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diff --git a/43297-0.txt b/43297-0.txt new file mode 100644 index 0000000..750ac83 --- /dev/null +++ b/43297-0.txt @@ -0,0 +1,4676 @@ +*** START OF THE PROJECT GUTENBERG EBOOK 43297 *** + +Trancriber's Note + +Text emphasis is displayed as _Italic_ and =Bold=. +Whole and fractional parts are displayed as 8-1/2. +Although in general, subscripts are denoted _{#}. The formulae for +the minerals are presented as K2OAl2O3·6SiO2 (Orthoclase) where the +number following the element would normally be subscripted. + + + + +The Cambridge Manuals of Science and Literature + + +THE NATURAL HISTORY OF CLAY + + +CAMBRIDGE UNIVERSITY PRESS + +London: FETTER LANE, E.C. + +C. F. CLAY, Manager + +[Illustration] + + +Edinburgh: 100, PRINCES STREET + +London: WILLIAM WESLEY AND SON, 28, ESSEX STREET, STRAND + +Berlin: A. ASHER AND CO. + +Leipzig: F. A. BROCKHAUS + +New York: G. P. PUTNAM'S SONS + +Bombay and Calcutta: MACMILLAN AND CO., Ltd. + +_All rights reserved_ + + + + +[Illustration] + + + THE NATURAL HISTORY OF CLAY + + BY + + ALFRED B. SEARLE + + Cantor Lecturer on Brickmaking, + Author of _British Clays, Shales + and Sands_; _The Clayworker's + Handbook_, etc., etc. + + Cambridge: + at the University Press + New York: + G. P. Putnam's Sons + 1912 + + +Cambridge: + +PRINTED BY JOHN CLAY, M.A. + +AT THE UNIVERSITY PRESS + +_With the exception of the coat of arms at the foot, the design on the +title page is a reproduction of one used by the earliest known Cambridge +printer, John Siberch, 1521_ + + + + +PREFACE + + +Both as raw materials and in the form of pottery, bricks, tiles, +terra-cotta and many other articles of use and ornament, clays are +amongst the most important rock products. Yet the origin of the +substances we know as 'clay,' the processes occurring in its formation +and the causes of some of the most important of its characteristics are +of such a nature that it is remarkable that its use should have become +so extended in the arts and sciences, while we know so little of its +properties when in a pure state. + +In the following pages an attempt has been made to state in a simple +form an outline of our present knowledge of the subject and to indicate +the problems which still lie before us. + +The experimental solution of these problems is rendered peculiarly +difficult by the inertness of the materials at ordinary temperatures and +the ease with which the clay molecule appears to break down into its +constituent oxides at temperatures approaching red heat or as soon as it +begins to react with alkaline or basic materials. + +Another serious difficulty is the highly complex nature of that property +known as 'plasticity' to which many clays owe their chief value. For +many years this has been regarded as an elementary property such as +hardness, cohesion or colour, but it is now known to be of so elusive a +nature as almost to defy measurement with any degree of accuracy. + +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. + +No industry exercises so great a fascination over those engaged in it as +do the various branches of clayworking; no other substance offers so +many problems of such absorbing interest to the artist, the craftsman, +the geologist, the chemist and the general student of nature, whilst the +differences in legal opinion as to the nature of clay could themselves +occupy a volume far larger than the present one. + + A. B. S. + + The White Building, + Sheffield. + _November 1911._ + + + + +CONTENTS + + + CHAP. PAGE + + Table of clay rocks viii + + I Introduction. The chemical and physical properties of clays 1 + + II Clay and associated rocks 48 + + III The origins of clays 70 + + IV The modes of accumulation of clays 84 + + V Some clays of commercial importance 103 + + VI Clay-substance: theoretical and actual 135 + + Bibliography 168 + + Index 170 + + + + +LIST OF ILLUSTRATIONS + + + FIG. + + 1 Quartz crystals 9 + + 2 Pyrite 14 + + 3 Marcasite 14 + + 4 Illustrating the structure of a 'clay crumb' 24 + + 5 Chart showing rates of drying 27 + + 6 Seger Cones indicating a temperature of 1250° C. 34 + + 7 Ludwig's Chart 36 + + 8 Coal Measures sequence in North Staffordshire 55 + + 9 Lias clay being worked for the manufacture of hand-made + sand-faced roofing tiles 58 + + 10 Oxford clay near Peterborough 60 + + 11 Cliffs of Boulder clay at Filey lying on Calcareous Crag 66 + + 12 China clay pit belonging to the North Cornwall China Clay Co. 72 + + 13 Orthoclase Felspar 75 + + 14 Illustrating the successive deposition of different strata 90 + + 15 Lacustrine clay at Skipsea 92 + + 16 Clay at Nostel, showing Marine Band 94 + + 17 Kaolinite and Mica 105 + + 18 Mining best Potter's clay in Devonshire 111 + + + + +THE CHIEF CLAY ROCKS (arranged geologically) + + + +--------------------------------------------------------+ + {|Recent (_alluvial clay_, _silt_, _brick earths_, | + {| _boulder clay_) | + {|--------------------------------------------------------| + Tertiary {|Pliocene } | + {|Miocene } (_brick earths_, _ball clays_, | + {|Oligocene } _coarse pottery clays_) | + {|Eocene } | + |--------------------------------------------------------| + {|Cretaceous (_cement clays_, _brick clays_) | + {|--------------------------------------------------------| + Secondary {|Oolitic (_brick and tile clays_) | + {|--------------------------------------------------------| + {|Triassic (_brick, tile and terra-cotta clays_) | + |--------------------------------------------------------| + {|Permian (_brick, tile and flower-pot clays_) | + {|--------------------------------------------------------| + {|Carboniferous (_brick clays_, _fireclays_, _ganister_) | + {|--------------------------------------------------------| + Primary {|Devonian } | + {|Silurian } | + {|Ordovician } (_clay schists, slates and clay shales_) | + {|Cambrian } | + {|Pre-Cambrian } | + |--------------------------------------------------------| + |Igneous Rocks occur on several horizons (_china clays_ | + | _and kaolins_) | + +--------------------------------------------------------+ + +(In the above Table only the clay-bearing strata are mentioned. The +formations named consist chiefly of other rocks in which the clays form +strata of variable thickness.) + + + + +CHAPTER I + +INTRODUCTION. THE CHEMICAL AND PHYSICAL PROPERTIES OF CLAY + + +The chief uses of clay have been recognized since the earliest periods +of civilization; the ancient Assyrian and Egyptian records contain +numerous references to the employment of clay for the manufacture of +bricks and for fulling or whitening cloth. + +Clays are distributed so widely and in many cases are so readily +accessible that their existence and some of their characteristics are +known in entirely uncivilized regions. The use of certain white clays as +a food, or at any rate as a means of staving off hunger, is common among +some tribes of very primitive peoples. The more important uses of clays +for building and other purposes are naturally confined to the more +civilized nations. + +The term _clay_ (A.S. _cloeg_; Welsh _clai_; Dutch _kley_) although used +in a scientific sense to include a variety of argillaceous earths (Fr. +_argile_ = clay) used in the manufacture of bricks, tiles, pottery and +ceramic products (Gr. _keramos_ = potter's earth) generally, is really a +word of popular origin and use. Consequently, it is necessary to bear in +mind, when considering geological or other problems of a scientific +nature, that this term has been incorporated into scientific terminology +and that its use in this connection not infrequently leads to confusion. +In short, whilst almost every dictionary includes one or more +definitions of clay, and most text-books on geology, mineralogy, and +allied sciences either attempt a definition or assume the reader's +knowledge of one, there is no entirely satisfactory limitation in regard +to the substances which may or may not be included under the term. + +_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 is commonly supposed that all clays are +plastic, but some of the purest china clays are almost devoid of this +property and some of the most impure earths used for brickmaking possess +it in a striking degree. Shales, on the one hand--whilst clearly a +variety of clay--are hard and rock-like, requiring to be reduced to +powder and very thoroughly mixed with water before they become plastic; +many impure surface deposits, on the other hand, are so highly plastic +as to necessitate the addition of other (sandy) materials before they +can be used for the manufacture of bricks and tiles. + +Attempts have been made to include in the term clay 'all minerals +capable of becoming plastic when moistened or mixed with a suitable +quantity of water,' but this definition is so wide as to be almost +impracticable, and leads to the inclusion of many substances which have +no real connection with clays. The limitation of the use of the word +'clay' to the plastic or potentially plastic materials of any single +geological epoch is also impracticable, for clays appear to have been +deposited in almost every geological period, though there is some +difference of opinion as to the time of the formation of certain clays +known as _kaolins_. + +Clay is not infrequently termed a _mineral_, but this does not apply at +all accurately to the many varieties of earths known as 'common clays,' +which, together with the 'boulder clays,' contain many minerals and so +cannot, as a whole, be included under this term. + +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_. Whatever mineral (if any) may give the chief +characteristic property to the clays as a class must be designated by a +special title, for the general term 'clay' will not serve for this +purpose. Geologically, the clays are sedimentary rocks, some being +unaltered, whilst others--the slates--are notably metamorphosed and can +seldom be used for the purposes for which clays are employed. + +Most clays may be regarded as a mixture of quartz grains, undecomposed +rock débris and various decomposition products of rocks; if the +last-named consists chiefly of certain hydrous alumino-silicates, they +may be termed 'clay substance' (see Chapter VI. The imperfections of +this statement as a definition are obvious when it is remembered that it +may include a mixture of fine sand and clay containing only 30 per cent. +of the latter substance. + +It is, at the present time, quite impossible to construct an accurate +definition of the term 'clay.' The most satisfactory hitherto +published defines 'clay' as 'a solid rock composed mainly of +hydro-alumino-silicates or alumino-silicic acids, but often containing +large proportions of other materials; the whole possessing the property +of becoming plastic when treated with water, and of hardening to a +stone-like mass when heated to redness.' + +From what has already been written, it will be understood that there is +no such entity as a standard clay, for the varieties are almost endless, +and the differences between them are sometimes so slight as to be +scarcely distinguishable. + +A further consideration of this branch of the subject may, however, +conveniently be deferred to a subsequent chapter. + +The best-known clays are the surface clays, loams and marls, the shales +and other sub-surface clays, and the pottery and china clays. The values +of these different materials vary enormously, some being almost +worthless whilst others are highly valued. + +The _surface clays_ are chiefly used for the manufacture of bricks and +tiles (though some are quite unsuitable for this purpose) and form the +soil employed in agriculture in many districts. + +The _sub-surface clays_ and _shales_ are harder, and usually require +mechanical treatment before they can be used for brick and terra-cotta +manufacture, or for the production of refractory and sanitary articles. + +The _pottery and china clays_ are usually more free from accessory +constituents, and are regarded as the 'purest' clays on the market, +though a considerable amount of latitude must be allowed in interpreting +the term 'pure.' China clays are by no means pure in the state in which +they occur, and require careful treatment before they can be sold. + +Further information with regard to the characteristics of certain clays +will be found in Chapter V. + + +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). + +All clays may, apparently, be regarded as consisting of a mixture of one +or more hydrous alumino-silicates with free silica and other non-plastic +minerals or rock granules, and their chemical properties are largely +dependent on the nature and proportion of these accessory ingredients. + +The purest forms of clay (china clays and ball clays) approximate to the +formula above-mentioned, but others differ widely from it, as will be +seen from the analyses on p. 16. The chemical properties of pure clay +are described more fully in Chapter VI. + +[Footnote 1: This formula is commonly written Al2O33·2SiO2·2H2O, +but although this is a convenient arrangement, it must not be understood +to mean that clays contain water in a state of combination similar to +that in such substances as washing soda--Na2CO3·24H2O, or zinc +sulphate crystals--ZnSO4·7H2O (see Chapter VI).] + +Taking china clay, which has been carefully purified by levigation, as +representative of the composition of a 'pure' clay, it will be found +that the chief impurities in clays are (_a_) stones, gravel and +sand--removable by washing or sifting; (_b_) felspar, mica and other +silicates and free silica--which cannot be completely removed without +affecting the clay and (_c_) lime, magnesia, iron, potash and soda +compounds, together with minute quantities of other oxides, all of which +appear to be so closely connected with the clay as to be incapable of +removal from it by any mechanical methods of purification. + +To give a detailed description of the effect of each of the impurities +just referred to would necessitate a much larger volume than the +present, but a few brief notes on the more important ones are essential +to a further consideration of the natural history of clay. + +_Stones_, _gravel_ and _sand_ are most noticeable in the boulder clays, +but they occur in clays of most geological ages, though in very varying +proportions. Sometimes the stones are so large that they may be readily +picked out by hand; in any case the stones, gravel and most of the sand +may be removed by mixing the material with a sufficient quantity of +water and passing the 'slip' through a fine sieve, or by allowing it to +remain stationary for a few moments and then allowing the supernatant +liquid to run off into a settling tank. Some clays contain sand grains +which are so fine that they cannot be removed in this manner and the +clay must then be washed out by a stream of water with a velocity not +exceeding 2 ft. per hour. Even then, the clay so removed may be found to +contain minute grains of silt, much of which may be removed by a series +of sedimentations for various periods, though a material perfectly free +from non-plastic granules may be unattainable. + +Most of the sand found associated with clays is in the form of fragments +of _quartz_ crystals (fig. 1), though it may be composed of irregular +particles of other minerals or of amorphous silica. + +_Felspar_, _mica_ and other adventitious silicates occur in many natural +clays in so fine a state of division that their removal would be +unremunerative. In addition to this they act as fluxes when the clays +are heated in kilns, binding the less fusible particles together and +forming a far stronger mass than would otherwise be produced. +Consequently, they are valuable constituents in clays used for the +manufacture of articles in which strength or imperviousness is +important. If these minerals are present in the form of particles which +are sufficiently large to be removed by elutriation in the manner +described on the previous page, the purification of the clay is not +difficult. Usually, however, the most careful treatment fails to remove +all these minerals; their presence may then be detected by +microscopical examination and by chemical analysis. For most of the +purposes for which clays are used, small proportions of these silicates +are unimportant, but where clays of a highly refractory nature are +required; and for most of the purposes for which china clays (kaolins) +are employed, they must not be present to the extent of more than 5 per +cent., smaller proportions being preferable. + +[Illustration: Fig. 1. Quartz crystals, natural size. (From Miers' +Mineralogy _by permission of Macmillan & Co._)] + +_Oxides_, _sulphides_, _sulphates_ and _carbonates_ of various metals +form the third class of impurities in clays. Of these, the most +important are calcium oxide (lime), calcium carbonate (chalk and +limestone), calcium sulphate (gypsum and selenite), the corresponding +magnesia, magnesium carbonate, and sulphate, the various iron oxides, +ferrous carbonate and iron sulphides (pyrite and marcasite) (p. 13). + +Potash and soda compounds are commonly present as constituents of the +felspar, mica, or other silicates present, and need no further +description, though small proportions of _soluble salts_--chiefly +sodium, potassium, calcium and magnesium sulphates--occur in most clays +and may cause a white scum on bricks and terra-cotta made from them. + +_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 act as a +flux, reducing the heat-resisting power of the clay and increasing the +amount of vitrification; they produce in extreme cases a slag-like mass +when the clay is intensely heated. If, on the contrary, the grains are +larger (as frequently occurs with limestone), they are converted into +lime or magnesia when the clay is 'burned' in a kiln, and the lime, on +exposure to weather, absorbs moisture (_i.e. slakes_), swells, and may +disintegrate the articles made from the clay. Limestone (except when in +a very finely divided state) is almost invariably objectionable in +clays, but chalk is frequently a valuable constituent. + +[Footnote 2: A _malm_ is a natural mixture of clay and chalk (p. 68).] + +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 compounds have the serious objection of acting as very rapid and +powerful fluxes, so that when clays containing them are heated +sufficiently to start partial fusion, a very slight additional rise in +temperature may easily reduce the whole to a shapeless, slag-like mass. +Magnesia compounds act much more slowly in this respect and so are less +harmful. + +_Gypsum_--a calcium sulphate--occurs naturally in many sub-surface +clays, often in well-defined crystalline masses. It reduces the +heat-resisting power of the clays containing it and may, under some +conditions, rise to the surface of the articles made from the clay, in +the form of a white efflorescence or scum, such as is seen on some brick +walls. + +_Iron compounds_ are highly important because they exercise a powerful +influence on the colour of the burned clays. 