lime painting 651 Lime white 19, 288 Limonene 681 Linocut 846 Linolenic acid 657 Linseed oil 657 – bleached 660 – blown 659 – boiled 660 – cold-pressed 658 – hot-pressed 658 – refined 658 – Stand oil 659 – sun-refined 660 – varnish 658 Linseed oil varnish 676 Linters 575, 621 Lissamine Green 372 Litharge 6, 232 Lithium cobalt phosphate 22, 293 Lithopone 19, 290 Litmus 379 LMCT transition 134 London force 540 Lorentz oscillator 39 Luster 61 Luster glaze 642 Luteolin 191, 322 LWC (paper) 616 Madder 631 Madder brilliant [SC] 419

915

Madder brown [SC][WN] 420 Madder lake 9, 22, 191, 194, 347, 349 – color formation 191, 194 Madder lake brilliant [SC] 419 Madder root red [SC] 420 Magenta [SC] 419 Magnetite 19, 247 – color formation 247 Mahogany brown [KR] 410 Malachite 6, 25, 125, 222, 631 – color formation 125 Malachite green 371, 372 – color formation 372 Malachite green dye 367 Malachite green pigment 367, 374 Maleic anhydride 729 Maleic resin 850 Malvidin 327 Manganese 291, 293, 629 Manganese black 629 Manganese blue 12, 23, 291 – color formation 291 Manganese blue hue [DS][WN] 383 Manganese chrome antimony titanium brown 262 Manganese ferrite black (oxide) 263 Manganese ferrite black (spinel) 259 Manganese niob titanium brown 262 Manganese oxide 631 Manganese rutile brown 262 Manganese titanium brown 20, 262 Manganese tungsten titanium brown 262 Manganese violet 22, 293 – color formation 293 Manganese zinc chromite brown 259 Manganese-black technique 629 Marker ink see ink, felt-tip pen, ink Mars black 247 Mars brown 246 Mars orange 243 Mars pigments 248 Mars red 22, 245 Mars yellow 20, 243 Mass sizing 607 Massicot 6, 20, 94, 232 – color formation 94 Mastic 681, 685 – aging 686 Masticadienoic acid 685 Masticonic acid 685

916 � Index

Matte color 68 Maxilon blue 379 Maya blue 360 Mayan blue genuine [DS] 358 Mayan dark blue [DS] 364 MDIP 568 Medium – acrylic 738 Mercury 6, 236, 278 Metal color 63, 65 Metal complex chromophore 199 Metal etching – reverse glass painting 643 Metal ink see ink, metal ink Metallic luster 62 Metallization – diazo chromophore 164 Metals – color 65 – luster 62 – oil paint 675 Metamerism 36 Methyl orange 388 Methyl red 387 Methyl violet 371 Methylene blue 379 MHPC 739 Mie scattering 59 Milk 713 Milori blue 285 Mineral green 222, 225 Minium 236, 238 Mitis green 25, 226 Mixed metal oxides 254 – ceramic colors 635 MMCT transition 138 MMO 254 MO see molecular orbital MO (designation of color mechanism) 143 MO theory 144 Molecular orbital 77, 143, 190 – bathochromic shift 147, 150 – n → π ∗ transition 146 – π → π ∗ transition 147 – σ → σ ∗ transition 146 Molybdate orange 283 Molybdate red 22, 283 Molybdenum 280 Monoazo yellow/orange pigments 395

Monohydrazone yellow/orange pigments 395 Mordant Black 3 520 Mordant Blue 7 520 Mordant Red 7 520 Morellic acid 342 Morin 191, 322 Morindon 348 Morning red 58 Moronic acid 685 Mountain blue 221 Mountain green 222 Mowilith 734 Mulberry 348 Munjistin 348 Mural painting see fresco MWC (paper) 616 Myrcene 681 Myricetin 323 N-Methyl-pyrrolidone 740 Naphthalene Fast Orange 527 Naphthamide maroon [DS] 410 Naphthol AS pigments 406 β-Naphthol pigments 405 Naphthol red light [WN] 407 Naphthol red medium [WN] 407 Naphthol red pigments 406 Naphthoquinone 156 Naples yellow 20, 234 Naringenin 321 Natural Black 1 335 Natural Blue 1 358 Natural Blue 2 363 Natural Brown 1 323, 325 Natural Brown 9 857 Natural Brown 11 855 Natural dye ink see ink, dye (nature) Natural Green 2 325 Natural ink see ink, natural ink Natural Orange 4 318 Natural Orange 6 346 Natural Red 3 350 Natural Red 4 350 Natural Red 6 347 Natural Red 8 347 Natural Red 9 347 Natural Red 14 347 Natural Red 16 347 Natural Red 18 347

