Chapter V: Some Clays of Commercial Importance

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.

Again, a clay deposit may be so far removed from human habitations as to make it practically valueless, but if, for any reason, the population of the district in which the clay is situated grows sufficiently, the clay may become of considerable value. It not infrequently happens, therefore, that the commercial importance of a clay deposit is one which fluctuates considerably, yet, in spite of this fact, there are certain kinds of clay which are nearly always of some commercial value. The most important of these are the kaolins (china clays), the pottery and stoneware clays, the refractory clays (fireclays), the brick and terra-cotta clays and shales, and the clays used in the manufacture of Portland cement. The origin and manner in which these clays have been accumulated have been described in the previous chapters; it now remains to indicate their characteristics from the point of view of their commercial value.

=Commercial china clays and kaolins= in the United Kingdom are not simple natural products but, in the state in which they are sold commercially, have all been subjected to a careful treatment with water, followed by a process of sedimentation whereby the bulk of the impurities have been removed. According to the extent to which this treatment has been carried out, they will contain 10 per cent. or more mica and quartz, with little or no tourmaline, felspar and undecomposed granite. In some parts of Europe and America, kaolins are found in a state of sufficient purity to need no treatment of this kind unless they are to be used for the very highest class of wares.

_Magnified 920 Diameters_

_Magnified 220 Diameters_

_Magnified 220 Diameters_

Crystals of Kaolinite

_Magnified 920 Diameters_

Crystals of Secondary Muscovite.

Fig. 17. Kaolinite and Mica. (_After G. Hickling_ (36).)]

Mica is usually the chief impurity as its particles are so small and their density resembles that of the purified china clay more closely than do the other minerals. In commerce the term _china clay_ is almost invariably used to denote the washed material obtained from the 'china clay rock,' but at the pits the word 'clay' is used indiscriminately for the carclazite (p. 78) and for the material obtained from it. As the term 'kaolin' is used indifferently abroad for the crude 'deposit' and for the purified commercial article, it should be understood that the following information relates solely to the substance as usually sold and not to the crude material.

Commercial china clay or kaolin is a soft white or faintly yellowish substance, easily reduced to an extremely fine powder, which when mixed with twice its weight of water will pass completely through a No. 200 sieve. Its specific gravity is 2·65, but the minuteness and nature of its smallest particles and their character are such that it will remain in suspension in water for several days; it thus appears to possess colloidal properties, at any rate so far as the smaller particles are concerned. It is almost infusible, but shows signs of softening at 1880° C. (Seger Cone 39) or at a somewhat lower temperature, according to the proportion of impurities present. When heated with silica or with various metallic oxides it fuses more readily owing to the formation of silicates.

China clays and kaolins are not appreciably affected by dilute acids, but some specimens are partially decomposed by boiling concentrated hydrochloric acid (26) and all are decomposed by boiling sulphuric acid, the alumina being dissolved and the silica liberated in a form easily soluble in solutions of caustic soda or potash. This has led to the conclusion that some kaolins may have been produced by weathering, as the bulk of true kaolinitic clays (such as Cornish china clay) is not affected by boiling hydrochloric acid (p. 81).

Owing to the exceptional minuteness of its particles, it is extremely difficult to ascertain whether pure china clay or kaolin is crystalline or amorphous. Johnson and Blake (21) found that all the specimens they examined 'consisted largely of hexagonal plates' and that in most kaolins 'these plates are abundant--evidently constituting the bulk of the substance.' This observation is contrary to the experience of most investigators, the majority of whom have found the bulk of the material to be amorphous and sponge-like, but a small portion of it to consist of hexagonal or rhombic crystals.

Mellor (22) has proposed the name _clayite_ for this amorphous material, the crystalline portion being termed _kaolinite_ as suggested by Johnson and Blake.

Both kaolinite (crystalline) and clayite (amorphous) yield the same results on analysis and correspond very closely to the formula H4Al2Si2O9 or Al2O3·2SiO2·2H2O, so that it is most probable that they are the same substance in different physical states.