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. + +It is a curious fact that red iron oxide does not appear to form any +compound with the other constituents of clay under ordinary conditions +of firing, and although a 'base' and capable of reducing the +heat-resisting power of clays, it does not appear to do so as long as +the conditions in the kiln are sufficiently oxidizing. It is this which +enables red bricks and other articles to be obtained with remarkable +uniformity of colour combined with great physical strength. In a +reducing atmosphere, on the contrary, ferrous oxide readily forms and +attacks the clay, forming a dark grey vitreous mass. If the iron +particles are separated from each other they will, on reduction, form +small slag-like spots, but if they are in an extremely fine state of +division and well distributed, the brick or other article will become +slightly glossy and of an uniform black-grey tint. The famous +Staffordshire 'blue' bricks owe their colour to this characteristic; +they are not really 'blue' in colour. The effect of chalk on the colour +of red-burning clays has already been mentioned. + +_Iron pyrite_ (fig. 2) and _marcasite_ (fig. 3)--both of which are forms +of iron sulphide--occur in many clays, particularly those of the Coal +Measures. _Mundic_ is another form of pyrites which resembles roots or +twigs, but when broken show a brassy fracture. When in pieces of +observable size the pyrite may be readily distinguished by its +resemblance to polished brass and the marcasite by its tin-white +metallic lustre and both by their characteristic cubic, root-like and +spherical forms; the latter only show a brass-like sheen when broken. +Even when only a small proportion of mundic, pyrite or marcasite is +present, it is highly objectionable for several reasons. In the first +place, half the sulphur present is given off at a dark red heat and is +liable to cause troublesome defects on the goods. Secondly, because the +remaining sulphur and iron are not readily oxidized, so that there is a +great tendency to form slag-spots of ferrous silicate, owing to the iron +attacking the clay at the same moment as it parts with its remaining +sulphur. For this reason, clays containing any iron sulphide seldom burn +red, but form products of a buff colour with black spots scattered +irregularly over their surface and throughout the mass--an appearance +readily observable on most hard-fired firebricks. If chalcopyrite +(copper-iron sulphide) is present the spots may be bright green in +colour. + +[Illustration: Fig. 2. Pyrite.] + +[Illustration: Fig. 3. Marcasite.] + +Slightly magnified. + +(_From Miers'_ Mineralogy _by permission of Macmillan & Co._) + +_Carbon_, either free or as hydrocarbons (chiefly vegetable matter) or +in other forms, is a constituent of most clays, though seldom reported +in analyses. Its presence exercises an important influence in several +respects. On heating the clay, with an ample supply of air, the +carbonaceous matter may distil off (as shale oil), but more usually it +decomposes and burns out leaving pores in the material. If the +air-supply is insufficient and the heating is so rapid and intense that +vitrification commences before the carbon is all burned away, the pores +become filled with the fused ingredients of the clay, air can no longer +reach the carbon particles and a black 'core' or heart is produced. +Under peculiarly disadvantageous conditions the material may also swell +greatly. This is a serious defect in many classes of clay used for +brickmaking, and its causes and prevention have been exhaustively +studied by Orton and Griffiths (1)[3] but, beyond the brief summary +given above, these are beyond the scope of the present work. + +_Water_ is an essential constituent of all unburned clays, though the +proportion in which it occurs varies within such wide limits that no +definite standard can be stated. This water is found in two conditions: +(_a_) as moisture or mechanically mixed with the clay particles and +(_b_) in a state of chemical combination. + +[Footnote 3: References to original papers, etc. will be found in the +appendix.] + +ANALYSES OF TYPICAL CLAYS + +_The samples were all dried at 105° C._ + + +-------------------+--------+-------+---------+--------+-------+-------+ + | Clay | China | Ball |Fireclay | Brick |Boulder| Marl | + | | Clay | Clay | | Clay | Clay | | + +-------------------+--------+-------+---------+--------+-------+-------+ + | Locality |Cornwall| Dorset|Yorkshire|Midlands| Lancs.|Suffolk| + +-------------------+--------+-------+---------+--------+-------+-------+ + |Ultimate Analysis: | | | | | | | + | Silica | 47·1 | 49·1 | 68·9 | 57·7 | 63·7 | 43·7 | + | Alumina | 39·1 | 33·7 | 19·3 | 24·3 | 20·4 | 15·5 | + | Ferric oxide | ·6 | 1·2 | 1·0 | 5·0 | 3·0 | 5·2 | + | Titanium oxide | -- | ·2 | 1·8 | ·1 | ·2 | -- | + | Lime | ·4 | ·8 | ·9 | 3·7 | 4·3 | 16·3 | + | Magnesia | ·2 | ·3 | ·3 | 2·5 | 2·7 | 2·1 | + | Potash and Soda | ·3 | 2·5 | ·9 | 2·8 | 2·9 | ·7 | + | Carbon | 2·6 | 4·3 | 1·8 | 1·6 | ·4 | 1·6 | + | Water | 9·3 | 7·7 | 4·8 | 2·0 | 2·2 | 2·4 | + | Other Matter | ·4 | ·2 | ·3 | ·3 | ·2 | 12·5 | + +-------------------+--------+-------+---------+--------+-------+-------+ + | Total | 100·0 | 100·0 | 100·0 | 100·0 | 100·0 | 100·0 | + +-------------------+--------+-------+---------+--------+-------+-------+ + |Proximate Analysis:| | | | | | | + | Gravel and Sand | -- | 8·4 | 4·6 | 22·1 | 23·1 | 9·2 | + | Silt | -- | 4·8 | 9·0 | 3·1 | 8·4 | 16·0 | + | Felspar- and mica-| | | | | | | + | dust | 5·2 | 15·4 | 10·3 | 24·3 | 18·5 | 8·9 | + | Silica-dust | 3·1 | 4·0 | 38·0 | 3·1 | 12·6 | 2·0 | + | Free calcium | | | | | | | + | carbonate | -- | -- | -- | 2·1 | ·2 | 28·4 | + | Free iron oxide | | | | | | | + | and pyrites | ·4 | ·9 | ·7 | 4·2 | 1·6 | 3·9 | + | 'True clay' | 91·3 | 66·5 | 37·4 | 41·1 | 35·6 | 31·6 | + +-------------------+--------+-------+---------+--------+-------+-------+ + | Total | 100·0 | 100·0 | 100·0 | 100·0 | 100·0 | 100·0 | + +-------------------+--------+-------+---------+--------+-------+-------+ + +For other analyses the books in the Bibliography at the end of the +present volume should be consulted, particularly No. 2, _i.e._ _British +Clays, Shales and Sands_. + +The amount of mechanically mixed water will naturally vary with the +conditions to which the clay has been subjected; it will be greatest in +wet situations and will diminish as the clay is allowed to dry. + +The 'combined water,' on the contrary, appears to be a function of the +true clay present in the material, and reaches its highest proportions +in the china clays and kaolins, which contain approximately 13 per cent. +On heating a clay to 105° C. the moisture or mechanically mixed water is +evaporated, but the combined water remains unaffected[4] until the +temperature is raised to more than 600° C., when it is driven off and +the clay is converted into a hard stone-like mass with properties +entirely different from those it previously possessed (see Chapter VI). + +[Footnote 4: Strictly, there is a slight loss at lower temperatures, but +it is too small to be important.] + + +The Physical Characters of Clays. + +The physical characters of clays are of far more interest and importance +than their chemical ones, though the two are naturally connected in many +ways, and just as the chemical composition of clays is a subject of +extreme complexity so is a study of many of their physical properties. +Hence only a few of the more important characteristics can be mentioned +here: for further details the reader must consult a larger treatise (2). + +Clays are moderately soft, solid bodies, particularly when moistened, +and can usually be cut with a knife, though some indurated clays and +shales are almost as hard as felspar. Their apparent specific gravity +varies greatly, some clays being much more porous than others, but the +true specific gravity is usually between 2·5 and 2·65; it is similar to +that of quartz and slightly lower than that of felspar and mica. Many +clays appear to be devoid of structure, but those obtained from a +considerable depth below the surface are frequently laminated and have a +structure not unlike that of mica. This will be discussed later. + +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 exact nature of both the crystals and the +amorphous material is still unknown in spite of many investigations; in +the purer clays both forms of substance appear to have the same chemical +composition, viz. that of _kaolinite_ (H4Al2Si2O9), which the +crystalline portion closely resembles. + +Clays emit a characteristic yet indefinable odour when moist; the cause +of this is very imperfectly understood, though it is not improbably due +to decomposing organic matter, as this occurs in most clays. + +The colours of freshly-dug clays are extremely varied and range from an +almost pure white through all shades of yellow, red and brown to black. +The predominating colours are grey or greyish brown and a peculiar +yellow characteristic of some surface clays. 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. + +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) +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 +extent to which the carbonaceous matter is burned out also determines +the colour of the fired ware. + +The presence of adventitious minerals in the clay may also affect its +colour, particularly when fired. + +The most obvious feature in a piece of moist clay is its _plasticity_[5] +or ability to alter its shape when kneaded or put under slight pressure +and to retain its new shape after the pressure has been removed. It is +this property which enables the production of ornaments, vessels of +various kinds, and the many other articles which are the result of the +application of modelling tools, of moulding or of the action of a +potter's wheel. So long as clay contains a suitable proportion of +moisture it is plastic and may be made into articles of any desired +shape, but if the amount of moisture in it is reduced or removed +completely, the material is no longer plastic. It may become so, +however, on adding a further suitable quantity of water and mixing, +provided that it has not been excessively heated. If, in the removal of +the moisture, the clay has been heated to 600° C. or more, it loses its +power of becoming plastic and is converted into a material more closely +resembling stone. + +[Footnote 5: 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. + +So far as clays are concerned, their plasticity appears to be connected +with the presence of combined water as well as of mechanically mixed +water, for if either of these are removed, plasticity--both actual and +potential--is destroyed. The part played by water is not, however, +completely known, for the many theories which have been advanced only +cover some of the conditions and facts. + +A number of observers agree that the molecular constitution of clay is +peculiar and that it is to this that plasticity is due. Yet the curious +fact that the purest clays--the kaolins--are remarkably deficient in +plasticity shows that molecular constitution is not, alone, sufficient. +Others hold that the remarkably small size of clay particles enables +them to pack together more closely than do particles of other materials +and to retain around them a film of water which acts partly as a +lubricant, facilitating the change of shape of the mass when under +pressure, and partly as an adhesive, causing the particles to adhere to +each other when the pressure is removed. + +Zschokke has laid much emphasis on the importance of molecular +attraction between clay and water as a cause of plasticity, and has +suggested that the absorption of the water effects a change in the +surfaces of the clay particles, giving them a gelatinous nature and +enabling them to change their form and yet keep in close contact. + +The fact that mica, fluorspar and quartz, when in a sufficiently finely +divided state, are also slightly plastic, appears to be opposed to the +molecular constitution theory. Smallness of grain undoubtedly has an +influence on the plasticity of clay, coarse-grained clays being notably +less plastic than others. + +Daubrée pointed out that felspar, when ground with water, develops +plasticity to a small extent, and Olschewsky carried this observation +further and has suggested that clays owe their plasticity to prolonged +contact with water during their removal from their place of formation +and previous to or during their deposition. A further confirmation of +this theory is due to Mellor (3) who showed that on heating china clay +with water under very considerable pressure its plasticity was increased +and that felspar and some other non-plastic materials developed +plasticity under these conditions. + +Johnson and Blake (21) supposed that plasticity is due to the clay being +composed of extremely minute plates 'bunched together,' a view which was +also held by Biedermann and Herzfield, Le Chatelier and others. +Olschewsky enlarged this theory by suggesting that the plasticity of +certain clays is dependent on the large surface and the interlocking of +irregular particles with the plates just mentioned. These theories of +interlocking are, however, incomplete, because the tensile strength of +clays should accurately represent the plasticity if interlocking were +the sole cause. Zschokke has shown that tensile strength is only one +factor which must be determined in any attempt to measure plasticity. + +E. H. L. Schwarz (35) has suggested that many clays are composed of +small globular masses of plates so arranged as to form an open network +(fig. 4) which is sufficiently strong not to be destroyed by pressure. +In the presence of water and much rubbing the plates are separated and +are made to lie flat on each other, thereby giving a plastic and +impermeable mass. If this is really the case it would explain the +porosity and large surface of some clays and might account for their +adsorptive power. + +[Illustration: Fig. 4. Illustrating the structure of a 'clay crumb.' +(_After Schwarz._)] + +A theory which was first promulgated in 1850 by Way (4), but which has +only received detailed attention during the last few years, attributes +plasticity to the presence of colloid substances in clay or to the fact +that clay particles possess physical characters analogous to those of +glue and other colloids. These colloid substances have a submicroscopic +or micellian structure; they are web-like, porous and absorb water +eagerly. This water may be removed by drying, only to be re-absorbed on +cooling, but if the heating temperature is excessive the structure of +the colloids is destroyed. This colloid theory explains many of the +facts noted by earlier investigators such as Aron, Bischof, Seger, +Olschewsky, etc., but it is not entirely satisfactory, though Rohland +(5)--to whom the present prominence of this theory in Europe is largely +due--persistently maintains the contrary. One great objection is the +fact that no characteristic _inorganic_ colloid substance has been +isolated from pure clay. It is possible that some of the so-called +'colloidal' properties of clay may be due to the smallness of its +particles and to their great porosity, as suggested by Olschewsky. + +Despite the present impossibility of producing a plastic material from +artificially prepared colloidal hydro-alumino-silicates of the same +ultimate composition as clay, and the fact that the addition of +colloidal substances does not necessarily increase the true plasticity +of clay, it cannot be denied that the presence of colloids has an +important influence on it. The addition of starches, glue, gums and +similar substances whilst apparently increasing the plasticity of clay +does not do so in reality. The addition of 1 per cent. of tannin, on the +contrary, has been found by Ries (6) to increase both plasticity and +binding power. + +Plasticity appears to be composed of a number of characteristics so that +it is scarcely likely that any single cause can be assigned to it. On +the contrary, a study of the binding power, tensile strength, +extensibility, adsorption, texture and molecular constitution of clays +suggests very strongly that all these properties are involved in the +production of plasticity and that it is due to the chemical as well as +the physical nature of clay. No clay is entirely colloidal--or it would +be elastic and not plastic--but all appear to contain both colloidal and +non-colloidal (including plate-like) particles, and it is not improbable +that materials in both these states are required, the colloidal matter +acting as a cement. Ries (6) has, in fact, pointed out that colloids +alone lack cohesiveness and solidity, and a fine mineral aggregate is +necessary to change them into a plastic mass resembling clay. The +relative proportions of the colloidal material and the sizes of the +non-plastic grains will exercise an important influence on all the +physical characteristics mentioned above, and therefore on the +plasticity. + +The manner in which slightly plastic clays become highly plastic in +nature is by no means certainly known. It has long been understood that +the increase of plasticity is due to changes undergone by the clay +during transportation. The most illuminating suggestion is that made by +Acheson in 1902, who concluded that it is due to impurities in the water +used in transporting the clay or remaining in contact with it during and +after its deposition. These impurities may be considered as derived from +the washings of forests, and after many experiments with plant extracts +Acheson believed the most important substance in this connection to be +tannin or gallo-tannic acid, a dilute solution of which he found +increased the plasticity of china clay by 300 per cent. From this he +further argued that the use of chopped straw by the Israelites in Egypt +in the manufacture of bricks was unconsciously based on the tannin +content of the straw increasing the plasticity of the material. + +[Illustration: Fig. 5. Chart showing rates of drying. (_After +Bleininger._)] + +Beadle has stated that 2 per cent. of dissolved cellulose will increase +the plasticity of china clay and make it equal to that of ordinary +clay. + +Plasticity is diminished by heating clays, and whilst much of it may be +recovered if the temperature has not risen above 400° C. it cannot be +completely restored. Moreover, a clay which has once been heated to a +temperature above 100° C. dries in a somewhat different manner to a raw +clay. This is well shown in fig. 5 in which are summarized the results +obtained by A. V. Bleininger on a sample of ball clay from Dorset before +heating and after portions of it had been heated for 16 hours to 200°, +250°, 300°, 350° and 400° C. respectively. It is not impossible that if +subjected to the influence of water for a sufficiently long time the +whole of the plasticity of a heated clay may be restored, providing that +the temperature has not been sufficient to cause a destruction of the +clay molecule, but as this resumption requires a certain amount of time, +Bleininger has proposed to use the reduction in plasticity effected by +the heating to enable excessively plastic clays to be worked without the +necessity of adding non-plastic material to them. If any destruction of +the clay-molecules has occurred, the plasticity of that portion of the +clay can never be restored. + +The _binding power_ of clays is a characteristic closely connected with +plasticity and occasionally confused with it. All plastic clays have the +power of remaining plastic when mixed with materials such as sand, +brick-dust ('grog') and other materials which are quite devoid of +plasticity. 