Index

Natural Red 19 347 Natural Red 20 346 Natural Red 22 337 Natural Red 24 335 Natural Red 25 352 Natural Red 26 338 Natural Red 31 337 Natural Violet 1 361 Natural Yellow 1 323 Natural Yellow 2 322, 323, 325 Natural Yellow 3 337 Natural Yellow 6 318 Natural Yellow 8 322 Natural Yellow 10 322, 323, 325 Natural Yellow 11 322, 325 Natural Yellow 13 322–325 Natural Yellow 14 324 Natural Yellow 20 341 Natural Yellow 24 342 NBHK 587 NBSK 587 Neoabietic acid 682 Nickel barium titanium yellow 264 Nickel ferrite brown 259 Nickel rutile yellow 262 Nickel silicate green 264 Nickel titanium yellow 20, 262 Nickel tungsten titanium yellow 262 Nonassociative thickeners 557 Nondrying oils 656 n → π ∗ transition 146 OCC 568 Ocher 118, 238 – blue 292 – color formation 50, 118, 136, 241 – red 245 – yellow 243 Ocher earth 9 Oil see binder, oil paint Oil paint 655, 703 – binder 656 – yellowing 671 – dammar see dammar – degradation 669 – grinding 656 – ingredients 655 – mastic see mastic – resin 685

– resins 681 – aging 686 – solvents 681, 687 – turpentine oil 679, 681 – turpentine substitute 687 – varnish 679, 689 – aging 686 – cyclohexanone resin 691 – dammar 690 – mastic 690 – natural resin 690 – yellowing 687 Oleanane 684 Oleanolic acid 685 Oleanonic acid 684 Oleic acid 657, 804 Olivine base 264 OMCT transition 134 Onglaze color 632 ONP 568 Opacifier 635 Opacity 66 – size dependence 60 Opaque glass 307 Opera pink [DS] 419 Opera rose [WN] 419 Orange pigments 19 Orcein 379 Organic vermilion [DS] 407 Oriental blue [WN] 383 Orotan 740 Orpiment 6, 20, 233 Orseille 379 Oscillator 39 Ovalbumin see albumin Ox gall 726 Oxazine 365, 379 Oxidation – polyphenol 776 Oxide chromophore 231 Oxynitride pigment 91 PAAE 603 PAC – color formation 356 Paeonidin 327 PAH – color formation 169

� 917

918 � Index

Paint see color lake, see glue paint – acrylic paint see acrylic paint – blackboard 746 – distemper see glue paint – egg white paint see egg white paint – fingerpaint 750 – glue paint see glue paint – gouache see glue paint – grinding 542 – oil paint see oil paint – pastel crayon 746 – plant-based 790 – poster paint see glue paint – retarder 738 – silicate paint 745 – size see glue paint – water-based paint see watercolors – watercolors see watercolors Paint system 27 Palmitic acid 657 Palustric acid 682 PAM 593, 595 PAmA 593 Pannetier’s green 269 Paper 503, 567 – calendering 613 – coating 605 – binder 613 – thickener 613 – coating color 611 – dispersing agent 612 – fillers 612 – pigments 612 – wetting agent 612 – colorants 592 – coloring 524 – composition 589 – decay 622 – dyeing 506 – filler 590 – fixative 602 – hydrophobic modification 609 – inkjet 826 – manufacture 589 – paper grades 614 – printing 855 – retention aid 592 – satinage 613 – sizing 605

– starch 595 – watercolor 726 – writing paper 614, 826 – yellowing 624 Paper pulp see pulp Paper types 616 Para Red 405 Paraloid 734 Paratacamite 6, 25, 218 Paris blue 285 Paris green 226 Pastel crayons 747 Patent blue 372 Patina (copper) 223 PB1 371, 523 PB15 23, 383 PB15:1 383 PB15:3 383 PB15:6 383 PB16 383 PB27 23, 285 PB28 23, 259, 260 PB29 23, 228 PB30 23, 221 PB31 23, 219 PB32 306 PB33 23, 291 PB35 23, 263, 266 PB36 23, 259 PB36:1 259 PB60 355 PB62 371 PB66 358 PB71 264, 637 PB72 259 PB73 263 PB74 264 PB81 259 PB84 360 PBk6 19, 210 PBk7 19, 211 PBk8 19, 215 PBk9 19, 215 PBk10 209 PBk11 19, 247 PBk12 259 PBk20 19 PBk22 259 PBk23 259, 264

