Chemical technology of ceramics: еducational manual 9786010436367

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AL-FARABI KAZAKH NATIONAL UNIVERSITY

G. A. Seilkhanova

CHEMICAL TECHNOLOGY OF CERAMICS Educational manual

Almaty «Qazaq University» 2018

UDC 663.3/7 LBC 35.41 S 44 Recommended for publication by the decision of the Academic Council of the Faculty of Chemistry and Chemical Technology, Editorial and Publishing Council of Al-Farabi Kazakh National University (Protocol №7 dated 05.07.2018) Reviewers: Doktor of chemlcal science, ass. professor D.N. Akbaeva Doktor of technical science, professor S.A. Efremov

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Seilkhanova G.A. Chemical technology of ceramics: еducational manual / G.A. Seilkhanova. – Almaty: Qazaq University, 2018. – 68 p. ISBN 978-601-04-3636-7 The training manual presents the theoretical foundations of ceramic production, physical and chemical properties of ceramics. The main technological stages for producing ceramic materials are discussed. This manual contains laboratory work for determining some characteristics of ceramics. There are test questions in order to improve the assimilation of theoretical material as well as to control students’ knowledge. The manual can be used in the study of disciplines “Chemical technology of silicate materials”, “Chemistry and technology of ceramics”, “Ceramic materials”, “Technology of composite silicate materials”. The manual is intended for use by students of chemical and technological specialties, and can also be used by teachers and staff working in the field of production of silicate materials. Published in authorial release.

UDC 663.3/7 LBC 35.41 ISBN 978-601-04-3636-7

© Seilkhanova G.A., 2018 © Al-Farabi KazNU, 2018

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INTRODUCTION Ceramics is the first artificial material in the history of humanity. People learned to use the useful properties of clay even in the primitive era – its plasticity when combined with water, the ability to dry on air and keep the desired shape, and then turn into a new material under the influence of fire, which is called ceramics. The unique properties of ceramics were evaluated in ancient times. That is why the widest application and popularity of ceramics were in the domestic sphere – in the manufacturing of various types of containers. According to modern science, the origin of the ceramic ware manufacturing is associated with East Asia. 13-10 thousand years ago, at the turn of two cultural and historical eras – the Paleolithic (the Old Stone Age) and the Neolithic (New Stone Age), the primitive inhabitants of the Japanese islands, the southern part of the Russian Far East and East China learned to sculpt clay vessels, burn them with fire and obtain sufficiently strong hermetic containers for the preparation and storage of food, products and other household purposes. The oldest pottery was primitive in form and decoration, brittle due to low-temperature firing, however it was already the first step in the long history of pottery, which has become an important element of human culture and still retains its importance. The art of making ceramics originated in East Asia in ancient times has passed the way of thousands of years of constant development and development of new technological boundaries. Currently, the production of ceramic materials is undergoing rapid development in connection with the active improvement of traditional technology for their production. The use of ceramic materials in new areas of technology has significantly increased. The main goal of the educational manual is to help students to form knowledge about ceramic materials and the technology for their production. The most widely used in practice classification of ceramic materials is given in this manual. The structure of ceramics, its microstructure, physical and chemical properties are described, raw 3

materials (plastic, non-plastic masses), the main stages of production are considered. The training manual presents laboratory work aimed at determining some of the physical and chemical properties of ceramics, as well as test questions allowing to monitor students' knowledge. The concept of ceramics The production of ceramic materials has been known since ancient times. The word "ceramics" comes from the Greek keramike – the art of making clay products. The main raw material used in production is clay. At the same time, man learned to give a shape to a raw clay and fix it by drying and high-temperature processing. Therefore, for a long time ceramic technology was understood as the production of clay products or clay-containing masses by molding, drying and subsequent calcination. Currently, the technology of ceramic production is understood as the manufacture of products from mineral raw materials or chemicals by mass preparation, molding, removal of the temporary bond and high-temperature firing in order to make them stone-like properties.

