PROPERTIES OF CERAMIC RAW MATERIALS 2nd Edition in SI I Metric Units W. R Y A N , Ph.D., FRIC, C.Chem., FICeram. Principal Lecturer Department of Ceramic Technology North Staffordshire Polytechnic PERGAMON OXFORD PARIS PRESS N E W YORK FRANKFURT TORONTO SYDNEY U.K. P e r g a m o n Press Ltd., H e a d i n g t o n Hill H a l l , Oxford 0 X 3 OBW, England U.S.A. P e r g a m o n Press I n c . , M a x w e l l H o u s e , F a i r v i e w Park, Elmsford, N e w York 1 0 5 2 3 , U.S.A. CANADA P e r g a m o n of C a n a d a Ltd., 7 5 T h e East M a l l , Toronto, Ontario, Canada AUSTRALIA P e r g a m o n Press ( A u s t . ) Pty. L t d . , 1 9 a B o u n d a r y S t r e e t , R u s h c u t t e r s B a y , N . S . W . 2 0 1 1 , Australia FRANCE P e r g a m o n Press S A R L , 2 4 r u e d e s Ecoles, 7 5 2 4 0 Paris, C e d e x 0 5 , F r a n c e P e r g a m o n Press G m b H , 6 2 4 2 K r o n b e r g - T a u n u s , FEDERAL REPUBLIC OF G E R M A N Y Pferdstrasse 1 , F e d e r a l R e p u b l i c of G e r m a n y Copyright © 1 9 7 8 W . Ryan All Rights Reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means: electronic, electrostatic, magnetic tape, mechanical, photocopying, recording or otherwise, without permission in writing from the publishers First e d i t i o n 1 9 6 8 S e c o n d e d i t i o n in S . I . U n i t s 1 9 7 8 Library of Congress Cataloging in Publication Data Ryan, William, 1 9 3 4 P r o p e r t i e s of c e r a m i c r a w materials. 1 . C e r a m i c m a t e r i a l s . I. Title TP815.R8 1978 666.028 ISBN 0 - 0 8 - 0 2 2 1 1 3 - 0 (Hardcover) ISBN 0-08-022114-9 (Flexicover) Printed in Great Britain by Biddies 77-24386 Ltd., Guildford, Surrey CHAPTER 1 Introduction A T ONE time a ceramic product was regarded as any article made wholly or partly from clay which during its manufacture had been shaped and then fired to a temperature high enough to produce the required strength. This was never an accurate definition of the term "ceramic", since it excluded such materials as glasses and cements which were and are regarded as ceramic materials. The advent and increasing importance of non-clay ceramics, simpler in composition than the traditional clay-based ceramics, often being composed of single materials, have widened the scope of the term. Today a ceramic may be defined as a product manufactured by the heat treatment of a material (or mixture of materials) which is inorganic and non-metallic. In spite of the expansion of the field where the term "ceramic" may be applied, the volume of clay-based ceramics manufactured is much greater than that of non-clay ceramics, and clay may still be regarded as the basic and most important of ceramic materials. The variety of ceramic products is enormous. The easiest brought to mind perhaps is tableware, i.e. cups, saucers, plates and so on. Tableware, however, is only a small part of the whole ceramic spectrum, which includes such products as bricks and tiles of all types, earthenware, bone china and porcelain, all types of glass, sewer pipes, sanitary ware, chemical porcelain, acid-resisting ware for chemical plants, and many others. In addition, an important type of ceramic product are those known as refractories, used because of their ability to withstand high temperatures. Ceramic products can be divided into those used at normal temperatures PROPERTIES OF CERAMIC RAW MATERIALS and those used at high temperatures, and these divisions may then be sub-divided into those products which have a porous body after firing and those which are non-porous. Such a division is shown in Fig. 1. CERAMICS ^GLASSES-CEMENTS P R O D U C T S U S E D AT ELEVATED TEMPERATURES P O R O U S BODY VITREOUS BODY 3.g. Alumino silicate e.g. Special refractories refractories such often made from as fireclay ware. pure single Insulating refractories materials. Alumina silicon nitride \ P R O D U C T S USED AT OR SLIGHTLY ABOVE R O O M TEMPERATURE POROUS BODY e.g. Heavy clayware such as Common bricks Tiles Conduits Pipes Sanitary fireclay Some types of pottery Earthenware Terra cotta Majolica VITREOUS BODY e g . Fine clay ware like bone china, porcelain. Chemical stoneware Electrical Porcelain FIG. 1. Classification of ceramic products. Fine tableware and ornamental ware depend for their appeal on appearance, i.e. on their quality of design, decoration, translucency and colour. Mechanical strength and resistance to temperature and chemical attack need not be very high in such products. On the other hand, refractory materials, heavy clay products and other functional products must have the physical properties to withstand the temperature, load or chemical attack to which they are subjected. Appearance in these cases is usually only of secondary importance. Glasses and cements may be regarded as intermediate products, being used both at elevated and room temperatures. Each product must possess the properties expected of it in use, 2 INTRODUCTION whether they are of beauty, mechanical strength, temperature resistance or any other property. The properties of the finished ceramic article will broadly depend on: (a) The types, purity and relative amounts of the materials from which it is made. (b) The methods and conditions used in its manufacture. We are here concerned with (a), but the two cannot be divorced, since the materials used will to a considerable extent dictate the manufacturing processes available. 3 CHAPTER 2 Manufacturing Processes I N ORDER that we may understand the importance of the properties of ceramic materials in the manufacture of the ceramic article, it is necessary to look briefly at the processes involved in manufacture. These vary depending on the type of article produced and the processes to which the shape to be made and the physical properties of the materials from which it is to be made lend themselves. Below is a generalised diagram of the major processes involved, although in particular cases some of these stages may be unnecessary, or others required. RAW MATERIALS WON I PURIFICATION I SIZE ADJUSTMENT MATERIALS MIXED IN REQUIRED PROPORTIONS γ PRODUCT SHAPED I γ DRIED γ PRODUCT FIRED Firing is usually the end of the process for refractory products, but products used at normal temperatures are often glazed, necessitating a second or glost fire to mature the glaze and make 4 MANUFACTURING PROCESSES it flow over the surface of the ware and adhere to the body. Fine ceramic ware may require more fires after application of decoration. Where glaze is applied to heavy clayware the main function is not as in fine ceramics to improve appearance, but to make the surface impermeable. The body underlying the glaze in these products is often porous, and if glaze were not applied it would readily adsorb liquids. The undesirability of this is evident in such products as sewer pipes and sanitary ware. Heavy clay products are in general only once fired. Either they do not have glaze applied to them—as in the case of building bricks, for example— or the glaze is applied after shaping and drying, the one fire serving to mature both the body and the glaze. 2.1. Body Preparation This process involves intimately mixing the various raw materials in the right proportions and in the right degrees of fineness, and producing a mixture which is of the water content and consistency required for the next stage of the process— shaping or making. Sometimes, as in common brick and in some special refractory bodies, only one material is used. Even in these cases, mixing is important, since definite proportions of coarse, medium and fine material must be incorporated in the body to give the necessary physical properties, and these must be thoroughly intermixed. More frequently several raw materials are mixed together to form the body. Thorough mixing in these cases is essential if the body is to have the same proportions throughout and hence behave consistently. Methods of Mixing 2.1.1. Dry Mixing This method has been used mostly for non-clay bodies, but is now becoming increasingly used in clay-containing bodies, 5 PROPERTIES OF CERAMIC RAW MATERIALS because it has the great advantage that water can be added to the dry mixed body to obtain the exact consistency required for making. This avoids the filter-pressing which is necessary where slip mixing is used. The disadvantage of dry mixing clay-containing bodies is that since no filter-pressing is carried out, any soluble salts present in the raw materials remain in the body, and can cause difficulties in manufacture by interfering with deflocculation (section 3.4) or reducing plasticity. Where the body is mixed in slip form and filter-pressed, much of the soluble salt impurity is removed in the press water. Troubles due to soluble salt impurities can often be overcome by adding barium carbonate to the dry mix (see section 3.4). In the case of non-clay bodies, binders and plasticisers or lubricating materials are added to the dry mix. These materials serve to aid the shaping process and to give the formed body some strength before firing. Small amounts of organic materials are usually used for this purpose, e.g. starch, waxes, polyvinyl alcohol; these burn away during firing, after they have fulfilled their function, and are thus not present in the finished product. Although the materials are said to be mixed dry, they will probably contain some adsorbed moisture, and before weighing out the various constituents this moisture content should be determined and an allowance made. 2.1.2. Plastic Mixing Plastic mixing is used particularly in heavy clayware production, where the plasticity developed when water is added to clay is turned to account. Water is added to the proportioned mixture of raw materials, and the whole is mixed together at the plastic consistency required for making. The addition of water to attain this consistency is known as tempering. The mixing process blends together not only all the raw materials, but also the coarse, medium and fine particles and the water to give an homogeneous body. After mixing, the body is usually pugged, during which process the material is further mixed, consolidated and extruded from the pug machine as a continuous column ready for making. 6 MANUFACTURING PROCESSES Bricks and other simply shaped articles can be made by this method simply by arranging for the extruded column to be of suitable cross-section and cutting the column as it is extruded. The pugging process is often done under vacuum, so that any air entrapped in the plastic mix can be removed in the shredding process which occurs in the pug before consolidation and extrusion. This improves plasticity and strength. 2.1.3. Slip Mixing This method is used mainly for pottery and fine ceramic bodies. A free-flowing suspension of a powdered ceramic material in a liquid (usually water) is termed a slip. If the various raw materials are separately made into slips by stirring with water, a slip of the body can be prepared by mixing the right amounts of the individual slips. This gives a very intimate mixture of materials, since small particles which might hold together in clusters in the dry or plastic state can be completely separated in the slip form. In order that the proportions of the various raw materials should be right in the body mix, a known weight or a known volume of each sup is put into the body mixture. It is necessary to know the weight of dry material present in a fixed volume or a fixed weight of slip, so that the required volume or weight of slip can be calculated. Knowing the weight of a fixed volume (usually a pint) of slip, and the specific gravity of the dry material, the weight of dry material in any volume or any weight of slip can be calculated using the relation known as Brogniart's equation : ^ = ( P - 2 0 ) ^ where Ρ is the weight of one pint of slip (in ounces), Wis the weight of dry material contained in one pint of slip (in ounces), and S is the specific gravity of the dry material in question. The figure 20 in the equation represents the weight of one pint of water in ounces. A more general form of the equation is 7 PROPERTIES OF CERAMIC RAW MATERIALS where Ρ is the weight of any volume of the slip in any units, W is the weight of dry material in that volume in the same weight units, A is the weight of the same volume of water in the same weight units, and S as before is the specific gravity of the dry material. The required volumes or weights of the individual slips of raw materials can then be calculated, measured out and mixed together to give the body composition required. The body slip must then be brought to a suitable condition for making, and it is here that the disadvantage of this method of body preparation becomes apparent, for most of the water must now be removed from the body slip. This is done by using the filter press in which the body slip is pumped under pressure onto cloth or nylon sheets. The water passes through, but the solid material is left behind and forms a filter cake of plastic body of about the same consistency as used in the plastic mixing process. The advantages of this method are that amounts of raw materials can be measured out accurately, and lumps of material (e.g. ball clay) difficult to break down by other means can be completely dispersed in the slip form, thus leading to intimate mixing. The disadvantage of the method is that the water required for mixing must be removed by filter pressing before the body can be used for making. The filter cakes obtained from the press may be pugged and the pugged body passed on for plastic making, or they may be again dispersed in water to form a slip, which is used in the slip casting process of manufacture. Both of these methods are briefly described in the next section. Before filter pressing, it is usual to pass the body slip through sieves to remove any coarse material which would blemish the finished ware; also it is passed- over strong electromagnets to remove any magnetic impurity. 8 MANUFACTURING PROCESSES 2.1.4. Direct Preparation of Casting Slip Is being increasingly used in the ceramic industry, particularly in the production of sanitary ware, where almost all the products are slip cast (see section 2.2.1), and in bone china body casting slips. The process involves mixing all the raw materials with water and deflocculant (see section 3.4), to prepare the casting slip directly at the correct concentration and with the correct fluid properties for use. The advantage over the traditional wet preparation method is that the filter pressing stage is eliminated. The disadvantage is that it is more difficult to remove impurity. Ball clays if used in the body composition are first mixed with water and \ to f of the total deflocculant demand, using a highspeed blunger. This enables the ball clay suspension to be made up to reasonably high concentration, yet maintaining it fluid enough to be passed over magnets and sieves. China clay is then added dry, and the various non-plastic materials used in the body composition (flint, stone, felspar, etc.) are added as concentrated suspensions, together with the remaining deflocculant. This procedure allows the slip to be prepared exactly to the concentration required for casting. 2.1.5. Spray Drying Where fabrication is carried out by pressing a powder (e.g. in wall tile manufacture), it is important that the powder is freeflowing and of the required moisture content. For wall tile manufacture, such powder used to be prepared by drying plastic body from the filter press and then crushing it to produce a suitably sized powder at a moisture content of 7 to 8%. The modern method of dust preparation avoids the filter pressing stage and prepares the dust directly from the aqueous body suspension. This is done by atomizing the body suspension into fine drops which are then subjected to hot air which evaporates off the excess water and leaves the powder at the moisture content desired. Powder prepared in this way gives spherical granules which are free-flowing and press better than powder prepared by drying plastic body. 9 PROPERTIES OF CERAMIC RAW MATERIALS 2.2. Making Methods The major methods of making may be divided according to the moisture content of the body at the time of making. Table 1 shows the approximate moisture contents used in these methods, and their general fields of application. TABLE 1. MAKING METHODS—TYPICAL MOISTURE CONTENTS AND FIELDS OF APPLICATION Approximate moisture content Physical state of body Slip casting 25-50% Fluid suspension Generally used for large or awkwardly shaped pieces, or small orders Plastic making 18-22% Plastic mass Relatively simple shapes, pottery, bricks, pipes Making method Typical fields of use Semi-dry pressing ^9% Damp powder Automatic processes, bricks, tiles, etc. Dry pressing 0-4% Dry powder Non-plastic materials; usually addition of a binder is needed 2.2.1. Slip Casting Slip casting is somewhat akin to wet mixing and filter pressing in that the process involves adding water which is subsequently 10 MANUFACTURING PROCESSES removed. It is for this reason a relatively slow method of production, and is in general only used when the size or shape of the article or the character of the body would make other more economical methods of shaping impracticable. It may also be used where the small numbers required do not warrant the "setting u p " of a plastic making method. The fluid slip is poured into a plaster of Paris mould of the required shape. Water from the slip is adsorbed into the porous mould, and as this process proceeds, a layer of the slip material is built up on the inside wall of the mould and takes the shape of the mould. Casting is allowed to continue until the layer of cast material is of the required thickness, when the mould is inverted and the excess slip poured away. The mould is left inverted to drain for a few minutes, then set upright again, and any excess slip which has cast on the face of the mould is removed or "scrapped off" with a knife. The cast is then allowed to partially dry in the mould. As the cast dries it shrinks and parts from the mould, thus enabling it to be removed, after which it is usually fully dried before having any blemishes removed with a knife or sponge, and passed on for biscuit firing. Since water is removed from the slip during the casting process, the level of the slip in the mould falls, and for this reason a "ring" is usually incorporated in the mould to hold the slip height above the height of the article being cast (Fig. 2). Excess cast to be mould FIG. 2. Plaster mould containing slip and cast. 11 PROPERTIES OF CERAMIC RAW MATERIALS The "ring" may be made of rubber or plaster. If made of plaster, casting will take place on the side of the "ring". This excess cast is removed with a "scrapping" knife when the cast is partly dry, and a damp sponge is run around the cut to give a smooth finish. The process described above is called hollow casting and is used to make hollow articles such as vases, basins, etc. Solid casts can be made by allowing casting to continue until the whole of the interior of the mould has cast up. For complex shapes it may be necessary to make the plaster mould in several parts to enable the cast to be removed from the mould. Each part of the mould, including the "ring", is located in its correct position by a "natch", i.e. a raised piece of plaster or plastic on one part of the mould which fits into a depression in the corresponding part. If the mould is in many parts it may be necessary to hold it together during casting by means of a wire ring or stout elastic band which fits about the outside of the mould and can be removed when unmoulding the cast. The concentration of solid material in a casting süp must be high to avoid the necessity for the mould to remove too much water and become saturated. At the high concentrations required, slips of most materials, particularly those high in clay content, would be too thick to pour easily, and faults would occur in casting. To make the slip fluid enough to handle, certain chemicals known as deflocculants are added in small amounts to the slip. The process of making the slip fluid by the addition of a deflocculant is called deflocculation. The importance of the fluid properties of a casting slip is discussed in section 3.4, and the mechanism by which deflocculation takes place in sections 5.1.11 and 5.1.12. 2.2.2. Plastic Making This term covers a wide variety of making methods all of which depend on the development of plasticity in the body being used. Where the body is of high clay content, plasticity is easily obtained by adding water, but where the body is of low clay content or does not contain clay at all, plasticising materials (highly plastic clay like bentonites or organic plasticisers) have to 12 MANUFACTURING PROCESSES be added. Plasticity is a complex property of a material which will be dealt with more fully in section 3.1. It will suffice here to define plasticity as the property which allows the material to be deformed (or shaped) without cracking or breaking, under the influence of an applied force, and to retain its new shape when the deforming force is removed or reduced below a certain value. The materials to which these shaping processes are applied must be plastic, and since it is the clay content of ceramic bodies which is responsible for most of their plasticity, bodies made by these methods usually contain a fairly high proportion of clay. The development of the plastic qualities of a body depend also on the amount of water in the body. In general the higher the proportion of clay in the body, and the more plastic the type of clay, the more water is required to produce a workable consistency. Consistent with good plasticity, the lower the moisture content at which the body can be worked the better, since a high moisture content leads to high shrinkage in drying, which may cause cracking or the setting up of strains in the dried article which may lead to cracking during firing. This is one of the advantages of the mechanical methods of making; since they use higher forces to shape the ware, the body can be used at a lower moisture content than if hand making were used, hence the shrinkage in drying is less, and the danger of cracking during drying is reduced. Ideally, the body should develop high plasticity at low moisture content; this would make for easy shaping and safe drying. This combination does not occur in practice, however, and a compromise must be made between having enough water present to develop the plasticity required for shaping and at the same time keeping the moisture content low enough to make drying safe. A detailed description of the methods of plastic making are outside the scope of this book, but briefly the main methods are as follows. THROWING This is the old method of shaping a plastic body by hand whilst it is rotating on a potter's wheel. Considerable skill and 13 PROPERTIES OF CERAMIC RAW MATERIALS experience are needed, and the method is scarcely used in industry today, although it is still practised by studio potters. JOLLYING A N D JIGGERING In jollying, the outside surface of a hollow-ware article is formed on a plaster of Paris mould, and the inside is shaped by a metal profile tool which is brought down manually or automatically onto the revolving mould. The body is squeezed between the mould and the tool whilst the mould is rotating, and both surfaces are formed. Jiggering is a similar process used for flat-ware production. A flat disc or bat of the body is first formed by spreading the body on a flat, revolving plaster head with a flat tool brought down onto the body from above. The flat bat is then thrown onto a plaster mould which forms the face shape. The mould is rotated in a jigger head and a profile tool is brought down onto the body to shape the back of the ware (see Fig. 3). The manual operation of these machines has been largely replaced by semi-automatic or automatic machines. Fio. 3. Jigger. 