Alfred B. Searle

“ A plastic substance is one with the characteristics of 'a fluid of so great a viscosity that it does not lose its shape under the influence of gravitation.'
The causes of plasticity appear to be somewhat numerous, though there is no generally accepted explanation of this remarkable quality which distinguishes clays from most other substances. It is true that wet sand, soap, wax, lead and some other materials possess a certain amount of plasticity, but not to anything like the same extent as clay. ”

“ Moreover, the fact that the purest 'clay' which can be isolated from plastic clays appears to be amorphous and to some extent colloidal greatly increases the difficulty of obtaining it in a pure state, especially as no liquid is known which will dissolve it without decomposing it. The fact that it is not an elementary substance, but a complex compound of silica, alumina and the elements of water, also increases the intricacy of the problem, for these substances occur in other combinations in a variety of other minerals which are clearly distinct from clay. ”

“ Examined under a microscope, clays are seen to consist of grains of a variety of sizes, the largest of which will usually be found to be composed of adventitious materials such as sand, quartz, felspar, mica, chalk and limestone. The smallest particles—to which clays owe their chief characteristics—are so minute as to make any examination of their shape very difficult, but they are usually composed of minute crystalline plates together with a much larger proportion of apparently amorphous material. ”

“ The extent to which a clay can thus bind other materials together into a plastic mass depends, apparently, on the plasticity of the clay itself and on the size and nature of the particles of the added material; the more plastic the clay the larger will be the amount of material it can thus 'bind,' and the finer the latter the more easily will it form a strong material when mixed with a plastic clay. ”

“ The thoroughness with which the methods of physical chemistry have been applied to geological and mineralogical problems during recent years has been of very great assistance to the student of clay problems, as will be seen on studying some of the works mentioned in the short bibliography at the end of the present volume. When the principles of hydrolysis, ionization, mass reaction and reactional velocity have been applied in still further detail to the study of clays, our knowledge of their natural history will increase even more rapidly than it has done during the past few years. ”

“ It is important not to overlook the enormous part played by variations in the level of the land relative to that of the ocean in past ages. For instance, there is abundant evidence to show that practically the whole of Great Britain has been repeatedly submerged to great depths and has been raised to heights far greater than its present average. These oft-repeated risings and settlings have caused great changes in the nature of the deposited materials so that in the Coal Measures, for example, there are deposits of obviously fresh-water origin sandwiched in between others undoubtedly marine. ”

“ If the heating is stopped before the fusion has begun, the clay will be porous and comparatively soft, but as more and more material fuses, the mass (on cooling) becomes harder and less porous, as the fused material occupies the pores and sets to a dense, firm glassy mass. The amount of vitrification, or partial fusion, which occurs is, therefore, of great importance in some industries, as by stopping it at an appropriate stage articles of any desired degree of porosity, translucency or strength may be obtained. ”

“ Brick clays are those which are not suitable—either from nature or situation—for the manufacture of pottery or porcelain and yet possess sufficient plasticity to enable them to be made into bricks. The term is used somewhat loosely, and geologists not infrequently apply it to clays which are quite unsuitable for brickmaking on account of excessive shrinkage and the absence of any suitable non-plastic medium. Large portions of the 'London clay' are of this nature and can only be regarded as of use to brick- and roofing-tile-manufacturers when the associated Bagshot sands are readily accessible. ”

“ The manufacturer of china-ware and porcelain requires china clay or kaolin which, in addition to the foregoing characteristics, shall be highly refractory. It must, therefore, be free from more than about 2 per cent. of lime, magnesia, soda, potash, titanic acid and other fluxes.
It is a mistake to suppose that all white clays of slight plasticity are china clays or kaolins. Some pipe clays have these characteristics, but they contain so large a proportion of impurities as to be useless for the purposes for which china clay is employed and are consequently of small value. ”

“ It is therefore essential that, for the manufacture of stoneware, [115] a clay should contain a large proportion of refractory material which will form a 'skeleton,' the interstices of which will be filled by the more fusible silicates produced by the firing.
It is generally found that of all the fluxes present in vitrifiable clays, soda and potash compounds—the so-called 'alkalies'—and all lime compounds are the most detrimental, as in association with clay they form a material with a very short range of vitrification. Magnesia, on the contrary, accompanies a long vitrification range. ”

“ To take full advantage of the refractory qualities of a clay it is necessary to select it with skill, prepare and mould it with care, to burn it slowly and steadily, to finish the heating at a sufficiently high temperature and to cool the ware slowly.
Rapidly heated fireclay is seldom so resistant to heat under commercial conditions as that which has been more steadily fired. Rapid or irregular heating causes an irregular formation and distribution of the fused material during the process of vitrification (p. 37) and so produces goods which are too tender to be durable. ”

“ Whatever may be the legal significance of the term 'mineral'—which has an important economic bearing on account of minerals being taxed or 'reserved' in some instances where non-minerals (including brick clay) are exempt—there can be no doubt that, scientifically, clay is not a mineral but a rock. ”