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1 CLASSIFICATION OF CERAMIC MATERIALS AND PRODUCTS The properties of ceramic materials are greatly influenced by their structure. Therefore, the pore structure of ceramic materials is the basis of the generally accepted classification. Ceramics can be divided into two classes: porous, giving an earthy fracture and waterpermeable (without glazing); sintered, giving shiny conchoidal fracture and water-resistant. Each of these classes, depending on the properties and structure of the shard, its external design, as well as the application of products is divided into groups (for example, coarse- and fine-grained, glazed and unglazed, refractory and acid-resistant, etc). Usually, ceramic products are classified according to the production and industry characteristic, which allows to reflect the properties of products, the field of application and, largely, the production method. According to this classification, the following groups of ceramic materials and products are distinguished: building ceramics – products intended for the laying of buildings and structures (brick, ceramic stones, blocks), ceramic tiles for exterior and interior wall cladding and flooring, products for underground communications (sewage and drainage pipes), sanitary ware, tiles, heat-insulating ceramic materials (expanded clay, agloporite); refractory materials – products used for the laying of industrial furnaces and apparatus operating at high temperatures, as well as products used as an ammunition for firing parts for various purposes; chemically resistant materials – products intended for the work in aggressive environments replacing or protecting metal parts of devices, machines in industry; fine ceramics – household porcelain and faience ware, artistic and decorative products, chemical vessels and other types of products; technical and special ceramics – materials and products with specific properties used by aviation, rocket and space technology, nuclear technology, radio electronics, electrical engineering. 5

2 THE STRUCTURE OF CERAMICS The structure of ceramics is determined by the mutual distribution and the combination of its constituent phases. Almost all ceramic materials are complex systems consisting of three main phases – crystalline, vitreous and gas. The crystalline phase is basic, it determines the phase composition, properties and group of ceramic material. The structure may have one or more crystalline phases in different ratios. The crystalline phase is mainly represented by mullite crystals, as well as quartz and kaolinite particles, which are not dissolved in the vitreous phase. Thus, the crystalline phase in porcelain accounts for about 30%. Mullite with a needle-like fibrous structure has high mechanical strength and chemical resistance, low thermal expansion and, in connection with this, high thermal stability. The vitreous phase is the interlayer between the crystalline constituents in the ceramic material that fulfills a cementing role in it. The amount and composition of the vitreous phase are determined by the presence of impurities and fluxes introduced into the mass composition. The vitreous phase is a feldspar melting with the grains of quartz and alumina partially dissolved in it. Sometimes there is feldspar glass with mullite crystals. The vitreous phase increases the translucency of the shard. The strength and thermal stability decrease with the increase of the vitreous phase. The gas phase is air or other gases contained in the pores of the material. Ceramic materials are combination of a solid (crystalline material) with voids – pores. The mutual distribution, the physical and chemical nature and the quantitative ratio of the crystalline, vitreous and gas phases determine the structure of the ceramic material. In the structure of ceramics, microstructure and texture are distinguished. Microstructure means the nature of crystalline phases, 6

the composition of the vitreous phase, and also their combination with pores. Texture determines the grain and pores in the material, their size, volume, relative position. The microstructure and texture of ceramics depend on many factors, mainly, on the type of raw material, the technological methods of their processing and mass preparation, molding methods, the physical and chemical processes occurring during sintering. To fully characterize the microstructure, in addition to the porosity data, it is necessary to know the quantity, size, shape and distribution of each of the components of a complex ceramic system.

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3 PHYSICAL AND CHEMICAL PROPERTIES OF CERAMICS 3.1. Porosity, density, permeability In the ceramic material, along with the main structural components (crystalline and vitreous phases), there are voids (pores) filled with air or gas. The size, total volume and pore distribution pattern affect the properties of ceramics. The pores of ceramics are variable in form and shape. The pores are divided into closed, i.e. inaccessible for penetration of liquid and gas, and open, which can be dead-end or channel-forming. The most complete and correct characteristics of porosity of ceramics are given by integral and differential curves of pore distribution by their size. However, it is difficult to obtain these characteristics, therefore the porosity of ceramics is usually determined by the following indirect indicators: true density ρt – the weight per unit of volume, excluding pores, kg / m3; apparent density ρa – density of a material, including pores kg / m3; true porosity Pt – the ratio of the total volume of the open and closed pores to the total volume of material, %; apparent, or open, porosity Pa – the ratio of pore volume filled with the liquid to the total volume of the material, %; closed porosity – is found from the expression Pc= Pt- Pa; water absorption W – the ratio of the mass of water filling the pores of the material during its boiling to the mass of dry material, %. Then the true and apparent porosity can be found from expressions Pt = (ρt – ρa) / ρa. 100 = (1- ρa/ρt) .100 and Pa=Wρt, where ρa/ρt – relative density. 8