According to Hickling (36) the general impression that the particles of china clay are amorphous is due to the use of microscopes of insufficient power. With an improved instrument, Hickling claims to have identified the 'amorphous' portion of china clay with crystalline kaolinite, the clay particles (fig. 17) being in the form of irregular, curved, hexagonal prisms or in isolated plates. The former show strong transverse cleavages. The index of refraction and that of double refraction agree with those of Anglesea kaolinite crystals, as does the specific gravity.

In spite of their great purity, commercial china clays and kaolins are almost devoid of plasticity, nor can this property be greatly increased by any artificial treatment. This has led to the conclusion that plasticity is not an essential characteristic of the clayite or kaolinite molecules, but is due to physical causes not shown by any investigation of the chemical composition of the material.

In addition to the specially purified kaolins just described, alkaline kaolins, siliceous kaolins and ferruginous kaolins are obtained from less pure rocks and do not undergo so thorough a treatment with water. Some of these varieties are not improbably derived from transported kaolins, as they occur in Tertiary strata, and so bear some resemblance to the white fireclays on the Carboniferous limestone of Staffordshire, Derbyshire and North Wales, though the latter are far more plastic.

To be of value, a china clay or kaolin must be as white as possible and must be free from more than an insignificant percentage of metallic oxides which will produce a colour when the clay is heated to bright redness. If the material is to be used in the manufacture of paper, paint or ultra-marine, these colour-producing oxides are of less importance providing that the clay is sufficiently white in its commercial state.

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.

Users of china clays and kaolins generally find it necessary to carry out a lengthy series of tests before accepting material from a new source, as such a material may possess characteristics not readily shown by ordinary methods of analysis, but which are sufficiently active to make it useless for certain purposes (see p. 143).

=Pottery clays= are, as their name implies, those used in the manufacture of pottery, and comprise the china clays already mentioned (p. 104), the ball clays and the less pure clays used in the manufacture of coarse red ware, flower pots, etc.

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.

The _ball clays_ (p. 64) form the basis of most ordinary pottery, though some china clay is usually added in order to produce a whiter ware. Flint is added to reduce the shrinkage--which would otherwise be inconveniently great--and the strength of the finished ware is increased, its texture is rendered closer and its capability of emitting a ringing sound when struck are produced by the inclusion of Cornish stone or felspar in the mixture. Small quantities of cobalt oxide are also added to improve the whiteness in the better classes of ware.

The ball clays are characterised by their remarkably high plasticity, their fine texture and their freedom from grit. They are by no means so pure as the china clays, and unless carefully selected can only be used for common ware.

The better qualities burn to a vitrified mass of a light brownish tint, but when mixed with the other materials used in earthenware manufacture they should produce a perfectly white ware. The inferior qualities are used for stoneware, drain pipes, etc. 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.

_Coarse pottery clays_[11] are usually found near the surface and whilst they may be derived from any geological formation, those most used in England are of Triassic or Permian origin, though some small potteries use material of other periods, including alluvial or surface clays. These clays are closely allied to those used for brickmaking, but are somewhat finer in texture and more plastic. In some cases they are prepared from brick clays by treating the latter in a wash-mill, the coarser particles being then removed, whilst the finer ones, in the state of a slip or slurry, are run into a settling tank and are there deposited.

[Footnote 11: Coarse pottery has been defined as that made from natural clay without the addition of any material other than sand and water.]

The presence of a considerable proportion of iron oxide results in the formation of red ware, which is necessarily of a porous nature, as the fluxes in the clay are such that they will not permit of its being heated to complete vitrification without loss of shape. To render it impervious the ware is covered with a glaze, usually producing red, brown or black ware (Rockingham ware).

The _stoneware_ or _drain-pipe clays_, are the most important of the _vitrifiable clays_ and owe their value to the fact that they can be readily used for the manufacture of impervious ware without the necessity of employing a glaze. They are, therefore, used in the manufacture of vessels for holding corrosive liquids such as acids and other chemicals, for sanitary appliances, sewerage pipes and in many other instances where an impervious material is required.