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. + +Rohland (5) has shown that the binding power of clay is not alone due to +its cohesion, but that it is closely associated with the colloidal +nature of plastic clays: 'fat' clays being those which are highly +colloidal, highly plastic and possessing great binding power, whilst +'lean' clays are those deficient in these characteristics. The fact +that, as a general rule, the dark coloured clays possess the most +binding power, confirms this suggestion, as the dark colour is largely +due to organic materials, probably in a colloidal state. + +The _shrinkage_ which all clays undergo on drying and when heated is +another important characteristic. It is due to the fact that as water is +removed the solid particles approach closer to each other, the volume of +the whole mass being thereby reduced. In a wet piece of clay each +particle is surrounded by a film of water, the thickness of which +depends on the nature of the clay. As this water evaporates from the +surface of the clay its place is taken by water from the interior which +rises to the surface by capillary attraction. So long as there is any +water between the particles of clay there will be shrinkage when this +water is removed, but a stage is eventually reached when the particles +of clay are in contact with each other and no more shrinkage can occur. +That this cessation of shrinkage may take place before all the water has +been removed from the clay is easily understood when it is remembered +that whilst the clay particles may be in contact, yet there are still +places (pores) where the contact is incomplete, and in these pores water +may be retained. The amount of shrinkage clays undergo on drying depends +partly on the proportion of water added to them and partly on the sizes +of the different particles of clay, sand, etc. present. An average +reduction in volume of 12 to 38 per cent. may be regarded as normal, but +coarse loams may shrink only 1 per cent. and very finely ground, highly +plastic ball clays may shrink as much as 50 per cent., though this is +unusual. + +As all coagulated colloids, which have absorbed water, shrink on drying, +this behaviour of clay appears to confirm the view as to its partially +colloidal nature held by some investigators. + +When a piece of dry clay is heated sufficiently a further shrinkage +(technically known as _kiln shrinkage_) occurs. This begins somewhat +below a red heat and increases in rough proportion to the temperature +and the duration of the heating. Prolonged heating at a lower +temperature will effect the same amount of shrinkage as a short exposure +to a higher temperature, but though the greater part of the shrinkage +occurs in a comparatively short time, continued heating will be +accompanied by a further reduction in volume. + +This is due to the fact that clays have no definite melting point, but +undergo partial fusion at all temperatures above 950° C. or, in some +cases, at even lower ones. As a portion of the material fuses, it fills +up the pores in the mass and attacks the unfused material, this process +being continued until either the heating is stopped or the whole +material is reduced to a viscous slag. + +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. Sand is sometimes added for the same purpose, though +its more frequent use is to reduce the shrinkage in drying. + +Quartz and other forms of free silica expand on heating, so that clays +containing them in large quantities shrink very slightly or may even +expand. + +As clays shrink equally in all directions it is usual to state the +contraction in linear instead of volume form. Thus instead of stating +that a certain clay when moulded into bricks, dried and burned, shrinks +18 per cent. by volume, it is customary to state that it shrinks 3/4 in. +per (linear) foot. For many purposes, it is sufficient to regard the +linear shrinkage as one-third the volume-shrinkage, but this is not +strictly accurate. + +The _fusibility_ of clays is a characteristic which has been very +imperfectly studied. Most clayworkers and investigators employ the term +'fusibility' in a special sense which is apt to be misleading. Owing to +the extremely high temperatures to which refractory clays can be heated +without even losing their shape, it is almost impossible to fuse them +completely. In addition to this, clays are not perfectly homogeneous +materials and some of their constituents melt at lower temperatures than +others. For this reason a clay may show signs of fusion at 1100° C., but +it may be heated for some hours at 1800° C. and yet not be completely +melted! Consequently no single 'fusing point' can be stated. + +In practice, a suggestion made many years ago by Seger (7) is used; the +clay to be tested is made into a small tetrahedron (fig. 6), heated +slowly until it bends over and the point of the test-piece is almost on +a level with the base. The temperature at which this occurs is termed +the 'fusing point' though it really only indicates the heat-treatment +which is sufficient to soften the material sufficiently to cause it to +bend in the manner described. In spite of the apparent crudeness of the +test this 'softening point' appears to be fairly constant for most +refractory clays. + +The bending of a test-piece in this manner is the result of the action +of all fluxes[6] in the clay, and as this depends on the size of grain +and the duration of the heating above incipient fusion and does not give +a direct measure of temperature, nor is the softening effect under one +rate of rise in temperature the same as that at another rate. +Nevertheless a study of the behaviour of various clays heated +simultaneously is valuable and the method forms a convenient means of +comparing different materials. + +[Footnote 6: For fluxing materials see p. 8.] + +The temperature may be measured by means of a pyrometer, but for the +reason just stated it is more convenient and in some respects more +accurate to use standard mixtures known as Seger Cones (fig. 6), and to +state the softening point in terms of the 'cone' which behaves like the +clay being tested. A medium fireclay will not soften below Seger Cone 26 +(1650° C.) and a really good one will have a softening point of cone 34 +or 35 (1750° to 1800° C.). + +[Illustration: Fig. 6. Seger Cones indicating a temperature of 1250° C.] + +The _refractoriness_ of a clay, or its resistance to high temperatures, +is an important requirement in bricks required for furnace linings, in +crucibles, gas retorts and other articles used in the metallurgical and +other industries. The term is much abused and is frequently understood +to mean resistance to the cutting action of flue gases and flame, the +corrosive action of slags, and the strains set up by the repeated +changes in temperature. This is unfortunate, for the term refractoriness +has a perfectly definite meaning and should be employed exclusively to +denote that a given clay is capable of retaining its shape at a given +temperature or under given conditions when heated alone and without +being subjected to any pressure. In Great Britain there is no officially +recognized standard of minimum refractoriness[7], but where one is +required the suggested minimum of Seger Cone 26 (1650° C.) made by E. +Cramer (8) is usually employed. This is the recognized minimum in +Germany for fireclays, and though objections may be urged against the +use of Seger Cones as a standard, equally forcible ones may be brought +against making a temperature-scale the basis of measurement. Under +present circumstances, however, it is necessary to adopt one or other of +these. + +[Footnote 7: See _Refractory Clays_, Chapter V.] + +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. of all these oxides. +Cramer, in 1895, found that free silica also interfered with the action +of these oxides and more recently Ludwig (9) has devised a chart (fig. +7), on the upright sides of which are plotted the equivalents of the +lime, magnesia and alkalies, whilst the silica equivalents are plotted +on the horizontal base. In each case the 'molecular formula' of the clay +is calculated from its percentage composition, and this 'formula' is +reduced so as to have one 'molecule' of alumina, thereby fixing the +alumina as a constant and reducing the number of variables to two--the +metallic oxides and the silica. Unfortunately Ludwig's chart is only +applicable to the more refractory clays and cannot be relied upon even +for these, though it is extremely useful for comparing clays from +identical or similar geological formations. + +[Illustration: Fig. 7. Ludwig's Chart.] + +Attempts to express the refractoriness of clays by means of formulae +proving abortive, there only remains the direct test of heating a clay +under definite conditions in the manner previously described. + +_Vitrification_ is closely connected with the fusibility and +refractoriness of clays, and, as a term, indicates the amount of fusion +which has occurred under certain conditions of heating. As already +mentioned, all clays, on being subjected to a high temperature, undergo +partial fusion, the more powerful bases attacking the finest particles +of clay and silica, forming molten silicates, and then slowly attacking +the more refractory portion; this slow fusion and solution continues +until the whole of the material is melted. 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. +Thus for common bricks, only sufficient vitrification is permitted to +bind the particles firmly together, but in engineering bricks--where +much greater strength is required--the vitrification is more complete. +Porcelain and earthenware may be similarly distinguished. + +The extent to which a given clay will vitrify depends on the amount of +fluxing material (metallic compounds, and oxides other than ferric oxide +and alumina) it contains, on the smallness of its particles and on the +duration and intensity of the heating. Clays containing alkalies and +lime compounds vitrify with great rapidity when once the necessary +temperature has been reached, so that unless great care is exercised the +action will proceed too far and the goods will be warped and twisted or +may even form a rough slag. Refractory clays, on the contrary, vitrify +more slowly and at much higher temperatures so that accidental +overheatings of them are far less common. + +The difference between the temperature at which sintering +or vitrification occurs and that at which the clay melts +completely--usually termed the 'vitrification range'--varies with the +nature of the clay. In some cases the clay melts as soon as +vitrification becomes noticeable, in others the vitrification occurs at +a dull red heat, but the material does not lose its shape until after a +prolonged heating at the highest temperature of a firebrick kiln or +testing furnace. + +Calcareous clays have the melting and sintering points close together, +so that it is almost impossible to produce vitrified and impervious ware +from them, as they lose their shape too readily. If, however, the +difference between the sintering and fusing temperatures can be +enlarged--that is, if the vitrification range can be extended--more +impervious ware can be made. The easiest means of extending the +vitrification range consists in regulating the proportion of large and +small particles. The former increase and the latter diminish the range. + +Basic compounds and fluxes cause a lowering of the melting-point and a +shortening of the vitrification range. + +The _porosity_ of raw clay is usually of small importance, but the +porosity of fired clay or ware is often a serious factor in determining +the suitability of certain articles for their intended purposes. In its +natural state, clay does not readily absorb much water; on the contrary +it becomes pasty and impervious unless it is disturbed and its texture +destroyed, when it may be mixed with water to form a paste or, with more +water, a thin 'cream' or 'slurry.' + +When heated moderately, clay forms a porous material and, unless the +heating is excessive, it will absorb about one-eighth of its weight of +water. Further heating at a higher temperature reduces its porosity--the +more easily fused material filling some of the pores--until a stage is +reached when the material is completely vitrified and is no longer +porous. + +Porosity may thus be regarded as the opposite of vitrification; porous +goods being relatively light and 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. + +Clays which are porous can be dried more readily and with less risk of +cracking than those which are more dense. For this reason, some +clayworkers mix non-plastic material such as sand or burned clay with +their raw material. + +The _impermeability_ of plastic clay to water is a characteristic which +is important for many purposes. + +The _absorptive power_ of clays is closely related to their porosity so +far as pure water is concerned, but if the water contains certain salts +in solution a selective absorption occurs, the bases being retained by +the clay in such a manner that they cannot be removed by washing. The +selective action is known as _adsorption_ and is most noticeable in +highly plastic clays. Bourry (10) has shown that the slightly plastic +china clays only exercise a small power of adsorbing calcium carbonate +from solution, but highly plastic clays may adsorb 20 per cent. of it. +The alkaline chlorides and sulphates do not appear to be adsorbed in +this manner, but the carbonates are readily removed from solution. All +calcium and magnesium compounds appear to be adsorbed, though in +variable quantities, the reaction being complicated when several soluble +salts are present. Ries (6) has found that gallo-tannic acid is adsorbed +readily and increases the plasticity of clay. + +Ashley (11) has endeavoured to measure the plasticity of clays by +determining their adsorption capacity for various aniline dyes, but his +untimely decease prevented the investigation being completed. There is +reason to suppose that the relation between adsorption and plasticity is +extremely close in many clays and that the former may, to an important +extent, be used as a measure of the latter. In some clays, however, this +relationship does not exist. + +Sand and burned clay only show faint adsorption phenomena; felspar shows +them to a slight and almost negligible extent and most of the other +non-plastic ingredients of clays are non-adsorptive. + +Selective adsorption being an important characteristic of colloidal +substances, the possession of this power by plastic clays supports the +claim that plasticity is due, at least in part, to the presence of +colloids. + +The addition of small quantities of a solution of certain substances to +a stiff clay paste usually reduces its stiffness, and in some cases +turns it into a liquid. The alkalies are particularly powerful in this +respect and their action may be strikingly illustrated by mixing a few +drops of caustic soda with a stiff clay paste. In a few moments the +mixture will be sufficiently liquid to pour readily, but it may be +rendered quite stiff again by adding sufficient acid to neutralize the +alkali previously used. Weber (12) has utilized this characteristic to +great advantage in the production of sanitary ware and crucibles for +glass-making by a process of casting which he has patented. + +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. If, however, a larger proportion +of water is added the particles of clay will be separated so widely from +each other that they lose their cohesion, and instead of a plastic mass, +the material will form a liquid of cream-like consistency. If a piece of +stiff clay paste is suspended in a large volume of water without +stirring, disintegration will still occur (though a much longer time +will be required) and the clay will be deposited as a sediment at the +bottom of the vessel. The leaner the clay or the larger the proportion +of non-plastic material it contains, the more rapidly will this +disintegration take place. A highly plastic clay will become almost +impervious and will retain its shape indefinitely. + +If a mixture of clay and water in the form of a cream or slurry be +allowed to rest, the larger and less plastic particles will settle, but +many of the particles of true clay will remain suspended for several +hours and some of them for several days. Some particles of clay are so +small that it is doubtful if they would ever settle completely unless +some coagulant were added, and as they readily pass through all ordinary +filtering media it is extremely difficult to collect them in a pure +state. These turbid suspensions of clay may be rapidly cleared by the +addition of sodium chloride which increases the surface tension of the +solution. The fine particles behave in the same way as colloidal +substances, _i.e._ as if they possessed an electrostatic charge. Hence +the addition of a salt (electrolyte), whose ions annul the opposite +charges of the electric double layer assumed by Helmholtz to be present, +enables the particles to coagulate in accordance with the ordinary laws +of surface tension (14). + +_Exposure_ to the action of air and frost has a marked effect on many +clays. When freshly dug these may be hard and difficult to crush, but +after exposure they break up readily into small fragments. Clays differ +greatly in the extent to which they are affected by exposure; some are +completely disintegrated by standing 48 hours in the open air, whilst +others are scarcely affected by exposure in bleak places through several +years of storm, sunshine and frost. Usually, however, the effect of a +couple of nights exposure to hard frost will produce a marked +disintegration of the material. + +This process of exposure is known as 'weathering' and its effects are so +important that it is employed whenever possible for clays requiring to +be crushed before use. All clays are rendered more workable by exposure, +but some of them are damaged by the oxidation of some impurities (_e.g._ +pyrites) in them, though in other clays this very oxidation, if followed +by the leaching action of rain, effects an important purification of the +material. + +Weathering appears to have no effect on the chemical composition of the +particles of true clay in the material, though it may decompose the +impurities present. On the clay itself its action is largely physical +and consists chiefly in separating the particles slightly from each +other, thereby enabling water to penetrate the material more readily and +facilitating the production of a plastic paste. The disintegrating +action of the weather on some 'clays' is so complete that they require +no crushing but can be converted into a homogeneous paste by simply +kneading them with a suitable proportion of water. + +It is possible that on exposure to the heat of the sun's +rays--particularly in tropical climates--some chemical decomposition of +the clay may occur, but compared with the purely physical action of +weathering the amount of such chemical decomposition must be relatively +unimportant in most cases. It may, however, account for the presence of +free silica and free alumina in some clays. + +The action of the weather on rocks, resulting in the formation of clays, +is described in Chapter III. + +_Heat_ effects remarkable changes in the physical character of clays; +the most important of these have already been noted. At a gentle heat, +the clay is dried and retains most of its power of becoming plastic when +moistened; very little, if any, decomposition occurs. At a higher +temperature it loses its 'combined water,' the clay molecule apparently +dissociating, and a hard stony mass--consisting of particles of free +silica and free alumina cemented together by the more easily fusible +impurities present--is formed. If the heating is continued the hardness +of the material is increased owing to more molten silicate having been +produced from the impurities present, and on cooling, its tensile +strength and resistance to crushing will be found to be enormously +greater than those of the original clay. All potential plasticity is +destroyed by heating to 700° C. and no method of restoring it has yet +been devised. As clays are abundant, this is not a serious disadvantage +for the specially desired characteristics of