Porosity – depending on the porosity, ceramic materials are divided into sintered and porous. The true porosity of the completely sintered ceramics is 3-5%, the apparent – less than 0,1. Porosity and water absorption are used in technology as criteria for the degree of sintering of ceramics and for different types of ceramics have different values. The density of ceramics is determined by the density of its constituent crystalline phases. It ranges from 2250-2800 kg / m3 for silicate ceramics to 4500-9000 kg / m3 for ceramics based on heavy oxides. The degree of crystallization of ceramics, the isolation of a particular crystalline phase, the presence of polymorphic transformations can be judged from the change in density. The permeability of ceramics is determined by the number of channel-forming pores. It is characterized by the permeability coefficient, which shows how much liquid or gas passes per unit time through unit area and thickness of the sample at a pressure drop. 3.2. Mechanical strength and thermochemical properties At normal temperatures, ceramics refers to brittle materials, which can be destroyed with a small elastic deformation. Plastic deformation is absent. Theoretical strength of ceramic materials of crystalline structure calculated by their interatomic bonds is approximately equal to (1-5).105 МPа. However, the actual strength of ceramics is much lower – 10.103 МPa due to the features of its structure. The crystalline phase of ceramics almost always has a defective structure at both macro- and micro-level due to the presence of impurity ions, dislocation, vacancies, grain boundaries, voids, inclusions of other phases. Strength reduction can also be caused by microcracks on the surface of the material which are stress concentrators, and can contribute to the destruction of the material. If it is possible to largely avoid lattice defects and surface microcracks, the strength of the material approaches the theoretical one. The nature of the destruction of ceramics depends on the type of applied load. Therefore, the strength properties of materials are evaluated by short-term strength in compression, flexure, tension, 9

twisting. It should be noted that the compressive and flexural strengths are most often evaluated, since they are very simple methodically. Ceramics of a crystalline structure, mainly oxide, has maximum strength. For example, the compressive strength of clay bricks is 10-20 MPa, porcelain 300-500 MPa, some varieties of technical oxide ceramics 1000-1800 MPa and more. Flexural strength is 1/3-1/5, and the tensile strength is 1/6-1/8 of the compressive strength. The strength properties of ceramics decrease with increasing temperature. The strength in some ceramics varies unevenly. The strength of ceramics of the crystalline structure (for example, corundum) decreases smoothly, mainly due to the structural weakening of bonds with increasing temperature. Clearly expressed maximum for mullite ceramics containing a significant amount of vitreous phase on the strength curve is formed (Figure 1). The increase in strength in the interval 1000-1200 °C is explained by the decrease in the viscosity of vitreous phase, the appearance of plasticity, which reduces the tendency of the material to brittle destruction. It is also possible that the vitreous phase tightens the microcracks present in the ceramic structure. In some cases, ceramics is used at high temperatures in the stressed state, for example under compression, flexure or tension. The ability of ceramic materials to withstand constant loads at high temperatures is evaluated by two methods: determination of destruction temperature under load and obtaining creep curves.

Figure 1. Change in strength of ceramics at high temperatures: 1 – ceramics of a crystalline structure; 2 – ceramics containing a significant amount of vitreous phase

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The deformation temperature is set at a load of 0.2 MPa, wherein 4 temperature points are recorded: deformation start, 4, 10 and 20% compression. This method is usually used to characterize refractory materials and determine its optimum operating temperature, which lies between the deformation start temperature and 4% compression. 3.3. Thermophysical properties Thermophysical properties of ceramics include heat capacity, thermal conductivity, thermal expansion. Heat capacity of ceramics is amount of heat required to raise material temperature by one unit. This heat increases vibrational motion of atoms in the crystal lattice sites. Heat capacity is a property of a substance that does not depend on the structural features of the ceramic material, its density, porosity, the size of the crystals and grains, and other factors. The heat capacity of ceramics depends on temperature. Thermal conductivity of ceramics is amount of heat passing through the material at a certain temperature gradient. In solid nonmetallic substances, in particular ceramics, heat is transferred by thermal elastic waves. Thermal conductivity of ceramics is characterized by the coefficient of thermal conductivity λ, measured in W/(m.K). This property of ceramics depends on temperature, so the coefficients are given in a certain temperature range. Thermal conductivity of ceramics of a crystalline structure strongly decreases with the increase in temperature. Thermal conductivity in ceramic materials with a large vitreous phase content increases monotonically under the same conditions, which is explained by the significant differences in their structure. Increasing the porosity reduce the thermal conductivity of the ceramic samples. The form of porosity plays an important role. The most effective pores are those that break the integrity of the material and cause numerous ruptures and cracks. Thermal expansion of ceramics is a reversible process and is caused by an increase in length between the atoms with an increase in the amplitude of their oscillations under the influence of tempera11

ture. Thermal expansion of structural constituents of ceramics (crystals and vitreous phase) is different and depends on their structure, as well as the strength of chemical bonds. The ability to thermal expansion is characterized by TCLE (temperature coefficient of linear expansion, α) true or average; related to a certain temperature range: αtrue=

; αav =

.