74 MANUFACTURING PROCESSES T H E ROLLER MACHINE The roller machine is similar in principle to the jigger, but instead of being formed by the type of profile tool used in the jigger, the back of the ware is shaped by a heated, revolving, circular profile which is operated automatically. The machine has several heads, each holding a mould, and these revolve on a base board to be placed in turn under the making head. The operative is required to feed the machine with fresh moulds, to supply the moulds with approximately the right amount of body (cut off for him automatically in slices from a pugged roll), and to remove the moulds carrying the made ware and transfer them to a dryer from which he also removes dried moulds and ware. EXTRUSION Where solid articles of constant cross-section are required, extrusion can be used, i.e. the plastic body is forced through a die of the appropriate cross-section, and the extruded column is cut into suitable lengths. Extrusion is used for the making of bricks, pipes, rods, etc. PRESSING This is a rapid and cheap method of producing large numbers of simply shaped articles. The body is fed into a metal die and pressure is applied. The formed article is then ejected from the die and the process is repeated. 2.2.3. Other Methods of Making D R Y AND SEMI-DRY PRESSING Both of these methods have the advantage that drying after the making stage is not necessary. In addition, since little or no drying shrinkage is involved, greater accuracy of size can be achieved than in methods involving the use of more water. Semidry pressing is used in brick and tile manufacture and can also be 15 PROPERTIES OF CERAMIC RAW MATERIALS applied to industrial porcelain products. The body must have plastic properties which enable the damp (lubricated) powder to flow and fill up the die when pressure is applied. Dry pressing is used in the manufacture of articles from low or zero clay content bodies—for example, insulators and capacitors for the electronics industry. An organic binder is usually required to lubricate the particles during pressing, and give strength to the unfired article. Particle size distribution of the material is particularly important, since this largely determines how the powder will pack under pressure. Spray drying is now beginning to be used to prepare suitable free flowing powder (see section 2.1.5). ISOSTATIC PRESSING Very high pressures are required for dry and semi-dry pressing, and even then the pressed density (i.e. particle packing) obtained is often not as good as can be obtained from plastic making or casting. Moreover, the use of these high pressures can lead to other faults. Due to frictional losses, the pressure drops from the pressed surface to the face farthest away from the pressed surface. The greater this distance the greater the pressure drop and the greater the difference in density throughout the pressed piece. This difference in density often shows itself as laminations throughout the formed piece. The situation can be improved if pressing is done from two directions, say from top and bottom. In this case there will be a low density layer in the centre of the article which is sandwiched between two high density layers at the top and bottom. If the article is not too deep the differences may be small and unimportant, but if it is thick, or it is important that its properties are uniform throughout, then the differences in density are likely to be troublesome. If pressure could be applied evenly to the whole exterior of the article, differences in density throughout the article would be minimised and it is this that isostatic pressing aims to achieve. Basically, a rubber bag of the desired shape is uniformly filled with the dry powder to be pressed, the whole is immersed in oil or some other suitable liquid contained in a pressure vessel and 16 MANUFACTURING PROCESSES pressure is applied to the liquid. The liquid transmits the pressure uniformly all over the surface of the bag, and the powder within it is uniformly compressed. The method is particularly suitable for complex shapes and for use with non-plastic materials which can be pressed to high and uniform densities. As shown in Fig. 4, (c) FIG. 4. Isostatic pressing. variations can be made in the pressing arrangements to suit the particular shape being made. In Fig. 4b an accurately made metal former is shown in the bag containing the powder. On the application of pressure, the powder is pressed onto the former, producing a closed-ended tube. In Fig. 4c the powder is contained between the outside of the bag and a metal former, and the pressure is applied from inside the bag. The bag then expands and presses 77 PROPERTIES OF CERAMIC RAW MATERIALS the powder against the former, the external shape of the article being determined by the shape of the former. The rate of increase of pressure, dwell time and rate of pressure release are important in affecting the qualities of the product, and the optimum values are found by trial. As for other dry forming methods, the addition of organic lubricant and binder is necessary. H O T PRESSING In this method of forming heat and pressure are applied simultaneously to the powdered material. The forming and firing of the article are therefore carried out at the same time. High densities can be obtained at lower temperatures than required for firing under atmospheric pressure. The complexity of shape that can be hot pressed is at the moment limited. To obtain maximum values of strength and thermal shock resistance, pores must be eliminated in the body during firing without grain growth (i.e. growth of large grains at the expense of small ones). This can be better achieved in hot pressing than in normal firing, and hence the method is used where particularly high quality properties are required in the product. FLAME SPRAYING Flame spraying has been used for two distinct purposes : (a) The application of a protective ceramic coating to metals. (b) The formation of a ceramic article by spraying onto a metal or graphite mould. A rod of the ceramic to be sprayed is fed into a very hot flame where it is melted and an air blast atomises the liquid and directs it onto the article to be coated or the mould. Refractory oxides like alumina and refractory silicates like zircon are among the materials which can be flame sprayed in this way. Very accurate thickness of coating can be made and production of complex shapes is possible. Metal moulds can be removed from the 18 MANUFACTURING PROCESSES sprayed layer either by cooling quickly when the metal contracts more than the ceramic and so frees the ceramic layer, or, if the shape is complex, the metal can be dissolved away with acid, leaving the ceramic layer unattacked. Graphite moulds can be removed by heating in a strongly oxidising atmosphere when the carbon is oxidised to carbon dioxide. 2.3. Firing and Finishing It is the firing process which converts the weak, soft article into a strong, hard product. In most bodies this is brought about by reaction between fluxing materials (section 5.3) and the other constituents of the body, forming a liquid which on cooling solidifies into a glass bonding together particles or crystals which have not melted. Formation of a liquid on firing does not, however, always occur, as, for example, in the sintering of pure oxide ceramic materials which are fired at temperatures well below their melting points. The reactions which occur during firing will be dealt with more fully in section 4. Firing is carried out in a great variety of kilns, depending on the temperature required for maturity, size and quality of the ware. The purpose is the same, however, in every case—to give strength and hardness to the shaped ware. For ware which is not glazed, like building bricks, only one fire is required. Other ware, e.g. sanitary ware, can have glaze applied to the unfired article, and one fire used to mature both the glaze and the body. Yet other types of ware (usually stoneware) may be salt glazed, i.e. salt is introduced into the kiln during firing and reacts with the body surface to form a glaze. Most pottery ware has glaze applied after the first or biscuit fire, and must then be fired again to mature the glaze. If no decoration is applied, biscuit or glost firing is the final operation in manufacture. Where decoration is applied in or on the body or in the glaze the same is true. If decoration is applied on top of the matured glaze, as is often the case in decorative or tableware, further firing is required to mature the colour and fix it firmly into the glaze surface. Since different colours mature at different temperatures and may be spoiled at higher temperatures, 19 PROPERTIES OF CERAMIC RAW MATERIALS it may be necessary to have several decorating fires, first applying and firing those colours which mature at high temperature, followed by those of lower maturing temperature and so on down to colours of lowest maturing temperature. 20 CHAPTER 3 Properties Important During Making the body should be capable of being shaped without cracking or breaking, and should retain its shape after the making operation, i.e. it should be plastic. The formed article should dry out without losing shape or cracking, and in the dry state it should be strong enough to enable it to be handled safely. In claycontaining bodies the clay is largely responsible for these properties ; where the body does not contain clay it is usually necessary to add some organic binder or plasticiser to produce sufficient strength and plasticity in the body. IDEALLY 3.1. Plasticity We have already defined plasticity in section 2.2.2. Note that this definition is entirely qualitative, i.e. it simply describes the property without defining any units in which it might be measured. The problems of understanding and measuring plasticity have occupied much time of the research ceramist, but this most important and fundamental property, without which many present shaping methods would be impossible, is still not completely understood. We are in fact unable to measure plasticity, and have no units to apply to it. There are numerous methods of comparing plasticity, however, and these usually depend on measuring some more tangible property of the material which can be associated with plasticity. It has been found that materials which are highly plastic usually also show high dry-strength and a high moisture content at their optimum working consistency. Measurements of these two 27 PROPERTIES OF CERAMIC RAW MATERIALS properties have therefore been made, high values being taken as indicating high plasticity. Although such comparative methods give some indication of the plasticity of a material, they are by no means entirely satisfactory, and the results can be misleading, as the following example shows. If we deflocculate a clay or claycontaining body (sections 5.1.11, 5.1.12), then its dry-strength will increase perhaps by as much as 100%, but its moisture content at its best workability will decrease. Hence, plasticity as indicated by dry-strength measurement will be higher, whilst that indicated by moisture content measurement will be lower than the plasticity of the undeflocculated material. The plasticity developed by clays when water is added to them is unique, and no other material gives anything like as high a plasticity. It is this property plus the fact that clays are a cheap source of the chemicals required to take part in high temperature reactions which has made clays the basis of the ceramic industry. The ultimate particles of the clay minerals are extremely fine, and it has been found that the finer the particles the greater the plasticity the clay can develop. Thus ball clays, for example, with a high proportion of very fine particles are more plastic than china clays where the proportion of very fine particles is relatively small. However, it is not this fineness of particle size alone which is responsible for plasticity, for sand or alumina or some other non-plastic material ground equally fine develops little or no plasticity. As well as size, the shape of the particles is important. Clay particles are thin plates, hexagonal in shape. Because the clay particles are so small, the surface area is very great, and many of the atoms making up the clay structure are at the surface, i.e. they are joined to other atoms only on one side, unlike atoms in the interior which are joined to other atoms on all sides. T o satisfy their unbalance, the surface atoms attract water molecules to the surface of the clay, with the result that the clay acquires a layer of adsorbed water at the surface of its particles. This adsorbed water can serve to lubricate the movement of one clay plate over another, and thus aids the deformation of a clay mass and promotes plasticity. Finely ground sand or alumina will also adsorb water at the surface of the particles, but since these 22 PROPERTIES IMPORTANT DURING MAKING particles are not plate-like they cannot slide easily over one another and little plasticity results. The adsorbed liquid also seems to be important in affecting plasticity. Water is a polar liquid, i.e. although the H 2 0 molecule is electrically neutral overall, the bonding electrons in the O - H bonds are not equally shared between the oxygen and hydrogen, but are more likely to be found in the vicinity of the oxygen than of the hydrogen, or they can be regarded as being displaced slightly towards the oxygen. In the water molecule the oxygen can be regarded as carrying a small negative charge, and the hydrogens a small positive charge. These small charges enable the water molecules to orientate themselves at the clay surface, satisfying the unbalance of the atoms at the surface of the clay particle and forming an adsorbed water layer. Other polar liquids like alcohols can also be adsorbed on clay surfaces and develop some plasticity in the clay, although none give it to the same extent as water. Non-polar liquids like benzene produce no plasticity when mixed with clay. The plasticity of a ceramic body containing clay and non-plastic material is determined by the ratio of clay to non-plastic and by the type of clay used. Additions of small amounts of ball clays or montmorillonites (an even finer and more plastic clay) are made to bodies and sometimes glazes where an increase in plasticity is required. However, the extent of addition of these fine clays is limited since (a) the highly plastic clays are not white firing and their addition to a white burning body is limited by the colour they introduce, (b) they can cause drying difficulties by increasing drying shrinkage if added in excess. Since it is the water in a clay body which develops plasticity, the water content should be even throughout the body for consistent working properties. The process of pugging helps to distribute moisture evenly throughout the body, though there is a tendency for the centre of a pugged roll to be of higher moisture content than the outside. Ageing of filter cakes or of pugged rolls is said to even out moisture distribution so that on pugging before use the body acquires a more even moisture distribution than if pugged directly from fresh filter cakes. 23 PROPERTIES OF CERAMIC RAW MATERIALS 3.2. Dry-strength By dry-strength we mean the strength of the material after it has been shaped and dried, but before firing. It is important that this strength should be high enough to enable the ware to be finished (i.e. seam marks removed, edges trimmed and smoothed) and placed for firing, without breakage of the article. The minimum dry-strength permissible in a body will depend on the shape and thickness of the articles made from it, and how much these need to be handled in the dry state. We have said that high plasticity and high dry-strength are normally associated with one another, so the higher the clay content of the body and the finer the particle size of the clay the greater will be the strength of the dry articles. China body is one of the most delicate bodies to handle in the dry state. The clay content of the body (about 25 %) is relatively low, and since a good white colour is essential in the fired body, this is made up almost entirely of china clay. Small additions of the more plastic ball clay or very small additions of montmorillonite can be made to improve plasticity and dry-strength, but the additions must be kept small or colour will be introduced. This problem does not arise in, say, sanitary ware bodies, where the poor colour of the biscuit can be covered with an engobe or an opaque glaze, allowing the introduction of a much higher percentage of plastic clay into the body. High dry-strength is, like plasticity, due to the fineness and plate-like shape of the clay particles (hence the association of the two properties). When dried, there is a large surface area of contact between particles, and the smaller and more plate-like the particles are the greater this contact area and the greater the strength. For example, a china clay may typically show a dry 2 2 strength of about 1400 k N m ~ (i.e. about 200 lb f i n ~ ) , whereas a good plastic ball clay or brick clay will show strengths of 2 2 6900 kNm ~ (i.e. about 1000 lb f in ~ ) or even higher. Dry-strength tests are usually carried out in the control laboratory on clay deliveries and on the prepared body. The test is simply done by extruding rods of the plastic material, drying these 24 PROPERTIES IMPORTANT DURING MAKING under specified conditions, then supporting a rod between two knife edges and applying an increasing load to the centre of the rod until it breaks. The load required to break the rod and its cross-sectional dimensions are noted, and the modulus of rupture can then be calculated. An average modulus for at least twelve test rods of the same material should be taken. Breakage in the dry state can arise through no fault of the material, but due to poor manufacturing conditions or poor design. If the article is made or designed in such a way that stresses are set up during making, or if it is dried too quickly so that stresses are introduced whilst the body is shrinking, then failure may occur under a very small load. If the stresses are large enough, cracking may occur during drying without the application of any external load, or if the article does survive to be fired, then it may crack during firing. In the plastic state, clay particles will be in a random arrangement, but on the application of a force, particles tend to align themselves with their long axes parallel to the direction of the force. On drying the plastic mass this leads to more flat faces of particles being in contact, and hence greater surface area contact and greater dry-strength. The larger the force used in plastic making, the greater will be the particle alignment and the greater the dry-strength. Where the article is made by slip-casting, particle alignment is improved since the clay particles in the deflocculated slip tend to exist as individual particles and not as groups or agglomerates of particles (see section 5.1.12). During casting considerable alignment of particles takes place and the cast is in general denser and of higher dry-strength than the same article made plastically. 3.3. Drying Shrinkage After making, either from plastic body or from casting-slip, the formed article is allowed to dry at least partially before the next stage of manufacture. During drying shrinkage occurs, and it is this which makes drying one of the most dangerous processes in ceramic manufacture. 25 PROPERTIES OF CERAMIC RAW MATERIALS In slip casting, the cast is allowed to shrink sufficiently to allow its removal from the mould, and is then dried out, usually in some form of drying chamber. It is important that any slip which has cast up on the top surface of the mould should be removed after draining, so that the cast can freely shrink away from the mould wall. If this is not done, cracking is likely to occur due to the shrinking cast being held to the mould by the overflow cast. The same is true where a "ring" is used : the excess must be trimmed off before too much shrinkage has occurred (see Fig. 5). Excess cast Fresh cast Dry Cast held to mould by excess. Drying leads to distortion and probably cracking Mould Excess cast "scrapped off" Dry Ϊ HI ^ Ml Cast shrunk away from mould without distortion or cracking FIG. 5. Effect of not "scrapping off" after casting. The fine, plate-like particles of clay which lead to the desirable properties of high plasticity and high dry-strength also result in high drying shrinkage. Because at their best working consistencies the water films adsorbed on clay are thick, when this water is removed on drying, shrinkage occurs. A small drying shrinkage is 26 PROPERTIES IMPORTANT DURING MAKING desirable in pottery bodies, since it allows the formed article to shrink away from the plaster mould, and makes its removal from the mould easy. Excess shrinkage and particularly uneven shrinkage, however, can lead to stressing and cracking of the ware. High shrinkage also means that the accuracy of dimension to which an article can be made is reduced, and where high accuracy is required the moisture content of the body must be kept low, or water eliminated. When a plastic body dries, water is removed from the surface, and as the water content is reduced the particles come nearer together. Eventually the particles will touch each other and although water is still present in the voids between particles no further shrinkage can occur (see Fig. 6). Shrinkage (b) Particles drawn closer together as water is removed (a) Plastic mass, particles separated by water Shrinkage No shrinkage (d) Completely dry (c) Particles touching water only in voids between particles Fio. 6. Shrinkage of a plastic body during drying. Further drying will now lead to removal of the water in the voids, but no further shrinkage can occur. If a piece of plastic clay or body is allowed to dry and measurements of volume and corresponding moisture content are taken on the sample at time intervals during drying, a plot of volume against moisture content can be made as shown in Fig. 7. 27 PROPERTIES OF CERAMIC RAW MATERIALS Volume C Moisture content (%) FIG. 7. Plot of volume against moisture content for a plastic body during drying. The value of the moisture content at the point C, i.e. the moisture content at which shrinkage ceases, is known as the critical moisture content ( C . M . C ) . This is an important value, since drying at a moisture content greater than the C.M.C. involves shrinkage and risk of distortion or cracking, hence drying down to the C.M.C. value must be done slowly to allow drying to proceed evenly. Once the C.M.C. has been reached, drying can be finished off more quickly without fear of cracking. At moisture contents greater than the C . M . C the rate at which water is lost will be constant under constant conditions, since drying simply involves evaporation of water from the surface, which is replaced by water flowing from the interior to the surface: this is known as the constant rate period. Once the C.M.C. has been passed there is no longer a continuous water layer about the particles, and water from the inside has to diffuse to the surface before it can be removed. The rate of removal of water therefore decreases as the moisture content is reduced below the C . M . C , and continues to decrease until dryness is reached; the interval between the C . M . C and dryness is therefore known as the falling rate period. Figure 8 shows the constant and falling rate periods diagrammatically by plotting rate of water removal against percentage moisture content. 