Index �

PBk24 262 PBk26 259 PBk27 259 PBk28 259 PBk29 259 PBk30 259 PBk31 421 PBk33 263 PBr6 244, 246 PBr7 20, 244, 246 PBr8 246 PBr11 259 PBr23 403 PBr24 20, 262 PBr25 410 PBr29 20, 263, 264 PBr33 259 PBr34 259 PBr35 259 PBr37 262 PBr39 259 PBr40 262 PBr41 403 PBr43 263 PBr45 262 PBr46 259 PCC 19, 288, 590 PEG 547 PEI 593, 595, 603 Pelargonidin 327 Pen – ballpoint pen 801 – felt-tip pen 801 Pencil 747 – chalk 746 – colored crayon 750 Penniman process 252 Pentaerythritol 730 Perinone orange [DS] 422 Permament brown [DS] 410 Permanent alizarin crimson [WN] 354, 421 Permanent blue 228 Permanent carmine [SC][WN] 418 Permanent magenta [WN] 418 Permanent orange [DS] 409 Permanent red deep [DS] 407 Permanent red [KR][DS] 407 Permanent rose [WN] 418 Permanent white 19, 273, 290

919

Permanent yellow deep [DS] 425 Permanganate – color formation 138 Persian berry 325 Persian red 242, 245 Perylene black [WN] 421 Perylene dark red [SC] 421 Perylene green [SC][DS][WN] 421 Perylene maroon [SC][DS][WN] 421 Perylene red [DS] 421 Perylene red [WN] 421 Perylene scarlet [DS] 421 Perylene violet [SC][DS][WN] 421 Petroleum gasoline 687 Petunidin 327 PG1 371 PG4 371, 523 PG7 25, 385 PG14 25, 278 PG15 25, 284 PG17 25, 268 PG18 25, 269 PG19 25, 264, 266 PG20 25, 223 PG21 25, 226 PG22 25, 225 PG23 25, 248 PG26 259, 261 PG36 25, 385 PG39 25, 222 PG48 25, 284 PG50 25, 259, 261 PG51 264, 636 PG56 264 PGW 577 Phellandrene 681 Phenazine 365, 379 Phenol-modified rosin resin 851 Phenolphthalein 367 Phlobaphene 786 Phloxine 367 Photopaper 826 Phthaleine 367, 370 Phthalic anhydride 729 Phthalo blue (green shade) [DS] 383 Phthalo blue (red shade and green shade) [WN] 383 Phthalo blue (red shade) [DS] 383 Phthalo blue [SC] 383

920 � Index

Phthalo blue turquoise [DS] 383 Phthalo green (blue shade) [DS] 385 Phthalo green [SC] 385 Phthalo green (yellow shade) [DS] 385 Phthalo green (yellow shade) [SC] 385 Phthalo green (yellow shade) [WN] 385 Phthalo sapphire blue [SC] 383 Phthalo turquoise [WN] 383 Phthalocyanine pigments 175, 382 – blue 23, 383 – green 25, 385 Pigment 27 – antiquity 6 – cause of color 2 – chronological overview 6, 18 – copper 217 – copper pigments 224 – DR (doped rutile) see DR pigment – for glaze 632 – Impressionism 12 – inorganic – colorspace 207 – iron oxide 238 – Middle Ages 10 – Modern 14 – ocher 238 – opacifier 635 – organic – color space 312, 315 – oxynitride 91 – Renaissance 10 – Romanticism 12 – solubility 191, 316 – texture 191 Pigment classification by CI 14 Pigment red, yellow, … 14 Pimarane 682 Pinene 681 Plant black 19, 215, 631 Plant ink 787 Plant-based paints see paint, plant-based Plasma frequency 62 Plasmon 45 Plextol 734 PM (phosphomolybdic acid) 375 PMA (phosphomolybdic acid) 375 PO5 388, 392 PO20 20, 277 PO21 20, 282