Thermal expansion can be expressed as a percentage: ∗ 100. The volume coefficient of thermal expansion for isotropic polycrystalline bodies, to which the majority of ceramic materials belong, is equal to the triple linear β=3α. 3.4. Thermal properties Thermal properties of ceramics include refractoriness, constancy of volume and mass at high temperatures, thermal resistance. Refractoriness is the property of ceramics to withstand high temperatures without melting. Refractoriness is quantitatively determined by the temperature, under the influence of which the sample in the form of a truncated pyramid is deformed, and the vertex touches the support. Based on refractoriness, ceramic materials are divided into fusible, infusible and refractory. Fusible materials include ceramics based on red clay (brick, tile, pottery, etc.), the refractoriness of which is not more than 1350 °С. Infusible materials are characterized by refractoriness in the range of 1350-1580 °С. These are products made from white clay – porcelain, faience, acid-resistant and some types of technical ceramics. Materials with refractoriness greater than 1580 °С, belong to the refractory, which are subdivided into the refractory (1580-1770 °С), high-refractory (1770-2000 °С) and the highest refractoriness (>2000 °С). The first group includes acid, 12

semi-acid and chamotte refractories, the second – high-alumina, dolomite, etc. Materials with the highest refractoriness are ceramics based on carbon, pure oxides, some non-oxygen compounds. Constancy of volume and mass at high temperatures is important for products of refractory and technical ceramics exposed to longterm exposure to higher temperatures than their firing temperatures, as a result of which physical and chemical processes unfinished during the firing process take place in the materials during the exploitation. At the same time irreversible changes in the volume of products occur. Most ceramic materials are condensed under the influence of temperature, which leads to their additional shrinkage. Thermal resistance of ceramic materials characterizes their ability to withstand without destruction sudden temperature changes. Under the influence of temperature differences, the product is destroyed due to the appearance of stresses exceeding the tensile strength of the material. Different layers of the material with sudden heating or cooling have different temperatures, and therefore undergo unequal volumetric changes, leading to the appearance of shear stress. Thermal resistance depends on many factors – mechanical strength and modulus of elasticity of the material, TCLE, thermophysical parameters, dimensions and product shape. The structure of ceramics has great importance for heat resistance. In dense sintered ceramic products, thermal cracks are generated and propagated at a very high rate, while in a coarse-grained shard with a microcrack structure, this process is slower due to the cushioning and dilution action of structural inhomogeneities. Therefore, to increase the thermal resistance of the material, a coarse-grained non-plastic material is introduced or microcrack structure is artificially created. 3.5. Chemical resistance Ceramic materials in the process of exploitation are exposed to aggressive action of various substances – solid (dust, raw materials, refractories), liquid (solutions of acids, alkalis, melts of salts, glasses, slags, metals), gaseous (fuel combustion products and chemical reactions). 13

The ability of ceramics to withstand the effects of aggressive media is called chemical resistance. Corrosive destruction of ceramics takes place under the influence of an aggressive medium, which leads to a partial or total loss of strength or other properties of the material. Insufficiently high chemical resistance significantly reduces a product’s service life, which leads to the failure of the whole system. Chemical resistance of ceramics depends on the nature of the interacting phases, their chemical and mineralogical composition. The rate of a chemical interaction depends on external factors, in particular, temperature, pressure, concentration and the medium viscosity. The chemical resistance of ceramics is also strongly influenced by its structural features – porosity, type and nature of pore size distribution, the state of the surface, quantitative ratio and volume distribution of crystalline and vitreous phases. Chemical destruction of ceramics is enhanced with the increase in porosity, especially penetrating, because of the increase in the interaction surface with the aggressive medium. Therefore, corrosion in porous ceramics spreads throughout the volume. Continuous corrosion is observed in single-phase ceramics, selective corrosion of one of the phases, less stable, mainly vitreous is possible in multiphase ceramics. When ceramics is exposed to acids and alkalis, as well as their solutions the most vulnerable phase is the vitreous one, and the degree of dissolution depends mainly on its chemical composition. It is known, acid phases and oxides are more resistant to acids, and bases – against alkalis. The persistence of ceramics to gaseous agents is due to both its chemical composition and structure, and the composition of the gas. When ceramics interact with gaseous products, chemical reactions are possible, as a result of which more fusible compounds are formed. An example is the destruction of arches of glass melting furnaces under the influence of volatile components from the glass mass and fuel combustion products. 3.6. Electophysical properties The main electrophysical characteristics of ceramics include electrical conductivity, dielectric constant, dielectric losses, electric strength. 14