“ The variety of silicates and other minerals which—in a partially decomposed condition—go to form 'clays' is so great that the complete separation of the smallest particles of them from those of the true clay present has never been accomplished and our knowledge of the mineralogical constitution of many of the best known clays is far from complete.
It is highly probable that the action of water does not cease with the formation of clay, but that it slowly [82] effects an increase in the plasticity of the clay. ”

“ Clay is a popular term for a variety of substances of very varied origins, of great dissimilarity in their composition and in many of their chemical and physical properties, and differing greatly in almost every conceivable respect. ”

“ It should be noted that the term 'ball clay' is used for clays of widely differing characteristics though all obtained from one geological formation; when ordering it is necessary to state the purpose for which the clay is required or an entirely unsuitable material may be supplied. For the same reason, great care is needed in any endeavour to sell a ball clay from an hitherto unworked deposit. ”

“ The red oxide (ferric oxide) is the most useful form in burned clay, but in the raw material ferrous oxide and ferrous carbonate may also occur, though they are converted into the red oxide on heating. The red iron oxide, which is closely related to 'iron rust,' occurs in so finely divided a state that its particles appear to be almost as small as those of the finest clays. Hence attempts to improve the colour of terra-cotta and bricks by the addition of commercial 'iron oxide' are seldom satisfactory, the finest material obtainable being far coarser than that occurring in clays. ”

“ The proportion of metal in an ore or of hydrocarbon in a coal can be ascertained without serious difficulty by some means of analysis, but with clay the difficulties are so great that, to some extent at least, they must be regarded as being, for the present, insurmountable. ”

“ As ordinary clays are never quite free from metallic [156] compounds other than alumina, this formation of a fused portion—technically known as vitrification (p. 37) —occurs at temperatures depending on the nature of the materials present, so that a wide range of products is obtained, the series commencing with the entirely unfused pure clay (china clay) , passing through the slightly vitrified fireclays, the more completely vitrified ball clays to the vitrifiable stoneware clays and ending with materials so rich in easily fusible matter as scarcely to be worthy of the name of clays. ”

“ Chalk is added to clay in the manufacture of malm-bricks to produce a more pleasing colour than would be obtained from the clay alone, to reduce the shrinkage of the clay to convenient limits and, less frequently, to form a more vitrifiable material. Chalk, on heating, combines with iron oxide and clay, forming a white silicate, so that some clays which would, alone, form a red brick, will, if mixed with chalk, form a white one. ”

“ Lime and magnesia compounds may occur as silicates (varieties of felspar, mica, etc.) , but their most important occurrence is as chalk or limestone. Chalk is a constant constituent of malms [2] and of many marls, but the latter may contain limestone particles. Limestone occurs in many marls and to a smaller extent in other clays. In the boulder clays it frequently forms a large portion of the stony material. If the grains are very small (as in chalk) , the lime compounds [11] act as a flux, reducing the heat-resisting power of the clay and increasing the amount of vitrification ”

“ From a consideration of a classification such as the foregoing, together with a detailed study of the physical and chemical properties of the material as a whole, and also of the various portions into which it may be divided—particularly that which has been isolated by mechanical methods of purification and separation—it is not difficult to gain a fairly accurate idea of the nature of any clay. ”

“ Providing a clay has a sufficiently long vitrification range, a suitable colour when burned, and that it is capable of being readily formed into the desired shapes, its composition and origin are of small importance to the stoneware manufacturer. In actual practice, however, the number of sources of good stoneware clay is distinctly limited, and many manufacturers are thus compelled to add suitable fluxes to refractory clays [116] in order to meet some of their customers' requirements. For this purpose a mixture of fireclay with finely powdered felspar or Cornish stone is used. ”

“ So long as the adventitious materials consist chiefly of silica and [127] chalk and the mixture is sufficiently plastic to make strong bricks, it may be used satisfactorily in spite of its low content of clay, but if the so-called 'brick clay' contains limestone, either in large grains or nodules, it will be liable to burst the bricks or to produce unsightly 'blow-holes' on their surfaces. If too much sand or other non-plastic material is present, the resulting bricks will be too weak to be satisfactory. ”

“ The Chemical Properties of Clay.
The chief constituents of all clays are alumina and silica, the latter being always in excess of the former. These two oxides are, apparently, combined to form a hydro-alumino-silicate or alumino-silicic acid corresponding to the formula H4Al2Si2O9 [1] , but many clays contain a much larger proportion of silica than is required to form this compound, and other alumino-silicates also occur in them in varying proportions (see Chapters V and VI) . ”

“ If the clay contains much ferric oxide it will produce red or brown bricks according to the temperature reached in the kiln, but if much chalk is also present (or is added purposely) a combined lime-iron-silicate is produced and the bricks will be white in colour. If only a small percentage of ferric oxide is present a clay will produce buff bricks, which will be spotted with minute black specks or larger masses of a greyish black slag if pyrites are also present or if ferrous silicate has been produced by the reduction of the iron compounds and their subsequent combination with silica. ”