bricks, terra-cotta, +pottery and porcelain are all such as to be incompatible with +plasticity. The latter is extremely valuable in the shaping of the wares +mentioned, but after the manufacture is completed, the destruction of +the plasticity is an essential feature of their usefulness. + +If the heating is very prolonged or is repeated several times, clays +change other of their physical characters and become brittle and liable +to crack under sudden changes of temperature. This is partly due to the +further fusion (vitrification) which occurs and partly to the formation +of crystalline silicates, notably _Sillimanite_ (13). + +The extent to which clays are ordinarily heated and the conditions under +which they are cooled do not usually induce the formation of crystals; +the object of the clayworker being to produce a homogeneous mass, the +particles of which are securely held together. The result is that burned +clay products are usually composed of amorphous particles cemented by a +glass-like material formed by the fusion of some of the mineral +ingredients of the original substance. The silicates formed are, +therefore, in a condition of solid, super-cooled solution in which the +tendency to crystallize is restrained by viscosity. + +On raising the temperature of firing or on prolonging the heating at the +previous maximum temperature the viscosity of the fused portion is +diminished and crystallization may then occur. The facility with which +crystallization occurs varies greatly with the composition of the fused +material, those silicates which are rich in lime and magnesia +crystallizing more readily than those containing potash or soda. Vogt +has stated that small quantities of alumina promote the formation of a +glassy structure, and Morozewicz has shown that a large excess of this +substance must be present if crystallization is to occur. + +The study of the reactions which occur when clays are heated is, +however, extremely complex, not only on account of the variety of +substances present, but also on account of the high temperatures at +which it is necessary to work, so that for a further consideration of it +the reader should consult special treatises on the fusion of silicates. +This subject has now become an important branch of physical chemistry. + + + + +CHAPTER II + +CLAY AND ASSOCIATED ROCKS + + +Clay, as already mentioned, is geologically a rock and not a mineral, +and belongs to the important group of sedimentary rocks which have been +derived from the igneous or primary ones by processes of weathering, +suspension in water and subsequent deposition or sedimentation. + +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. + +In some cases they have again been transported and re-deposited and not +infrequently clay deposits are found which show signs of subsequent +immersion at considerable depths and have every appearance of having +been subjected to enormous pressures and possibly to high temperatures. + +Some clays have only been carried by small streams and for short +distances; these are seldom highly plastic and resemble the lean china +clays and kaolins. Others have been carried by rapidly moving rivers and +have been discharged into lakes or into the sea; they have thus +undergone a process of gradual purification by elutriation, the sand and +other heavier particles being first deposited and the far smaller +particles of clay being carried a greater distance towards the centre of +the lake or the quieter portions of the ocean. The nature of such +deposits will, naturally, differ greatly from each other, the materials +at first associated with the clay, or becoming mixed with it at a later +stage, exercising an important influence on its texture, composition and +properties. 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 destroying the nature of +the clay. In addition to this it must be remembered that the land is +continually rising or sinking owing to internal changes in the interior +of the earth, and that these subterranean changes bring about tilting, +folding, overturning and other secondary changes, which, later, cause a +fresh set of materials to be mixed with the clays. Further than this, +the action of the weather, of rivers and of the sea never ceases, so +that a process of re-mixing and re-sorting of materials is continuously +taking place, and has been doing so for countless ages. It is, +therefore, a legitimate cause for wonder that such enormous deposits of +clays of so uniform a character should occur throughout the length and +breadth of Europe, and practically throughout the world. For although +the composition of many of these beds is of a most highly complex +nature, the general properties such as plasticity, behaviour on heating, +etc., remain remarkably constant over large areas of country, and the +clays of each geological formation are so much alike in different parts +of the world as to be readily recognized by anyone familiar with the +material of the same formations in this country. Considerable +differences undoubtedly exist, but these are insignificant in comparison +with the vastly different circumstances under which the deposits were +accumulated. + +Leaving the consideration of the modes of formation of the various clay +deposits to later chapters (III and IV), it is convenient here to +enumerate some of the chief characteristics of the different clay +deposits and their associated rocks. In this connection it is not +proposed to enter into minute details, but rather to indicate in broad +outline the chief characteristics of the clays from the different +deposits. This general view is the more necessary as clay occurs in each +main geological division of the sedimentary rocks and in almost every +sub-division in various parts of the world. + +The =Precambrian, Cambrian, Silurian and Devonian= 'clays' are chiefly +in the form of shales or slates, the latter being clays which have +undergone a metamorphic change; the latter resulted in the production of +a hard and partially crystalline material with but little potential +plasticity and therefore of small importance for the ordinary purposes +of clay working. + +_Slates_ are distinguished from shales by their splitting into thin +leaves which are not in the plane of original deposition, but are due to +the deposited material being subjected to great lateral pressure. The +re-arrangement of the particles thus produced has imparted to the +material a cleavage quite independent of the original lamination. + +The shales in these formations are occasionally soft and friable and are +then termed _marls_, but this name is misleading as they contain no +appreciable proportion of finely divided calcium carbonate as do the +true marls[8]. + +[Footnote 8: Readers desiring more detailed information on the +occurrence of the clays mentioned in this chapter should refer to the +author's _British Clays_ (No. 2 in Bibliography).] + +The clays in the =Carboniferous Limestone= are not, as a whole, of much +importance, but the occurrence in this formation of pockets of white +refractory clays in Staffordshire, North Wales (Mold) and Derbyshire is +interesting, especially as these are used for the manufacture of +firebricks and furnace linings. These clays are highly silicious and in +composition are intermediate between the Yorkshire fireclays and +ganister. Their origin is uncertain, but it is generally considered that +they have been produced by the action of the weather and streams on the +shales and grits of the Coal Measures which formerly occupied the higher +ground around them, though Maw (16) states that 'it is scarcely open to +question that they are the remnants of the subaerial dissolution of the +limestone' (see 'Fireclays,' Chapter V). + +In the =Upper Carboniferous System= the clays are highly important +because of their general refractory nature, though they differ greatly +in this respect, some red-burning shales of this formation having no +greater power to resist heat than have some of the surface clays. + +Those of the Coal Measures are of two main kinds--shales, or laminated +rocks which readily split along the planes of deposition, and +unstratified underclays. The _shales_ usually occur above the seams of +coal and are either of lacustrine or marine origin, differences in their +fossils and lithological character supporting one origin for some +deposits and the other for the remainder. Some of them are fairly +uniform in composition, but others vary so greatly in their physical +characters, that they are divided by miners into 'binds' or relatively +pure shales, 'rock-binds,' or sandy shales, and sandstones. They also +vary greatly in thickness in different localities, and whilst they form +the main feature in some districts, in others they are replaced by +sandstones. + +The _underclays_ are so called from their usually lying beneath the coal +seams. They are not noticeably stratified and vary greatly in character +from soft unctuous materials to hard, sandy rocks. In composition they +vary enormously, the percentage of silica ranging from 50 per cent., or +less, to as high as 97 per cent. + +The mode of formation of the underclays is not certainly known. They do +not appear to be soils or of terrestrial origin, but according to Arber +(24) correspond closely to the black oozes of marine and semi-marine +estuarine deposits of tropical swamps, or to the muds surrounding the +stumps of trees in the buried forests of our coast-lines. They thus +appear to be quite distinct from the shales above them, both in origin +and physical characters. The more silicious portions, known as +_Ganister_[9], possess comparatively few of the characteristics of clay +though used, like all the more refractory clays of the Coal Measures, +for all purposes for which fireclay is employed. The term _fireclay_ is, +in fact, frequently applied to all the refractory deposits in the Coal +Measures, without much regard to their composition (see Chapter V). + +[Footnote 9: The Dinas rock used in the Vale of Neath (Wales) is an even +more silicious material found in the Millstone Grit immediately below +the Coal Measures. It is largely employed for firebricks.] + +Valuable Coal Measure clays occur in enormous quantities in +Northumberland, Durham, Yorkshire, Nottinghamshire, Derbyshire, +Staffordshire, near Stourbridge, in Warwickshire, Shropshire, North and +South Wales and South West Scotland. In Ireland, on the contrary, the +Coal Measure clays are of little value except in the neighbourhood of +Coal Island, co. Tyrone. The position of the 'Sagger Marls' of North +Staffordshire (Keele Series and Etruria Marls), relative to the +'Farewell Rock' or Millstone Grit, is shown in fig. 8 in which the +horizontal lines represent coal-seams and ironstone veins. + +[Illustration: + +---------------------+ + | _Keele Series_ | + | | + +---------------------+ Newcastle + | | + +---------------------+ Coal + | | + | | + | _Etruria | + | Marls_ | + | | + Top Red Mine +---------------------+ + | | + | | + Gubbin Ironstone +---------------------+ + | | + | | + | | + | | + +---------------------+ Knowles Coal + | | + | | + Burnwood Ironstone +---------------------+ + | | + | | + +---------------------+ Mossfield Coal + +---------------------+ 5 ft. Coal + | | + | | + | | + | | + +---------------------+ Hard mine Coal + | | + | | + +---------------------+ Cockshead Coal + | | + | | + +---------------------+ Crabtree Coal + +---------------------+ + | _Millstone Grit_ | + | | + +---------------------+ + + Fig. 8. Coal Measures sequence in North Staffordshire.] + +The dissimilarities in the fossils of the Coal Measure clays and shales +in the Northern and Southern Hemispheres suggest that there is a +considerable difference in their formation, but the number of clays and +shales which have been examined is too small for any accurate conclusion +to be drawn. + +For many industrial purposes, particularly for the manufacture of +refractory goods, the clays and shales of the Carboniferous System are +highly important. The less valuable burn to a reddish colour, often +spoiled with many grey spots of ferrous silicate derived from the +pyrites in the clay, but the purer varieties burn to a delicate primrose +or pale buff tint and are amongst the most heat-resisting materials +known. The Coal Measure clays of Yorkshire are particularly esteemed for +their refractory properties; for the manufacture of glazed bricks and +for blocks for architectural purposes somewhat ambiguously termed +'glazed terra-cotta.' The inferior qualities are largely used for the +manufacture of red engineering bricks, some of them competing +successfully with the more widely known 'blue bricks' of Staffordshire. + +The Coal Measure clays of Shropshire are noted for the manufacture of +red roofing tiles, especially in the neighbourhood of Broseley. + +Agriculturally, the Coal Measure clays are usually poor, but are +occasionally of good quality. The shales produce heavy, cold clays and +the yellow subsoil produces soils of a light, hungry character so that +the two should, if possible, be mixed together. + +=Permian clays= are of little value except for the manufacture of red +building bricks. The Nottinghamshire Permian clays make excellent +roofing tiles, flower pots and red bricks. + +Agriculturally, the Permian clays are a free working loam yielding large +crops of most of the ordinary farm products. + +=Triassic clays= are of great importance in the Midlands, those upper +portions of them known as the Keuper Marls being much used for the +manufacture of bricks. + +They are specially known amongst clayworkers as the material from which +the Midland red bricks of Nottinghamshire and Leicestershire and the +Somersetshire tiles are prepared. + +=Jurassic clays= are an important group, of marine origin, occurring in +close association with limestone. For this reason they form a valuable +source of material for the manufacture of Portland cement, but are of +less value to the brick and tile manufacturer. The Jurassic System +contains so large a variety of clays, of such widely different ages and +characteristics, that no general description of them can be given in the +present volume. + +[Illustration: Fig. 9. Lias clay being worked for the manufacture of +hand-made sand-faced roofing tiles. (_By courtesy of Messrs Webb Bros. +Ltd., Cheltenham._)] + +The '_Lias clays_'--the lowest of the Jurassic formation--are chiefly +dark, bituminous shales, including the 'alum shales,' and are often +seriously contaminated with pyrites and ironstone. When carefully +selected they may be used to advantage in the production of most red +articles such as bricks, tiles, chimney pots, etc. They shrink less in +the kiln than do most clays, and are easily fusible on account of the +lime they contain, but on the whole this formation is of great value for +the manufacture of the articles just mentioned. + +Agriculturally, the Lias clays are laid down for grass, but the lighter +soils are useful for arable purposes. + +The '_Oolitic clays_,' which are also Jurassic, usually contain +limestone in the form of nodules, but are nevertheless important. They +form a broad belt above the Lias from Dorset to Yorkshire, and include +the blue clays of the Purbeck beds, stiff blue bituminous Kimeridge +clays, the irregular, sandy Coral Rag clays, the famous Oxford clay +(from which the Peterborough and Fletton bricks are made), the Kellaways +blue clay, and the Fuller's Earth deposits. + +The '_Kimeridge clays_' are dark, stiff laminated clays, closely +resembling gault, and are much used in the West and Midlands for +brickmaking. A well-known deposit of this character has long been used +at Pickering in Yorkshire, but the most typical deposits are in +Huntingdonshire. The Kimeridge clays contain a bituminous shale, or +Sapropelic Coal, which evolves a characteristic odour on burning. + +Agriculturally, the Kimeridge clays resemble gault and are difficult to +work as arable land, though they form first-rate pasturage. + +[Illustration: Fig. 10. Oxford clay near Peterborough. (_By courtesy of +Messrs Ruston, Proctor & Co. Ltd._)] + +The '_Oxford clays_' are valuable for brickmaking when their use is +understood, but to the uninitiated they are very troublesome. Their +colour is dark blue or grey and they are usually stiff or somewhat shaly +in texture with layers of variable composition. The closely associated +Cornbrash (limestone) is a source of trouble unless great care is taken +in the selection of the material. 'Oxford clays' are not infrequently +traversed by seams of poor coal or by oil-shales. + +Agriculturally, Oxford clay is difficult to work and, while much of it +is valuable, large portions are poor and cold. When well exposed to +frost it is made much lighter, but even then is not very suitable for +wheat and autumn sown crops. + +The '_Kellaway blue clays_' are often included in the Oxford clays, +though they form irregular bands above them and are of fresh-water +origin, whilst the Oxford clays are marine deposits. They are chiefly +used commercially for domestic firebricks near Oundle and Stamford. + +=Cretaceous clays= occur, as their name implies, in association with +chalk. The chief clay in this System is the _gault_, a stiff, black, +calcareous clay of marine origin chiefly used for brickmaking. When used +alone, gault burns to a reddish colour, due to the iron present, but if, +as is more usual, it is mixed with chalk, it burns perfectly white. Some +gaults contain sufficient chalk to render the addition of a further +quantity unnecessary. + +Agriculturally, the Cretaceous clays form good arable soil where they +are not too exposed, but they suffer from drought. + +The '_Wealden clay_' is a stiff yellowish grey or blue clay extensively +used for brickmaking in Kent, Sussex and Surrey. It has been subdivided +by geologists into a number of other clays, such as the Wadhurst, +Fairlight, etc., but the differences between them lie more in the +fossils occurring in them than in the characters of the clays +themselves. They are usually contaminated with ironstone, gypsum and +some limestone. + +Agriculturally, the Wealden clay produces stiff, yellowish soils of a +wet and poor character, but sometimes loams of a highly productive +nature occur. + +The =Tertiary clays= include all those deposited after the Chalk and +previous to the close of the Glacial period. They are usually mixed with +sand and gravel, and though the deposits are often thin and irregular +they are the most generally important of all clays. They vary greatly in +character; some, like the London clay, being almost useless unless mixed +with other materials, whilst others like the ball clays of Devonshire +and Dorset are amongst the purest and most valuable of the plastic +clays. The Tertiary clays are divided by geologists into Pliocene, +Miocene and Eocene formations; of these the first are commercially +unimportant and the second do not exist in Great Britain. At one time +the Bovey Tracey clays were considered to be Miocene, but they have +recently been classed as Oligocene by Clement Reid. + +Agriculturally, the most important of the Tertiary formations is the +Eocene, particularly near London, though it is much covered by sand or +gravel. The _London clay_, which produces a heavy brown soil, is of +slight value, though when properly drained it produces good crops of +wheat, beans, and cabbages and other market-garden produce. For this +purpose it is greatly improved by the addition of lime and of town +manure. The South Hampshire Eocene beds of clay are cold, wet and of +small agricultural value. + +The Eocene clays are composed of a variety of clays, many of which are +only distinguishable by the different fossils they