28 PROPERTIES IMPORTANT C.M.C. Moisture content DURING MAKING (%) FIG. 8. Plot of rate of water removal against moisture content. Where the article is formed dry or at a moisture content less than the C.M.C. drying is either not necessary or can be done quickly and safely, since removal of water does not involve shrinkage. This is generally true where non-plastic materials are concerned (although they are sometimes slip cast), and in dry, semi-dry, hot and isostatic pressing methods. The shrinkage which occurs when a body or clay is dried from its normal plastic working consistency is called the wet-to-dry shrinkage of the material, and is usually expressed as a percentage of either the original wet length, or of the final dry length, i.e. wet l e n g t h - d r y lengthy wet length o / ^ wet length - dry length dry length χ o / This shrinkage is easily measured by hand moulding a block of the material at its working consistency in a plaster mould. As soon as the block is made, a line is drawn across it with a scriber, and marks are made at some fixed distance apart (conveniently 5 or 10 cm) on the line. The block is then allowed to dry sufficiently for it to be removed from the mould, and it is then air dried and finally dried in an oven at 100°C. When the block is dry the distance apart of the marks is again measured and the wet-todry contraction is calculated. 29 PROPERTIES OF CERAMIC RAW MATERIALS As well as depending on the material involved, the rate of drying will depend on the shape of the article concerned. The greater the surface area in relation to the volume of the article the quicker it will dry. Orientation of particles will affect the amount of shrinkage undergone in drying. As we have already said, such processes as pugging, plastic making and slip casting tend to orientate clay particles with their faces parallel to the force exerted on them. Shrinkage is then less in the direction parallel to the force, and greater at right angles to this direction. This fact probably accounts for the observation that shrinkage over the diameter of a pugged roll of clay-containing body is greater than the shrinkage over the length of the roll. The softer the body the more particle alignment will be produced and the greater the effect on differential shrinkage. Alignment is also increased the greater the forming force, the more clay the body contains, and the more plastic the clay. 3.4. Suspension Properties and Slip Casting Where a body is made into a slip either for slip casting or to achieve an homogeneous mix before filter pressing and pugging, it is necessary for the material in suspension to remain suspended and not to settle out. If the suspension is to be used for slip casting, it is also necessary that the suspension should be of high concentration and yet be fluid enough to pump through pipe lines, to pour into the plaster moulds, to flow into every corner of the mould and so faithfully reproduce the shape intended, and to drain cleanly from the mould after casting. If the body slip is to be filter pressed, the particles are kept in suspension by agitation with a large slowly revolving paddle stirrer until it is filter pressed. Casting-slips have to be made up at a higher concentration (measured as pint weight) so that the plaster mould is not required to remove too much water during the casting process, and so become saturated with water. The pint weight used for a casting-slip may be between 30 and 40 ounces per pint (i.e. relative density of 1-5 to 2Ό) or more, depending on the type of body in question. Knowing the pint weight of the slip 30 PROPERTIES IMPORTANT DURING MAKING and the specific gravity of the solid material, the weight of dry material in the pint can be calculated from Brogniart's equation (section 2.1.3). At the high pint weights required for slip casting, the suspension would be extremely viscous and impossible to use if some deflocculant (section 2.2.1) were not added to reduce the viscosity. The most commonly used deflocculants for clay containing bodies are sodium silicate and sodium carbonate, and these may be used either singly, or more frequently together. Other deflocculants such as sodium oxalate, sodium tannate, sodium phosphates and certain organic materials are also used to some extent, whilst for non-clay bodies hydrochloric acid or organics are usually used. The mechanism by which deflocculation occurs is dealt with in sections 5.1.11 and 5.1.12, but the practical importance of the process is that it produces a suspension of high pint weight which is at the same time fluid enough to allow it to be slip cast. The deflocculant acts chiefly on the clay in the body and the nature of the clay is important in determining the effectiveness of any particular deflocculant. As well as pint weight and viscosity, there is a third important property of a casting-slip, which has to be controlled for successful casting, i.e. thixotropy. If a clay suspension is well stirred, it may be fluid and easily pourable. If the suspension is then left undisturbed for some time it will gradually thicken, and in extreme cases it may thicken to such an extent that the container may be inverted without loss of the suspension. On vigorous stirring the suspension again becomes free flowing. This property of some suspensions to become more viscous with time when left at rest is known as thixotropy and is an important property of suspensions used in slip casting. Addition of a deflocculant to a casting-slip as well as drastically reducing the viscosity also reduces thixotropy. It is desirable to leave a small amount of thixotropy in the slip to give extra firmness to the cast and to keep a reasonably high rate of casting. Too high a value of thixotropy produces a "flabby" cast which is easily distorted, and on shaking may return to the fluid state and flow. The amount of deflocculant added is therefore 31 PROPERTIES OF CERAMIC RAW MATERIALS Zero " adjustment Lock nut" Phosphor-bronze - torsion wire Circular /fly-wheel Circular scale graduated 0 - 360° Cylindrical bob Sample holder Level adjustment FIG. 9. The torsion viscometer. adjusted to give a high fluidity, but to leave a small amount of thixotropy in the slip. The common use as deflocculant of mixtures of sodium silicate and sodium carbonate is due to the fact that in general the silicate gives high fluidity to a slip but tends to completely destroy thixotropy, whilst the carbonate leaves a considerable thixotropy in the slip on achieving the same fluidity as the silicate. If the two deflocculants are mixed, slips can usually be prepared with high fluidity and a small residual thixotropy, and these sups give good casting properties. 32 PROPERTIES IMPORTANT DURING MAKING The instrument usually employed for factory control of fluidity and thixotropy of casting-slips is the torsion viscometer, shown in Fig. 9. The instrument is set up vertically by use of the adjustment screws on the base, and the pointer carried on the flywheel is adjusted to read zero on the scale. The flywheel is then rotated through 360 degrees and locked in position. The well-stirred sup is placed in position so that the bob is immersed, and the flywheel is released. The torsion in the wire will make the flywheel swing through 360 degrees and its momentum will then make it continue swinging until it is brought to rest by the viscous drag of the suspension on the bob. The maximum reading indicated by the flywheel pointer on the second revolution is noted. This represents the fluidity of the slip and is expressed as degrees overswing. The more viscous the suspension the greater the drag on the revolving bob, and the smaller the reading. Thus fluidity is inversely related to viscosity, the higher the fluidity reading obtained the lower is the viscosity of the slip. Immediately after this first reading of fluidity has been obtained, the flywheel is again rotated and locked, and the suspension is left at rest for a fixed time—usually one or five minutes—before a second reading is taken. If the slip is thixotropic, it will thicken up on standing and the second reading will be less than the first. The first reading minus the second reading is then taken as the one minute or five minute thixotropy, and is again expressed in degrees. For most casting-slips the fluidity is adjusted to between about 280-320 degrees overswing, and the thixotropy to about 10-40 degrees, the exact figures being arrived at by trial or experience of previous slips which have given good results in practice. Thixotropy is believed to be due to a build-up of a structural arrangement of particles in the suspension. It can occur in plastic bodies as well as in suspensions, but because particle movement is more limited in the plastic state, the structure is not so easily built up and the effect is not so pronounced as in suspensions. In the completely deflocculated state, particles in suspension exist as individuals, each particle being separated from each other particle, whilst in the undeflocculated or flocculated state 33 PROPERTIES OF CERAMIC RAW MATERIALS particles form floes or aggregates containing a number of particles. On casting or drying out a deflocculated suspension the clay particles can pack together in a "face-to-face" manner, giving a dense structure which is not easily penetrated by water. Particles from a flocculated suspension pack in a more open manner, often in an "edge-to-face" formation which is more permeable to water. FIG. 10. Reproduced from Rheology of Ceramic Systems by F . Moore, by kind permission of the author, (a) Packing of particles from a flocculated suspension in "edge-to-face" or "house of cards" structure, (b) Packing of particles from a deflocculated suspension in "face-to-face" or "pack of cards" structure. Since the rate of casting is largely determined by the rate at which water can penetrate the cast layer, faster casting is obtained from flocculated than from deflocculated suspensions. Other properties of the cast and of the slip are affected by deflocculation, as shown in Table 2. The higher viscosities shown in flocculated suspensions is due to the fact that "edge-to-face" structure which exists also in the suspension must be broken down before flow can occur; this requires a greater force than is needed in a deflocculated suspension where there are no attractive forces between particles and no aggregation occurs. In addition, in flocculated suspensions water 34 PROPERTIES IMPORTANT DURING MAKING TABLE 2. SOME PROPERTIES AFFECTED ON DEFLOCCULATION Property Effect of deflocculation on property Bulk density of cast or dried-out slip Increase Viscosity of slip Decrease Thixotropy of slip Decrease Casting rate of slip Decrease Dry-strength of cast Increase Drying-shrinkage of cast Decrease Critical moisture content Decrease Rate of sedimentation of particles from suspension Decrease is locked up in the voids between aggregated particles, thus increasing the effective concentration of the suspension. Once a casting-slip has been adjusted to the required values of pint weight, fluidity and thixotropy, it is desirable that these properties should remain reasonably constant until the slip has been used. As well as producing the required fluidity and thixotropy in the slip, the deflocculant should therefore also produce a stable slip in which the fluidity and thixotropy do not vary excessively with time. Some deflocculants like the sodium metaphosphates, although excellent deflocculants in other respects, are little used, since they give unstable slips whose fluidities and thixotropies change rapidly with time. The presence of soluble salts, particularly sulphates in the raw materials, can cause difficulty in deflocculation. These are often found in clays, and tend to use up deflocculant in side reactions, e.g. CaSO. + Na.SiO, soluble salt > CaSiO, | + Na.SO* deflocculant 35 PROPERTIES OF CERAMIC RAW MATERIALS The sodium sulphate formed as a result of this reaction does not act as a deflocculant, but its presence reduces the effectiveness of further deflocculant additions (see the "overdoping" effect, section 5.1.12). It is common practice to remove soluble sulphates by precipitation as barium sulphate before deflocculant is added. This is achieved by addition of barium carbonate to the suspension: C a S 0 4 + BaCO, > B a S 0 4 + CaCO, Both the barium sulphate and the calcium carbonate produced are of low solubility and do not interfere with deflocculation when the deflocculant is added. 36 CHAPTER 4 Reactions Occurring on Firing FOR a typical ceramic body containing clay, silica and fluxing material we can summarise in general the reactions which occur during firing as follows : 1. At a temperature of 100°C any moisture left in the ware after drying and any hygroscopic moisture picked up from the atmosphere are driven off. 2. At about 450-500°C the clay mineral starts to decompose. Hydroxyl groups present in the clay structure are driven off as water in this reaction, which is known as dehydroxylation. Al208-2Si022H20 kaolinite • AhOa-2Si08 + 2 H a O t metakaolin steam This reaction results in a weakening of the body, since the metakaolin formed has no binding power like that of the clay. If after this reaction had taken place the body were to be cooled and water added, little or no plasticity would be developed i.e. once the clay structure has been broken down by this reaction its ability to become plastic when mixed with water is lost. 3. Organic matter present in the body may burn off at any temperature between about 300°C and 700°C or even higher, the temperature and ease of removal depending on the type of organic material present and the rate of heating. It is important that the kiln atmosphere should be strongly oxidising to enable oxidation of carbon to carbon dioxide to take place as early as possible. 37 PROPERTIES OF CERAMIC RAW MATERIALS The carbon must be burnt off before the surface of the ware vitrifies, since this can prevent oxygen reaching carbon within the body, with the result that the carbon will remain unburnt even at higher temperatures. Where iron is present in the body, ineffective removal of carbon can be serious, leading to faults known as "black cores" or "red hearts". It is usually desirable that iron compounds should be oxidised to the ferric state, the colour of which is bleached by certain oxides present in the body such as calcium oxide and alumina. Failure to burn off carbon results in reduction of iron to the ferrous form and this causes "black cores", particularly in heavy clay ware. "Black cores" which have undergone oxidation too late for the ferric iron to be bleached result in "red hearts". Similarly in tableware bodies, the colouring effect of iron impurities can be brought out by inability to remove carbon. To ensure removal of carbon at low temperatures the kiln atmosphere must be strongly oxidising and the rate of heating slow. Other decompositions which occur over this temperature range are those of carbonates and sulphides which may be present as impurities; these give carbon dioxide and oxides of sulphur. Some crystalline changes take place over this range also, notably the α-β quartz inversion (section 5.2) at 573°C. Since this change involves an expansion the rate of temperature rise should be slow near the inversion temperature for bodies containing quartz. Rapid temperature increase can cause cracking in such bodies. 4. Vitrification, i.e. glass formation, may start at any temperature above about 900°C, depending on the composition of the body. Particles of fluxing material react with particles of other body constituents with which they are in contact to form liquid, and the proportion of liquid increases as the temperature increases. The body contracts due to the formation of liquid (firing shrinkage) and the porosity is reduced. If vitrification is allowed to go too far, so much liquid may be formed that the body may lose shape. New materials may crystallise from the liquid as the temperature is increased, e.g. needle-like crystals of mullite 3Al»Os-2SiO» may separate from the melt. A soaking period at top temperature is usually given to allow temperature gradients in the ware to even out. On cooling the liquid solidifies to a glass 38 REACTIONS OCCURRING ON FIRING which cements together the unmelted particle and crystals formed during heating, and give strength to the fired body. Reaction can occur between particles on heating without formation of a liquid. In fact, such solid phase reactions are the first step towards densification even in the cases where solid phase reaction is followed by the formation of a liquid. In other cases densification may take place, resulting in a dense, hard product without the formation of a liquid at all. Such a case is that of pure alumina ceramics, which are fired at about 1800°C, more than 200°C below the melting temperature. The process is one of mutual diffusion of atoms between touching particles, and is known as sintering. As a result of sintering, particles become fewer and larger and pores are eliminated. Strength in the fired body is in these cases not due to glass formation, but to interlocking of crystals. Whether a liquid is formed or not, we start the firing with a compacted powder of high porosity, and finish with a body of relatively low or zero porosity. Since the process of densification depends on contact between particles, the more particle-particle contacts we have in our unfired material, the more readily the reactions leading to densification will take place. Thus the finer the particle size of our starting material, i.e. the greater its surface area per unit weight, the more readily it will fire. Atoms at the surface of the material have a higher energy than those in the bulk, since they are combined to other atoms only on one side and so are in a state of unbalance. The greater the surface area of the powder the greater the ratio of surface atoms to atoms in the bulk, and the greater the surface energy of the powder. After firing the surface area is very much reduced, i.e. the surface energy is reduced. It is this reduction in energy which is the "driving force" behind sintering. Where fluxing materials are incorporated in the body composition, the ratio of flux to refractory material will greatly affect the temperature to which the body must be fired. Table 3 shows the firing temperatures required to produce non-porous bodies from various mixtures of china clay, quartz and felspar. As the flux content is increased the required temperature is decreased. 39 PROPERTIES OF CERAMIC RAW MATERIALS TABLE 3. FIRING TEMPERATURES REQUIRED TO PRODUCE VITRIFICATION IN MIXTURES CONTAINING CHINA CLAY, QUARTZ AND FELSPAR Firing temperatures (°C) 1250 1300 1400 China clay 25 35 48 Quartz 40 40 30 Felspar 35 25 22 The effect of firing on a body depends not only on temperature, but also on time, i.e. a body fired rapidly to a high temperature may receive less heat treatment than one fired more slowly to a lower temperature. For this reason the measurement of temperature of firing alone is not sufficient, and the effect of firing on various standard bodies of standard shape, fired with the ware, gives a better guide to the amount of heat work done on the ware during firing. Some types of these pyroscopes as they are called are observed during firing, e.g. the "squatting" of cones of standard composition (Segar or Orton cones) or the sagging of standard bars supported at each end during firing (Holdcroft bars) can be observed during firing. Other types like the Buller's rings are usually examined after firing. In the Buller's ring method, the contraction of a ring of standard composition is measured. This is usually done after firing, although in some cases the rings are removed through trial holes during firing. The viscosity of the liquid formed during firing will affect the firing range of the body. If the liquid remains viscous over a large temperature range, then firing over that range is unlikely to lead to distortion of the ware. On the other hand, if the liquid formed rapidly becomes fluid as the temperature is increased, then the firing range will be short, and distortion during firing more likely. Another factor which clearly influences the firing required for 40 REACTIONS OCCURRING ON FIRING a body is the properties required in the product. For example, if translucency is required, then porosity must be as nearly eliminated as possible in the fired material; this will necessitate either a higher firing temperature or more flux in the body than is needed for a similar body which is to be porous after firing. Kiln atmosphere, as we have already said, is important in affecting the reactions which occur during firing. Normally an oxidising atmosphere is required so that organic matter can be burnt out and iron kept in the ferric state. There are, however, some instances where a reducing atmosphere is needed. A reducing atmosphere is usually obtained by allowing insufficient air into the kiln to enable all the carbon from the fuel (coal, oil or gas) to be converted to carbon dioxide. Some carbon is converted only to carbon monoxide CO, a reducing gas. By extracting oxygen from other materials, carbon monoxide can become oxidised to carbon dioxide, whilst reducing the material providing the oxygen. Other reducing gases like hydrogen can be introduced into laboratory kilns for special purposes. Instances where a reducing atmosphere is required at least for part of the fire are: (a) In the glost firing of hard porcelain where the small amount of impurity iron present is purposely converted to the ferrous state, giving the fired porcelain a blue tint which is preferred to the yellow colour obtained from ferric iron. This is achieved by using a reducing atmosphere in the kiln from about 1000°C to the top temperature of about 1400°C. (b) Blue engineering bricks have their firing completed in a reducing atmosphere, converting the iron to the ferrous form and producing the blue colour. (c) Certain colours depend on obtaining a low oxidation state of the metal involved and require reducing conditions, e.g. cupric oxide, which gives green colours under oxidising conditions, can under reducing conditions give a brilliant red known as rouge flambé. 47 CHAPTER 5 Ceramic Raw Materials 5.1 Clays 5.1.1. Formation In general, clay minerals have been formed from the decomposition of igneous rocks such as granite, which were themselves formed by solidification of molten materials from the interior of the earth. Granite is composed of approximately equal proportions of the minerals mica (K,0-3AliO,-6SiO,-2HiO), quartz (SiO,), and felspar (K,0-Al,0,-6SiO,), of which the least stable when exposed to the action of water and air is felspar. It is from the decomposition or kaolinisation of felspar in the presence of air and water over long periods of time that the kaolinitic clays have been formed. All the potash and part of the silica in felspar have been dissolved away, the residue combining with water to give the clay mineral kaolinite. K.O-A1.0.-6SÎO. felspar +2HsO > Al,0,.2SiO,.2H,0 — KtO kaolinite -4SiO, Kaolinite is a crystalline material, individual crystals being flat and hexagonal in shape. Although extremely small (most particles lying in the range 1-10 micrometret diameter) the crystals of kaolinite are relatively large compared to those of other clay minerals. r t 1 micrometre (μπι) = 10"*m = 3 937 χ 10"Μη. 42 CERAMIC RAW MATERIALS The small size, plate-like nature of crystals and high specific surface areaf of the clay minerals are responsible for many of the important properties of the clay minerals, as we have already seen. Because the particle size is so small, individual particles cannot be clearly distinguished under the optical microscope, but they can be seen under the higher magnification available with the electron microscope. 5.1.2. Residual and Sedimentary Clays The clay once formed from the parent rock may be deposited at its place of origin, or may be transported by water and redeposited at some distance from its place of origin. In the first case, the clay is known as a residual or primary clay, and in the second as a sedimentary or secondary clay. Sedimentary clays depend on their fine particle size for remaining in suspension whilst they are being transported, large particles are likely to be lost en route, and only the very fine particles will be carried to the final deposit. Sedimentary clays are thus usually of finer particle size than residual clays. During their transportation, sedimentary clays become contaminated with other material and the variety of non-clay material found in them is greater than in residual clays. These impurities are usually fine, and are much more difficult to separate from the clay mineral than impurities found in residual clays. The nature and amount of the impurities found with the clay determine to a large extent the uses to which the clay can be put. 5.1.3. The Structure of the Kaolins The structure of the kaolin minerals is based on the combination or condensation of two layer structures. One layer, known as the silica layery is composed of silicon and oxygen atoms, and the second layer, known as the gibbsite layer, is composed of aluminium atoms and hydroxyl groups. t Surface area per unit weight of material. 