Owing to the lime, magnesia, potash and soda they contain, the stoneware clays undergo partial fusion at a much lower temperature than is required by some of the purer clays. The fused portion fills the pores or interstices of the material, making--when cold--a ware of great strength and impermeability.

The chief difficulty experienced in the manufacture of stoneware is the liability of the articles to twist and warp when heated. For this reason it is necessary to burn them very carefully and to select the clays with circumspection. Some clays are quite unsuitable for this branch of pottery manufacture because of the practical impossibility of producing ware which is correct in shape and is free from warping.

What is required are clays in which the partial fusion will commence at a moderate temperature and will continue until all the pores are filled with the fused material without the remaining ingredients being attacked or corroded sufficiently to cause the ware to lose its shape. As the temperature inside a potter's kiln is continually rising, the great tendency is for the production of fused material to take place at an ever-increasing rate, so that the danger of warping becomes greater as the firing nears completion. Some clays commence to vitrify at a moderate temperature and can be heated through a long range of temperature before an appreciable amount of warping occurs; such clays are said to possess a 'long range of vitrification' (p. 38). In other clays the difference between the temperature at which vitrification commences and that at which loss of shape occurs is only a few degrees; such clays are useless for the manufacture of stoneware, as their vitrification range is too short. It is therefore essential that, for the manufacture of stoneware, 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.

The clays used in Great Britain for the manufacture of the best stoneware are the Devonshire and Dorset ball clays, the upper portions of these deposits being used for this purpose as they are somewhat less pure than the lower portions used in the manufacture of white ware. For coarser grades of stoneware, clays of other geological formations are employed, especially where the finished ware may be coloured, as the purity of the clay is of less importance. 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 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. Chalk--which is a cheaper and more powerful flux--or powdered glass cannot be used as the range of vitrification of the mixture would be too short.

Some manufacturers take the opposite course and add fireclay, flint, or other refractory material to a readily fusible clay. This is satisfactory if the latter clay is relatively low in lime and owes its fusibility to potash, soda or magnesia in the form of mica or felspar. The mica and felspar grains enter so slowly into combination with the clay that a long range of vitrification occurs, whereas with lime, or with some other soda and potash compounds, the combination occurs with great rapidity and the shape of the ware is spoiled.

The =refractory clays= are commonly known as _fireclays_ on account of their resistance to heat. The china clays and kaolins are also refractory, but are too expensive and are not sufficiently plastic to be used commercially in the same manner as fireclays, except to a very limited extent, though bricks have been made for many years from the inferior portions of china clay rock at Tregoning Hill in Cornwall.

The geological occurrence of the fireclays of the Coal Measures has already been described on p. 53. In addition, there are the refractory clays occurring in pockets or depressions in the Mountain Limestone of North Wales, Staffordshire, Derbyshire and Ireland, which consist of siliceous clays and sands, the insoluble residue of the local dissolution of the limestone, intermixed with the débris of the overlying Millstone Grit (see p. 54). These clays and sands can be mixed to produce bricks of remarkably low shrinkage, but the pockets are only large enough to enable comparatively small works to be erected and the clays are so irregular both in composition and distribution as to render their use somewhat speculative.

A third type of refractory clay--termed _flint clay_--is used in large quantities in the United States, but is seldom found in Great Britain. When moistened, flint clays do not soften, but remain hard and flint-like with a smooth shell-like fracture. For use they are ground extremely fine, but even then they develop little plasticity. They are considered by Ries (6) to have been formed by solution and re-precipitation of the clay subsequent to its primary formation, in a manner similar to flint. They are somewhat rich in alumina and many contain crystals of pholerite (Al2O3·2SiO2·3H2O).

The Coal Measure fireclays (p. 53)--which are by far the most important--are divided into two sections by the coal seams, those above the coal being shaly and fissile in structure whilst those below (_underclays_) are without any distinct lamination. Both these clays may be equally refractory, but the underclays are those to which the term fireclay is usually applied. The lowest portions are usually more silicious and in some areas are so rich in silica as to be more appropriately termed silica rock or _ganister_. Fireclays may, in fact, be looked upon as a special term for the grey clays of the Coal Measures, interstratified with and generally in close proximity to the seams of coal. They are known locally as _clunches_ and _underclays_ and were at one time supposed to represent the soil that produced the vegetation from which the coal was formed, but are now considered by many authorities to be of estuarine origin.