contain. The most +important are the Reading clays, the London clay and the Bagshot clays. + +The _Reading clays_ extend over a considerable area in the South of +England and are most valuable near the town from which they derive their +name. The best qualities are mottled in a characteristic manner and are +particularly suitable for the manufacture of roofing tiles and small +terra-cotta--an industry for which Reading is famous. + +The _London clay_ is always a treacherous material and is best avoided +in the manufacture of bricks and other articles except under highly +skilled technical advice. + +The _Bagshot clays_ in Dorsetshire are famous for the ball and pipe +clays shipped from Poole, whilst at Bovey Tracey and in several parts of +Devonshire equally valuable ball clays are found and are shipped from +Teignmouth. + +These _ball clays_ are of variable composition and colour and require +careful selection and testing. They are closely associated with sands, +but the lower beds of clay are remarkably stiff, plastic and +white-burning. The colour of the raw clay varies from a pale yellow to a +dark brown or even to black, but this is little or no criterion of the +colour of goods made therefrom, as the colour is due to carbonaceous +matters, 4 per cent. or more carbon being usually present. + +The 'blue' and 'black' ball clays are the most valued by potters, but +the quality is usually ascertained by a burning test. + +The value of these ball clays both in Devonshire and Dorset is due to +their comparative freedom from iron and alkalies and to their remarkable +unctuousness and plasticity. They are, therefore, largely used in the +manufacture of all kinds of earthenware of which they form the +foundation material. + +In composition, ball clays appear to consist chiefly of a +hydro-alumino-silicate corresponding to the formula H4Al2Si2O9, +and in this they very closely resemble the china clays (kaolins). The +latter are, however, but slightly plastic whilst the ball clays are +amongst the most plastic clays known. The china clays are also much more +refractory than the ball clays owing to the somewhat larger proportion +of alkalies in the latter. + +_Pipe clays_ are an inferior quality of ball clay; they contain rather +more iron and alkalies and considerably more silica. For this reason +they can only be used for cheaper wares where colour is of less +importance and where their excessive contraction can be neutralized by +the addition of other substances such as flint. + +The =Boulder clays= occur in a blanket-like covering of Drift which lies +over the greater part of Northern and Central England, and over a +considerable portion of Scotland and Ireland. They are a product of the +Ice Age and, whilst varying greatly in character, may usually be +distinguished by the occurrence in them of rounded stones and gravel, +some of the former bearing clear indications of glacial action. The +boulder clays are largely used for the manufacture of building bricks, +but the strata in which they occur are so irregular that very careful +supervision of the digging is necessary. In some localities these clays +form beds 12 ft. or more in thickness and relatively free from gravel; +in other districts the clay is interspersed with lenticular deposits of +gravel or sand (commonly known as 'pockets'), and if these are mixed +with the clay considerable difficulty in manufacture may be experienced. +The total thickness of the drift deposits is often very great, as in +the cliffs at Filey (fig. 11) which are 200 ft. high. + +[Illustration: Fig. 11. Cliffs of Boulder clay at Filey lying on +Calcareous Crag.] + +The boulder clays--considered apart from the stones, gravel and sandy +materials occurring with them--are usually red-burning, stiff and very +plastic, but the gravel, sand and crushed stones mixed with them in the +formation of the material usually render them of medium plasticity. By +careful washing, most boulder clays may be purified sufficiently to +enable coarse brown pottery to be made from them. Clean deposits of +sufficient size to be worked without any purification are occasionally +found. Usually, however, the boulder clay formation is somewhat +treacherous as it is difficult to ascertain its nature; boreholes are +apt to be quite misleading as the formation is so irregular in +character. + +Agriculturally, drift or boulder clays are poor soils, but by judicious +management and careful mixing they may be made more fertile. Where it +contains chalk--as in Norfolk and Suffolk--boulder drift forms an +excellent arable soil. + +=Pleistocene or Recent clays= are amongst the most important brickmaking +materials in the South of England. They are of remarkably varied +character, having been derived from a number of other formations. +Usually the deposits are somewhat shallow and irregular in form, but +beds of considerable thickness occur in some localities. + +Agriculturally, they are of considerable importance. + +Most of the =brick earths= used in the south-east of England are of +Recent formation, those of the Thames Valley being of special importance +in this connection, particularly where they are associated with chalk; +thus forming natural _marls_ or enabling artificial marls to be +produced. + +The brick earths--in the sense in which this term is used in the +south--comprise three important types of clay: (_a_) _Plastic clays_ not +particularly differentiated from those already described, (_b_) _Loams_ +or sandy clays which are sufficiently plastic for satisfactory use, have +the advantage of shrinking but slightly in drying, and are largely used +in the manufacture of red facing bricks and as light soils, and (_c_) +_Marls_ or calcareous clays, used for the production of light coloured +or white bricks, the chalk they contain combining with any iron +compounds present and, at the same time, reducing the contractility of +the clay. On burning, they form a cement which binds the particles into +a strong mass. These are the 'true marls' or 'malms' composed of clay +and chalk and must not be confused with the so-called marls of +Staffordshire and elsewhere which are almost free from lime compounds. +There is, at present, no definition of 'marl' which is quite +satisfactory; a maker of London stock bricks understanding by this term +a clay containing at least 10 per cent. of chalk; a maker of white +Suffolk bricks a material containing at least twice this amount; an +agriculturalist any soil, not obviously sandy, which will make his clay +land less sticky; and many geologists any friable argillaceous earths. A +general consensus of opinion is, however, being gradually reached that +the term 'marl' should be limited, as far as possible, to clays +containing calcium carbonate in a finely divided state. + +=Alluvial deposits=--which are also of Recent formation, though still of +sufficient age for skeletons of mammoths to be found in them--are of so +variable a nature as to render any brief, general description +impossible. Many of them are so contaminated with sand and crushed +limestone as to be useless for manufacturing purposes and of small value +agriculturally, but others are important in both these respects. + +Further details of the occurrence of clays in the various formations +described will be found in the _Maps and Memoirs of the Geological +Survey_ and in the author's _British Clays_ (2). + + + + +CHAPTER III + +THE ORIGINS OF CLAYS + + +The terms 'primary' and 'residual' are applied to those clays which are +found overlying or in close association with the rocks from which they +have been derived, and distinguish them from the 'secondary' or +'transported' clays which have been carried some distance away from +their place of origin. + +=Residual clays= may be formed by the simple removal of other materials, +the clay remaining behind, as in the decomposition of some argillaceous +limestones, in which the calcareous matter has been removed by solution +whilst the clay is unaffected. Such a clay is not a primary one as it +has probably been derived from some distant source and, having been +deposited along with the limestone ooze, has formed an intimate mixture +from which the limestone has, at a later geological epoch, been removed +in the manner indicated. Residual clays are seldom pure, being often +rich in iron compounds, though the white clays of Staffordshire and +Derbyshire are highly refractory. + +It is seldom necessary to distinguish residual clays from other +secondary or transported ones (Chapters II and IV). + +=Primary clays=, on the contrary, have been derived from rocks which +have undergone chemical decomposition, one of the products being clay. +The most important primary clays are the kaolins, which are derived from +the decomposition of felspar, but other primary clays derived from other +minerals are known, though less frequently mentioned. + +The _kaolins_ are primary clays[10] formed by the decomposition of +felspar and occur in many parts of the world. In Great Britain the most +important are the china clays found in Devon and Cornwall, which occur +in association with the granite from which they have been formed. The +kaolins in Germany are, apparently, of similar origin, though some are +derived from porphyry and not from granite; they are the chief material +used in the manufacture of Dresden, Meissen, Berlin and other +porcelains. The French kaolins from St Yrieux and Limousin are said by +Granger (17) to be derived from gneiss amphibole. The American kaolins +have, according to Ries (6), been chiefly formed from the weathering of +pegmatite veins, but the origin of some important deposits in Texas and +Indiana has not yet been fully explained. + +[Footnote 10: Some kaolins in central Europe appear to have been +transported and of secondary origin.] + +[Illustration: Fig. 12. China clay pit belonging to the North Cornwall +China Clay Co. (_By courtesy of W. H. Patchell Esq._)] + +The corresponding material used by the Chinese for the manufacture of +porcelain bears a name which is really that of the place from whence it +was originally obtained; the term _Kao-ling_ indicates merely a high +ridge. According to Richthofen (18) the rock from which Chinese +porcelain is made is not a true kaolin, but is allied to the _jades_. +The term 'kaolin' is therefore a misnomer when applied to white-burning, +primary clays generally, but its use has become so firmly established as +to render it permanent. + +Kaolins are seldom found in a sufficiently pure state to be used direct, +but must be freed from large amounts of undecomposed rock, quartz, mica, +etc., by a process of washing and sedimentation. When purified in this +manner, the best qualities of china clay yield, on analysis, alumina, +silica and water in the proportions indicated by the formula +H4Al2Si2O9 together with about 5 per cent. of mica and other +impurities. Some high class commercial kaolins contain over 30 per cent. +of mica and 10 per cent. of quartz. + +The chief constituents of rocks which take part in the production of +kaolins appear to be the felspars, but the natural processes by which +these felspars are decomposed are by no means perfectly understood. Some +kaolins appear to have been formed by weathering and others by subaerial +action. Thus Collins (19) has stated very emphatically that the +kaolinization of Cornish felspar has been chiefly effected by fluorine +and other substances rising from below and not by carbonic acid and +water acting from above. Ries (6) and other American observers are +equally convinced that certain kaolins they have examined are the result +of 'weathering.' German and French investigators are divided in their +opinions, and Fuchs has found that the Passau (Saxony) kaolin is derived +from a special mineral, not unlike a soda-lime felspar deficient in +silica, to which he has given the name 'porcelain spar.' + +The _felspars_ form a class of minerals whose chief characteristic is +the combination of an alkaline or alkaline-earth base with silica and +alumina. Orthoclase (K2OAl2O3·6SiO2)--the chief potassium +felspar--is typical of the whole class. When treated with water under +suitable conditions, the felspar appears to become hydrolysed and some +of the water enters into combination, the potash being removed by +solution. Attempts to effect this decomposition artificially have proved +abortive though several investigators appear to have effected it to a +limited extent by electrolysis or by heating under great pressure (3). + +The effect on felspars of waters containing carbon dioxide in solution +has been studied by Forschammer, Vogt, and others, and they have +concluded that kaolinization may occur with this agent though it does +not appear to be the chief cause in the formation of Cornish china +clays. + +[Illustration: Fig. 13. Orthoclase Felspar, natural size. (_From Miers_' +Mineralogy _by permission of Macmillan & Co._)] + +The probable effect of fluoric vapours has been studied by Collins (19) +who confirmed von Buch's observation that fluorides (particularly +lepidolite and tourmaline) are constantly associated with china clay; he +found by direct experiment that felspar is decomposed by hydrofluoric +acid at the ordinary temperature without the other constituents of the +granite in which it occurs being affected. This theory is confirmed by +the great depths of the kaolin deposits in Cornwall and in Zettlitz +(Bohemia) which appear to be too great to render satisfactory any theory +of simple weathering though kaolins in other localities, especially in +America, appear to be largely the result of weathering. According to +Hickling (36) the product of the action of hydrofluoric acid 'has not +the remotest resemblance to china clay.' + +Kaolin, when carefully freed from its impurities, as far as this is +possible, is peculiarly resistant to the action of water. This +resistance may be due to its highly complex constitution, as the simpler +hydro-alumino-silicates, such as collyrite, show an acid reaction when +ground with water. Rohland (5), therefore, suggests that kaolinization +is effected by water first hydrolysing the felspar and forming colloidal +silica and sodium or potassium hydroxides which are removed whilst the +complex alumino-silicate remains in the form of kaolin. Hickling (36), +on the contrary, believes that the action of the weather on felspar +produces secondary muscovite--a form of mica--and that this is, later, +converted into kaolinite or china clay (fig. 17, p. 105). + +The various theories which have been propounded may be summarized into +three main classes, and whilst it is probable that any one of them, or +any one combination, may be true for a particular kaolin, yet the whole +process of kaolinization is so complex and the conditions under which it +has occurred appear to be so diverse that it is doubtful if any simple +theory can be devised which will satisfactorily meet all cases. + +(_a_) The decomposition of the granite, and particularly of the felspar +within it, may be ascribed to purely chemical reactions in which the +chief agents are water and carbon dioxide. + +(_b_) Other substances--possibly of an organic nature and derived from +the soil--may have played an important part. + +(_c_) Wet steam and hot solutions of fluorine, boron or sulphur +compounds may have effected the decomposition. + +The recent progress made in the application of the laws of physical +chemistry to geological problems is continually throwing fresh light on +this interesting subject. Thus, studies of the dissociation pressures +and transition points between the anhydrous and the hydrous states of +various substances and the effect of water as a powerful agent of +decomposition (hydrolysis) have shown that hydration is a +characteristic result of decompositions occurring in the upper portions +of the earth's crust and not in the lower ones, and that it is usually +checked, or even reversed, when the substance is under great pressure. +At great depths kaolins and other complex hydrous silicates give place +to anhydrous ones such as muscovite, andalusite and staurolite. There +is, therefore, good reason to believe that the kaolinization of Cornish +felspar has occurred at only moderate depths from the surface and that +it has been chiefly produced by the action of water containing acid +gases in solution. The acid in the water may have been absorbed from the +atmosphere, or it may be due to vapours rising from below through the +felspathic material. + +In Great Britain, china clay occurs in the form of powdery particles +apparently amorphous, but containing some crystals, scattered through a +mass of harder rock, the whole being known as china clay rock or +'carclazite.' The softer portions of this china clay rock are known as +'growan' and the china clay in it represents only a small proportion of +the whole material. + +The finer particles of clay and other materials are removed by treatment +with water, whereby one-third to one-eighth of the material is +separated. This small proportion is then subjected to further washing +and sedimentation in order to obtain the china clay in a state of +commercial purity. It will thus be understood that the Cornish china +clays are not 'deposits' in the usual acceptation of that term, the soft +growan from which they are obtained being almost invariably the result +of decomposition _in situ_ of some species of felspar in disintegrated +granite. + +The commercial kaolins of France, Germany, America and China very +closely resemble the Cornish china clays in composition, but when used +in the manufacture of porcelain they create differences in the finished +material which are clearly noticeable, though microscopical examination +and chemical analysis, at present, fail to distinguish between them in +the raw state on account of their great resistance to ordinary chemical +and physical forces. + +In addition to the breaking up of felspathic rocks with the formation of +china clay or kaolin (kaolinization), other decompositions which occur +may result in the formation of clays, and an examination of a +considerable number of clays by J. M. van Bemmelen (26) has led him to +suppose that several different clay-forming forces have been at work in +the production of clays. He classifies these under four heads: + +(1) _Kaolinization_, or the decomposition of felspathic and similar +rocks by the action of telluric water containing active gases in +solution. + +(2) _Ordinary weathering_ in which the action is largely mechanical, but +is accompanied by some hydrolysis owing to the impurities contained in +the water which is an essential factor. + +(3) _Lateritic action_--or simple decomposition by heat--which occurs +chiefly under tropical conditions, but may also occur in temperate +climates, and has for its main product a mixture of free silica and +alumina, the latter being in the form of (amorphous) 'laterite.' It may +not improbably be a result of the decomposition of the clay molecule +similar to that which occurs when china clay is heated, as there is no +temperature below which it can be said that china clay does not +decompose into free silica and alumina (29). + +(4) _Secondary reactions_ in which the products of one of the reactions +previously described may undergo further changes, as the conversion of +amorphous clayite into crystalline kaolinite, or amorphous laterite into +crystalline hydrargillite. + + +Weathering. + +The action of the forces conveniently classed under the term +_weathering_ are of two main kinds: + +(_a_) The _mechanical grinding_ of sandstone, quartzite, limestone, and +other rocks, causes an addition of adventitious material to clay, the +proportions being sometimes so large as to render it necessary to