43 PROPERTIES OF CERAMIC RAW MATERIALS T H E SILICA LAYER Each silicon atom is surrounded by four oxygen atoms, the oxygens being at the corners of a regular tetrahedron. Looking down on such a tetrahedron it may be represented as shown in Fig. 11a. The three oxygens forming the base triangle of the tetrahedron are below the level of the centrally placed silicon, and the oxygen forming the apex of the tetrahedron is above it. Figure l i b shows a side elevation of the same tetrahedron, and indicates the position of the silicon more closely. • Silicon Ο Oxygen (a) Plan (b) Side elevation 4 Fio. 11. (Si04) "" tetrahedron. The silicon carries four positive valencies, and the oxygens each carry two negative valencies, so that the tetrahedron as a 4 whole carries a net charge of minus four, i.e. [ S i 0 4 ] " . In certain silicates (known as the orthosilicates) these [Si0 4]*~ tetrahedra exist as individuals, their charge being satisfied by cations with which they combine, e.g. ZrSiO* zircon or zirconium orthosilicate. In other structures, tetrahedra join together, sharing oxygens and forming chains or sheets. In the clay mineral structures three of the four oxygens in each tetrahedron are shared with other tetrahedra, giving a continuous sheet as shown in Fig. 12. This structure can be extended indefinitely in the a and b directions, i.e. in the plane of the paper. Three of the four oxygens in every tetrahedron are now valency satisfied, receiving one 44 CERAMIC RAW MATERIALS — b — 1 1 FIG. 12. The ^ O e f ê - tetrahedral silica layer. valency from each of the two silicons to which they are linked. The fourth oxygen in each tetrahedron (those at the apexes of the tetrahedra) is joined to only one silicon, and so has one valency unsatisfied. It is through these valency unsatisfied oxygens that the silica layer can join via cations to similar structures. Each tetrahedron contains one silicon atom, one oxygen with one valency unsatisfied, and a half share in three more oxygens which are valency satisfied since they are joined to adjacent tetrahedra. The basic formula for the sheet structure is therefore Si070a/» or 2 [ S i O H ] - or [Si,O e ] -. Since the oxygens are disposed tetrahedraUy about the central silicon, this sheet is often called the tetrahedral layer. Note (Fig. 12) that the finked tetrahedra form a hexagonally shaped ring structure which is repeated many times. T H E GIBBSITE LAYER In the kaolin minerals the tetrahedral silica layer is joined via the valency unsatisfied oxygens to a layer of aluminium hydroxide 45 PROPERTIES OF CERAMIC RAW MATERIALS Al(OH)s, called the gibbsite layer. The basic "building block" of the gibbsite layer is an aluminium atom surrounded by six hydroxyl groups, the hydroxyls being at the corners of a regular octagon (see Fig. 13). The gibbsite layer is therefore often referred to as the octahedral layer. FIG. 13. [Al(OH)e]»- octahedron. Like the silica tetrahedra, these A l ( O H ) e - octagons can join together, every O H group being shared by two aluminiums. Every O H group receives half a valency share from two trivalent aluminiums, and since the O H group is monovalent, the hydroxyl groups are all valency satisfied and there are no unsatisfied valencies in the structure (Fig. 14). Each octagon contains one aluminium atom and a half share in six hydroxyl groups, i.e. the basic formula is A l ( O H ) e / 2 or Al(OH) 8 . Note that half the hydroxyl groups are above the plane of the aluminium atoms and half are below it. Like the tetrahedral silica sheet, the octahedral gibbsite sheet is capable of infinite extension in the a and b directions, and the linked octahedra form a repeated hexagon ring shaped structure similar to that in the tetrahedral layer. 8 46 CERAMIC RAW MATERIALS ® Aluminium © Hydroxyl a Bonds above the plane of the aluminium atoms Bonds below the plane of the aluminium atoms FIG. 14. The n[Al(OH) 8] octahedral gibbsite layer. a CONDENSATION OF THE [Si a0 5] ~ AND Al(OH), LAYERS The dimensions of the hexagonal rings in the silica and gibbsite sheets are very similar, and it is possible for the two sheets to condense together. This occurs by eUmination of hydroxyl groups in the gibbsite structure, their places being taken by the valency unsatisfied oxygens of the silica sheet. Since the hydroxyls in the gibbsite structure receive half a valency share from each of the two aluminiums to which they are joined, and the valency unsatisfied oxygens in the silica structure have one valency unsatisfied, it is necessary for each valency unsatisfied oxygen to combine to two aluminium atoms, receiving half a valency share from each, 2+ as did the hydroxyl. This leads to electrical neutrality [ A l a ( O H ) 4 ] 2 [Sia05] "" or the structural formula of the kaolin minerals Al a (OH)« •Si a 0 5 . This is more commonly though less accurately written as Al a O,-2SiO a -2H a O. It is important to note that the 2 H a O in this last formula is present in the kaolin structure not as water molecules, but as hydroxyl groups. A section through the joined layers is shown in Fig. 15. 47 PROPERTIES OF CERAMIC RAW MATERIALS Charge Gibbsite octahedral layer Silica tetrahedral layer 30 H - 3 2 AI + 6 20. ΙΟΗ - 5 2 Si* +8 30 - 6 Net charge Zero FIG. 15. The combined tetrahedral and octahedral layers. The structure of the kaolin minerals Al 2(OH)4.Sia0 6. All the kaolin minerals have this same basic structure, the difference between them being in the way that these pairs of layers are stacked on top of one another to form the crystal. Since the structure in Fig. 15 is electrically neutral and all valencies are satisfied, there can be no valency bond between the kaolin sheets. There are two types of forces which can hold the sheets together: (a) weak van der Waals' attractive forces which exist between all particles of matter in close proximity (these are short range forces, i.e. they fall off rapidly with increasing distance of separation); / / / / / / / / / / / / / / / / . ^ \ \ \ \ \ \ \ \ \ ( \ \ \ \ \ ^ A . 2( O H ) 4 ( S i 20 5 I Strong ionic bonds holding A I 2( 0 H ) 4 and S i 2O s sheets together Weak hydrogen bonds and van der Waals forces holding kaolin sheets together WWWWWWsiA FIG. 16. Stackings of kaolin layers. 48 CERAMIC RAW MATERIALS (b) weak hydrogen bonds between the hydrogens of the hydroxyl groups in the gibbsite layer of one kaolin sheet and the oxygens in the silica layer of the next kaolin sheet. The hydrogen here acts as a bridge, having some attraction for both oxygens as shown diagrammatically in Fig. 16. Because the bonds between kaolin layers are weak, the number of layers which can be stacked together is limited and the clay crystals tend to be thin in the c direction. The bonds between octahedral and tetrahedral layers and in the a and b directions are strong, thus the crystals of the clay minerals are long and wide but thin. The hexagonal structures in both the tetrahedral and octahedral layers result in flat, hexagonal crystals (Fig. 17). a FIG. 17. Hexagonal clay crystallite composed of many stacked layers of kaolin sheets. The crystal is long and wide compared to its thickness. Although the forces between kaolin layers are weak, they are strong enough to hold the layers in fixed positions and give a constant basal spacing of 7-2 A.f The basal spacing is the distance between corresponding layers of atoms, e.g. from the silicon atoms in one kaolin sheet to the corresponding silicon atoms in the next kaolinite sheet, or between aluminium atom layers in the two sheets, etc. All the kaolin minerals contain one silica layer and one gibbsite layer, and are therefore sometimes called one-to-one, or single layer minerals. Some minerals like the montmorillonites, as we t 1 a n g s t r o m unit ( 1 À ) = 1 0 " 10 metres. 49 PROPERTIES OF CERAMIC RAW MATERIALS shall see in section 5.1.5, contain two silica layers combined to one central layer, which may be gibbsite or some similar structure, and these are termed two-to-one or double layer minerals. In the kaolin mineral nacrite, corresponding atoms in the kaolin sheets he directly over one another, e.g. an aluminium atom in one kaolin sheet is directly over the corresponding aluminium atoms in all the other kaolin sheets of which the crystal is composed. In the minerals dickite and kaolinite some displacement takes place, i.e. corresponding atoms in each kaolin sheet are not directly over one another, but are displaced to a fixed extent. In halloysite the displacements are completely random. These are all members of the kaolin group of minerals, and have the same structural formula Al2(OH)4-Si 2 0 5 . They differ only in the way the kaolin sheets are stacked on top of each other to form the crystal. One other member of the kaolin group exists : this is the hydrated form of halloysite, having the formula Al 2 (OH)4-Si a 05-2H a O. There is again a random stacking of layers, but water molecules exist between the kaolin layers. The crystal size of these minerals differs because the difference in stacking of layers influences the stability of the structure. In nacrite where there is no displacement the structure is stable, and crystal size is relatively large. In dickite the displacement is small, and crystal size is still relatively large. In kaolinite more displacement occurs, the structure is less stable than in nacrite or dickite, and the crystal size is therefore smaller. The random displacement in halloysite makes for instability and hence small crystals. These four members all show the hexagonal plate like crystals, but in the hydrated halloysite where water enters between layers, interlayer attraction is very small and the layers tend to roll up, producing tube-like crystals. On heating, the interlayer water can be driven off, and the tubes then unroll. 5.1.4. The Kaolinitic Clays These are the clays containing a kaolin as the main clay mineral. The term "clay" as opposed to "clay mineral" indicates a mixture 50 CERAMIC RAW MATERIALS of clay mineral with a variable amount and type of non-clay mineral material. CHINA CLAYS These are residual clays, and are essentially composed of the clay mineral kaolinite contaminated with silica, mica, felspar and partly decomposed felspar, all from the native rock from which the clay was formed. Large deposits of china clay are found in Devon and Cornwall. The clay mineral is much finer than the contaminating materials, and thus is relatively easily purified by sedimentation, giving a white clay which is also white burning due to its low content of colouring impurities, particularly iron and titanium compounds. Because of its low impurity content, china clay is also refractory, it is therefore useful both in the production of white decorative ware and as a refractory clay. Large quantities of china clay are used in paper manufacture as well as in the ceramics industry. BALL CLAYS These clays are named from the cubes or balls in which the clay was cut. They are found in this country in Devon and Dorset. Ball clays are sedimentary clays, though they have been transported over a relatively short distance. They are characterised by their fine grain size and often high content of organic material. Although the clay mineral is essentially kaolinite, it is much finer than that found in china clay (see section 5.1.10), giving ball clays a greater plasticity and dry-strength than china clays. They contain a greater quantity and variety of impurity than china clays and because the impurities are either very fine or to some extent part of the clay composition, they are not easily removed. Iron and titanium impurities give the clay a fired colour which may vary from off white to dark brown depending on the amounts, and fluxing impurities reduce the refractoriness of the clays. Much of the colour of an unfired ball clay can be due to organic materials, so unfired colour is often a poor guide to colouring impurities, 51 PROPERTIES OF CERAMIC RAW MATERIALS and a dark clay may fire to a much lighter colour. The advantages of ball clays are their high plasticity and dry-strength; their disadvantages are fired colour and low refractoriness. FIRECLAYS These also were formed from felspar, and the clay mineral is essentially kaolinite. They are found widespread over the Midlands and North of England and in Scotland. In some cases they are residual, but the bulk are sedimentary and may have been transported over long distances before being deposited. Fireclays are found in association with coal measures, and it may have been the growth of vegetable life which later formed these coal measures which extracted the alkali compounds from the fireclays and thus gave them their refractory nature. They are of fine particle size and therefore plastic, but, like ball clays, they contain colouring impurities and fire to a buff colour. Due to their low alkali content they are chiefly used for their refractory properties. BRICK CLAYS These comprise a wide variety of clays of varying composition, the clay mineral being of the kaolinitic or illitic type. They are invariably high in iron content (hence their fired colour) and often contain gross amounts of other impurity, notably calcium compounds. Because of the high impurity content, fluxing additions are not normally necessary and the clay can be fired at a relatively low temperature. Some deposits are high in organic matter, which ignites on firing and reduces the amount of fuel necessary to fire the ware. 5.1.5. The Structure of the Montmorillonites In the gibbsite sheet structure as we have seen there are two planes of hydroxyl groups, one above and one below the central layer of aluminium atoms. In the one-to-one layer minerals, one layer of these hydroxyls is condensed with a silica layer with 52 CERAMIC RAW MATERIALS replacement of hydroxyl groups. It is possible for both the upper and lower hydroxyl layers in the gibbsite structure to condense with silica layers, leading to a two-to-one layer structure, i.e. one containing two silica layers sandwiching between them a gibbsite layer. The central layer need not be gibbsite, but can also be brucite Mg(OH) 2 or ferrous hydroxide Fe(OH) 2 which, form structures similar to the gibbsite structure. When gibbsite is the central layer, condensation of a silica layer on each side of it leads to the formula Al 2 (OH) 2 -2Si 2 O e or A l 2 0 8 - 4 S i 0 2 - H 2 0 . This is the ideal formula for pyrophyllite, one of the montmorillonite group of minerals. Diagrammatically we can represent the condensations leading to the kaolin and montmorillonite type structures as follows: (OH), Al, (OH), Si.CM OH or A l 2 0 s - 2 S i 0 2 . 2 H 2 0 2 + [Si205] " - > 2 ( O H ) - + Al, kaolinite tetrahedral replaced (OH) 8 J silica layer gibbsite + 2[Si 2 0 5 ] 4(OH)replaced tetrahedral silica layer + Si 2 OA OH A l 2 or A l 2 0 s 4 S i 0 2 H 2 0 pyrophyllite OH When brucite is the central layer, the ideal formula becomes M g 8 ( O H ) 2 - 2 S i 2 0 6 or 3 M g O - 4 S i 0 2 H 2 0 . This is the ideal formula for talc (also known as steatite, soapstone and French chalk), another member of the montmorillonite group. The structures of pyrophyllite and talc are shown in Fig. 18 (a) and (b). Note that in the talc structure three divalent magnesiums are required to maintain electrical neutrality whereas only two trivalent aluminiums are required in the pyrophyllite structure. Pyrophyllite and talc can be regarded as the two parent minerals of the montmorillonite group. Other members can be 53 FIG. 18. (a) Structure of pyrophyllite. (b) Structure of talc. PROPERTIES OF CERAMIC RAW MATERIALS 54 CERAMIC RAW MATERIALS formed as we shall see (section 5.1.10) by replacement of some of the silicon in the tetrahedral layers by aluminium, and the replacement of aluminium in the octahedral layer by magnesium or iron. In the kaolins we have seen that there are two forces acting between the kaolinite sheets, van der Waals' forces and hydrogen bonds. In the montmorillonites there is no possibility of hydrogen bonding, since both the outer layers are silica layers and there are no exposed hydroxyl groups at the surface. The montmorillonite sheets are therefore held together only by van der Waals' forces, and are even more weakly held together than are the kaolinite sheets. In liquids, montmorillonite sheets can be separated by penetration of liquid molecules between the sheets. This leads to the characteristic swelling of montmorillonites in certain liquids, particularly in water, and a variable basal spacing. The basal spacings found in montmorillonite may vary between about 11 and 60 Â depending on the type and amount of liquid adsorbed between the layers. Recognition of materials by X-ray diffraction depends on their characteristic basal spacings, and since glycerol produces a constant spacing of 17 Â in montmorillonites, addition of glycerol is used in the identification of montmorillonite minerals by X-ray. In the kaolins the available surface area of a crystal is simply its external surface, i.e. the two flat hexagonal faces and the edges. Because the individual sheets within the crystal can be separated in the montmorillonites, not only the external area, but also an internal surface area, is available. This in conjunction with their small particle size gives them a much greater overall surface area than kaolins, and makes them valuable adsorbents and catalysts. 5.1.6. The Micas Although the micas are not classified as clays, since they do not produce a plastic mass on addition of water, it is convenient to deal with the micas here, since structurally they closely resemble the montmorillonites. The general formula of the micas is that of pyrophyllite with one-quarter of the silicon replaced by aluminium. If we write pyrophyllite Al.SiiOioiOH)^ then the basic mica formula is 55 PROPERTIES OF CERAMIC RAW MATERIALS M [Al 2(SisAl)Oio(OH)a]-. Since the silicon is four valent, and aluminium is only three valent, replacement of one silicon by one aluminium results in a deficiency of positive charge. This is made + up by the monovalent cation M+. Where M is potassium, we have potash mica or muscovite. + K . A l 2 [ S i A l ] O 1 0 ( O H ) 2 or K 2 0 3 A l 2 0 , 6 S i 0 2 2 H 2 0 . The potassium ions are found between the montmorillonitelike layers. They serve to hold the negatively charged layers together more firmly than is the case in the montmorillonites, and this results in a constant basal spacing of 10 Â in muscovite and there is no swelling in water. The interlayer cation cannot be replaced in the laboratory by another type of cation without breaking down the mica structure. In calcium mica or margarite half the silicon in the pyrophyllite structure is replaced by aluminium, and the double deficiency in positive charge is balanced by the divalent calcium cation, giving Ca +{Al 2 [Al 2 Si 2 ] O 1 0 ( O H ) 2 } " or C a 0 2 A l 2 0 2 . 2 S i 0 2 H 2 0 2 2 The calcium appears to strongly and margarite is flakes as are the micas potassium. Margarite is mica". bind the layers together particularly for this reason not easily cleaved into containing monovalent cations like therefore sometimes called "brittle SiijAli o 5 A I 2( 0 H ) 2 /Si^AljOs 10A 7 FIG. 19. Potassium ions between layers in muscovite structure. 56 CERAMIC RAW 5.1.7. The Mites or Hydrous MATERIALS Micas These are mica like materials, but contain less potassium and more water than muscovite. They occur as contaminants in clays, particularly fireclays, and are difficult to separate from the clay mineral since they are present in similar size to the clay particles. The separation of hydrous mica from fireclays would be important, since its presence introduces alkali and reduces the refractoriness of the clay. It has been suggested that the H 8 0 + cation replaces K+ in muscovite, thus giving the lower potassium and higher water contents found in the hydrous micas. Where large flakes of hydrous mica have been isolated, it has been found that fissures are present in the flakes. Material in these fissures has been shown to absorb dye which is not adsorbed on the mica itself, and it is possible that the material in the fissures is clay mineral and that hydrous micas are in fact mixtures of true micas and clay mineral. 5.1.8. The Chlorites These are clay minerals of small particle size and greenish colour. Basically they consist of alternate layers of talc Mg 8Si 4Oio (OH) a and brucite Mg(OH),, but the brucite layer carries a positive charge due to some replacement of magnesium by aluminium, and this is balanced by the talc layer which is negatively charged due to some replacement of silicon by aluminium. A typical chlorite ispenninite, which may be written {MgsAl(OH)e}+ {Mg,(AlSi,)O 1 0(OH),}-. 5.1.9. The Vermiculites These minerals show large flakes rather like those of the micas. When heated they undergo considerable volume increase, and this property makes them useful thermal insulation materials. Structurally they resemble the chlorites, but they are more hydrated and carry magnesium cations between the layers. 57 OPERTIES OF CERAMIC RAW MATERIALS 5.1.10. Isomorphous Substitution in the Clay Minerals Where the sizes of cations are similar, it is possible for them to replace one another in the clay mineral structures. For example, minerals are found where calcium has replaced magnesium or ferrous iron in the structure, similarly ferric iron can replace aluminium. Although such replacements result in a change in chemical composition, they do not result in a change of charge; since the valencies of the replaced and replacing ion are the same, the structure remains electrically neutral. It is, however, possible for substitutions to occur in which the valencies of the replaced and replacing ions are not the same, for example trivalent aluminium can replace quadrivalent silicon. Because of the differences in valency of the ions, this replacement leads to a deficit of positive charge in the structure, i.e. the clay structure is negatively charged. The charge is balanced by adsorption of cations external to the clay structure. This substitution of one cation in the lattice for another of similar size is called isomorphous substitution, arid is particularly common in the montmorillonite minerals. In the mineral montmorillonite, t one-sixth of the aluminium in the pyrophyllite structure is replaced by divalent magnesium. This results in a net negative charge on the clay, which is balanced by the adsorption of cations external to the structure, i.e. AUSi.OxoiOH),, [AlxjMgJSi.OxoCOH), pyrophyllite \ montmorillonite where M is some monovalent cation such as Na+, H+, etc. adsorbed externally to the structure to satisfy the negative charge of the structure. If a divalent cation such as Ca++ is adsorbed to satisfy the charge, then only half the quantity would be needed, i.e. [Al^ftJSi.O^OH), 1 \ Ca* the calcium form of montmorillonite t Montmorillonite is the name of one member of the montmorillonite group of minerals as well as the general name of the group. 