It is important to notice that whilst the coals almost invariably occur in association with underclays, some fireclays are found at a considerable distance from coal.

The fireclays of the Coal Measures have a composition varying within comparatively wide limits even in contiguous strata; those chiefly used having an average of 20 to 30 per cent. of alumina and 50 to 70 per cent. of silica. They appear to consist of a mixture of clay and quartz with a small proportion of other minerals, but in some of them a portion of the clay is replaced by halloysite--another hydro-alumino-silicate with the formula

H6Al2Si2O10 or Al2O3.2SiO2.3H2O.

Their grey colour is largely due to vegetable (carbonaceous) matter and to iron compounds. The latter--usually in the form of pyrites--is detrimental to the quality of the goods as it forms a readily fusible slag. Unlike the iron in red-burning clays it can seldom be completely oxidized and so rendered harmless. The fireclays must therefore be carefully selected by the miners.

On the Continent, and to a much smaller extent in Great Britain, refractory articles are made from mixtures of grog or burned fireclay with just sufficient raw clay to form a mass of the required strength. For this purpose a highly plastic, refractory clay is required and the Tertiary ball clays of Devon and Dorset (p. 64) are particularly suitable.

The most important characteristics of a fireclay are that it shall be able to resist any temperature to which it may be exposed and that the articles into which it is made shall not be affected by rapid changes in temperature. Other characteristics of importance in some industries are the resistance to corrosive action of slags and vapours, to cutting and abrasion by dust in flue-gases or by the implements used in cleaning the fires. For those purposes it is necessary that a fireclay should possess high infusibility (p. 32), a low burning shrinkage (p. 29) and a high degree of refractoriness (p. 34), and before it is used these characteristics should be ascertained by means of definite tests, as they cannot be determined by inspection of a sample or from a study of its chemical analysis.

Several grades of fireclay have long been recognized on the Continent and in the United States of America, but the recent Specification of the Institution of Gas Engineers is the only official recognition in Great Britain of definite grades. This specification defines as No. 1 grade a fireclay which shows no signs of fusion when heated to 1670° C. or Cone 30 at the rate of 10° C. per minute, and as No. 2 grade fireclay those which show no signs of fusion when similarly heated to 1580° C. or Cone 26.

It is regarded as a sign of fusion if a test piece with sharp angles loses its angularity after heating to a predetermined temperature (see p. 35).

It is customary to regard as 'fireclay' all clays which, when formed into the shape of a Seger Cone (fig. 6) do not bend on heating slowly until a temperature of 1580° C. (Cone 26) is reached. Any clays comprised within this definition and yet not sufficiently refractory to be of the No. 2 grade just mentioned may be regarded as No. 3 grade fireclays. Many of the last named are well suited for the manufacture of blocks for domestic fireplaces, for glazed bricks and for firebricks not intended to resist furnace temperatures.

To resist sudden changes in temperature the material must be very porous--the article being capable of absorbing at least one-sixth of its volume of water. For this reason it is customary to mix fireclays with a large proportion of non-plastic material of a somewhat coarse texture, the substance most generally employed being fireclay which has been previously burned and then crushed. This material is known as _grog_ or _chamotte_ and has the advantage over other substances of not affecting the composition of the fireclay to which it is added, whilst greatly increasing its technical usefulness. The addition of grog also reduces the shrinkage of the clay during drying and ensures a sounder article being produced.

The most serious impurities in refractory clays are lime, magnesia, soda, potash and titanium and their compounds as they lower the refractoriness of the material. Iron, in the state of ferric oxide is of less importance, but pyrites and all ferrous compounds are particularly objectionable. Pyritic and calcareous nodules may, to a large extent, be removed by picking, and by throwing away lumps in which they are seen to occur. There is, at present, no other means of removing them.

Fireclays may be ground directly they come from the mine, but it is usually better to expose them to the action of the weather as this effects various chemical and physical changes within the material, which improves its quality as well as reduces the power required to crush it.