Electrical conductivity of ceramics is usually determined from the inverse value of conductivity – resistance. In this case, the specific volume ρv (Ohm.m) and surface ρs (Ohm) resistances are distinguished, which are experimentally determined. By their values, the specific volume χs=1/ρs (Ohm-1) conductivity can be calculated. Electrical conductivity of a material depends on the nature of charge carriers, their concentration and mobility. Most ceramic materials are dielectrics due to their composition and structure. Crystalline phases in ceramics mainly have the ionic structure of the lattice. Advantageously, covalent bonds are inherent only in ceramics based on anoxic compounds. Free electrons in ceramics are almost completely absent. An exception is semiconductor ceramics. The electrical conductivity of ceramics obeys the additivity rule, i.e. is composed of the electrical conductivities of its constituent phases. Charge carriers in ceramics are ions – vitreous phase ions, which are more mobile than ions located in the lattice sites. A definite contribution to the conductivity is made by ions in the defective position, as well as ions of impurity compounds. The alkali metal ions, especially Na+, Li+, have the highest mobility. Therefore, the content of alkali oxides in insulating and, especially, radiotechnical ceramics, is excluded or reduced. It is expediently to introduce large divalent metal cations into the material composition to reduce the electrical conductivity, which have a retarding effect on the movement of alkali metal ions. Pores in ceramics have a low electrical conductivity and are taken into account only at high field strength. The electrical conductivity of ceramics containing a vitreous phase increases with increasing temperature, since the concentration and mobility of charge carriers are in an exponential dependence on temperature: χ = χ0.еαT, where χ0 – electrical conductivity at 0 °С, α – temperature coefficient. The electrical conductivity of a purely crystalline ceramic with a temperature varies slowly, and keeps relatively low values up to very high temperatures. Dielectric constant characterizes the measure of dielectric polarization, and is expressed by the ratio of the capacitor’s charge to the dielectric between the plates Qd to the capacitor’s charge with a vacuum or air gap Qv: ε = Qd/ Qv. 15

Polarization is the process of moving the structural elements of a dielectric from its optimal position under the action of an electric field. In this case, the electrical neutrality of crystals remains. The mechanism of polarization is affected by the composition and structure of the ceramic dielectric. There are electronic polarization, ionic and spontaneous. The electronic polarization is caused by the displacement of the center of gravity and the deformation of the electron cloud with respect to the positive nucleus of the atom. The ionic polarization is the mutual displacement of elastically bound ions of different charges. Dielectric losses characterize the energy expended on the displacement of structural elements of a dielectric material under the influence of an electric field. Dielectric losses in ceramic dielectrics are added from total energy consumption associated with the processes of polarization, through conductivity, ionization of the gas phase, heterogeneity of structure. Dielectric losses in ceramics increase with the increase in temperature. Electric strength characterizes the ability of ceramics to withstand the breakdown effect of the applied electric field. Quantitatively, the electric strength is determined by the breakdown voltage (i.e., the voltage at which the product or sample breaks) and the breakdown tension, which is expressed by the ratio of the breakdown voltage to the thickness of the sample: Es = Ubr /h (kV/cm, kV/mm).

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4 RAW MATERIALS Raw materials used for most ceramic materials for construction, household, technical purposes, can be divided into plastic (or clay) and non-plastic. 4.1. Plastic materials Clay and kaolins belong to plastic clay materials. Clays are mountain rocks, mainly consisting of clay-forming minerals – layered aluminosilicates. In technical terms, clays are earthy rocks, capable of forming a plastic dough which after drying has some strength, and after calcination it acquires stone-like properties. Clays are products of decomposition and hydration of some rocks (granites, pegmatites, micas, etc.) under the influence of atmospheric agents. In the simplest case, this process can be expressed as follows: R2O .А12O3. 6SiO2 + СO2+2Н2O → Feldspar

→ А12O3. 2SiO2 .2H2O +R2CO3+4SiO2. Clay

Sand

Clays and kaolins, which remain in place of their formation, are called primary. Rocks that are transferred to other places belong to secondary, or turn-down, deposits. Depending on the transfer agent, alluvial, glacial and loess-like clays are distinguished. They respectively were transferred by snow or rain water, glacier and winds. 4.1.1. Composition of clay Clays are characterized by material, chemical and granulometric composition. 17