PO23 20, 278 PO34 402 PO42 243 PO43 422 PO45 282 PO48 420 PO49 420 PO61 425 PO62 409 PO66 426 PO71 423 PO73 423 PO75 20, 267 PO78 20, 267 PO82 262 Poisonous green 226 Polyacrylamide 593, 595 Polyacrylate 814 Polyamidoamine 593 Polycadinen 684 Polychrome firing 628 Polycyclic aromatic hydrocarbons – chromophore 169 – color formation 169 Polycyclic aromatics – color formation 356 – quinones – chromophore 172 – color formation 356 Polyene chromophore 164, 310, 313, 317 – annulene 174 – donor-acceptor substituted 168 – linear 166 – phthalocyanine 175 – polycyclic 169 – polycyclic quinone 172 – porphine 175 Polyethylene glycol 547, 812 Polyethyleneimine 593, 595, 603 Polymer 814 Polymethine chromophore 183, 310, 313 – bathochromic shift 186 Polymyrcene 685 Polyphenol 776, 783 – oxidation 776 Polyporic acid 345 Polyvinylalcohol 595, 740 Polyvinylamine 593, 603 Polyvinylpyrrolidone 813, 814

Index

Ponceau 4R 527 Poppy seed oil 657 Porphine 175 Porphyrazine 175 Porphyrin 175 Posnjakit 218 Poster paint see glue paint π → π ∗ transition 147 PR1 405 PR3 388 PR38 392, 402 PR53 405 PR57 406 PR81 371, 523 PR83 22, 354 PR88 362 PR101 22, 244–246, 263 PR102 22, 245, 246 PR103 22, 282 PR104 22, 283 PR105 22, 238 PR106 22, 236 PR108 22, 277 PR112 407 PR113 22, 278 PR122 419 PR144 403 PR149 421 PR166 403 PR168 355 PR170 407 PR175 410 PR176 410 PR177 354 PR178 421 PR179 421 PR181 362 PR184 407 PR185 410 PR187 407 PR188 407 PR202 419 PR206 420 PR207 419 PR209 419 PR224 421 PR230 263 PR231 263 PR232 264, 637

� 921

PR233 264 PR235 259 PR236 263 PR242 403 PR254 423 PR255 423 PR259 22, 230 PR260 426 PR264 423 PR265 22, 267 PR275 22, 267 Precipitated calcium carbonate 19, 288, 590 Preservatives 566 Priderite base 264 Primal 734 Primrose yellow 280 Print – copier 833 – laser 833 Printing ink 836, 845, 855 – artistic 841 – binder 846 – colorant 843 – commercial 843 – solvents 853 Privet 332 PRMP 577 Proanthocyanidin 781 Procion blue MX-R 512 Procion yellow 512 Propanol 806 Propylene glycol ether – acrylate 740 – Acrylic 740 Propylene glycols – acrylate 738, 740 – ink 812 Protective colloid 555 Protein see binder, protein Protein color – binder 706 – ingredients 706 Prussian blue 23, 140, 141, 285 – color formation 140, 141 Pseudopurpurin 348 PT (phosphotungstic acid) 375 PTA (phosphotungstic acid) 375 Pulp 576, 581 – BAK 587

922 � Index

– BEK, BEKP 587 – BHK 587 – BHS 589 – BKP 583, 587 – BSK 587 – BSS 589 – ECF 587, 589 – mechanical 577, 581 – NBHK 587 – NBSK 587 – SBHK 587 – SBSK 587 – TCF 587, 589 – UKHP 584 – UKP 583, 584 – UKSP 584 – USS 588 – wood pulp 577 – BCTMP (SW,HW) 580 – CMP 577, 579 – CRMP 577 – CTMP 577 – GW, GWD 577 – PGW 577 – PRMP 577 – RMP 577 – SGW 577 – TMP 577 – TRMP 577 Pulp paper 614 Pure yellow [SC] 409 Purple 9, 22, 361 – color creation 361 Purple gold 634 Purple [KR] 410 Purple magenta [SC] 419 Purple of Cassius 634 Purple pigments 21 Purpurogallin 798 PV1 371, 523 PV3 371 PV5 354 PV14 22, 292 PV15 22, 230 PV16 22, 293 PV19 418 PV23 381 PV27 371 PV29 421