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. It is, therefore, necessary to exercise great care in the firing.

=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.

Clay-shales are valued according to (_a_) the proportion of oil which can be distilled from them, those rich in this respect being termed _oil shales_; (_b_) the colour when burned, as in _brickmaking and terra-cotta shales_; (_c_) the refractoriness, as in _fireclay shales_ and (_d_) the facility with which they are decomposed on exposure or on heating and form sulphuric acid as in _alum shales_.

_Oil shales_ contain so much carbonaceous matter that on distillation at a low red heat they yield commercially remunerative quantities of a crude oil termed _shale tar_. In composition they are intermediate between cannel coal and a purely mineral shale. To be of value they should not yield less than 30 gallons of crude oil per ton of shale, with ammonia and illuminating gas as by-products. They are of Silurian, Carboniferous or Oolitic origin, the Kimeridge shale associated with the last-named being very valuable in this respect.

The most important oil shales occur in Scotland.

The _fireclay shales_ have already been described on pages 53 and 116.

The _brickmaking shales_ are those which are sufficiently rich in clay to form a plastic paste when ground and mixed with water. They can be made into bricks of excellent colour and great strength, but for this purpose require the use of powerful crushing and mixing machinery. They are usually converted into a stiff paste of only moderate plasticity and are then moulded by machinery in specially designed presses, though some firebricks are made from crushed shale mixed into a soft paste with water and afterwards moulded by hand. Some shales, such as the _knotts_ at Fletton near Peterborough are not made into a paste, the moist powdered shale being pressed into bricks by very powerful machinery.

Brickmaking shales may be found in any of the older geological formations, though they occur chiefly in the Silurian, Permian, Carboniferous and Jurassic systems. The purer shales of the Coal Measures burn to an agreeable cream or buff colour, the less pure ones and those of the other formations mentioned produce articles of a brick-red or blue-grey colour.

Where the shales are of exceptionally fine grain and their colour when burned is very uniform and of a pleasing tint they are known as _terra-cotta_ shales, the red terra-cottas being chiefly made from those occurring in Wales and the buff ones from the lower grade fireclays of the Coal Measures.

_Alum shales_ are characterised by a high proportion of pyrites, which, on roasting, form ferrous sulphate and sulphuric acid. The latter combines with the alumina in the shale and when the roasted ore is extracted with water a solution of iron sulphate and aluminium sulphate is obtained. From this solution (after partial evaporation) alum crystals are obtained by the addition of potassium or ammonium sulphate.

The chief alum shales are those of the Silurian formation in Scotland and Scandinavia. The Liassic shales of Whitby were at one time an equally important source of alum.

During recent years a large amount of alum has been obtained from other sources or has been made from the lower grade Dorset and Devonshire ball clays by calcining them and then treating them with sulphuric acid. These clays being almost free from iron compounds yield a much purer alum at a lower cost.

=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. Similarly, some of the very tough surface clays of the Northern and Midland counties are equally valueless, though designated 'brick clays' in numerous geological and other reports. It is, therefore, necessary to remember that, as ordinarily used, the term 'brick clay' merely indicates a material which appears at first sight to be suitable for brickmaking, but that more detailed investigations are necessary before it can be ascertained whether a material so designated is actually suitable for the purpose.

It is also important to observe that local industrial conditions may be such that a valuable clay may be used for brickmaking because there is a demand for bricks, but not for the other articles for which the clay is equally suitable. For instance, a considerable number of houses in Northumberland and Durham were built of firebricks at a time when it was more profitable to sell these articles for domestic buildings than for furnaces.

In many ways the bricks used for internal structural work form the simplest and most easily manufactured of all articles made from clay. The colour of the finished product is of minor importance and so long as a brick of reasonably accurate shape and of sufficient strength is produced at a cheap rate, little else is expected.

Impurities--unless in excessively large proportions--are of small importance and, indeed, sand may almost be considered an essential constituent of a material to be used for making ordinary bricks. It is, therefore, possible to utilize for this purpose some materials containing so little 'clay' as to make them scarcely fit to be included in this term. So long as the adventitious materials consist chiefly of silica and 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.