58 PLATE 1. U n l o a d i n g a filter p r e s s . By courtesy and Sons Ltd. of Josiah Wedgwood PLATE 2. A p u g mill. By courtesy ofJosiah Wedgwood and Sons Ltd. PLATE 3. T h r o w i n g by h a n d . By courtesy Sons Ltd. of Josiah Wedgwood and PLATE 4 . Semi-automatic cup jollying machine. Josiah Wedgwood and Sons Ltd. By courtesy of PLATE 5. Single-head rollerflatwaremaking machine with turntable. By courtesy of Service Engineers Lid. PLATE 6. Apparatus for testing dry strength. By courtesy of English China Clays Co. Ltd. PLATE 7. C h i n a c l a y w o r k i n g s in C o r n w a l l . By courtesy China Clays Co. Ltd. PLATE 8. of Ball c l a y w o r k i n g at N e w t o n A b b o t , D e v o n . By of English China Clays Co. Ltd. English courtesy PLATE 9. A c r a z e d p l a t e . PLATE 1 0 . R e f r a c t o r y p u r e a l u m i n a w a r e . By courtesy Royal Porcelain Co. Ltd. of Worcester CERAMIC RAW MATERIALS In nature, there is usually a mixture of adsorbed cations satisfying the charge on the clay mineral structure, the cations usually found being Ca, Mg, H, N a and Κ. In nontronite, another member of the montmorillonite group, all the aluminium of the pyrophyllite structure is replaced by ferric iron, and some of the silicon is replaced by aluminium. Since both aluminium and ferric iron 8 a+ are trivalent, the substitution of Al + by F e results in no change in charge on the structure, however the replacement of some 8+ Si*+ by A l leads to a negative charge which is satisfied by the adsorption of external cation, giving: Fe^tSi^AyOxoiOH), Ψ nontronite Similar isomorphous substitution occurs to a lesser extent in other clay minerals. The clay minerals found in the ball clays and fire clays are essentially of the kaolinite structure, but some isomorphous substitution has taken place in the mineral lattice. This substitution although small in extent has a considerable effect on the properties of the clays. The relatively pure kaolinite found in china clays is white, whereas the clay minerals of the ball clays and fireclays are buif coloured. This colour is not due to simple admixed impurity, since the colour cannot be removed without breakdown of the clay structure. It is probably due to isomorphous substitution by iron, and since this iron is present in the structure of the clay, its removal would be expected to lead to breakdown of the mineral. Although the replacing ion must be of similar size to the ion it replaces, the sizes of the two ions will not be identical, and the replacement will result in some strain in the lattice, making the structure less stable. Clay minerals which show some isomorphous substitution are referred to as being disordered. This disorder can be recognised in X-ray diffraction patterns obtained from the materials. Disorder then leads to lower stability in the structure, and this in turn leads to smaller particle size. The relatively " p u r e " kaolinite of the china clays results in much greater particle size 59 PROPERTIES OF CERAMIC RAW MATERIALS than shown by the disordered forms of kaolinite found in the ball clays and fireclays. The smaller particle size and hence greater surface area in the disordered clays results in greater plasticity, dry-strength and wet-to-dry shrinkage. 5.1.11. Cation Exchange Where isomorphous substitution involves the replacement of IV a cation of higher valency by one of lower valency (e.g. S i m replaced by A l ) the structure becomes negatively charged, and this charge is satisfied by adsorption externally to the structure of some cation M. We may then write the clay which has adsorbed cations of type M as M clay. The charge on the clay can be demonstrated by putting two electrodes into an aqueous suspension of the clay and applying a direct current voltage between the electrodes. Some ionisation of the clay occurs in water, i.e. M clay ^ M + + clay and the negatively charged clay particles can be seen to migrate towards the positive electrode, whilst the M+ cations move towards the negative electrode. It is possible to replace the M+ cation by some other cation, say N+, by treating the clay with a solution of an N+ salt, say NA, e.g. M clay + N+ A - ^ Ν clay + M+ A~ in solution The amount of M+ which is replaced by N+ will depend on the concentration of the N A solution we use, the sizes of the two cations M+ and N+, the valencies of the two cations, and the solubility of the product MA. In general, preference of adsorption is given to small cations of high valency. An approximate order of preference can be compiled, which is in order of preferred adsorption as follows: H+ > Mg*+ > Ca»+ > Li+ > Na+ > K+ Thus in general, Na+ would be readily replaced by H or C a , but would not readily replace them from a clay. If we wished to 2 replace adsorbed Ca + on a clay with Na+ by using NaCl solution, 60 + , + CERAMIC RAW MATERIALS we would need to use a high concentration of NaCl. On the other hand, if N a + was adsorbed on the clay, we could replace it with Ca*+ using a dilute solution of CaCl a . The exchange of one type of cation for another is known as cation exchange, or base exchange, and is particularly important in flocculation and deflocculation of ceramic suspensions. The amount of cation which a clay will adsorb and exchange in this way is known as the cation exchange capacity (c.e.c.) or base exchange capacity of the clay. The amounts of cation adsorbed are small, and the c.e.c. is expressed as milli-equivalentst of cation adsorbed per 100 g of clay. Because isomorphous substitution is extensive in the montmorillonites, they require relatively large amounts of adsorbed cation to satisfy the charge, and so have high c.e.c values (about 70-150 m.e./lOOg). In the kaolin clays, the amount of substitution is less, and they show lower c.e.c. values, china clays having values of between about 3-6 m.e./100 g, and the clay minerals of the ball clays and fireclays between about 15-40 m.e./100 g. The c.e.c. values quoted for china clays could result from the substitution of only about 1 silicon in every 300 by aluminium. Although isomorphous substitution is undoubtedly the main cause of cation exchange in the montmorillonites, in the kaolins there may be another important reason. At the edges of clay particles there must be unsatisfied valency bonds where the clay lattice comes to an end. It is possible that these unsatisfied valencies can become satisfied by combining with ions, and these ions may then be exchangeable for other ions. If this is so, then the clay would be expected to adsorb both cations and anions since there would be broken negative and positive bonds at the crystal edges (see Fig. 20). Published figures show relatively high anion exchange in the kaolins compared to the montmorillonites, . , .„. » « . 1 gram equivalent 1 1 milli-equivalent = j^QQ + Hence 1 m.e. N a = 0Ό23 g of sodium. + 1 m.e. Ca+ = 0020 g of calcium. 1 m.e. H+ = 0010 g of hydrogen. 61 PROPERTIES OF CERAMIC RAW MATERIALS -il ® = Al OH • - Si 0= ο + 1 FIG. 20. Possible "broken bond" adsorption sites in kaolinite. so this broken bond mechanism of ion exchange may be of importance in the kaolins, whereas isomorphous substitution is the major factor in the montmorillonites. We have said that clays will preferentially adsorb small cations + 2 like H or ions of high valency like Ca + in preference to large 2+ monovalent ions like Na+, and that replacement of C a or H+ by Na+ would generally require a high concentration of Na+. In fact, the exchange can be carried out if we can remove the H+ or 24 Ca * as some insoluble or unionised product. Suppose we treat a hydrogen clay with sodium carbonate solution, then the reaction is,f H - clay + N a 2 C 0 3 ^ N a clay + H 2 C O carbonic acid % 2H+ + CCVThe hydrogen ions produced from the carbonic acid on the right hand side of the equation, are free to compete with sodium ions for adsorption on the clay, and due to their smaller size, the hydrogen ions will compete successfully unless the sodium ion concentration is high. The reaction above therefore lies to the left, and the dotted arrow indicates that the reaction to the right t In these equations no valency is ascribed to the clay, and where a cation M is adsorbed on the clay, the clay is written M clay no matter what the valency of the ion M. For this reason these equations may not "balance" on the left- and right-hand sides. 62 CERAMIC RAW MATERIALS only occurs to a limited extent. If we use sodium hydroxide, the reaction is H - clay + N a O H N a clay + H 2 0 The water formed on the right-hand side of this equation is only very weakly ionised. Very few hydrogen ions are now present to compete with sodium ions for adsorption, and the reaction lies to the right, i.e. the sodium clay is formed and the exchangeable hydrogen from the clay is removed as weakly ionised water. For a calcium clay the position is reversed, the effectiveness of sodium carbonate being greater than that of sodium hydroxide, i.e. Ca clay + N a O H ^ N a clay + Ca(OH), Ca 2+ % + 2(OH)~ but Ca clay + N a 2 C 0 3 N a clay + C a C 0 3 φ In the case of the hydroxide, the calcium hydroxide produced is 2+ + ionised to give C a ions in water, which can compete with N a , but if the carbonate is used, the calcium is precipitated as the low solubility calcium carbonate, there is no competition between sodium and calcium for adsorption, and the sodium clay is formed. The formation of a sodium clay is the usual means of achieving deflocculation of a clay suspension, and the above reactions are therefore important in deciding whether or not low viscosity suspensions of high concentration, as used in süp casting, can be achieved with a particular deflocculant and a particular clay. 5.1.12. The Clay-Water System The weight of exchangeable cation adsorbed on a clay expressed as a percentage of the clay weight is very small. 10 milli-equivalents of Na+ represents 0-23 g of sodium, so for a sodium clay of c.e.c. + 10 m.e./100 g, only 0-23 % of the total weight is exchangeable N a . Nevertheless the type of exchangeable cation associated with the clay very markedly affects the properties of the clay. 63 PROPERTIES OF CERAMIC RAW MATERIALS In water, clay behaves like the anion of a weak acid (e.g. acetate) and some ionisation takes place, water M clay ^ M+ + clayThe extent of this ionisation depends on the electropositivity of the cation M ; the more electropositive is M the more ionisation will occur. The more M clay ionises, the more clay- is produced and the less clay is left as unionised M clay. Charged clay particles will repel one another whereas the uncharged M clay will not repel similar particles, and coagulation of particles into aggregates containing many particles will result. Coagulation will result in a lower energy in the suspension as a whole, and will occur if there is not a sufficiently high energy barrier to coagulation due to forces of repulsion between particles. Repulsion between particles depends on the type of exchangeable cation adsorbed on the clay, repulsion being high when the exchangeable cation is highly electropositive (e.g. Na+), and low when it is not so electro2+ positive (H+ or C a ) . As well as repulsive forces between particles, there will be attractive forces due to van der Waals' attraction. These are short range forces which are powerful when particles are close together, but whose effect falls off rapidly with increasing distance of separation. When particles are close together (i.e. at high suspension concentration) van der Waals' attractive forces tend to predominate, but at larger particle separation distances (i.e. at lower suspension concentration) there will be a balance between attractive and repulsive forces, and the resultant force may be one of repulsion if repulsive forces between particles are high enough. In deflocculation, the repulsive forces are made a maximum (usually by making Na+ the exchangeable cation), whereas 2+ in the flocculated state (poorly ionised H+ or C a as exchangeable cation) the repulsive forces are reduced, and the predominating force may be one of attraction for all concentrations of suspension. Figure 21 shows the variation of repulsive force jR, attractive force A, and resultant force r with increasing separation of particles (i.e. increased dilution of suspension), for (a) a deflocculated suspension and (b) a flocculated suspension. 64 CERAMIC RAW MATERIALS cο "ΊΟ Separation distance between particles gc ο (0 (a) Deflocculated suspension (b) Flocculated suspension FIG. 2 1 . V a r i a t i o n o f inter-particle forces w i t h v a r y i n g c o n c e n t r a t i o n o f s u s p e n s i o n . R e p r o d u c e d f r o m Rheology of Ceramic Systems by F . M o o r e , b y k i n d p e r m i s s i o n o f the a u t h o r . For the deflocculated suspension we have a net force of attraction between particles for low separations, but since the repulsive force is high and the attractive force falls off more rapidly with increasing separation distance, the resultant force r becomes repulsive at higher separation. In the flocculated suspension the repulsive force is low, and never overcomes the force of attraction at any separation distance. The change in properties observed on deflocculation of a flocculated suspension were shown in Table 2 (section 3.4), and are due to the increased force of repulsion obtained on deflocculation. T H E DOUBLE LAYER THEORY As well as adsorbing exchangeable cations, clay surfaces adsorb water molecules. Because the concentration of electrical charge is high at the clay surface, and because the water molecules are polar (section 3.1), the water molecules are orientated and rigidly held close to the surface of the clay. As the distance from the surface is increased, the degree of orientation and the rigidity 65 PROPERTIES OF CERAMIC RAW MATERIALS with which the water molecules are held decrease. The exchangeable cations are also high in concentration near to the clay surface, the concentration falling off with increasing distance from the surface as shown in Fig. 22. Free water c— >) c>ay| Water ^ — ^ molecules & + <£3> Cations + FIG. 22. Adsorbed water molecules and exchangeable cations in a clay-water system. A — A is the limit of the zone of rigidly held water molecules. At the clay surface the charge will be negative and at a small distance from the surface the negative charge will be partly neutralised by the cations inside the rigidly held layer of water. With increasing distance from the clay surface the negative charge will become more and more reduced, until at some point in the free water zone it will be reduced to zero (curve I, Fig. 23). For some materials, the concentration of cations in the zone of rigidly held water is sufficient to neutralise and reverse the surface charge (curve II, Fig. 23). The charge will then again fall to zero in the free water zone. The magnitude of the charge at the limit of the rigidly held water layer, i.e. at A — A, is known as the electrokinetic or zeta-potential (ζ), and it is the value of the zeta-potential which largely determines whether particles will repel one another or floe together to form -agglomerates, i.e. it determines whether the 66 CERAMIC RAW MATERIALS suspension will be flocculated or deflocculated. When the exchangeable cation is highly electropositive, it ionises well and the zetapotential is high (la, Fig. 23). When the exchangeable cation is not so electropositive, ionisation is limited, the potential at the particle surface is comparatively small, and the charge is reduced to zero at a lesser distance from the surface (lb, Figure 23), i.e. both e, the surface density of charge at the particle surface, and d the thickness of the double layer, are reduced from the a values to the b values in Fig. 23. This results in a decrease in zetapotential from a to b. 9 FIG. 23. Change of potential with distance from particle surface. It was shown by Muller and Abramson in 1934 that the value of the zeta-potential could be approximately expressed by the equation æ = ~D~ 67 PROPERTIES OF CERAMIC RAW MATERIALS where ζ is the zeta-potential, e is the surface density of charge on the particle, d is the double layer thickness, and D is the dielectric constant of the liquid medium. For a colloidal suspension to be stable (i.e. not to undergo aggregation of particles followed by settling of the aggregates), it is necessary for the zeta-potential to be above a certain minimum value; this value is thought to be about 0 0 2 volt (positive or negative). If the zeta-potential is less than this value, repulsive forces between particles are low, and particles can approach near enough to one another for van der Waals' attractive forces to operate and aggregation to occur. The effect of the type of exchangeable cation adsorbed on a clay on its zeta-potential in aqueous suspension is shown in Table 4. The figures are all for the same fireclay, the only difference between samples being that the exchangeable cation was changed by chemical treatment. TABLE 4. ZETA-POTENTIALS OF DIFFERENT CATIONIC FORMS OF A FIRECLAY Exchangeable cation carried by clay Ca H ++ + Mg Zeta-potential (volts) -001 -002 ++ Na+ -004 -006 T H E EFFECT OF ADDITION OF ELECTROLYTE O N ZETA-POTENTIAL If we have a suspension of a clay with the exchangeable cation hydrogen, then the zeta-potential of the hydrogen clay suspension 68 CERAMIC RAW MATERIALS will be low. If sodium hydroxide is added to the suspension, then sodium will replace hydrogen according to the equation H clay + N a O H ^ N a clay + H a O The sodium clay so formed will have a high zeta-potential, and so the zeta-potential of the clay suspension will rise as sodium hydroxide is added. When all the exchangeable hydrogen has been replaced by sodium the zeta-potential will be at its maximum value. This situation will occur when the cation exchange capacity ZetaPotential (Volts) Electrolyte added (m.e. Na per 100g clay) FIG. 24. Effect on zeta-potential of a hydrogen clay suspension, of additions of NaOH and NaCI. value of sodium hydroxide has been added. If now further sodium hydroxide is added it cannot be adsorbed by the clay, for the clay is saturated with sodium ions. As the sodium ion concentration is increased, these will now crowd the double layer and reduce the negative potential, i.e. " d " will be reduced. Reduction in " d " will result in a decrease in zeta-potential, so addition of excess sodium hydroxide will lead to a reduction in zeta-potential (see Fig. 24). This effect is well known, and is called "overdoping". The effect can be more easily demonstrated by making small additions of a deflocculant, such as sodium carbonate, to a concentrated clay suspension, and measuring the fluidity of the suspension after each addition with a torsion viscometer. The fluidity first increases 69 PROPERTIES OF CERAMIC RAW MATERIALS as the clay becomes deflocculated, but a point is reached where further addition of the deflocculant decreases the fluidity due to "overdoping". Another way of looking at "overdoping" is by way of the common ion effect, which is well known in chemistry. An example of the effect can be easily demonstrated by adding concentrated hydrochloric acid to a concentrated solution of sodium chloride, when solid sodium chloride is precipitated. In solution, sodium chloride ionises to give sodium ions and chloride ions, Η,Ο NaCl — > Na+ + Cl~ The product of the ionic concentrations of sodium and chloride ions has a maximum value (the solubility product) which we may call K, i.e. in any solution of sodium chloride [Na+] [C1"]<K, the square brackets indicating concentration of the ions. When the value of Κ is reached, no more sodium chloride will dissolve, i.e. we have a saturated solution. On adding concentrated hydrochloric acid to a concentrated salt solution we increase the chloride ion concentration [CI"]. This may result in the product [Na+] [ Q - ] exceeding the value of K, and in order to reduce the value of the product to K, unionised NaCl must be precipitated. The ionisation of the sodium chloride is said to be suppressed by the chloride ion of the acid which is common to both the acid and the salt We have said that high zeta-potential in a clay suspension results from high ionisation of the exchangeable cation. Addition of excess sodium hydroxide to a sodium clay may be regarded as suppressing the ionisation of the clay, i.e. it makes the exchangeable sodium ions behave as though they were less electropositive. We can write the effect, HaO + N a clay — > N a + c l a y Na+ + clay- + N a O H ^ N a clay + Na+ + OH~ excess unionised Thus the sodium clay is made to behave more like a calcium or hydrogen clay by suppressing its ionisation, the zeta-potential is 70 CERAMIC RAW MATERIALS reduced, the particles agglomerate and the viscosity of the suspension is increased. If instead of adding sodium hydroxide to the hydrogen clay we were to add sodium chloride, then the zeta-potential would drop from the outset. Because the exchangeable hydrogen cannot be removed in an unionisable form, sodium does not replace hydrogen on the clay, and even the first additions of NaCI serve only to "overcrowd" the double layer and reduce zeta-potential (see Fig. 24), i.e. the reaction H clay + NaCI ^ N a clay + HCl % H+ + C i liés to the left, since the hydrogen ions produced on the right of the equation can compete with sodium for the exchange positions on the clay. It is true that if enough sodium chloride is added so that the sodium ion concentration is increased, hydrogen will eventually be replaced by sodium. However, at the high concentration of sodium chloride required, the "overcrowding" will be so great that the zeta-potential will not be increased, nor the viscosity of the suspension lowered. PROTECTIVE COLLOIDS Some deflocculants, notably sodium silicate and Calgon, which produce large complex anions in solution can deflocculate a clay, and further additions will not lead to the "overdoping" effect found with simpler deflocculants. The large colloidal anions of these deflocculants are said to enter into the double layer and prevent a high concentration build-up of cations in the double layer which would otherwise lead to "overdoping". As such a deflocculant is added to a clay suspension, fluidity increases to a maximum which is maintained for further additions of the deflocculant. The large anions responsible for the prevention of "overdoping" are called protective colloids since they make the suspension insensitive to the presence of excess deflocculant. 77 PROPERTIES OF CERAMIC RAW MATERIALS Some clays like ball clays which contain organic material may show the effect even when a simple deflocculant like sodium carbonate is added. The deflocculant can react with the organic material to give a complex organic anion which can then act as a protective colloid. 5.1.13. Practical Uses of the Cation Exchange Properties of Clays The vast changes in the physical properties of a clay which occur when a flocculating exchangeable cation (H, Ca, etc.) is replaced by a deflocculating cation (Na, K) or vice versa, are made use of in ceramic practice. The most important application is in slip casting, where deflocculation reduces the viscosity of the suspension enabling it to be used at high solids concentration and workable fluidity. Adjustment of fluidity and thixotropy by deflocculant addition allows casting-slips to be controlled, giving the required characteristics. Flocculation of a suspension can also be useful. When applying glaze to biscuit vitreous ware by hand dipping for example, the amount of glaze picked up by the biscuit depends mainly on the viscosity of the glaze suspension. In order to obtain a sufficiently thick glaze covering, it is frequently necessary to flocculate the glaze suspension, and thereby increase the viscosity and glaze pick-up. This can be done by addition of hydrochloric acid or calcium chloride. Filter pressing can be carried out more quickly with a flocculated suspension than with a deflocculated suspension, since the filter cakes formed are more open-packed, and therefore more permeable to water. Where it is necessary to keep coarse material (e.g. "grog") suspended in a clay containing suspension, flocculation may so increase the suspension viscosity that settling out is prevented. On the other hand, flocculation of a dilute suspension of fine particles will cause coagulation of particles and will increase the rate of settling. 72 CERAMIC RAW MATERIALS Ion exchange in clays is not limited to inorganic cations, organic cations can be exchanged onto clays producing organo-clays with interesting and useful properties. Montmorillonites saturated with organic amine cations become organophilic, i.e. they will adsorb organic liquids in the same way that the normal clays will adsorb water. Such organo-montmorillonites have been used to adsorb organic effluents. 