PV32 410 PV37 381 PV39 371 PV42 419 PV47 22, 293 PV49 22, 293 PV55 419 PVAm 593, 603 PVOH 595, 740 PVP 813, 814 PW1 19, 289 PW4 19, 273 PW5 19, 290 PW6 19, 270 PW18 19 PW18:1 19 PW21 19, 290 PW22 19, 290 PW25 19 PY1 396 PY3 396 PY12 401 PY13 401 PY24 262 PY31 20, 283 PY32 283 PY33 283 PY34 20, 280 PY35 20, 275 PY36 20, 283 PY36:1 283 PY37 20, 275 PY39 20, 233 PY40 20, 291 PY41 20, 234 PY42 20, 243 PY43 20, 243 PY46 20, 232 PY47 263 PY53 20, 262 PY65 396 PY74 396 PY83 401 PY97 396 PY108 355 PY110 425 PY119 20, 259 PY120 409 PY126 401

Index �

PY127 401 PY128 402 PY139 426 PY151 409 PY153 342 PY154 409 PY155 402 PY157 264 PY158 263 PY159 264, 637 PY160 264 PY161 262 PY162 262 PY163 262 PY164 20, 262 PY171 410 PY175 409 PY180 409 PY181 409 PY184 20, 279 PY189 262 PY213 411 PY216 262 PY220 823 PY227 264 PY43 244 Pyrazolone pigments 395 Pyrochlore base 264 Pyrrol crimson [DS] 423 Pyrrol orange [DS][WN] 423 Pyrrol red [DS] 423 Pyrrol scarlet [DS] 423 Pyrrole red light [WN] 423 Pyrrole red [WN] 423 Pyrrolidone 812 Quark 713 Quercetin 191, 322 Quinacridon red light [SC] 419 Quinacridone 418 Quinacridone burnt orange [DS] 420 Quinacridone burnt orange [WN] 420 Quinacridone burnt scarlet [DS] 420 Quinacridone coral [DS] 419 Quinacridone fuchsia [DS] 419 Quinacridone lilac [DS] 419 Quinacridone magenta [DS][SC] 419 Quinacridone magenta [WN] 418 Quinacridone magenta [WN][DS][SC] 419

Quinacridone purple [SC][DS] 419 Quinacridone quinone 420 Quinacridone red [DS][SC] 418 Quinacridone red [WN] 419 Quinacridone rose [DS] 418 Quinacridone violet [DS][SC] 418 Quinacridone violet [WN] 419 Quinizarin 348 Quinoline yellow 527 Quinones – cause of color 172 – color formation 156 Quinoxalindione pigments 411 Radical chromophores – sulfide 189 Rag paper 614 Raw Sienna 240, 244 Raw umber 240, 246 Rayleigh scattering 58 RB4 512 RB19 512 RBK 568 RBk31 512 Read lead 238 Realgar 20, 236 Red chalk 245 Red earth 245 Red lakes 347 Red lead 6, 22 Red ocher 22, 118, 238, 245 – color formation 118, 241 Red orpiment 236 Red pigments 21 – ceramics 630 Red wine 333 Redwood 191, 335 – color formation 335 Refractive index 42, 51 Relbun root 348 Reseda 191 Resin balsam 679, 681 Resinate 850 Resins 679 – aging 686 – oil paint 681 Retarder 738 Retention aid see paper, retention aid

923

924 � Index

Reverse glass painting see stained glass, reverse glass Rhamnetin 191, 322 Rheology modifier – ink 813 Rhodamine 6G 371 Rhodamine B 371 Riebeckite 9, 23 – color formation 142 Rinmann’s green 25, 264, 266 RMP 577 Rollerball pen ink see ink, rollerball pen Root 345 Rose madder 347, 349 Rosin 681 – ester 850 – phenol-modified 851 – salt 850 RR23 512 RR24 512 RR180 512 Rubiadin 348 Ruby madder alizarin [WN] 407 Ruby red deep [SC] 423 Ruby red [SC] 418 Ruby [SC] 423 Rutile 101 Rutile base 262 Rutile mixed oxides 261 – color formation 261 RY3 512 Safflower oil 657 Saffron 318 Saflor 191 Saflor yellow 338 Saflorcarmine 338 Sandalwood 337 Sandarach 236 Sanguine 245, 648 Santalin 337 Sap color see color, sap color Sap green 191, 325 Saponification – lead white 289 – oil paint 671 Satinage see Paper, calendering Saturn red 238 SBHK 587