“ Shales are rocks which have been subjected to considerable pressure subsequent to their deposition and are, consequently, laminated and more readily split in one direction than in others. Some shales are almost entirely composed of silica or calcareous matter, but many others are rich in clay, the term referring to physical structure and not to chemical composition. The clay-shales occur chiefly in the Silurian and Carboniferous formations, the latter being more generally used by clayworkers. ”

“ If care is taken to make a microscopical examination identifying the chief impurities present the calculation from the analysis may usually be accepted as sufficiently accurate, but it is very unsatisfactory to assume, as some chemists do, that the alkalies and lime in the clay are all in the form of felspar and that the silica remaining in excess of that required to combine with the alkalies, lime and alumina is free quartz. ”

“ Although clays occur in deposits of almost all geological periods, many of them are of little or no commercial value. This may be due to their situation or to their composition and other characteristics. Thus, a Coal Measure clay is ordinarily quite inaccessible, and to sink a shaft specially to obtain it may be an unprofitable undertaking; if, however, a shaft is sunk for coal the clays in the neighbourhood of the coal seams are rendered accessible and, usually, a certain amount of such clays is brought to the surface in order to remove it out of the way of the coal miners. ”

“ The colour of burned ware depends upon the iron compounds in the clay—these producing buff, red, brown or black (usually termed 'blue') articles—on the presence of finely divided calcium carbonate (chalk) [20] which can destroy the colouring power of iron compounds and produce white ware, and on the treatment the clay has received in the kiln. A clay which is white when underfired will usually darken in colour if heated to vitrification, and one which burns red in an oxidizing atmosphere may turn blue-grey or black under reducing conditions. ”

“ The natural colour of a clay is no criterion as to its purity, for some of the darkest ball clays produce perfectly white ware on burning, whilst some of the paler clays are useless to the potter on account of the intensity of their colour when they come out of the kiln. The colour of raw clays is largely due to the carbonaceous matter they contain, and as this burns away in the kiln, the final colour of the ware bears no relation whatever to that of the original clay. ”

“ If the transporting stream flows through valleys whose sides are formed of limestone, chalk, sandstone or other materials, these will become mixed with the clay, and to so great an extent has the mixing occurred that very few clays occur in a state even approximating to purity. The majority of clays are contaminated with iron oxide, lime compounds and free silica in such a fine state of division that it is impossible to purify them completely without [50] destroying the nature of the clay. ”

“ Various attempts have been made to ascertain the relationship (if any) between the refractoriness of clays and their chemical composition. If attention is confined strictly to the more refractory clays, some kind of relationship does appear to exist. Thus Richter found that the refractoriness of clay is influenced by certain oxides in the following order: magnesia, lime, ferrous oxide, soda and potash, but this only applies to clays containing less than 3 per cent. ”

“ The effect of adding water to a dry clay is curious. At first the particles in contact with the water become sticky and plastic, and if the proportion of water added is suitable and the mixing is sufficiently thorough a plastic mass will be produced, the characteristics of which will depend on the nature of the clay used. This process of mixing clay with a limited amount of water is known as 'tempering.' The proportion of water required to make a paste of suitable consistency for modelling appears to be constant for each clay. ”

“ The reduction in the volume of commercial articles made of clay and placed in kilns varies greatly. With bricks, terra-cotta and pottery it must not, usually, exceed 40 per cent. or the warping and cracking which occur will be so great as to make the articles useless. The fineness of the particles exercises an important influence on the kiln shrinkage of a clay, and the latter is frequently reduced in commercial clayworking by adding burned clay ground to a coarse powder to the plastic clay before it is used. ”

“ The china clays (p. 104) are not used alone in pottery manufacture as they lack plasticity and cohesion. In the production of china-ware or porcelain they are mixed with a fluxing material such as Cornish stone, pegmatite, or felspar, together with quartz or bone ash. Thus, English china ware is produced from a mixture of approximately equal parts of bone ash, china clay and Cornish stone, whilst felspathic or hard porcelain is made from a mixture of kaolin, felspar and quartz, a little chalk being sometimes added. ”

“ Whatever may be the primary origin of clay, its chief occurrence is in geological formations which have undoubtedly been formed by aqueous action. The materials resulting from the exposure of primary rocks to the action of the elements have been carried away by water—often for long distances—and after undergoing various purifications have been deposited where the speed of the water has been sufficiently reduced. ”

“ Porosity may thus be regarded as the opposite of vitrification; porous goods being relatively light and [40] soft whilst vitrified ones are dense and hard. For some purposes, porosity is an important characteristic: for example, building bricks which are moderately porous are preferable to those which are vitrified. The manufacture of porous blocks for the construction of light, sound-proof partitions, etc. has increased rapidly of late. They are made by adding sawdust or other combustible material to the clay. The added substances burn out on firing the goods in a kiln. ”