5.2. Silica Of the elements found in the earth's crust, silicon is second only to oxygen in abundance. It does not occur in nature as the element, but as the oxide SiO a and in a great variety of silicate minerals. Silica occurs as quartzite, ganister and silica sand, in the crypto crystalline forms (i.e. small crystals) of chert, flint and chalcedony, and also in hydrated forms such as opal. All these are essentially of the same chemical composition, although the crystal size and impurity content may vary. In pottery bodies the addition of flint, like that of other nonplastics, reduces the drying shrinkage of the body and also reduces the plasticity. It "opens u p " the body allowing escape of gases during firing without distortion of the ware. During the vitrification period, it combines with the basic oxides of the fluxing ingredients to form a glass which is responsible for the strength of the fired ware. Although quartz and sand can be used, flint has been the most popular form of silica for the manufacture of pottery ware in this country, since its small crystal size makes for easy conversion during firing to another crystal form of silica known as cristobalite. This conversion is an important factor in influencing the properties of the fired product as we shall see. Flint pebbles either from the sea-shore or found in chalk deposits are calcined to make them easier to grind. Calcination is carried out at about 900°C and during this process the pebbles disintegrate into small pieces and the colour is changed from black to white. The calcined flint is then ground in the presence of water to about 55% less than 10 micrometre diameter (as measured by the hydrometer method of grain size determination), and used 73 PROPERTIES OF CERAMIC RAW MATERIALS in the slop form in compounding the body. Grinding to the correct grain size is important, since this also influences the conversion of quartz to cristobalite during firing. INVERSIONS A N D CONVERSIONS OF SILICA There are three main crystalline forms of silica, quartz, tridymite and cristobalite. These three forms all have the same chemical formula SiO«, but differ in the ways in which the silicon and oxygen atoms are arranged in the structure. In cristobalite and tridymite the atoms are less closely packed than in quartz, and hence they show lower specific gravities: Specific gravity Quartz 2-65 Cristobalite 2-32 Tridymite 2-28 Tridymite and cristobalite are the high temperature forms of silica, whilst quartz is the stable form u p to 870°C. The temperature stability range of tridymite is between 870°C and 1470°C, and that of cristobalite between 1470°C and the melting point at 1710°C, above which temperature silica glass or fused quartz, a non-crystalline form of silica, is formed. The changes from one crystalline form to another are very sluggish, and the reverse changes on cooling, i.e. from cristobalite and tridymite back to quartz, are so slow as to be inappreciable. The change from quartz to tridymite is also extremely slow, and may not take place at all without the presence of some impurity. In practice, we must first form cristobalite by heating quartz to 1470°C or more, and then cool down to between 870°C and 1470°C and maintain the temperature until tridymite is formed. It is possible to speed up the formation of cristobalite and tridymite by the addition of small amounts of impurity materials, known as mineralisers. Thus the presence of lime can catalyse the conversion of quartz to cristobalite, whilst iron compounds favour the formation of tridymite. The rate of conversion of one 74 CERAMIC RAW MATERIALS form to another is also increased by increasing the fineness of the material. These changes involving quartz, tridymite and cristobalite are major changes in atomic structure, involving breaking of valency bonds and considerable amounts of energy. These changes are called conversions. In addition to conversions from one major crystalline form to another, each of the three major forms can undergo minor structural changes known as inversions. The inversions involve only a rotation of valency bonds between the silicon and oxygen atoms, only small energy changes are involved, and the essential structures are unaltered. In each case, the lower temperature form is known as the alpha form, and the higher temperature form as the beta form. The main inversions which occur are as follows: 573°C ^ ß-quartz 117°C α-tridymite ^ ßi-tridymite 163°C ßi-tridymite ^ ß a-tridymite 220-280°C a-cristobalite ^ ß-cristobalite α-quartz Although the changes in structure involved in inversions are slight, the different forms show different physical properties, the most important of these in ceramics being the difference in thermal expansion. When α-quartz changes to ß-quartz there is a linear expansion of 0-45 %. The cristobalite change produces an expansion of about 1% and the α-βχ tridymite inversion involves a 0 1 % linear expansion. Unlike conversions, inversions occur almost instantaneously, so at the inversion temperatures size changes occur very quickly. These rapid changes in size can lead to cracking during firing if the temperature is taken up or dropped too quickly through the inversion temperatures during firing. The α-β cristobalite inversion can be used to advantage to prevent crazing (i.e. the cracking of glaze due to failure in tension). During the biscuit fire, ß-quartz is partially converted to ß-cristobalite. On cooling the β form will invert to α-cristobalite at 220°C, 75 PROPERTIES OF CERAMIC RAW MATERIALS accompanied by a contraction. On cooling down from the glost fire, the inversion and contraction will again occur. At the temperature of the inversion the glaze will have solidified, and the contraction of the body on cooling will therefore put the glaze into compression (see Fig. 25). Unfired glaze Heat Biscuit 'V/////////////Ä e Body expands at 220 C \ Λ due to α-|3 Cristobalite inversion vHeat Glaze in compression "*? \ Body contracts due to β - α Cristobalite inversion. Solid glaze is left in compression Cool Cool Glaze solidifies on cooling Glaze melts and flows to cover body Fio. 25. Glaze put into compression by "cristobalite squeeze". This process of putting the glaze into compression is often termed the "cristobalite squeeze". The glaze is capable of withstanding compressive forces very much greater than those required to crack it in tension, thus the glaze is less likely to fail if it is left in compression. Tensile forces on the glaze can result in practice when the ware becomes hot, or through adsorption of moisture into the biscuit (if porous) through unglazed parts or through micro glaze imperfections. This adsorption of water results in expansion of the body (known as moisture expansion) and the glaze layer is put into tension when it is likely to fail and produce crazing. The effectiveness of the "cristobalite squeeze" depends on production of cristobalite in the fire, and this is favoured by adequate firing of the biscuit and the use of finely ground quartz in the body. An important non-crystalline form of silica is fused silica, formed by heating any form to above the melting point and then 76 CERAMIC RAW MATERIALS cooling. Fused silica has a very low and almost linear thermal expansion, which makes it particularly useful where resistance to rapid changes in temperatures is required (i.e. it has good thermal shock resistance). Due to the presence of fluxes, silica present in glazes melts at a much lower temperature than its normal melting point, and forms a glass with the other constituent materials of the glaze. The higher the silica content of the glaze, the more the matured glaze resembles fused silica, and the lower its thermal expansion. As the silica content of the glaze is increased, the refractoriness also increases, so that for a glaze to "fit" a low expansion (thermal shock resisting) body like porcelain, the silica content and the firing temperature of the glaze must be high. Silica in the body composition increases the thermal expansion of the body, since it does not all enter into the glassy phase on firing. In vitreous bodies a high proportion of glass is formed, and these bodies show relatively low thermal expansion, particularly if the flux content is low and they are fired at a high temperature (the body composition then becomes similar to that of the glaze). Vitreous bodies with high flux content, fired at lower temperatures, or porous bodies where the proportion of glass formed in the biscuit is small, show higher expansions. In the latter case much of the silica remains in the crystalline form and in this state has a high expansion. C R A Z I N G A N D PEELING Crazing results from tensile forces set up in the glaze. These may be caused by differences in the thermal contraction of the body and glaze (i.e. a poor body/glaze "fit") or to moisture expansion of a porous body. Other factors which affect craze resistance are the elasticity and mechanical strength of the glaze and body, and the formation of a "buffer layer" between the glaze and body. When the glaze melts during firing, some biscuit is dissolved in the glaze with the result that an intermediate zone is formed which has neither the composition of the body or the glaze, but is somewhere between the two. The properties of this inter-layer are also intermediate between those of the body and 77 PROPERTIES OF CERAMIC RAW MATERIALS the glaze, and it thus tends to "buffer" any effects due to differences in body and glaze properties. Crazing due to differences in thermal expansion between body and glaze can be cured by increasing thermal expansion and contraction of the body and decreasing that of the glaze, thus ensuring that the glaze is in compression. This can be done by ensuring that the body has a sufficiently high silica content, and that quartz is converted as far as possible to cristobalite during the biscuit fire. The silica content of the body may be increased by reducing the clay content or adding more flint or quartz, and the cristobalite conversion assisted by grinding the flint more finely or firing at a slightly higher temperature. Decrease in the thermal expansion of the glaze may be achieved by increasing the silica content of the glaze, or reducing the flux content, either will result in a "harder" glaze, i.e. one which requires a higher maturing temperature. Peeling is a fault associated with too high a compression in the glaze. Since the glaze is much stronger in compression than in tension, the compression must be severe for the fault to occur, and it is therefore less common than crazing. The fault involves the cracking and peeling away of the glaze from the biscuit, and often occurs at particular places on an article, such as at rounded corners. It is cured either by re-design of the shape, or by remedies which are the exact opposite of those for crazing, i.e. lowering the body contraction by using coarser silica or reducing the silica content of the body, or increasing glaze contraction by reducing the silica content or increasing in flux content of the glaze. 5.3. Fluxes A flux is a material which lowers the fusion temperature of the material or mixture to which it is added. It is not strictly possible to divide materials into fluxes and refractory materials, since whether or not a material acts as a flux depends not only on the material, but also on the material to which it is added. Nevertheless, there are certain materials which are generally regarded as fluxes, and these are high in alkali or alkaline earth content. 78 CERAMIC RAW MATERIALS Pure kaolinite A l 2 0 8 - 2 S i 0 2 - 2 H 2 0 is a refractory material. On heating, water is lost at about 450 to 500°C, giving metakaolin as the dehydration product. A l 2 0 8 - 2 S i 0 2 2 H 2 0 -* A l 2 0 8 2SiO. + 2 H 2 0 t kaolinite metakaolin steam Liquid is first formed at 1545°C and the whole does not become molten until 1785°C is reached. Clays are not pure kaolinite, and invariably contain impurities some of which act as fluxes and lower the vitrification temperature. For refractory clay products, clays must be chosen which are low in natural fluxing impurity. Fireclays, for example, contain essentially clay mineral, quartz and hydrous mica (section 5.1.7), plus minor impurities such as organic material, oxides of iron and titanium, carbonates and sulphides. The fireclays found in the Scottish Lowlands contain about 8 5 % clay mineral, 10% hydrous mica and 5 % quartz, whereas the fireclays found in the English Midlands contain about 4 5 % clay mineral, 2 5 % hydrous mica, and 3 0 % quartz. Clearly the Scottish fireclay containing less of the natural fluxing hydrous mica is a more valuable refractory clay than the English fireclay. For refractory products the raw materials are selected to be low in fluxing impurities. For non-refractory products used at room temperature or slightly above, fluxes must be added to the batch composition to reduce the firing temperature required. If fluxes were not added, and the body was fired at its normal temperature, little or no glass would be formed, and the resulting article would be weak. Hard paste porcelain used in the production of laboratory crucibles, basins, etc., may be required to withstand temperatures of 1100-1200°C, so a relatively small amount of flux is added, and the body is fired to a high temperature (about 1400°C). On the other hand, vitreous earthenware used for production of hotel tableware and sanitary ware is not required to withstand high temperatures, so more flux can be added, enabling glass to be formed in the body and provide the necessary strength at a lower firing temperature (about 1150°C). Some clays contain enough natural flux to allow them to be used for their purpose without addition of extra fluxes, e.g. 79 PROPERTIES OF CERAMIC RAW MATERIALS stoneware clays contain natural fluxes in mica, felspar, lime, magnesia and iron oxides, and can be fired at 1200-1300°C to give a vitreous body without addition of further flux. Similarly building bricks are made from clay containing enough natural fluxes to give strength to the bricks when fired at about 1100°C. As we have said, the materials normally regarded as fluxes are rich in alkali or alkaline earth elements. They may be present as impurity in the raw material or more frequently may be deliberately added to the body. Of the fluxes added deliberately, the most important are the felspars, Cornish stone, nepheline syenite, and bone ash (used in bone china bodies). 5.3.1. Felspars These are of essentially the same structure as silica, but some silicon is replaced by aluminium and the resultant negative charge is satisfied by potassium, sodium, calcium or barium. Thus potash felspar or orthoclase is KAlSi 8 O e or K 8 O A l 8 O e - 6 S i O t soda felspar or albite has the similar formula N a A l S i 8 0 8 . If half the silicon in silica is replaced by aluminium, and the negative charge is satisfied by calcium we get lime felspar or anorthite C a A l 8 S i 8 0 8 , and similarly the barium felspar celsian B a A l 8 S i 8 0 8 . None of these are found pure, invariably potash felspar will contain some soda felspar, and iron oxide and quartz are also found in commercial felspars. For use in whiteware bodies the iron content must be low, or colour will be imparted to the fired body. Generally, high soda felspars make for lower vitrification temperatures in a body than high potash felspars, and are more prone to produce warpage during firing, since the liquid formed is of lower viscosity than that formed when potash felspar is used. The flux should be evenly distributed throughout the body, and so must be finely ground before mixing. Felspars are used as fluxes not only in ceramic body compositions, but also in glazes, glasses and porcelain enamels. The chief sources of felspars are Norway, Sweden, Russia, U.S.A. and Canada. 80 CERAMIC RAW MATERIALS 5.3.2. Cornish Stone This represents an intermediate stage in the breakdown of felspar into clay mineral. It consists of felspar, quartz, clay mineral, mica, fluorspar and other minor impurities. Cornish stone is graded on a colour basis, the varieties being known as hard purple, midpurple and white. The purple varieties are richest in felspar and the white variety poorest, hence the hard purple is the strongest flux. The colour is due to fluorspar or calcium fluoride (CaF,) which contains colouring impurity. The use of Cornish stone can involve the emission of fluorine compounds during firing, these are injurious to health, and so the stone used to be treated to remove the fluorspar, the product being known as defluorinated stone. This process unfortunately proved uneconomic and was discontinued. Since no workable deposits of felspar are found in Britain, Cornish stone was used extensively in whiteware bodies in this country, the stone replacing the felspar and some of the clay used in similar bodies in other countries. In recent years, the use of Cornish stone has declined, and more fluxing materials are now imported into this country. 5.3.3. Nepheline Syenite Is a mineral containing nepheline { 3 ( ^ 2 0 · Κ 2 0 ) · 4 Α 1 2 θ 3 · 9 8 Ί Θ 2 } felspar, mica, and other minor constituents including magnetite F e 8 0 4 . It has a higher alkali content than felspar and is thus a more vigorous flux. It has been used in place of felspar in body compositions, allowing lower firing temperatures and greater firing ranges than obtainable with felspar, resulting in less warpage of the ware during firing. It has also been used as a replacement for felspar in glass and porcelain enamel compositions. Nepheline syenite is found in Canada, U.S.A., Norway, India and Russia. 5.3.4. Bone Ash Calcined bone is essentially tricalcium phosphate Cat(P0 4 )i, but also contains some residual organic matter. It is used as a flux in bone china bodies in conjunction with felspar or Cornish stone, and enhances the translucency of the fired body. As well as 81 PROPERTIES OF CERAMIC RAW MATERIALS acting as a flux, the small amount of organic material present increases the rather poor plasticity of the body. Originally only bones of oxen were used, but today the bones of many other animals are calcined for this purpose. The temperature of calcination of the bone is important, since it very much affects the behaviour of the bone; in particular it determines how much organic material is left in the bone, and therefore the amount of plasticity the bone will contribute to the body. Over-calcined bone will have little or no organic material remaining, and will contribute little to the plasticity of the body. On the other hand, bone which is under-calcined may contain so much organic material that frothing may result in the body slip, leading to faults in the product. 5.4. Refractory Materials A refractory material is one which has the ability to withstand high temperature without breaking or deforming. Refractory products are used wherever high temperatures are encountered, and include refractory bricks for furnace linings, tubes for electric furnaces, crucibles, thermocouple sheaths, refractory cements, etc. The more important characteristics which are required of a refractory are: (a) High melting point. (b) Mechanical strength at high temperature. (c) Resistance to chemical attack in the particular situation in which it is used. Of the materials with high melting points, metals are usually expensive and often oxidise at high temperatures. The problem of oxidation can be overcome by using oxides, and many materials used as refractories are oxides, either pure or in combination with other oxides. Refractory materials may be classified according to their chemical nature into: (a) Basic refractories. (b) Neutral refractories. (c) Acid refractories. 82 CERAMIC RAW MATERIALS A basic refractory is one which is stable to alkaline materials, but reacts with, and is therefore attacked by, acids; an acid refractory is stable to acids but is attacked by alkalis, and a neutral refractory is chemically stable to both acids and bases. Table 5 shows the main materials used, together with their approximate melting points. TABLE 5. REFRACTORY MATERIALS—TYPES AND APPROXIMATE MELTING POINTS Material Type Silica SiO a Clay Al 2 0 3 2Si0 2 2H a O \ I Alumina A1 2 0 3 Mullite 3Al 2 0 3 -2Si0 2 Graphite Chromium oxide C r 2 0 3 \ | / Zirconium oxide ZrOa Magnesium oxide MgO Calcium oxide CaO Barium oxide BaO \ 1 | / 5.4.1. Aluminosilicate Refractories Approximate melting point (°C) Acid 1710 1770 (pure kaolinite) Neutral 2050 1810 3700 2430 Basic 2720 2800 2570 1920 Clays are one of the oldest refractories known, and are still much used. Because they become plastic when mixed with water, shaping is much easier than with other refractory materials, and this property allied to their cheapness makes them a popular general purpose refractory material. The refractoriness of an alumino-silicate depends on (a) the ratio of silica to alumina, and (b) the nature and amount of fluxing material present as impurity. Figure 26 shows the phase diagram for mixtures of silica and alumina. Note that small additions of silica to alumina or of alumina to silica result in reduction of the melting point. Addition 83 PROPERTIES OF CERAMIC RAW MATERIALS of 5 % alumina to silica reduces this temperature from 1710°C, the melting point of pure silica, to 1595°C; further additions of alumina then cause an increase in the melting point. The composition 9 5 % S i 0 2 : 5 % A 1 2 0 3 is the composition of the eutectic, i.e. the composition with a minimum melting point. At the composition 7 2 % A l 2 O a : 2 8 % S i 0 2 we have the composition of mullite 3Al,Oa-2Si0 2 , which has a melting point of 1850°C. All mixtures containing less than 7 2 % A 1 2 0 , will form liquid above 1595°C, compositions containing more than 7 2 % A l 2 O e first form liquid at 1850°C or higher temperature. As the A 1 2 0 , content is increased from 7 2 % the temperature at which liquid first forms increases until the melting point of A l 2 O a is reached at a composition of 100% A 1 2 0 8 . 2100 ρ -i 2000 2100 2000 Liquid /Corundum ^ (J 1900 / 1800 „Cristobalite + liquid 2. 1700 Ε liquid 1800 Mullite S + liquid 1700 Corundum + mullite 1600 1600 Cristobalite + mullite Si02 AUOo 100 0 90 10 I , 80 20- I 70 30 I I I I 60 40 50 50 40 60 30 70 I I 20 80 10 90 0 100 % composition FIG. 26. Phase diagram for the silica-alumina system. The silica : alumina ratio of an alumino-silicate is then a good guide to the likely refractoriness of the material, though the melting point indicated by the phase diagram will not be exact, since the alumino-silicates are never pure, and the presence of 84 CERAMIC RAW MATERIALS small amounts of impurity, particularly fluxes, will reduce refractoriness. The true refractoriness of an alumino-silicate can be found by the Seger Cone Test, in which the material to be tested is made TABLE 6. SQUATTING TEMPERATURES AND COMPOSITIONS OF SOME SEGER CONES Seger Approximate cone squatting temp. No. (°C) C omposition (faiolecular parts) AI2O3 SiO* CaO F e 2 0 3 Na a O PbO B,O f 42 2000 1 40 1920 1 0-66 t35 1770 1 2 30 1670 1 6 20 1530 3-9 39 0-7 0-3 — —- — _ 15 1435 21 21 0-7 0-3 — — — — 10 1300 10 10 0-7 0-3 — — — — 1180 0-5 5 0-7 0-3 — — — — 01 1080 0-3 3-95 0-7 0-3 0-2 — — — 05 1000 0-3 3-75 0-7 0-3 0-2 — — — 010 900 0-3 3-50 0-7 0-3 0-2 — — — 015 790 0-6 3-2 — — — 0-5 0-5 1 020 670 0-2 2-4 — — — 0-5 0-5 1 022 600 — 2 — — — 0-5 0-5 1 5a — t Pure kaolinite composition. 