No brick clay can be regarded as 'safe' if it contains nodules of limestone--unless these can be removed during the preparation of the material--or if the resulting bricks will not show a crushing strength of at least 85 tons per square foot.

The introduction of machinery in place of hand-moulding and of kilns instead of clamps has greatly raised the standard of strength, accuracy in shape and uniformity in colour in many districts, and many builders in the Midlands now expect to sort out from the 'common bricks' purchased, a sufficient number of superior quality to furnish all the 'facing bricks' they require. Apart from this, and in districts where buildings are faced with stone or with bricks of a superior quality, the 'stock' or 'common brick' may be made from almost any clay which will bear drying and heating to redness without shrinking excessively or cracking. A linear shrinkage of 1 in. per foot (= 8-1/2 per cent.) may be regarded as the maximum with most materials used for brickmaking. Clays which shrink more than this must have a suitable quantity of grog, sand, chalk, ashes or other suitable non-plastic material added.

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.

Further information on brick earths will be found on page 67.

A description of the processes used in the manufacture of bricks being outside the scope of the present work, the reader requiring information on this subject should consult _Modern Brickmaking_ (25) or some similar treatise.

_Roofing tiles_ require clays of finer texture than those which may be made into bricks. Stones, if present, must be removed by washing or other treatment, as it is seldom that they can be crushed to a sufficiently fine powder, unless only rough work is required. If sufficiently fine, the clay used for roofing tiles may be precisely the same as that used for bricks and is treated in a similar manner. It must, however, be of such a nature that it will not warp or twist during the burning; it must, therefore, have a long range of vitrification (p. 38).

_Terra-cotta_ is an Italian term signifying baked earth, but its meaning is now limited to those articles made of clay which are not classed as pottery, such as statues, large vases, pillars, etc., modelled work used in architecture, or for external decoration. Although the distinction cannot be rigidly maintained, articles made of clay may be roughly divided into

(_a_) Pottery (_faience_) and porcelain (glazed),

(_b_) Terra-cotta (unglazed),

(_c_) Bricks and unglazed tiles devoid of decoration.

In this sense, terra-cotta occupies an intermediate position between pottery and bricks, but no satisfactory definition has yet been found for it. Thus, bricks with a modelled or moulded ornament are, strictly, terra-cotta, yet are not so named, and some pottery is unglazed and yet is never classed as terra-cotta, whilst glazed bricks are never regarded as pottery. Again during the past few years, what is termed 'glazed terra-cotta' has been largely used for architectural purposes, yet this is really 'faience.'

Although this overlapping of terms may appear confusing to the reader, it does not cause any appreciable amount of inconvenience to the manufacturers or users, as it is not difficult for a practical clayworker to decide in which of the three classes mentioned a given article should be placed.

Partly on account of the lesser weight, but chiefly in order to reduce the tendency to crack and to facilitate drying and burning, terra-cotta articles are usually made hollow.

It is necessary that clays used in the manufacture of terra-cotta should be of so fine a texture that the finest modelling can be executed. Such clays occur naturally in several geological formations, and some may be prepared from coarser materials by careful washing, whereby the larger grains of sand, stones, etc., are removed. Some shales, when finely ground, make excellent clays for architectural terra-cotta, portions of all the better known fireclay deposits being used for this purpose. It is, however, necessary to use only those shales which are naturally of fine texture, as mechanical grinding cannot effect a sufficient sub-division of the particles of some of the coarser shales.

The finer Triassic 'marls' are also admirable for terra-cotta work, the most famous deposit being the Etruria Marl Series in the Upper Coal Measures near Ruabon.

The most important characteristics required in terra-cotta clays are (_a_) fine texture, or at any rate the ability to yield a fine, dense surface, (_b_) small shrinkage with little tendency to twist, warp or crack in firing, (_c_) pleasing and uniform colour when fired, and (_d_) a sufficient proportion of fluxes to make it resistant to weather without giving a glossy appearance to the finished product.