The material composition is represented by a clay substance and impurities. The clay substance consists of a complex of clay-forming minerals which impart plasticity to clay. All clay minerals have a typical layered structure, similar to the structure of mica. When clay is mixed with water the latter enters the interlayer spaces of the clay mineral, and its layers are able to move one against the other along the water film and be fixed in a new position. This ability of minerals explains the most important property of clay – its plasticity. The most common clay mineral is kaolinite А12O3.2SiO2 .2H2O. Kaolinite and its analogs are characterized by the presence of packets consisting of tetrahedral and octahedral layers. Kaolinite, due to this structure, contains the largest amount of alumina compared to other clay minerals, and therefore has a high refractoriness. The crystal lattice is distinguished by its dense structure, strong adhesion of packets. That is why kaolinite cannot attach and hold a large amount of water, and when drying it easily gives it. Montmorillonite Al2O3.4SiO2.H2O.nН2O (the simplified formula) has a crystal lattice consisting of three-layered packets. Outside, there are two layers of silicon-oxygen tetrahedron, and in the center there is a layer of alumina-oxygen octahedron, where an isomorphous substitution of aluminum ions by other ions is possible. The common material in low-melting clays is illite, or hydromica K2O.MgO.4Al2O3.7SiO2.2H2O, which is a product of hydration of micas. The crystal lattice of this mineral is represented by threelayered packets similar to the structure of montmorillonite, but alkaline and alkaline-earth metals take part in its structure, and there is also an isomorphous substitution of silicon and aluminum cations. It is known that in nature there is almost no monomineral clays. An exception is kaolin, which contains mainly kaolinite mineral, as well as bentonites, which are represented mainly by montmorillonite. The most common are polymineral clays with the predominance of a mineral the name of which is determined by the name of the prevailing mineral. In addition to clay minerals, clays contain impurities – quartz, carbonate, feldspar, glandular and organic. Quartz impurities are included in the composition of clays in the form of quartz sand or dust. They reduce the plasticity of the rock, improve its drying properties, but increase fissuring of products after firing. 18

Carbonate compounds (mainly СаСO3) can be fine-dispersed, loose and dense stony inclusions. The most harmful are stony inclusions, which cause the formation of voids during the firing and sometimes lead to the rupture of products. Impurity feldspar sands, the remains of undecomposed rock play the role of fluxes and reduce the refractoriness of clays. Iron impurities are present in the form of minerals (limonite, siderite), hydroxide, pyrite. Iron compounds give the clay and burned products a creamy to dark-red color, and therefore are extremely harmful to white burning products, such as porcelain. The content of iron impurities is also limited for electrotechnical ceramics, since they lead to a decrease in electrical properties. Pyrite inclusions (FeS2) cause during firing the appearance of black smelters (flies). Organic, which is part of the clays, color them from gray to black. When firing, organic impurities burn out creating inside the reducing medium and increasing the porosity of the materials. They are useful in the production of heat-insulating and unfavorable for the production of dense ceramic materials. Chemical composition of clays is mainly determined by oxides SiO2, А12O3, CaO, MgO, Fe2O3, TiO2, К2O and Na2O. Silicon dioxide is present in clays only in the bound state in clay minerals and in free – in the form of silica impurities. The content of SiO2 in clays is 60–65%, and in sanding ones is 80‒85%. Alumina is found in clays only in the bound state of clay and impurity minerals in an amount of 10 to 38%. The higher the content of А12O3, the higher the quality of clays. Calcium and magnesium oxides are part of carbonates, and also participate in the structure of some clay minerals. The content of CaO is 2-3% and only in some varieties, clay reaches 20-25%, MgO in clays is not more than 3%. Iron oxide is contained in impurities in a bound state and in an amount of a fraction of a percent in high-quality, white burning clay, and in red brick clays up to 10-15%. Titanium dioxide in impurities contains no more than 1.5%. It gives greenish tones to the burnt ceramics. Alkaline oxides К2O, Na2O are sometimes included in the composition of clay minerals, but more often present in impurity feldspar sands, as well as in the form of soluble salts. They contribute to the formation of fusible eutectics. 19

Chemical composition of clays determines their industrial application. Clays in the content of А12O3 and TiO2 are divided into highly basic (> 40%), basic (30-40%), semi-acidic (15-30%) and acidic (