85 PROPERTIES OF CERAMIC RAW MATERIALS into a small cone of standard dimensions, and mounted on a refractory base along with several Seger cones of the same dimensions and standard compositions. The cones are heated until the test cone "squats" or bends under its own weight. After cooling the test cone is compared with the standard cones, some of which being less refractory will have squatted at a lower temperature than the test cone, and some more refractory cones will not have squatted. The test material is said to have the pyrometric cone equivalent of the standard cone whose behaviour most resembles the test cone. The numbers, compositions, and squatting temperatures of some standard Seger cones are given in Table 6. There is a whole range of alumino-silicate refractories, varying in composition from pure silica on one hand to pure alumina on the other. The refractoriness decreases as AhO* is increased from 0 to 5 % , then increases with increasing A h O , content. Silica is an acidic oxide, i.e. it is little attacked by acids (except hydrofluoric acid), but is readily attacked by alkalies to form silicates. Hence, as the silica content of an alumino-silicate refractory increases, resistance to alkalies decreases. Although pure silica and alumina refractories are strictly pure oxide refractories, they form the two end-members of the alumino-silicate refractories series, and are so dealt with under this heading. SILICA BRICKS These are usually manufactured from quartzite rock, although ganister is sometimes used. The raw material is crushed and ground, then graded into coarse, medium and fine fractions by sieving. The production mix is made by proportioning these fractions to give an optimum packing density. Water is added to give a "stiff m u d " consistency and about 1-5% of slaked lime and a small amount of sulphite lye is added. The lime produces some glass on firing which acts as a bond, and also aids the conversion of quartz to cristobalite during firing. The purpose of the sulphite lye is to give the bricks some dry-strength before firing. The body has very low plasticity, low dry-strength and low 86 CERAMIC RAW MATERIALS drying shrinkage. The bricks are moulded and dried (usually in a tunnel dryer) at which stage they are fragile and require careful handling. Firing is a long process due to the danger of damage as the α-β quartz inversion temperature is passed, and as the α-β cristobalite inversion temperature is passed in cooling. The bricks are "soaked" or held at the top temperature for some time to ensure maximum conversion to cristobalite. The fired bricks are mechanically strong, and retain their strength at temperatures almost up to the melting point. This is because strength is derived not from a glassy bond, but from interlocking of crystals. Silica bricks are used mainly as a steel plant refractory where they form the roofs of open-hearth furnaces. They are also used in coke ovens and in the roofs of glass furnaces. Their outstanding properties are the ability to withstand loads at high temperatures and their resistance to attack by iron oxide. Failure is usually due to their poor thermal shock resistance. " S U P E R D U T Y " SILICA BRICKS Normal silica bricks contain about 95 or 9 6 % SiO,. The presence of other materials, e.g. small amounts of alumina, reduces refractoriness. By removal of impurities the refractoriness of the material can be increased, and this is done in the production of super duty silica bricks, by washing the crushed rock to remove clay. The main difference between normal and super duty silica bricks is, however, not in their refractoriness, but in their load bearing ability at high temperatures. SEMI-SILICA REFRACTORIES These contain less silica and more alumina than silica bricks, and can be made either from natural mixtures of clay and sand, or by deliberate mixing. They contain about 10-12% A 1 2 0 , and are less refractory than silica bricks, having a softening point of about 1600°C. Their load bearing and abrasion resistance, although good, is inferior to silica brick, but their thermal shock resistance is better and they are cheaper. 87 PROPERTIES OF CERAMIC RAW MATERIALS FIRECLAY REFRACTORIES The fireclay refractories are the most common refractory in general use, because of their cheapness. They cover a range of alumina content of between about 30 to 45 %, the remainder being essentially silica, with small amounts of iron oxide, lime, magnesia and alkalies. Their refractoriness increases as the alumina content increases, but is generally within the range 1600-1750°C. Since fired strength in fireclay refractories is due to a glassy bond, they do not bear loads well at high temperatures, and their maximum working temperature is considerably less than their refractoriness. Resistance to slag attack is reasonably good and thermal shock resistance is better than silica brick, hence they find general use at moderate temperatures and where they are not required to carry a large load. Fireclay goods may be made by semi-dry pressing, plastic extrusion followed by pressing, or by slip casting. Whatever the method of shaping, "grog" is usually added to the mix as ground, pre-fired fireclay (often from faulty fired ware). If the refractoriness is to be improved, high alumina material like bauxite, sillimanite, etc., may also be added to increase the alumina content of the body. The purpose of the "grog" is to open up the plastic mass of clay, making drying easier and reducing drying shrinkage. The grog is usually size graded, and the appropriate amounts of each grade is added. After making and drying, the product is fired to a temperature dependent on the alumina content of the body, about 1200°C for siliceous fireclay bodies up to about 1400°C for those high in alumina. SILLIMANITE REFRACTORIES These are manufactured from sillimanite, kayanite and andalusite, all of formula A l t O , S i O i . The pure material has an alumina content of 6 2 % and a melting point of 1850°C; it can be used as a refractory up to about 1700°C, i.e. its working range is similar to that of silica brick. Refractoriness under load, though good, is not as good as silica brick, but again thermal shock resistance is better. 88 CERAMIC RAW MATERIALS The raw materials are non-plastic, so after crushing, grading and mixing a binder must be added, which may be sulphite lye or clay. Any clay additions must be kept to a minimum, for high refractory purposes, since they increase the silica content and hence reduce the refractoriness of the body. Firing must be carried out up to about 1500°C, and this together with the cost of the raw material makes sillimanite ware more expensive than fireclay. Sillimanite is used where temperature is in excess of that which firebrick will stand and where temperature fluctuations rule out the use of silica refractories, or where its slag resistant properties can be made use of, e.g. in glass melting tanks. M U L L I T E REFRACTORIES Mullite has the formula 3Al,0,-2SiO„ i.e. it contains 7 2 % AliO,, and its refractoriness is 181O-1850°C. It occurs rarely in nature, but is found in many fired ceramic bodies. It forms long, needle-like crystals interlocking of which gives good mechanical strength at high temperatures. Mullite also shows a uniform thermal expansion, and its formation in a body makes for good thermal shock resistance. Mullite bricks are manufactured by fusing high alumina materials like bauxite or diaspore with clay or silica sand in proportions to give the mullite composition. Because of their high refractoriness, load-bearing capacity and slag resistance, mullite refractories are much used as refractories in glass-making plants. A L U M I N O U S REFRACTORIES These are refractories of the fireclay type, but with addition of extra alumina in the form of bauxite, diaspore, e t c , to increase refractoriness. With increasing alumina content the body becomes more refractory, but cost also increases because of the need for higher firing temperatures and cost of the high alumina material. 89 PROPERTIES OF CERAMIC RAW MATERIALS A L U M I N A REFRACTORIES These contain 90 to 100% A1»0 8, and have melting points up to 2050°C, i.e. that of pure alumina. The maximum working temperature lies within the range 1700°C-1900°C depending on the alumina content. Alumina contents in this range can be obtained by adding alumina to materials already high in alumina like kyanite. Small additions of plastic clays like bentonites are sometimes used, allowing making by extrusion or pressing. The pure oxide is obtained by purification and heating of the naturally occurring hydrates, the source material being bauxite. In addition, fired bauxite of lower purity is used in the manufacture of bauxite refractories, in mixture with silica sand in the production of fused mullite, in the preparation of refractory calcium aluminate cements by fusing with lime, and in the preparation of abrasives. Although expensive compared with alumino-silicate refractories, pure alumina is the most used pure oxide refractory because of its high refractoriness, hardness, mechanical strength, thermal shock resistance, resistance to chemical attack and its electrical properties. I N S U L A T I N G REFRACTORIES T o prevent damage to refractories from liquid slag or gas attack or from erosion, and to give them strength, refractories are made dense, i.e. of low porosity. In order to prevent heat loss through refractories by conduction and radiation it is desirable that the material should be of high porosity. High porosity, however, lowers resistance to attack, since slag or gas penetrate the pores; it also reduces resistance to erosion and load bearing capacity. T o overcome these conflicting requirements it is common practice to have a dense refractory at the hot surface to carry load and resist erosion and chemical attack, and to back this with an insulating refractory of high porosity which provides the necessary thermal insulation. Where no attack is likely, hot face insulating 90 CERAMIC RAW MATERIALS refractories can be used at the working face. Porosity can be achieved by: (a) Changing the ratio of coarse : medium : fine in the body. Where low porosity is required the ratio is chosen so that voids between large particles can be filled with smaller particles, i.e. a good packing density is obtained. By controlling the proportions of these fractions some variation in porosity can be obtained. (b) Adding organic material such as sawdust to the mix. This burns out on firing leaving a porous texture. (c) Addition of a foaming agent. Insulating bricks can be manufactured from the refractory materials already discussed, and in addition diatomite insulating bricks are made from the naturally occurring diatomaceous earth or kieselguhr. This is a highly siliceous material derived from the skeletons of microscopic organisms called diatoms. The material is particularly suitable for insulation purposes, because the structure is highly porous and consists of many small, closed pores. 5.4.2. Oxide Refractories Most of these are basic refractories which are used where the presence of alkaline conditions inhibits the use of silica or other acid refractories. Such conditions are met in the linings of steel plant furnaces. These are in contact with basic slags formed by the addition of lime for the purpose of removing phosphorus and sulphur from the iron. MAGNESITE Magnesite is the common name for magnesium carbonate which occurs naturally. In this country it occurs as the mixed carbonate dolomite MgCOa-CaCO,. On heating at above 800°C the carbonate breaks down to the oxide with evolution of carbon dioxide. MgCOs -> MgO + CO, t 91 PROPERTIES OF CERAMIC RAW MATERIALS It is the oxide which is the major component of magnesite refractories, and this may be derived from sources other than the carbonate, e.g. from the mineral brucite Mg(OH)« or from magnesium salts (chloride and sulphate) obtained from sea water. In the extraction from sea water, dolomite is first calcined, then slaked with water to give calcium and magnesium hydroxides. calcine CaCO.MgCO, —* slake CaO + MgO — > C a ( O H ) , + M g ( O H ) , dolomite with water The slaked slurry is then mixed with sea water when soluble magnesium salts are precipitated as the hydroxide by reaction with calcium hydroxide Ca(OH), + MgCl, -+ Mg(OH), | in slaked slurry in sea water + Cad, in solution The calcium salts produced are soluble, whilst the magnesium hydroxide is only sparingly soluble. The magnesium hydroxide is separated by settling followed by filtration of the slurry. The net result of the process is the conversion of dolomite to magnesium hydroxide, calcium being replaced by magnesium from the sea water. Although magnesium oxide is formed at relatively low temperatures from both the carbonate and the hydroxide, it is necessary to fire to at least 1500°C to form "dead burned" magnesite. If lower temperatures are used the oxide tends to réhydrate to the hydroxide, and this rehydration is accompanied by an expansion which can lead to cracking of the formed bricks. Bricks are manufactured in the usual way for non-plastic materials, i.e. the calcined oxide is ground and graded, and the appropriate amounts of each grade are mixed with water and sulphite lye to act as a binder. Bricks are hydraulically pressed, dried and fired to about 1600°C. The product is highly resistant to attack by basic slags, but fails by shear under load and hence cannot be used as a load bearing refractory. Magnesite bricks and cements 92 CERAMIC RAW MATERIALS are used for high temperature furnace linings and in the hearths of steelmaking furnaces. High purity magnesium oxide (periclase) refractories are also manufactured for special purposes, e.g. crucibles for metal purification processes and thermocouple insulation. Shapes can be fabricated from pure fused magnesia by pressing or slip casting methods. DOLOMITE Dolomite refractories are manufactured from the calcined material, i.e. they contain effectively calcium and magnesium oxides. Like magnesite this is a basic refractory, and its uses are similar, i.e. in open hearth and oxygen steelmaking furnaces, electric furnaces, glass tank regenerators, etc. Dolomite refractories have a greater tendency to rehydration than magnesite, even after calcination, and this can lead to disintegration of the fired refractory. T o prevent rehydration before the refractory is put into service, dolomite refractories are often covered with tar. Stabilised dolomite refractories less prone to rehydration are made from dolomite and serpentine. CHROMITE These are neutral refractories and can be used as a "buffer" between acid and basic refractories to prevent mutual attack between them. Chromite refractories are made from naturally occurring ferrous chromite F e O C r a 0 8 . Chromite is a member of II m the spinel group of minerals of general formula M O v M , O , , 11 m where M is a divalent and M a trivalent metal. Chromite refractories consist essentially of the spinel, with alumina, silica and other minor impurities. Manufacture is similar to that of magnesite refractories, except that the initial calcination of the raw material is not necessary. Chromite refractories are chemically inert and therefore resistant to attack, but their thermal shock resistance is poor. 93 PROPERTIES OF CERAMIC RAW MATERIALS CHROME-MAGNESITE Addition of magnesite to chrome ore produces a refractory which has better thermal shock resistance than chromite and better load-bearing capacity than magnesite. Manufacture is again similar to that of magnesite refractories. Chrome-magnesite refractories can replace silica brick in the roofs of steel furnaces, where their load bearing qualities are utilised. Failure of chromemagnesite refractories is commonly due to "iron oxide bursting" where the refractory expands and finally bursts. This is probably due to replacement of chromium by ferric iron in the spinel structure. ZIRCONIA, ZrO, Zirconia occurs to a limited extent in nature, but is usually obtained from the silicate zircon Z r S i 0 4 , which is found in silica sands. The oxide has a melting point of 2700°C, a low thermal conductivity, and is used as a high-temperature refractory. Since it is not wetted by many molten metals it is used to manufacture refractory crucibles. A thin layer of the oxide is often applied to kiln furniture by painting with a zirconia slip. Pure zirconia refractories cannot be made, because of a phase change occurring at about 1000°C. Zirconia is therefore stabilised by the addition of lime which produces a crystal form which does not undergo the inversion. BERYLLIUM OXIDE, BeO This is obtained from the mineral beryl 3BeOAltO»-6SiOi and because of its refractoriness, resistance to chemical attack and good thermal shock resistance, it is used in super refractory applications where its cost is justified. These include rocket and missile components and crucibles for melting refractory metals. It can be formed by hot pressing, dry pressing, extrusion or slip casting, but care is needed in its handling because the material is toxic. 94 CERAMIC 5.4.3. Other Refractory SILICON CARBIDE, RAW MATERIALS Materials SiC Silicon carbide or carborundum is made by heating silica sand and coke. It is a very hard, refractory material which has good load-bearing capacity at high temperatures and is chemically stable, although it oxidises slowly in air at high temperatures. Because of its great hardness, it is much used as an abrasive. Its high thermal conductivity together with its load-bearing capacity make it suitable for use in kiln furniture and refractory bats, and because it is so hard it can also be used where resistance to erosion is important. A further use is as a heating element material in electric furnaces. BORON CARBIDE, B 4C This has a melting point of 2450°C and is even harder than silicon carbide. It is used as an abrasive and in special applications where great erosion resistance combined with refractoriness is required, e.g. in nozzles and bearing liners. It is used also in nuclear reactors in control elements and radiation shields, because of its neutron absorption capacity. GRAPHITE This is a naturally occurring form of carbon which is soft, but very refractory. It is very resistant to chemical attack, but tends to oxidise at high temperatures to C 0 2 and must therefore be protected from oxidising atmospheres at high temperatures. Unlike most ceramic materials it is a good conductor of both heat and electricity. Thermal expansion is low, and thermal shock resistance good. Strength is high, and increases with increasing temperature. This unusual combination of properties accounts for the wide variety of uses to which graphite is put. Because of its refractoriness, low chemical reactivity and strength, it is used in the manufacture of crucibles, moulds for hot pressing, thermocouple sheaths and other refractory applications, whilst its electrical 95 PROPERTIES OF CERAMIC RAW MATERIALS properties make it suitable for electrodes, brushes for electric motors, crucibles for induction heating and many other applications. In addition, it is used as a moderator in nuclear reactors and as a lubricant. Z I R C O N I U M SILICATE, ZIRCON, ZrSiO* Zircon is found in some silica sands, from which it is extracted. It is refractory and has alow and almost linear thermal expansion, hence it shows good thermal shock resistance and is used as a high-temperature kiln refractory, and is also a constituent of chemical and electrical porcelains. It is resistant to attack by molten glasses and glazes and is used in furnace linings where these are melted. Because of zircon's low solubility in molten glasses and glazes, it is used as an opacifier. It is also used as a host lattice in making some colours used in decoration. B O R O N NITRIDE, BN This is prepared by heating elementary boron in an atmosphere of nitrogen or ammonia gas. It is similar to carbon in structure, and gives two forms. One of these resembles graphite, being highly refractory but soft, with a low coefficient of friction ; the other is like diamond, very hard and brittle. Its properties are made use of in the production of crucibles and seals and gaskets for high temperature use. It is a good neutron absorber and has been used as a control rod material for nuclear reactors. Because it is not wetted by molten glass it has been used as a mould lubricant. SILICON NITRIDE, S i 8N 4 This is prepared by heating silicon powder in nitrogen or ammonia atmosphere under pressure. It is hard like silicon carbide, but is more oxidation-resistant. It has excellent thermal shock resistance, high strength, corrosion and abrasion resistance. It is used for wear resistant parts, and is a possible material for use in gas turbine blades. 96 CERAMIC RAW MATERIALS CERMETS If the properties of ceramic materials are compared generally with those of metals, it is found that ceramics have greater hardness and corrosion resistance and can withstand higher temperatures. On the other hand, ceramics lack the ductility and impact resistance of metals and have lower mechanical strength and thermal conductivity. Due to their superiority in the last two properties, metals have much better thermal shock resistance than ceramics, i.e. they do not crack or shatter when the temperature is suddenly changed. If a material could be produced which showed the hardness, high melting point and corrosion resistance of ceramic materials, whilst also possessing the high impact resistance, strength and conductivity of metals, it would clearly be a most useful material. In particular, gas turbine blades and rocket nozzles could be constructed of such material, enabling higher working temperatures to be used than are possible at present. In attempts to obtain these desirable properties, the so-called cermets have been investigated. These are mixtures of metal and ceramic powders which are pressed and sintered in vacuum or inert gas atmosphere to prevent oxidation. Some advance has been made in obtaining these properties with cermets using such systems as metal-metal carbide, chromium-alumina, and cermets based on borides, nitrides and silicides. 5.5. Materials Used in Low Expansion Bodies, Glasses and Glazes Although other properties like strength and elasticity have some effect, the main physical property effecting thermal shock resistance, and the one most easily controlled is thermal expansion. Fired bodies of low thermal expansion are subjected to relatively small stresses when their temperatures are rapidly changed, and hence they show good thermal shock resistance. 97 PROPERTIES OF CERAMIC RAW MATERIALS PYROPHYLLITE A N D TALC CONTAINING BODIES The use of talc often in conjunction with pyrophyllite has produced wall tile and tableware bodies of lower thermal expansion than conventional clay-flint-flux bodies, and these bodies also show a wider firing range. In addition, talc-containing bodies show reduced moisture expansion (and therefore less delayed crazing) and desirable electrical properties (see section 5.9). Low thermal expansion is due to the formation of cordierite 2 M g O - 2 A l 2 0 8 - 5 S i 0 2 in the body. This has a very low thermal expansion and so gives bodies of excellent thermal shock resistance, used for example in the production of oven-to-table ware, where the same container is used both to cook and to serve the food. Bodies high in cordierite suffer from the disadvantage of having a narrow firing range, and due to their low thermal expansions they are difficult to glaze and decorate. T H E U S E O F L I T H I A , Li 02 Lithium oxide can be introduced either as a chemically prepared compound (usually the carbonate), or as one of the naturally occurring minerals lepidolite ( L i F K F A i 2 0 8 - 3 S i 0 2 ) , spodumene ( L i 2 O A l 2 0 8 - 4 S i 0 2 ) , or petalite ( L i 2 O A l 2 0 8 - 8 S i 0 2 ) . When used to partially replace felspar or Cornish stone, lithia acts as a powerful flux giving low alkali content and low thermal expansion at the same maturing temperature in ceramic bodies, glazes, glasses and vitreous enamels. Introduction of small amounts of lithia into alumino silicate refractory bodies can lead to greater thermal shock resistance, whilst whiteware bodies of low, zero, or even negative thermal expansion can be prepared. The advantages of these bodies are that thermal shock resistance is increased in the product, and the firing cycle can be carried out more quickly in manufacture, since the risk of dunting is decreased. Inclusion of lithia in glass, glaze and enamel compositions results in greater fluidity in the molten state, giving better gloss or a reduction of maturing temperature. 