term +the material an argillaceous sand, rather than a sandy clay. Some of +these grains of mineral matter are so minute and so resistant to the +ordinary chemical reagents as to make it extremely difficult to +distinguish them from clay. + +(_b_) The _chemical decomposition_ due to the action of very dilute +solutions. By this means simple silicates are decomposed with the +formation of colloidal silica which may either remain in solution or may +be deposited in a coagulated form. At the same time, some +alumino-silicates will be similarly decomposed into colloidal +alumino-silicic acids or clays. + +The ultimate results of the action of ordinary weathering on silicate +rocks are, therefore, sands and clays, the latter being in some ways +quite distinct in their origin and physical properties from the china +clays. According to J. M. van Bemmelen (26) such clays also contain an +alumino-silicate soluble in boiling hydrochloric acid followed by +caustic soda, whereas pure china clays are unaffected by this treatment. + +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 effects an increase in the +plasticity of the clay. There thus appear to be at least three kinds of +primary clay, viz.: + +_Kaolinic_ or _china clays_ which are chiefly derived from felspar and +can be isolated in a relatively pure state. They are highly refractory, +but only slightly plastic. + +_Epigenic_ or _colloidal clays_ derived from kaolinic clays, as a +secondary product, or directly from felspar, mica, augite and other +alumino-silicates by 'weathering.' They are usually less refractory and +much more plastic than the china clays and contain a large percentage of +impurities--sometimes in the form of free silica (sand) or of metallic +oxides, carbonates, sulphides, sulphates, silicates, or other compounds. +Many so-called secondary clays such as pipe clays, ball clays and +fireclays may be of this type, though their origin is difficult to trace +owing to their subsequent transportation and deposition. + +_Lateritic_ or _highly aluminous clays_, of a highly refractory +character, but low plasticity. They are usually somewhat rich in iron +oxide which materially affects their plasticity. Unlike the china clays, +pure lateritic clays are completely decomposed by hydrochloric acid. +Bauxite and some of the highly aluminous clays of the Coal Measures +appear to be of this type. + +Unfortunately these different types of clay are extremely difficult to +distinguish and in many instances they have become mixed with each other +and with other materials during the actions to be described in the next +chapter, that it is often almost impossible to decide whether the true +clay in a given specimen possessed its characteristics _ab initio_ or +whether it has gained them since the time when it ceased to be a primary +clay. + +=Secondary clays= are those which have been produced by the action of +the weather and other natural forces on primary clays, the changes +effected being of a physical rather than a chemical nature (see Chapter +IV). + +The essential constituent of secondary clays has not been positively +identified. In so far as it has been isolated it differs from the true +clay in the primary clays in several important respects, and until its +nature has been more fully investigated great caution must be exercised +in assigning a definite name to it. For many purposes the term +_pelinite_ (p. 149) is convenient, being analogous to the corresponding +one used for material in china clays (_clayite_, p. 107). These terms +are purely provisional and must be discarded when the true mineralogical +identities of the substances they represent have been established. + + + + +CHAPTER IV + +THE MODES OF ACCUMULATION OF CLAYS + + +From whatever sources clays may have been originally derived, the manner +in which they have been accumulated in their present positions is a +factor of great importance both in regard to their chemical and physical +characters and their suitability for various purposes. + +As explained in Chapter III, the china clays or kaolins may usually be +regarded as primary clays derived from granitic or other felspathic or +felsitic rocks by chemical decomposition. Such clays are found near to +their place of origin, are usually obtainable in a comparatively pure +state and are generally deficient in plasticity. They may occur in beds +of small or great depth, but these are not 'accumulations' in the +ordinary meaning of that term. + +Residual clays (p. 70) also form a distinct class, as unlike the +majority of argillaceous materials they are left behind when other +substances are removed, usually by some process of solution. In many +cases, however, the residual clays are really secondary in character, +having been transported from their place of origin, together with +limestone or other minerals, the mixture deposited and subjected to +pressure and possibly to heat, whereby a rock-like mass is formed. This +mass has then been subjected to the solvent action of water containing +carbon dioxide or other substances which dissolve out the bulk of the +associated minerals and leave the residual clay behind. + +The chief agents in the transport and accumulation of clays are the +_air_, in the form of wind; _water_, in the form of rain, streams, +rivers, floods, lakes and seas, or in the form of ice and snow as in +glaciers and avalanches; _earth-movements_ such as the changes wrought +by volcanoes, earthquakes and the less clearly marked rising and falling +of various portions of the earth's crust which result in folded, +twisted, sheared, cracked, inclined, laminated and other strata. + +These agents have first moved the clay from its original site and have +later deposited it with other materials in the form of strata of widely +varying area and thickness, some 'clay' beds being several hundred feet +in depth and occupying many square miles in area, whilst others are in +the form of thin 'veins' only a few inches thick or in 'pockets' of +small area and depth. These deposits have in many places been displaced +by subsequent earth-movements and have been overlain by other deposits +so as to render them quite inaccessible. Others have been covered by +deposits several miles thick; but the greater part of the covering has +since been removed by glacial or other forces, so that clays of +practically all geological ages may be found within the relatively small +area of Great Britain. + + +The Transportation of Clays. + +By the action of wind or rain, or of rain following frost, the finer of +particles clay are removed from their primary site and as the rain drops +collect into streamlets, these unite to form streams and rivers and the +clay with its associated minerals is carried along by the water. As it +travels over other rocks or through valleys composed of sandstone, +limestone and other materials, some of these substances are dislodged, +broken into fragments of various sizes and with the clay are carried +still further. In their journey these materials rub against each other +and against the banks and bed of the stream, thereby undergoing a +prolonged process of grinding whereby the softer rocks are reduced to +very fine sand and silt which becomes, in time, very intimately mixed +with the clay. If the velocity of the stream were sufficiently great, +the mixed materials--derived from as many sources as there are rocks of +the districts through which they have passed--would be discharged into a +lake or into the sea. Here the velocity of the water would be so greatly +reduced that the materials would gradually settle, the largest and +heaviest fragments being first deposited and the finer ones at a greater +distance. + +With most streams and rivers, however, the velocity of the water is very +variable, and a certain amount of deposition therefore occurs along the +course, the heavier particles only travelling a short distance, whilst +the finer ones are readily transported. If the velocity of the stream +increases, these finer particles (which include the clay) may become +mixed with other particles of various sizes and the materials thus +undergo a series of mixings and partial sortings until they are +discharged at the river mouth or are left along its sides by a gradual +sinking of the water level. The clay will be carried the whole course of +the river, unless it is deposited at some place where the velocity of +the water is reduced sufficiently to permit it to settle. + +If floods arise, the area affected by the water will be increased. The +_alluvial clays_ have, apparently, been formed by overflowing streams +and rivers, the material in suspension in the water being deposited as +the rate of flow diminished. Such alluvial deposits contain a variety of +minerals--usually in a very finely divided state--clay, limestone-dust +or chalk, and sand being those most usually found. + +_River-deposited clays_, _i.e._, those which have accumulated along the +banks, are characterized by their irregularity in thickness, their +variable composition and the extent to which various materials are mixed +together. This renders them difficult to work and greatly increases the +risks of manufacture as the whole character of a fluviatile clay may +change completely in the course of a few yards. + +According to the districts traversed by the water, the extent to which +the materials have been deposited and re-transported and the fresh +materials introduced by earth-movements, river-deposited clays may be +(_a_) _plastic_ and sufficiently pure to be classified as 'clays,' (_b_) +_marls_ or clays containing limestone-dust or chalk thoroughly mixed +with the clay, and (_c_) _loams_ or clays containing so much sand that +they may be distinguished by the touch from the clays in class (_a_). +Intermediate to these well-defined classes there are numerous mixtures +bearing compound names such as sandy loams, sandy marls, argillaceous +limestone, calcareous sands, and calcareous arenaceous clays, to which +no definite characteristics can be assigned. + +To some extent a transportation of clays and associated materials occurs +in _lakes_, but the chief processes there are of the nature of +sedimentation accompanied by some amount of separation. On the shores +of lakes, and to a much larger extent on the sea coasts, extensive +erosion followed by transportation occurs continuously, enormous +quantities of land being annually removed and deposited in some portion +of the ocean bed. The erosion of cliffs and the corresponding formation +of sand and pebbles are too well known to need further description. It +should, however, be noticed that the clay particles, being much finer, +are carried so far away from the shore that only pebbles and sand remain +to form the beaches, the finer particles forming 'ocean ooze.' + +The action of the _sea_ in the transport of rock-materials is more +intense than that of rivers, the coasts being worn away by repeated +blows from the waves and the pebbles and sand grains the latter contain. +The ocean currents carry the materials dislodged by the waves and +transport them, sometimes to enormous distances, usually allowing a +considerable amount of separation to take place during the transit. In +this way they act in a similar manner to rivers and streams. + +_Glaciers_ may be regarded as rivers of ice which erode their banks and +bed in a manner similar to, but more rapidly than, streams of water. +Owing to their much greater viscosity, glaciers are able to carry large +boulders as well as gravel, sand and clay, so that the materials +transported by them are far more complex in composition and size than +are those carried by flowing water. + +[Illustration: Fig. 14. Illustrating the successive deposition of +different strata.] + + +Separation and Sedimentation. + +The clay and other particles having been placed in suspension in water +by one or more of the natural forces already mentioned, they soon +undergo a process of sorting or separation, previous to their +deposition. The power of water for carrying matter in suspension depends +largely on its velocity, and when this is reduced, as when a river +discharges into a lake or sea, the larger and denser particles at once +commence to settle, the smaller ones remaining longer in suspension, +though if the velocity of the water is reduced sufficiently all the +particles will be deposited. Hence, the deposits in lakes (_lacustrine_) +and at the mouths of rivers (_estuarine_) increase more or less +regularly in fineness according to their distance from the point at +which the water enters, the gravel and stones being deposited first, +then the coarse sand, next the finer sand and finally the silt and clay +(fig. 14). If cross-currents are present, the deposits will, naturally, +be made more irregular, and in some cases variations in the flow of the +transporting water may cause the coarser particles to be carried further +than usual so that they may cover some of the finer deposits previously +formed; but as the clay and silt particles are so much finer than sand +and gravel they usually travel so far before settling that their +deposits are very uniform if the area over which they are spread is +sufficiently large. Lake-deposited clays are for this reason more +uniform than estuarine beds, whilst beds deposited at considerable +depths in the sea and at a great distance from land are still more +uniform. + +A _lacustrine clay_ is usually more persistent and uniform than +fluviatile beds though sometimes difficult to distinguish from the +latter. Some of the most valuable clay deposits are of lacustrine +formation; their comparative purity and great uniformity enabling ware +of excellent colour and texture to be produced without much difficulty. +Thus the Reading mottled clays of the Hampshire basin, on the outskirts +of the London basin and in Northern France are well known for the +admirable red bricks, tiles and terra-cotta produced from them. Still +purer clays deposited at Bovey Heathfield in Devonshire are also of +lacustrine origin, though they differ in many respects from the ordinary +lake-deposited clays and are of unusual thickness for deposits formed +in this manner. + +[Illustration: Fig. 15. Lacustrine clay at Skipsea. (_By courtesy of T. +Sheppard Esq._)] + +The greater purity of lacustrine clays, as compared with fluviatile +ones, is attributed to the much larger area over which the deposit is +spread,--enabling variations in the deposits to be much less noticeable +than when a smaller area is covered--and to the very small velocity of +the water in lakes, whereby all the coarser particles are deposited a +considerable distance away from the clays and silts. + +Ries (6) has pointed out that many (American) lake-clays are of glacial +origin, having been laid in basins or hollows along the margin of the +ice-sheet or in valleys which have been dammed by an accumulation of +drift across them. Such clay beds are usually surface deposits of +variable thickness and frequently impure. Like all lacustrine deposits +they show (though in a more marked degree than in the older and larger +lakes) alternate layers of sand and clay, though the former are usually +too thin to be noticeable except for their action in enabling the +deposited material to be easily split along the lines of bedding. + +_Estuarine deposits_ partake of the nature of both fluviatile and marine +beds, according to their position relative to the river from which they +originate. They are usually uncertain in character and are often +irregular in composition owing to the variations in the flow of the +water. The Estuarine clays of Great Britain--with the possible exception +of the Jurassic deposits in Northamptonshire and Lincolnshire--are of +minor importance, but in some countries they form a valuable source of +clay. + +[Illustration: Fig. 16. Clay at Nostel, showing Marine Band. (_By +courtesy of T. Sheppard Esq._)] + +_Marine clays_ are, as their name implies, those deposited from sea +water. They are frequently found at a considerable distance from the +shores of the ocean in which they were laid down, and subsequent risings +and fallings of the surface of the earth have so altered the areas +occupied by sea water, that a large number of marine deposits now form +dry land. Though usually of enormous size and of generally persistent +character, marine clay deposits vary considerably in the composition of +the material at different depths, as well as in different areas. This is +only to be expected from the manner of their deposition, from the varied +sources of the material and from the numerous river- and ocean-currents +taking part in their formation. For this reason it is generally +necessary to mix together portions of the deposit drawn from various +depths in order to secure a greater uniformity than would be obtained if +a larger area were to be worked to a smaller depth. + +The Oxford clay which extends from the centre of England to the centre +of France is a typical marine clay. + +At the bottom of all oceans at the present day is a deposit, of unknown +thickness, of red calcareous clay or _ooze_ which is steadily increasing +in amount and is thereby forming a fresh marine deposit, though at +present its inaccessibility deprives it of all economic value. + +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. It can readily be understood, as +stated by Arber (24), that if, at a given period, the dry land during +the formation of the Coal Measures gradually subsided, it would first be +covered with clear water, whilst from those portions of the area which +occupied the higher ground the rivers and streams continued to pour into +their estuary a large amount of fresh-water material. Later, a stage +would be reached when mud of marine origin invaded the area and covered +the previous deposits. When, after an indefinitely long period, the +ground again rose, fresh-water deposits might again form, and this +alternation of marine and fluviatile deposits appears to have been +repeated with great frequency during the Carboniferous period. + +In the Lower Coal Measures of Yorkshire and Lancashire, Stopes and +Watson (23) have shown that the shales forming the roof of the Upper +Foot Coal were derived from drifted sediments of marine origin. + + +Precipitated Clays. + +If the plasticity of some clays is really due to the colloidal nature of +their particles, it is obvious that they must have been formed by a +process of coagulation or precipitation at a distance from the site of +the minerals from which they have been derived. According to the +'colloid theory,' felspar and other alumino-silicates are decomposed by +'weathering,' the chief effect of which is the formation (by hydrolysis) +of a colloidal solution of 'clay.' This apparently clear solution flows +along in the form of a small streamlet, joins other streamlets and +continues its journey. So long as it is quite neutral or contains free +alkali the solution will remain practically clear, but as soon as acids +enter the stream, or are formed in it by the decomposition of organic +matter, a coagulation of the colloidal matter will commence and the +amount of 'clay' thus thrown out of solution will depend on the amount +of such free acid. + +If the coagulation or precipitation occurs in still water, the 'clay' +will be deposited almost immediately, otherwise it will be carried +forward until it reaches a place where it can be deposited in the +manner already described. + +Such precipitated clays need not necessarily be pure, as other +substances may be present in colloidal form and may be coagulated at the +same time as the clay. In addition to these, the admixture of sand and +other minerals present in suspension in the solution may become mixed +with