SBk27 821 SBSK 587 SC (designation of color mechanism) 82 SC (paper) 616 Scarlet lake [WN] 407 Scarlet lake [WN 423 Scarlet red [SC] 423 Scattering 56 – size dependence 60 Scheele’s green 25, 225 Schellolic acid 859 Schmincke Violet [SC] 381 Schweinfurt green 25, 226 – color formation 226 Sealing wax 858 Sebacic acid 729 Seladonite 9 Selection rules 111 Semiconductor 77, 82, 102 – band gap 84 – band structure diagram 85 – chromophore 86 – color 86 – formation of bands 83 Sepia 857 SGW 577 Shell white 288, 289 Shell white (japanese) 289 Shellac see binder, shellac Siccatives 566 – oil paint 675 Sienna see terra di Sienna Sienna burnt 240 Sienna raw 240 Silica 593 Silicate paint 745 Silver 49, 62, 301, 633, 641 Silver ruby glass 49, 301 Silver solder 648 Silver yellow 301, 648 Sinoper 245 Sirius Blue 415 Sirius Red 415 Size dependence 71 Size paint see glue paint, 709 Sizing see paper, sizing Sky blue 58 Smalt 23, 306 – color formation 306

Index �

Snow white 273 SNP (paper) 616 Solubility 191, 316, 499 Solvents – ink 806, 852 – oil paint 681, 687 – printing ink 853 Soot 19, 210 SP 45 Spanish green 25, 223 Spanish red 242, 245 Sphene base 264 Spin-orbit coupling 112 Spinel 254 – mixed oxide pigments 257 Spinel base 259 Spinel black 19, 259 σ → σ ∗ transition 146 St. John’s white 19, 288 Stabilization – electrostatic 553 – steric 554 Stain dyeing 520 Stained glass 640, 646, 649 – binder 646 – black solder 646 – colorant 646 – enamel paint 648 – iron red 648 – reverse glass 642 – reverse glass painting – amelioration 643 – binder 643, 644 – color etching 643 – colorant 643, 644 – Eglomisé 644 – metal etching 643 – silver solder 648 – silver yellow 648 Stand oil 659, 672 Starch see paper, starch, 750 – cationic 593, 595 – modified 595, 607 Starch paste 595, 607 Stearic acid 657 Steric stabilization 554 Stick – pencil 747

Stil de Grain 10, 191, 324 – color formation 191 Succinic acid 729 Sugar 814 Sulfide chromophore 189, 231 Sun-refined linseed oil 660 Surface – matte 68 Surface light 69 Surface plasmon 45 Surface sizing 607 Surface tension – grinding paints 542 – ink 813 Surfactant 545 Surfactants – ink 813 Surfynol 739 Swedish green 225 SY2 388 Synacril Red 523 Tafigel 739 Talcum 630 Tamol 740 Tannin 321, 333, 775 – condensed 781 – nonhydrolyzable 781 Tannin-like tanning agents 783 Tanning agent 775 – hydrolyzable 779 Tartrazine 342, 392 TCF 587, 589 Tea 321, 323 Tempera 717 – egg 719, 720 – fatty 720 – egg yolk 718 – egg-oil emulsion 720 – emulsifier 719 Tempera painting 652 Terephthalic acid 729 Term 108 Terpinene 681 Terpinolene 681 Terra di Sienna 20, 244 Terre verte 248 Texanol 740 Theacitrin 785

925

926 � Index

Theaflavin 322, 783 Theanaphthoquinone 785 Thearubigin 322, 786 Theasinensin 784 Theasinensin, -naphthoquinone 322 Thermal black 214 Thiazine 365, 379 Thickener – acrylic 738 – coating (paper) 613 – ink 813 Thioindigo 362 Thioindigo [WS] 362 Thioxanthene 365 Thorn ink see ink, thorn ink Thénard’s blue 23, 259, 260 Tin antimony gray 263 Tin chrome violet 263 Tin vanadium yellow 263 Tin-niobium yellow 264 Tin-zinc rutile 262 Tirucallol 685 Titanium vanadium gray 262 Titanium white 19, 89, 270 – color formation 89 TMP 577 Toluidine red 388 Toner 833 – black 835 – color 835 – lake pigment 397, 405 tragacanth see binder, gum tragacanth Transparency 66 Transparent brilliant yellow [SC] 402 Transparent brown [SC] 403 Transparent copper green 225 Transparent cyan [SC] 383 Transparent iron oxide 253 Transparent magenta [SC] 419 Transparent maroon [WN] 410 Transparent orange [SC] 423 Transparent oriental blue [SC] 383 Transparent pyrrol orange [DS] 423 Transparent red deep [SC] 403 Transparent turquoise [SC] 383 Transparent yellow [WN] 402 Tree resin 607 Triazine 510 Trimellic acid 729