98 CERAMIC RAW MATERIALS 5.6. Plaster of Paris Calcined gypsum C a S 0 4 £ H 2 0 is prepared by heating the naturally occurring gypsum C a S 0 4 - 2 H 2 0 . If mixed with water, the hemihydrate rehydrates to the dihydrate, the reaction being accompanied by evolution of heat, expansion and a stiffening of the mass to rigidity. Plaster is used in making models and moulds, since it is cheap and can easily be converted from a powder into a hard mass of the desired shape. The setting time of a plaster-water mixture can be lengthened or shortened by addition of various chemicals to the plaster. Where an original model of an article is to be made prior to making working moulds, it is desirable that the setting time should be extended so that more time is available for modelling; this can be done by making suitable retarder additions to the plaster. In considering the dimensions of these master models, it is necessary to take into account the expansion of the plaster in setting (which can also be adjusted by chemical addition), and the drying and firing contractions of the body to be used. The ratio in which plaster is mixed with water is most important in affecting not only the setting time, but also the hardness and porosity of the set plaster. In general, the lower the moisture content the quicker the set, the stronger the set plaster and the lower its porosity. In the production of original models, the set plaster is required to be hard, strong, and of low porosity. These properties are attained by using specially prepared, retarded plasters and low water : plaster ratios. Working moulds also need to be strong and wear-resistant, but are required to have a fairly high porosity. A compromise must therefore be made between mixes giving great hardness with low porosity and softness with high porosity. The wafer : plaster ratio used for mould making is usually that which will give the minimum permissible porosity. To obtain consistent moulds, it is necessary that the water: plaster ratio should be kept constant, and that the blending time (i.e. the time between first mixing the powdered plaster with water, and pouring the mix at a creamy consistency) should also be maintained constant. The blending operation may be carried 99 PROPERTIES OF CERAMIC RAW MATERIALS out by hand or automatically by machine. To avoid air bubbles due to entrapped air in the plaster, the mix is sometimes de-aired by subjecting it to a vacuum during mixing. After pouring the plaster into the working case (which has the shape of the finished article, and from which the working moulds are made), the plaster is allowed to set before the lubricated case is removed. Before use, the shaped mould must then be dried out when its strength and water absorbing capacity increase. Drying must be done carefully at a temperature less than about 60°C so that only excess water is removed, i.e. not water of crystallisation. During use, either in slip casting or plastic making, plaster moulds absorb water. This water must be removed, or the mould will become saturated and will no longer perform its function. Mould drying is usually done overnight at a gentle heat, and must again be carefully controlled to avoid removal of water of crystallisation which would lead to cracking of the mould. With the water absorbed from plastic body or slip, moulds will absorb deflocculant and soluble salts. Insoluble calcium carbonate and calcium silicate, and soluble sodium sulphate will be formed by reaction between the mould material and the deflocculants, sodium carbonate and sodium silicate. The sodium sulphate is carried by water to the surface of the mould, where the water evaporates and leaves a deposit of sodium sulphate behind (a process known as efflorescence). The use of organic deflocculants can reduce this attack on the mould, and prolong the mould's working life. 5.7. Materials Used in Ceramic Glazes The purpose of applying a glaze to a ceramic article is (a) To provide an impermeable surface to a body which is otherwise porous and permeable. (b) To provide a smooth surface which is easily kept clean. (c) To enhance appearance. (d) To protect underglaze decoration from abrasion or chemical attack or to provide a surface on which a glaze decoration may be applied and shown to best advantage. 700 CERAMIC RAW MATERIALS Glaze is usually applied in aqueous suspension to the biscuit fired ware or to the dried clayware in the case of once fired ceramics. Application may be by hand dipping, or some mechanical means such as spraying. After drying, the ware is fired when the glaze melts and on cooling solidifies into a glass. The composition of the glaze must be such that it "matches" the body for thermal expansion as already discussed (section 5.2). A glaze composition is best expressed in terms of the constituent oxides, although these will not be present as oxides, but as complex silicates or borates in the fired glaze. By convention the oxides present are classified as basic oxides, amphoteric oxides and acidic oxides. A basic oxide is one which will react with an acid to form a salt and water. An acidic oxide is one which will react with a base to form a salt and water, and an amphoteric oxide is one which can react with both acids and bases. Amounts of each oxide are expressed as molecular parts, i.e. parts by w e i g h t s molecular weight of the oxide, and by convention the sum of all the basic oxides is arranged to be unity (Table 7). TABLE Basic oxides Oxide Molecular parts Na aO 01 K 20 01 Amphoteric oxides Oxide A I 2O 3 CaO 0-4 PbO 0-4 7 Molecular parts 0-2 Acidic oxides Oxide Molecular parts B 20 8 0-3 SiO a 3 0 Sum of basic oxides 1 -0 707 PROPERTIES OF CERAMIC RAW MATERIALS If we wished to express the oxides as parts by weight, then the molecular proportions would have to be multiplied by the corresponding molecular weight of the oxide (Table 8). TABLE 8 Oxide Molecular proportion Molecular weight of oxide Parts by weight molecular proportion % χ molecular weight by weight Na aO 01 62 6-2 1-8 K aO 01 94 9.4 2-7 CaO 0-4 56 22-4 6-4 PbO 0-4 223 89-2 25-6 AI2O3 0-2 102 20-4 5-9 B 20 3 0-3 70 21 0 60 Si02 30 60 1800 51-6 Total parts by weight = 348-6 1000 The ingredients are not necessarily introduced into the glaze as the oxides. N a a O , for example, may be introduced as the carbonate N a 2 C O s , or both N a a O and B 2 0 3 may be introduced by using borax N a 2 B 4 0 7 . Alumina would be an expensive ingredient, and A 1 2 0 3 is usually introduced with silica as clay. Silica is introduced as the oxide as flint or sand, or in combination as clay, felspar or lead silicates. FRITTING It is usual for some of the glaze constituents to be heated together to form a glass which is then ground and added to the 702 CERAMJC RAW MATERIALS remainder of the glaze constituents. This pre-fired glassy part of the glaze is known as a frit, and the process of making the glass is called fritting. A glaze may contain one or more frits plus unfritted components. The main reasons for fritting are: (a) To convert water-soluble ingredients to an insoluble form. This is necessary since the glaze is suspended in water and any soluble material would dissolve. (b) Although the lead oxides are only slightly soluble in water, they are more soluble in the acidic gastric juices, and the use of raw lead compounds has resulted in the past in lead poisoning of pottery workers. Fritting lead oxides with silica gives lead silicates which are much less soluble and therefore safer to use. (c) Any gas evolution (e.g. C a C O s -> CaO + C O a t ) can be completed during fritting, with the result that the gas is not evolved during the glost fire and bubbles in the glaze are avoided or reduced. (d) High clay content in a glaze can lead to excessive shrinking during drying, leading to cracking of the glaze layer. A proportion of the clay is therefore fritted, although some is retained unfritted to give good suspension properties to the glaze. A C I D I C OXIDES These may be silica or silica and boric oxide. Increase of the silica content at the expense of boric oxide results in increased maturing temperature of the glaze, hence low firing glazes are usually low in silica, whilst high-temperature glazes are high in silica. Boric oxide is introduced as borax or boric acid, both of which are soluble and require fritting. As well as reducing the maturing temperature of the glaze, boric oxide can be useful in promoting craze-resistance, since small additions of B a 0 8 lower the thermal expansion of a glaze. ALUMINA As we have said, alumina is not introduced as the oxide, but in the combined form as clay, felspar or Cornish stone. The main 703 PROPERTIES OF CERAMIC RAW MATERIALS function of the alumina is to prevent devitrification (i.e. formation of crystals, leading to a milky appearance). The quantity is usually kept low, since alumina increases the viscosity of the glaze at its maturing temperature. BASIC OXIDES These are the fluxing oxides which react with the acidic oxides during firing to form silicates or borates. Lead oxide is now introduced as a lead silicate frit to reduce the chances of lead poisoning. Lead borates are considerably more soluble than the silicates, so it is usually necessary to frit lead and borax components separately, although in some cases lead borosilicate frits can be made. Lead oxide gives brilliance to the glaze and brings up underglaze colours to the best advantage. It also produces fusibility and fluidity in the molten glaze and allows a safe firing range. The lead compounds tend to volatilise seriously above about 1200°C, so high-temperature glaze compositions do not contain lead. Soda and potash may be introduced as felspars, stone or as the carbonates, and in addition N a A O may be introduced as borax. They are used in relatively small proportions as secondary fluxes. Their proportions must be kept low, since their compounds are reactive and would give an unstable glaze if present in too high a proportion. High alkali contents also make for high thermal expansion in glazes, which would make crazing likely. Due to their solubility, the alkali materials are usually incorporated in a frit. Lime is usually introduced as the carbonate, which up to a certain addition acts as a flux in the glaze. Further additions increase the viscosity of the molten glaze and can lead to devitrification due to crystals of calcium silicate. Zinc oxide, added as the oxide, is sometimes used as a flux, particularly in leadless glazes. It also helps opacity, and is a usual ingredient of opaque glazes. OPACIFIERS To achieve opacity in a glaze, the opacifying material must remain undissolved in the glaze after firing; opacifiers are there704 CERAMIC RAW MATERIALS fore chemically unreactive materials. Stannic oxide, SnO a , can be used to produce opaque white glazes, the amount required depending on the composition of the glaze. Glazes high in alkalies are better solvents for the tin oxide, so more opacifier is required than for glazes low in alkalies. Zirconia added as the oxide Z r O a or as zircon Z r S i 0 4 can substitute part or all the stannic oxide, but more is required to give the same effect, and the fired colour is often not as good. Pure titania, T i O a , can also be used, but tends to give buff rather than white glazes since it intensifies the colouring effect of traces of iron present in the glaze. Impure titania is used to produce ivory glazes, and is particularly used in tile glazes where it is responsible for the "break-up" effect caused by crystallisation of the glaze on cooling. SPECIAL DECORATIVE EFFECTS Matt glazes have a rough textured surface, and are produced by addition of CaO, ZnO, BaO, SrO or M g O to the glaze. Either the oxides are converted to the silicates which crystallise out on cooling, or some undissolved material remains after firing; in either case a matt surface results. Crystalline glazes are somewhat similar, the decorative effect depending on the crystallisation of some component, e.g. during cooling of the glaze. Lustre glazes are achieved by the use of colouring metal oxides like copper oxide or iron oxide; these can either be incorporated into the glaze or applied to the surface of a fired glaze. In either case a reducing atmosphere is required during the fire to maintain the metal in its lower valency state, unless a liquid lustre containing carbonaceous material is used. SALT GLAZING This is a method by which stoneware bodies, e.g. sewer pipes, can be glazed by fluxing the surface of the body. N o glaze is applied to the body, but at a temperature of about 1250-1300°C in the biscuit fire, salt is introduced into the kiln. The salt vaporises and reacts with water vapour in the kiln: 2NaCl + H a O > 2HC1 + N a 2 Q 705 PROPERTIES OF CERAMIC RAW MATERIALS The soda then reacts with the surface of the ware, fluxing it and forming a glazed surface which is similar in composition to the body. Because of this similarity, salt glazes are highly crazeresistant. They are also resistant to chemical attack, and serve to make the body impermeable to liquids. 5.8· Colouring Materials Used in Decoration Colouring materials may be added deliberately to body or glaze compositions or applied to the biscuit or the fired glaze. They may also occur naturally as impurities in the raw materials. The most common of the naturally occurring colorants is iron, which may be present as the oxide, hydroxide, carbonate or sulphide. Many clays contain appreciable amounts of iron compounds, and may fire to an ivory, buff or red colour depending on the amount of iron present. Other materials which are themselves not colorants can alter the affectiveness of iron in colouring a fired body. Calcium, magnesium and aluminium oxides reduce the colouring effect of iron, whilst the presence of titanium oxide enhances it. Iron-containing clays are used to produce decorative bodies both in building products and tableware; fine ceramic ware, however, usually has a white body to which specially prepared colours are applied. The higher the temperature which the colouring material must withstand, the smaller the choice of colours available. Decoration applied on-glaze therefore provides the greatest choice, since these can be fired just to the temperature required to mature the colour and "fix" it in the glaze. Decoration applied in the body, in the glaze or on the biscuit surface must withstand higher temperatures, and this limits the choice. On the other hand, such decoration is protected by the glaze from mechanical and chemical attack in use, whereas on-glaze decoration is more prone. A wide variety of methods is available for applying decoration, including hand painting, spraying, stamping, printing, lithography and so on. The materials used in colours are usually calcined before use to remove any gas which may be evolved when the material is heated, 106 CERAMIC RAW MATERIALS and to convert the colorant to an insoluble, unreactive form. These materials are often chemically complex, but they are based on the salts of metals whose general colouring behaviour we can briefly discuss by consideration of the oxides. A N T I M O N Y OXIDE Introduced as lead antimonate gives in-glaze or on-glaze yellows in lead glazes. CHROMIUM OXIDE This gives a wide variety of colours. Used in-glaze it produces greens, whilst the chromâtes of lead and barium give reds and yellows. In the presence of tin, pinks are obtained. Chrome is also a constituent of browns and blacks. C O B A L T OXIDE This produces very strong blue colours due to the formation of cobalt alumino silicate; these are stable even at high temperatures. C O P P E R OXIDE Greens are obtained in most glazes, but in special glazes high in alkalies blues can be produced. In reducing atmospheres in-glaze and on-glaze reds can be obtained. GOLD In the presence of tin, purples and pinks are given. Metallic gold can be applied either in a form which fires bright ("liquid gold"), or mixed with a flux in a form which requires burnishing after firing to bring up the lustre. 707 PROPERTIES OF CERAMIC RAW MATERIALS I R O N OXIDE In-glaze yellows and browns are produced, and in reducing atmospheres greens are obtained. On-glaze reds, browns and blacks also contain iron. M A N G A N E S E DIOXIDE This gives in-glaze browns. In glazes high in alkalies, where manganates are formed, violets or pinks are obtained. It is also a constituent of some browns and blacks. N I C K E L OXIDE In the presence of cobalt, greens are obtained, whereas the presence of zinc oxide produces blues. It is also a constituent of some brown and yellow stains. PLATINUM Like gold, platinum can be applied to give a metallic decoration. The lustre is more stable than that of silver and does not tarnish. T I T A N I U M DIOXIDE This is used in conjunction with iron to produce yellows and browns, and is responsible for the crystalline "break-up" effect obtained on wall tiles. U R A N I U M OXIDE Introduced as the oxide or as sodium uranate it gives in-glaze yellows, oranges and blacks. In high lead glazes red crystals are produced. 705 CERAMIC RAW MATERIALS 5.9. Materials Used in Electrical Applications INSULATORS Porcelain is the most common ceramic insulating material, and is used for most ordinary insulating purposes because of its high electrical resistance and mechanical strength. Variations on the normal composition of the body may be made for special purposes. M A G N E T I C CERAMICS Ferrites or ferro-spinels are materials having the spinel type structure of F e 3 0 4 with part or all of the ferrous iron replaced by some other divalent metal, such as Ba, Ni, Μ η or Zn. The oxide of these metals is mixed with F e 3 0 4 , shapes being extruded or pressed with the addition of a binder to the body. On firing, ferrous iron can be replaced by the other divalent metal. The materials used must be pure, since small amounts of impurity can seriously affect the properties of the product. The magnetic ferrites are used in electric motors and transformers, and in high-frequency applications where they are more efficient than metals. Both permanent and electromagnets can be made. L O W - L O S S CERAMICS When any insulating material is used, some electrical energy is adsorbed by the insulator and converted into heat energy. This results not only in a loss of electrical energy, but also in heating up of the insulator. The percentage of electrical energy lost in this way is called the power factor. A low power factor is desirable in an electrical insulator, since it indicates low loss of electrical energy. Although the power factor of porcelain is satisfactorily low for low-frequency currents, as the frequency of the current is increased, so the power factor increases, until at high frequencies the loss of electrical energy becomes high, and porcelain is no longer a satisfactory insulator. Insulating materials used in high-frequency electronic equipment such as television, radar, 705 PROPERTIES OF CERAMIC RAW MATERIALS etc., must have low-loss characteristics at high frequencies. Steatite (talc) bodies containing small amounts of clay and flux were found to have low power factors at high frequencies, whilst later zircon porcelains consisting essentially of zirconium silicate and clay, gave even lower power factors. H I G H DIELECTRIC CONSTANT MATERIALS A condenser consists of two metal plates separated by an insulating material. If an electrical charge is put on one plate, providing the insulation between the two plates is good enough, there will be no leakage of current between the plates and the electrical energy is stored in the condenser. The dielectric constant of an insulating material is a measure of its ability to prevent leakage of electricity between the plates; the higher the dielectric constant the better the insulation and the more electrical energy can be stored. As electronic components become smaller and smaller, there is need for materials of higher and higher dielectric constant to allow sufficient storage of electricity. Air has a dielectric constant of unity, and insulators like mica, porcelain and alumina have values of about 5-10, i.e. used as insulating materials in condensers they allow a storage capacity of 5-10 times that which would be obtained if air separated the plates. Rutile bodies consisting almost entirely of titanium dioxide produce insulators with dielectric constants of about 100, whilst values in the thousands are obtained from certain titanates. The titanates important in this field are those of calcium, barium and strontium. Titanate bodies although showing very high dielectric constants tend also to have high power factors at low frequencies. Addition of zirconia to rutile and titanate bodies has been found to reduce the power factor whilst only slightly reducing the dielectric constant. These are non-plastic materials, and the bodies must as usual be plasticised by addition of small amounts of clay or organics. Shaping is done by pressing or extrusion, after which the articles are fired. Again the properties of the product can be seriously affected by small amounts of impurity. 770 Index Ageing 23 Ball clay 23, 24, 51, 59 Beryllia 94 Binders and plasticisers 6, 16, 18, 21 Black cores 38 Body mixing 6-7 preparation 5 Bone ash 81 Bone china 9, 24 Boron carbide 95 Boron nitride 96 Bricks 7, 15 blue engineering 41 silica 86-7 Brogniart's equation 7, 31 Buller's rings 40 Casting slip control of properties 30-5 direct preparation 9 Cation exchange 60 capacity 61 use of 63 Ceramic classification 2 definition 1 Cermets 97 Chlorites 57 Chromite 93 Clays ball 22, 23, 24, 51, 59 brick 52 china 22, 51 fireclays 52, 57, 59, 88 isomorphous substitution in 60 kaolinitic 42, 50-2 plasticity of 57 residual and sedimentary 43 structure of 4 3 - 5 0 Clay-water system 63 Colours 106-8 Cones 40 Cornish stone 81 Cracking 13 Critical moisture content 28, 29 Deflocculation (and flocculation) 6, 9, 12, 22, 31, 32, 35, 61-5, 71 Dickite 50 Disorder 59 Dolomite 93 Double layer theory 65 Dry mixing 5 Dry pressing 15, 29 Dry strength 21, 24, 25 Drying shrinkage 13, 15, 23, 25-30 Edge-to-face particle packing 34 Electrical applications of ceramics 109-10 Electro-kinetic potential 66-70 Extrusion 15 Face-to-face particle packing Felspars 42, 80 Ferrites 109 34 111 INDEX Filter pressing 6, 8 Firing 19 reactions occurring during 37 41 Flame spraying 18 Fluidity 33, 35 Fluxes 19, 38, 39, 40, 78-82 Gibbsite layer 4 5 - 6 Glazes compositions 101 compression in 76 crazing and peeling in 77-8 decorative 105 fritting 102 heavy clay ware 5 low expansion 97-8 materials used in 100-6 opacifiers in 104 salt 105 Granite 42 Graphite 95 Halloysite 50 Holdcroft bars 40 Hot pressing 18, 29 Mites 57 Isomorphous substitution Isostatic pressing 16, 29 Jollying and jiggering 98 Magnesite 91 Making methods 112 10-19 58, 61 Nacrite 50 Nepheline syenite 81 Non-clay bodies 5, 6, 12, 16, 31 Nontronite 59 Opacifiers 104-5 Orton cones 40 Overdoping 36, 69, 71 Plaster of Paris 99-100 moulds 11,30 Plasticity 6, 7, 12, 13, 21-3 Plastic making methods 12-15 Porcelain 41, 109 Porosity 39, 41 Power factor 109 Pressing 15 Protective colloids 70 Pugging 7, 15, 23 Pyrophyllite 53, 98 Pyroscopes 40 14 Kaolinite action of heat on 37 geological formation 42 structure 43-50 Kiln atmosphere 37, 41 Lithia 58 Margarite 56 Metakaolin 37 Micas 55-6 Moisture expansion 76 Montmorillonite 23, 24, 55 isomorphous substitution in structure of 52 Mullite 38, 89 Muscovite 56 Red hearts 38 Refractories 82-97 acid 82 alumina 90 alumino-silicate 83 aluminous 89 basic 82 fireclay 88 insulating 90 mullite 89 neutral 82 oxide 9 1 - 4 silica 86-7 INDEX sillimanite 88 Roller machine 15 Sanitary ware 9, 28, 29 Segar cone test 40, 85 Semi-dry pressing 15 Silica 73-8 bricks 86-7 Silicon carbide 95 Silicon nitride 96 Sillimanite 88 Sintering 39 Slip casting 8, 10-12, 25, 26, 30-6 mixing 7 Soluble salts 6, 8, 35 Spray drying 9, 16 Talc 53, 98 Tempering 6, 7, 23 Thermal shock resistance 18 Thixotropy 31, 32, 33, 35 Throwing 13 Titanates 110 Torsion viscometer 33 Van der Waals' forces Vermiculites 57 Viscosity 31 Wall tiles 48, 55, 64 9, 15 Zeta potential 66-70 Zircon 44, 96 Zirconia 44, 96 113