the particles during coagulation and be deposited with them. + +Clays formed in this manner are extremely difficult to identify on +account of the highly complex nature of the reactions occurring in their +vicinity both during and subsequent to their formation. + + +Re-Deposited Clays. + +Although many clays and other materials have been transported and +accumulated in the manner described, the majority of those now available +have been subjected to repeated transportation and deposition, owing to +the frequent and enormous changes in the relative levels of land and +water during the various geological epochs. So far as can be +ascertained, it is during these changes of position and the recurrent +exposure to air and to water containing various substances in solution, +together with the almost incessant grinding which took place during the +transportation and deposition, that most secondary clays became +plastic. If this is the case, it explains the impossibility of +increasing the plasticity of clay by artificial means, at any rate on a +large scale. + +The simplest of the agents of re-deposition are rain-storms and floods +which, forming suddenly, may cause the water of a stream or river to +flow with unwonted velocity and so carry away previously formed deposits +of various kinds. Clays transported in this way are termed by Ries (6) +_colluvial_ clays, the term 'diluvial' is generally employed in this +country. If these are derived from a primary clay which has not +travelled far since it left the original granite from which it was +formed, they will usually be white-burning and of only slight +plasticity, but if the flood affects materials which have already been +re-deposited several times, the colluvial clays may be of almost any +imaginable composition. Floods of a different character--due to the +subsidence of the land so that it is partially covered with lake- or +sea-water, which beats on its shores and erodes it in the manner already +described--are also important factors in the transportation of clays. + +So far as clays are concerned, the action of the sea is both erosive and +depository, though the sedimentation in it being that of the pelagic +ooze at great depths the clayey material is quite inaccessible. Under +certain conditions, however, the sea may erode land in one area and may +return the transported material to the land in another area. The +diluvial clay-silt known as _warp_ in the valley of the Humber is of +this character. + +Quite apart from the action of water, however, much denudation, +transportation and re-deposition of clays and associated materials has +been due to the action of ice in the form of glaciers, though these do +not appear to have had much effect in increasing the plasticity of the +clays concerned. + +_Glacially deposited clays_ are characterised by their heterogeneous +composition, some of them containing far more sand than true clay, +whilst yet retaining a sufficient amount of plasticity to enable them to +be used for rendering embankments impervious and for the manufacture of +common bricks, and, occasionally, of coarse pottery; others contain so +much sand as to be useless for these purposes. Most glacial deposits +contain a considerable proportion of stones and gravel which must be +removed before the clay can be used. + +The large proportion of adventitious matter is due in great part to the +much greater erosive force and carrying power of ice as compared with +water, resulting in much larger pieces of material being carried, and as +the whole of the ice-borne material is deposited almost simultaneously +when the glacier melts, only a very small amount of separation of the +material into different grades takes place. The comparative freedom from +coarse sand of some glacial clays shows that some sorting does occur, +but it is very limited in extent as compared with that wrought in +materials which have been exclusively transported by water. + +For the manufacture of bricks, tiles and coarse pottery in Yorkshire, +Lancashire and some of the more northern counties of Great Britain, +glacially deposited clays are of great importance in spite of their +irregular composition. They are frequently termed 'boulder clays' or +'drift clays' (p. 65), but in using these or any other terms for clays +transported by glacial action it is important that they should not be +understood to refer to the whole of the deposited matter. Large +'pockets' of coarse sand and gravel frequently occur in deposits of this +character and veins of the same materials are by no means uncommon. The +custom of some geologists of referring to the _whole_ of a glacial +deposit as 'boulder clay' has, in a number of cases, led to serious +financial loss to clayworkers who have erroneously assumed that, because +some 'boulder clays' are used for brick and tile manufacture, all +deposits bearing a similar title would be equally suitable. This +difficulty would largely be avoided if, as is now increasingly the case, +the term 'drift' or 'glacial deposit' were used for the deposits as a +whole, the term 'boulder clay' being restricted to the plastic portions +and not including pockets of sand, gravel and other non-plastic +materials. + +_Boulder clays_--using this term in the limited sense just +mentioned--consist of variable quantities of sand and clay, stones and +gravel being generally associated with them. The stones may usually be +removed by careful picking, and the gravel by means of a 'clay cleaner' +which forces the plastic material through apertures too small to allow +the gravel to pass. The plastic material so separated is far from being +a pure clay and may contain almost half its weight of sand, the greater +part of which is readily separated by washing the material. + +Boulder clays, when freed from stones and gravel, are sufficiently +plastic to meet the needs of most users, without being so highly plastic +and contractile as to necessitate admixture with sand or similar +material. + +Some boulder clays contain limestone in the form of gravel or as a +coarse powder produced by the crushing of larger fragments. These are +less suitable for manufacturing purposes as the lime produced when the +articles are burned in the kilns is liable to swell and to disintegrate +them on exposure. + +Owing to their origin and the nature of the impurities they contain, +boulder clays are never pure and when burned are irregular in colour and +somewhat fusible unless subjected to some process of purification. + + + + +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. + +[Illustration: _Magnified 220 Diameters_ + + _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. + +[Illustration: Fig. 18. Mining best Potter's clay in Devonshire. (_Photo +by Mr G. Bishop._)] + +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). + + + + +CHAPTER VI + +CLAY SUBSTANCE: THEORETICAL AND ACTUAL + + +Having indicated the origin, modes of accumulation and general +characteristics of the numerous materials known as 'clay,' it now +remains to ascertain what substance, if any, is contained in all of them +and may be regarded as their essential constituent, to which their +properties are largely due. Just as the value of an ore is dependent to +a very large extent on the proportion of the desired metal which it +contains, and just as coal is largely, though not entirely, esteemed in +proportion to the percentage of carbon and hydrogen in it, so there may +be an essential substance in clays to which they owe the most important +of their characteristics. + +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. This is +in no small measure due to the general recognition of all minerals or +rocks which become plastic when kneaded with water as 'clays' without +much regard being paid to their composition. Consequently materials of +the most diverse nature in other respects are termed clays if they are +known to become plastic under certain conditions. + +There is, in fact, at the present time, no generally accepted definition +of clay which distinguishes it from mixtures of clay and sand or other +fine mineral particles. The usual geological definitions are so broad as +to include many mixtures containing considerably less than half their +weight of true clay or they avoid the composition of the material +altogether and describe it as a finely divided product of the +decomposition of rocks. + +Many attempts have been made to avoid this unfortunate position, which +is alike unsatisfactory to the geologist, the mineralogist and the +chemist as well as to the large number of people engaged in the purchase +and use of various clays; and, whilst the end sought has not been +reached as completely as is desirable, great progress has been made and +much has been accomplished during the last twenty years. + +One of the earliest attempts to ascertain whether there is an essential +constituent of all clays was made by Seger (7) who used two methods of +separating some of the ingredients of natural clays from the remaining +constituents. The first of these methods consists in an application of +the investigations of Schulze, Schloesing and Schoene on soils, viz. +the removal of the finest particles by elutriation; the second is an +extension of the method of Forschammer and Fresenius, viz. the treatment +of the material with sulphuric acid. + +To the product containing the clay when either of these methods is used +Seger gave the name _clay substance_, but the material so separated is +by no means pure clay. The term clay substance must, therefore, be +confined to the crude product containing the clay together with such +other impurities as are in the form of extremely small particles or are +soluble in sulphuric acid. + +It has not yet been found possible to isolate pure clay from ordinary +clays, so that in investigating the nature of what Seger was +endeavouring to produce when he obtained the crude clay substance, +indirect methods are necessary. + +It has long been known that if a sample of 'clay'--using this word in +the broadest sense--is rubbed in a considerable quantity of water so as +to form a thin slip or slurry, it may readily be divided into a number +of fractions each of which will consist of grains of different sizes. +This separation may be effected by means of a series of sieves through +which the slurry is poured, or the slurry may be caused to flow at a +series of different speeds, the material left behind at each rate of +speed being kept separate; or, finally, the slurry may be allowed to +stand for a few seconds and may then be carefully decanted into another +vessel in which it may remain at rest for a somewhat longer period, +these times of resting and decantation, if repeated, providing a series +of fractions the materials in which are more or less different in their +nature. + +'Clays' containing a considerable proportion of coarse material are most +conveniently separated into a series of fractions by means of sieves, +whereby they are divided into (i) stones, (ii) gravel, (iii) coarse +sand, (iv) medium sand, (v) fine sand and (vi) a slurry consisting of +such small particles that they can no longer be separated by sifting. If +the residues on the sieves are carefully washed free from any adhering +fine material and are then dried, they will be found on examination to +be quite distinct from anything definable as clay. They may consist of a +considerable variety of minerals or may be almost entirely composed of +quartz, but with the possible exception of some shales of great +hardness, they are undoubtedly not clay. This simple process therefore +serves to remove a proportion of material which in the case of some +'clays' is very large but in others is insignificant; thus 40 per cent. +of sand-like material may be removed from some brick-clays whilst a ball +clay used for the manufacture of stoneware or pottery may pass +completely through a sieve having 200 meshes per linear inch. + +The material which passes through the finest sieve employed will contain +all the true clay in the material; that is to say, the coarser portion +will, as already mentioned, be devoid of the ordinary characteristics of +clay. At the same time, this very fine material will seldom consist +exclusively of clay, but will usually contain a considerable proportion +of silt, extremely fine mineral particles and, in the case of calcareous +clays, a notable proportion of calcium carbonate in the form of chalk or +limestone particles. Only in the case of the purest clays will the +material now under consideration consist entirely of clay, so that it +must be again separated into its constituents. This is best +accomplished, as first suggested by Schoene, by exposing the material to +the action of a stream of water of definite speed. H. Seger (7) +investigated this method very thoroughly and his recommendations as to +the manner in which this separation by elutriation should be carried out +remain in use at the present time. Briefly, all material sufficiently +fine to be carried away by a stream of water flowing at the rate of 0·43 +in. per minute was found by Seger to include the whole of the clay in +the samples he examined, but, as was later pointed out by Bischof, it is +not correct to term the whole of this material 'clay substance,' as when +examined under the microscope, it contains material which is clearly not +clay. + +Processes of decantation of the finest material obtained after +elutriation still fail to separate all the non-clay material, and Vogt +has found that when the material has been allowed to stand in suspension +for nine days some particles of mica are still associated with the clay. + +It would thus appear that no process of mechanical separation will serve +for a complete purification of a clay; indeed, there are good reasons +for supposing that extremely fine particles of quartz and mica render +physical characteristics an uncertain means of accurately distinguishing +clays from other rock dust. + +When chemical methods of investigation are employed the problem is not +materially altered, nor is its solution fully attained. It is, of +course, obvious that any chemical method should be applied to the +product obtained by treating the raw material mechanically as above +described, for to do otherwise is to create needless confusion. Yet by +far the greater number of published analyses of 'clays' report the +ultimate composition of the whole material, no attempt being made to +show how much of the various constituents is in the form of sand, stones +or other coarse particles of an entirely non-argillaceous character. + +If the particles of a 'clay' which are sufficiently small to be carried +away by a stream of water with a velocity of only 0·43 in. per minute +are analysed, it will be found that their composition will vary +according to the origin of the clay and the subsequent treatment to +which it has been subjected during its transport and deposition. If the +clay is fairly free from calcareous material and is of a white-burning +nature it may be found to have a composition like china clays. +Red-burning clays, on the contrary, will vary greatly in composition, so +that it becomes difficult to find any close analogy between these kinds +of clay. This difference is partly due to the extremely fine state of +division in which ferric oxide occurs in clays, the particles of this +material corresponding in minuteness to those of the purest clays and so +being inseparable by any mechanical process. + +In 1876 H. Seger (7) published what he termed a method of 'rational +analysis,' which consisted in treating the clay with boiling sulphuric +acid followed by a treatment with caustic soda. He found that the purer +china clays (kaolins) and ball clays were made soluble by this means and +that felspar, mica and quartz were to a large extent unaffected. Later +investigators have found that this method is only applicable to a +limited extent and that its indications are only reliable when applied +to the clays just named, but the principle introduced by Seger has +proved invaluable in increasing our knowledge of the composition of +clays. By means of this so-called rational analysis Seger found that the +purer clays yielded results of remarkable similarity and uniformity, +the material entering into solution having a composition agreeing very +closely with the formula Al2O3·2SiO2·2H2O which is generally +recognized as that of the chief constituent or constituents of china +clay (kaolin) and the purer ball clays. This crude substance, obtainable +from a large number of clays by the treatment just described, was named +_clay substance_ by Seger, who regarded it as the essential constituent +of all clays. + +Red-burning clays when similarly treated do not yield so uniform a +product, and the ferric oxide entering into solution makes the results +very discordant. Moreover, even with the china clays or kaolins a small +proportion of alkalies, lime and other oxides enter into solution and a +number of minerals analogous to clay, but quite distinct from it, are +also decomposed and dissolved. For these reasons the 'rational analysis' +has been found insufficient; it is now considered necessary to make an +analysis of the portion rendered soluble by treatment with sulphuric +acid in order to ascertain what other ingredients it may contain in +addition to the true clay present. + +As the china clays (kaolins) and ball clays on very careful elutriation +all yield a product of the same ultimate composition, viz. 39 per cent. +of alumina, 46 per cent. of silica, 13 per cent. of water, and 2 per +cent. of other oxides, they are generally regarded as consisting of +practically pure clay with a variable amount of impurities. Many years +ago Fresenius suggested that these non-clayey constituents of clays +should be calculated into the minerals to which they appeared likely to +correspond so as to obtain a result similar to that obtained by Seger +without the disadvantages of the treatment with sulphuric acid and as +supplementary to such treatment in the case of red-burning and some +other clays. More recent investigators have found that if a careful +microscopic examination of the clay is made the results of estimating +the composition from the proportion of the different minerals +recognizable under the microscope and by calculation from the analysis +of the material agree very closely and are, as Bischof (28) and, more +recently, Mellor have pointed out, more reliable than the 'rational +analysis' in the case of impure clays. 