Trimethylolpropane 730, 812 Tristimulus function 34 Triton X100 739 TRMP 577 Turkish red 196, 198, 282, 354 Turkish red oil 198 Turmeric 337 Turpentine oil 679, 681 Turpentine substitute 687 Tusche 855 – binder 858 – bister 856 – black 856 – brown 856 – colorant 855 – Sepia 857 Tylose 739 UKHP 584 UKP 583, 584 UKSP 584 Ultramarine blue 23, 189, 227, 228 – color formation 189, 227 Ultramarine pink 22, 227, 230 – color formation 227 Ultramarine violet 22, 227, 230 – color formation 227 Ultramarine yellow 283 Umber 20, 246 Umber burnt 240 Umber raw 240 Underglaze color 632 Universal gasoline 687 Ursane 684 Ursonic acid 684 USS 588 Valence band 84 Valonic acid 779 Van der Waals force 502, 539, 705 Vanadium yellow 263 Varnish 69, see oil paint, varnish Varnish linseed oil 658 Vat Blue 1 358 Vat dyeing 344 Vat Red 41 362 Vat Yellow 20 355 VB theory 148 Vegetable black 19, 215

Index

Velates 740 Venetian red 245 Verdigris 25, 223 Vermilion 22, 89, 236 – color formation 89 Vermilion light [SC] 407 Vermilion [SC] 423 Vermillion red [SC] 423 Veronese green 226 Victoria green 264 Victoria Rein Blau 371 Victoria Yellow 527 Vienna green 226 Viennese red 282 Viktoriagrün 636 Vine black 19, 215 Vinylsulfonic acid 512 Violet 332 Violin varnish 420 Viridian 25, 268, 269 Vivianite 9, 23, 292 – color formation 141 Vulcanization 344, 364 Walnut oil 657 Wash gasoline 687 Water 806 Water blue 372 Water glass 745 Water-based paint see watercolors Watercolor – wetting agent 726 Watercolor paper 726 – cold-pressed 727 – hot-pressed 727 – rough 727 Watercolors 721, 727 – binder 722 – glycerol 723 – humectant 723 – ingredients 721 Weld 325 Wetting agent 739 – watercolor 726 Wetting agents 542 WFC (paper) 616 WFU (paper) 616

� 927

White pigments 18, 67, 87 – ceramics 630 White wine 323 Winsor blue (red shade and green shade) [WN] 383 Winsor green (blue shade) [WN] 385 Winsor green (phthalo) [WN] 385 Winsor green (yellow shade) [WN] 385 Winsor lemon [WN] 396, 409 Winsor orange red shade [WN] 423 Winsor orange [WN] 409, 423 Winsor red deep [WN] 421, 423 Winsor red [WN] 423 Winsor violet [WN] 381 Winsor yellow deep 396 Winsor yellow deep [WN] 396 Winsor yellow [WN] 396, 409 Winther symbol 388 – A 389 – D 390 – E 390 – M 415 – Z 398, 400 Woad 359 Wood 345, 569 Wood pulp see pulp Wood-containing paper 614 Wood-free paper 614 Woodcut 846 Writing ink see ink, writing ink Writing paper 614, 826 Xanthene 365, 367 Xanthophylls 318 Xylindein 752 Yellow earth 243 Yellow lakes 324 Yellow ocher 20, 118, 238, 243 – color formation 118, 241 Yellow Orange S 405, 815 Yellow orange [SC] 425 Yellow pigments 19 Yellow root 337 Yellowberry 325 Yellowing – oil paint (binder) see oil paint, binder, yellowing – paper see paper, yellowing – varnish see oil paint, varnish, yellowing Yellowwood 325

928 � Index

Yellowwood lake 324 Zinc aluminum pink 259 Zinc chrome cobalt aluminum blue 259 Zinc ferrite brown 259 Zinc green 266, 284, 287 Zinc iron brown 20, 259 Zinc iron chromite 259 Zinc iron chromium brown 259 Zinc resinate 850

Zinc white 19, 89, 273 – color formation 89

Zinc yellow 20, 283

Zirconium cadmium red 264

Zirconium iron pink 264, 637 Zirconium pigments 637

Zirconium praseodymium yellow 264, 637 Zirconium vanadium blue 264, 637 Zirconium vanadium yellow 264