In large pieces of terra-cotta some irregularity of shape is almost unavoidable, but, if care is taken in the selection and manipulation of the material, this need not be unsightly.

The durability of terra-cotta is largely dependent on the nature of the surface. The most suitable clays, when fired, have a thin 'skin' of vitrified material which is very resistant to climatic influences, and so long as this remains intact the ware will continue in perfect condition. If this 'skin' is removed, rain will penetrate the material and under the influence of frost may cause rapid disintegration.

In the manufacture of very large pieces of terra-cotta a coarse, porous clay is used for the foundation and interior, and this is covered with the finer clay. By this means a greater resistance to changes in temperature is secured, the drying and the burning of the material in the kiln are facilitated and the risks of damage in manufacture are materially reduced.

=Cement clays= are those used in the manufacture of Portland cement and of so-called natural cements. They are largely of an alluvial character and are of two chief classes: (_a_) those which contain chalk or limestone dust and clay in proportions suitable for the manufacture of cement and (_b_) those to which chalk or ground limestone must be added.

They vary in composition from argillaceous limestones containing only a small proportion of clay to almost pure clays.

The manufacture of Portland cement has assumed a great importance and owing to the large amount of investigations made in connection with it, it may be said to represent the chief cement made from argillaceous materials, the others being convenient though crude modifications of it.

The essential constituents are calcium carbonate (introduced in the form of chalk or powdered limestone) and clay, the composition of the naturally occurring materials being modified by the addition of a suitable proportion of one or other of these ingredients. The material is then heated until it undergoes partial fusion and a 'clinker' is formed. This clinker, when ground, forms the cement.

In Kent, the Medway mud is mixed with chalk; in Sussex, a mixture of gault clay and chalk is employed; in the Midlands and South Wales, Liassic shales and limestone are used; in Northumberland a mixture of Kentish chalk and a local clay is preferred, and in Cambridgeshire a special marl lying between the Chalk and the Greensand is found to be admirable for the purpose because it contains the ingredients in almost exactly the required proportions.

For cement manufacture, clays should be as free as possible from material which, in slip form, will not pass through a No. 100 sieve, as coarse sand and other rock débris are practically inert. The proportion of alumina and iron should be about one-third, but not more than one-half, that of the silica, and in countries where the proportion of magnesia in a cement is limited by standard specifications, it will be found undesirable to use clays containing more than 3 per cent. of magnesia and alkalies.

Whilst calcareous clays usually prove the most convenient in the manufacture of cement, it is by no means essential to use them, and where a clay almost free from lime occurs in convenient proximity to a suitable chalk or limestone deposit an excellent cement may usually be manufactured.

The 'clays' from which the so-called 'natural' or 'Roman cements' are made by simple calcination and crushing, usually fuse at a lower temperature than do the mixtures used for Portland cement, and unless their composition is accurately adjusted they yield a product of such variable quality as to be unsuitable for high class work.

=Fuller's earth= is a term used to indicate any earthy material which can be employed for fulling or degreasing wool and bleaching oil. True fuller's earth is obtained chiefly from the neighbourhood of Reigate, Surrey, Woburn Sands, Bedfordshire and from below the Oolite formation near Bath, but owing to the scarcity of the material and the irregularity of its behaviour, china clay is now largely used for the same purpose. True fuller's earth is much more fusible than the white clays usually substituted for it, and when mixed with water it does not form a plastic paste but falls to powder. As the chief requirement of the fuller is the grease-absorbing power of the material there is no objection to the substitution of other earths of equal efficiency.

Fuller's earth does not appear to be a true clay, though its constitution and mineralogical composition are by no means clearly known. T. J. Porter considers that it is chiefly composed of montmorillonite (Al2O3·4SiO2H2O), anauxite, (2Al2O3·9SiO2·6H2O), and chalk with some colloidal silica and a little quartz. It therefore appears to resemble the less pure kaolins, but to contain little or no true clay, though in many respects it behaves in a manner similar to a kaolin of unusually low plasticity.

=Other clays= of commercial importance, with further details of the ones just mentioned, are described in the author's _British Clays, Shales and Sands_ (2).