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. Some clays are almost +destitute of felspar but comparatively rich in mica, whilst others are +the reverse, so that some means of identifying the extraneous minerals +is essential. When this is not used, the curious result is obtained +that German chemists calculate the alkalies, etc. to felspar whilst the +French chemists, following Vogt, calculate them to mica; English ceramic +chemists appear undecided as to which course to follow, and some of them +occasionally report notable amounts of felspar in clays quite destitute +of this mineral! + +A statement of the composition of a 'clay' based on a mechanical +separation of the coarser ingredients followed by an analysis of the +finer ones and a calculation of the probable constituents of the latter, +as already described, is known as a _proximate analysis_ in order to +distinguish it from an _ultimate analysis_ which states the composition +of the whole material in terms of its ultimate oxides. A proximate +analysis therefore shows the various materials entering into the +composition of the clay in the following or similar terms: + + Stones per cent. + Gravel " + Coarse sand " + Medium sand " + Fine sand " + Silt " + Felspar or mica dust " + Silica dust " + 'True clay[12]' " + Moisture " + Carbon " + Other volatile matter " + +[Footnote 12: In analytical reports a note should be appended stating +that the figure under this term shows the proportion of the nearest +approximation to true clay at present attainable.] + +For some purposes it is necessary to show the proportion of calcium, +iron and other compounds as in an ordinary ultimate analysis. + +A comparison of the foregoing with an ultimate or 'ordinary' analysis of +a clay (p. 16) will show at once the advantage of the former in +increasing our knowledge of the essential constituent of all clays, if +such a substance really exists. Its absolute existence is by no means +proved, for, as will have been noticed, its composition is largely based +on assumption even in the most thorough investigations, particularly of +the admittedly less pure clays. + +In the purer clays the problem is much simpler and in their case an +answer of at least approximate accuracy can be given to the question +'What is clay?' + +Even with these purer clays it is not sufficient to study an analysis +showing the total amount of the silica, alumina and other oxides +present; it is still necessary to effect some kind of separation into +the various minerals of which they are composed. When, however, the +accessory minerals do not exceed 5 per cent. of the total ingredients +their influence is less important and the nature and characteristics of +the 'clay substance' itself can be more accurately studied. By careful +treatment of well selected china clays, for example, it is possible to +obtain a material corresponding to the formula Al2O3·2SiO2·2H2O +within a total error of 1 per cent., the small amount of impurity +being, as far as can be ascertained, composed of mica. So pure a +specimen of clay is found on microscopical examination to consist of +minute irregular grains of no definite form, together with a few +crystals of the same composition and identifiable as the mineral +'kaolinite' (p. 107). This 'amorphous' material, which appears to be the +chief constituent of all china clays and kaolins, has been termed +_clayite_ by Mellor (22). + +Johnson and Blake, Aron and other observers have stated that the +majority of the particles in china clays and kaolins are crystalline in +form. Owing to their extreme smallness it is exceedingly difficult to +prove that they are not so, though for all ordinary purposes they may be +regarded as amorphous, the proportion of obviously crystalline matter +present in British china clay of the highest qualities being so small as +to be negligible. + +Hickling (36), using an exceptionally powerful microscope, claims to +have identified this 'amorphous' substance in china clay as 'worn and +fragmental crystals of kaolinite,' and recently Mellor and Holdcroft and +Rieke have shown that the apparently amorphous material shows the same +endo- and exothermal reactions as crystalline kaolinite. + +So far as china clays or kaolins are concerned, kaolinite or an +amorphous substance of the same composition appears to be identical with +the 'ideal clay' or 'true clay' whose characters have so long been +sought. + +This term--clayite--is very convenient when confined to china clays and +kaolins, but it is scarcely legitimate to apply it, as has been +suggested, to material in other clays until it has been isolated in a +sufficiently pure form to enable its properties to be accurately +studied. This restriction is the more necessary as in one very important +respect clayite obtained from china clay and some kaolins differs +noticeably from the nearest approach to it obtainable from the more +plastic clays: namely, in its very low plasticity. This may be explained +by the fact that it is only obtainable in a reasonably pure form in +clays of a primary character, whilst the plastic clays have usually been +transported over considerable areas and have been subjected to a variety +of treatments which have had a marked effect on their physical +character. 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. + +Ever since the publication of Seger's memorable papers (7), and to a +small extent before that time, it has been generally understood that +china clay or kaolin represented the true essential constituent of +clays, but several investigators have been so imbued with the idea that +all true clay substance must have a crystalline form that they have +frequently used the term 'kaolinite' to include the 'amorphous' +substance in plastic clays. This is unfortunate as it is by no means +proved that the latter is identical with kaolinite, and a distinctive +term would be of value in preventing confusion. Other investigators have +used the word 'kaolin' with equal freeness, so that whilst it originally +referred to material from a particular hill or ridge in China[13] it has +now entered into general use for all clays whose composition +approximates to that of china clay (p. 16) in which the plasticity is +not well developed. Thus, in spite of the difference in origin between +many German and French kaolins and the china clays of Cornwall, it is +the custom in Europe generally to term all these materials 'kaolin.' Yet +they are very different in many respects from the material originally +imported from China. + +[Footnote 13: _Kao-ling_ is Chinese for a high ridge or hill.] + +As the essential clay substance has not yet been isolated in a pure form +from the most widely spread plastic clays, but is largely hypothetical +as far as they are concerned, the author prefers the term +_pelinite_[14] when referring to that portion of any plastic clays or +mixtures of clays with other minerals which may be regarded as being the +constituent to which the argillaceous portion of the material owes its +chief properties. In china clay and kaolin the 'true clay' is identical +with clayite--or even with kaolinite (p. 108)--and there is great +probability that this identity also holds in the case of the more +plastic clays of other geological formations, but until it is +established it appears wisest to distinguish the hypothetical or ideal +clay common to all clays (if there is such a substance) by different +terms according to the extent to which its composition and characters of +the materials most closely resembling it are experimentally known. + +[Footnote 14: From the [Greek: pêlinos] = made of clay.] + +The substances most resembling this 'ideal clay' which have, up to the +present been isolated, are: + +(_a_) _Kaolinite._ Found in a crystalline form in china clays and +kaolins (p. 107). + +(_b_) _Clayite._ A material of the same chemical composition as +kaolinite, but whose crystalline nature (if it be crystalline) has not +been identified--chiefly obtained from china clays and kaolins. + +(_c_) _Pelinite._ A material similar to clayite, but differing from it +in being highly plastic and, to some extent, of a colloidal +nature--obtained from plastic clays. + +(_d_) _Laterite._ A material resembling clayite in physical appearance, +but containing free alumina and free silica (p. 80). + +(_e_) _Clay Substance._ A general term indicating any of the foregoing +or a mixture of them; it is also applied (unwisely) to the material +obtained when a natural clay is freed from its coarser impurities by +elutriation (p. 7). + + +The Chief Characteristics of 'True Clay' from Different Sources. + +In so far as it can be isolated _true clay_ appears to be an amorphous, +or practically amorphous, material which may under suitable conditions +crystallize into rhombic plates of kaolinite. The particles of which it +is composed are extremely small, being always less than 0·0004 in. in +diameter. They adsorb dyes from solutions and show other properties +characteristic of colloid substances though in a very variable degree, +some clays appearing to contain a much larger proportion of colloidal +matter than do others. To some extent the power of adsorption of salts +and colouring matters appears to be connected with the plasticity (p. +41) of the material, but this latter property varies so greatly in +clayite or pelinite from different sources as to make any generalization +impossible. + +True clay substance appears to be quite white, any colour present being +almost invariably traceable to ferric compounds or to carbonaceous +matter. The latter is of small importance to potters as it burns away in +the kiln. The specific gravity of clay substance is 2·65 according to +Hecht, the lower figures sometimes reported being too low. Its hardness +is usually less than that of talc--the softest substance on Mohs' +scale--but some shales are so indurated as to scratch quartz. It is +quite insoluble in water and in dilute solutions of acids or alkalies, +but is decomposed by hydrofluoric acid and by concentrated sulphuric +acid when heated, alumina entering into solution and silica being +precipitated in a colloidal condition. + +It absorbs water easily until a definite state of saturation has been +reached, after which it becomes impervious unless the proportion of +water is so large and the time of exposure so great that the material +falls to an irregular mass which may be converted into a slurry of +uniform consistency by gently stirring it. With a moderate amount of +water, pelinite develops sufficient plasticity to enable it to be +modelled with facility, but clayite and some specimens of pelinite are +somewhat deficient in this respect. The pelinitic particles usually +possess the capacity to retain their plasticity after being mixed with +considerable proportions of sand or other non-plastic material and are +then said to possess a high binding power (p. 28). + +If a large proportion of water is added to a sample of clayite or +pelinite and the mixture is stirred into a slurry it will be found to +remain turbid for a considerable time and will not become perfectly +clear even after the lapse of several days. Its power of remaining in +suspension is much influenced by the presence of even small amounts of +soluble salts in either the water or the clay substance, its +precipitation being hastened by the addition of such salts as cause a +partial coagulation of the colloidal matter present. Some specimens of +clayite and pelinite retain their suspensibility even in the presence of +salts, but this is only true of a very limited proportion of the +substance. In most cases the presence of soluble salts causes the larger +particles to sink somewhat rapidly and to carry the finer particles with +them. + +The rate at which a slip or 'cream' made of elutriated clay and water +will flow through a small orifice is dependent on the viscosity of the +liquid and this in turn depends on the amount of colloidal material +present, _i.e._ on how much of the clay (pelinite) is in a colloidal +form. Its viscosity is greatly affected by the addition or presence of +small quantities of acid or alkali or of acidic or basic salts. Acids +increase the viscosity; alkalies and basic salts, on the contrary, make +the slip more fluid. Neutral salts behave in different ways according to +the concentration of the solution and to the amount of clay (pelinite) +present in the slip. If the slip contains so little water as to be in +the form of a thin paste, neutral salts usually have but a small action, +but when the slip contains only a small proportion of clay (pelinite) +the presence of neutral salts will tend to cause the precipitation of +the clay. In this way salts act in two quite different directions +according to the concentration of the slip. + +On drying a paste made of clay and water the volume gradually diminishes +until the greater part of the water has been removed; after this the +remainder of the water may be driven off without any further reduction +in volume of the material. This is another characteristic common to +colloidal substances such as gelatin. The material when drying attains a +leathery consistency which is at a maximum at the moment when the +shrinkage is about to cease; on further drying the material becomes +harder and more closely resembles stone. + +Providing that wet clay is not heated to a temperature higher than that +of boiling water it appears to undergo no chemical change and on cooling +it will again take up water[15] and be restored to its original +condition except in so far as its colloidal nature may have been +affected by the heating. If, however, the temperature is raised to about +500° C. a decomposition of the material commences and water is evolved. +This water--which is commonly termed 'combined water'--is apparently an +essential part of the clay-molecule and when once it has been removed +the most important characteristics of the clay are destroyed and cannot +be restored. The reactions which occur when clay is heated are complex +and are rendered still more difficult to study by the apparent +polymerization of the alumina formed. Mellor and Holdcroft (29) have +recently investigated the heat reactions of the purest china clay +obtainable and confirm Le Chatelier's view (10) that on heating to +temperatures above 500° C. clay substance decomposes into free silica, +free alumina and water, the two former undergoing a partial +re-combination with formation of sillimanite (Al2O3SiO2) if a +temperature of 1200° C. is reached. Mellor and Holdcroft point out that +there is no critical point of decomposition for clay substance obtained +from china clay, as it appears to lose water at all temperatures, though +its decomposition proceeds at so slow a rate below 400° C. as to be +scarcely appreciable. + +[Footnote 15: Some clays are highly hygroscopic and absorb moisture +readily from the atmosphere. According to Seger (7) this hygroscopicity +distinguishes true clay from silt and dust.] + +After the whole of the 'combined water' has been driven off, if the +temperature continues to rise, it is found that at a temperature of 900° +C. an evolution of heat occurs. This exothermal point, together with the +endothermal one occurring at the temperature at which the decomposition +of the clay seems to be most rapid, has been found by Le Chatelier, +confirmed by Mellor and Holdcroft, to be characteristic of clay +substance derived from kaolin and china clay, and the two last-named +investigators state that it serves as a means of distinguishing +kaolinite or clayite from other alumino-silicates of similar +composition. These thermal reactions have not, as yet, been fully +studied in connection with plastic clays; with china clay, as already +noted, they probably indicate a polymerization of the alumina set free +by the decomposition of the clay substance, as pure alumina from a +variety of sources has been found by Mellor and Holdcroft to behave +similarly. + +On still further raising the temperature of pure clay (pelinite or +clayite) no further reactions of importance occur, the material being +practically infusible. If, however, any silica, lime, magnesia, +alkalies, iron oxide or other material capable of combining with the +alumina and silica is present as impurities in the clay substance, +combination begins at temperatures above 900° C. This causes a reduction +of the heat-resisting power of the material; the silicates and +alumino-silicates produced fuse and begin to react on the remaining +silica and alumina, first forming an impermeable mass in place of the +porous one produced with pure clay substance, and gradually, as the +material loses its shape, producing a molten slag if the 'clay' is +sufficiently impure. As ordinary clays are never quite free from +metallic 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. + +The constitution of the clay molecule is a subject which has attracted +the attention of many investigators and is being closely studied at the +present time. It is a subject of peculiar difficulty owing to the +inertness of clay substance at all but high temperatures, and to the +complexity of reactions which take place as soon as any reagent is +brought into active connection with it. + +Without entering into details regarding the various graphic formulae +which have been suggested, it is sufficient to state that the one which +is most probably correct, as far as present knowledge goes, is Mellor's +and Holdcroft's re-arrangement of Groth's formula (30) + + HO\ /OSiO\ + \Al2/ \O + / \ / + HO/ /\ \OSiO/ + / \ + HO OH + +which on removal of the hydroxyl groups might be expected to give the +anhydride + + O\\ /OSiO\ + \\Al2/ \ + // \ / + O// \OSiO/ + + +though in practice this substance--if formed at all--appears to be +instantly split up into Al2O3 and SiO2. + +By regarding the aluminium as a nucleus, as above, and some aluminium +silicates as hypothetical alumino-silicic acids, as suggested by +Ulffers, Scharizer, Morozewicz (29) and others, clay substance may be +conveniently considered, along with analogous substances, as forming a +special group quite distinct from the ordinary silicates. In this way +Mellor and Holdcroft (29) consider that clay substance is not a hydrated +aluminium silicate--as is usually stated in the text-books--but an +alumino-silicic acid, the salts of which are the zeolites and related +compounds. From this hypothesis it naturally follows that clay substance +is analogous to colloidal silica which has been formed by the +decomposition of a silicate by means of water and an acid. + +If this view be correct, pure clay substance or true +clay is a tetra-basic alumino-silicic acid H4Al2SiO9 or +Al2Si2O5(0H4). That its acid properties are not readily +recognizable at ordinary temperatures is due to its inertness; at higher +temperatures its power of combination with lime, soda potash and other +bases is well recognized, though the reactions which occur are often +complicated by decompositions and molecular re-arrangements which occur +in consequence of the elevated temperature. + +There are a number of minerals which closely resemble clayite +or pure clay substance in composition, the chief difference +being in the proportion of water they evolve on being heated. +Thus _Rectorite_ H2Al2Si2O8, _Kaolinite_ H4Al2Si2O9, +_Halloysite_ H6Al2Si2O10 and _Newtonite_ H10Al2Si2O12. In the +crystalline form these minerals may be distinguished from each other by +means of the microscope, but as the chief materials of which clays are +composed appears to be amorphous it is impossible to ascertain with +certainty whether a given specimen of clay substance is composed of a +mixture of these analogous minerals in an amorphous form or whether it +consists entirely of clayite, _i.e._ the clay substance obtained from +china clay. As already stated, the thermal reactions which occur on +heating clayite appear to be characteristic of kaolinite whilst +halloysite is completely decomposed at a temperature somewhat below 200° +C.; but the not improbable presence of two or more of these +alumino-silicic acids in clays of secondary or multary origin makes it +almost impossible to determine whether clayite is an essential +constituent of all clays or whether the purest clay substance (pelinite) +obtained from some of the more plastic clays does not possess a +different chemical composition as well as different physical properties. + +The view that clays may be regarded as impure varieties of clayite is +considered erroneous by several investigators for various reasons. For +instance, felspar is rarely found in china clays, but is a common +constituent of secondary (plastic) clays. J. M. van Bemmelen (26), who +has found that the alumina-silica ratio of clays produced by weathering +is always higher than that in clays of the china clay type produced by +hypogenic action. In a number of clays examined he found that a portion +was soluble in boiling hydrochloric acid whereas clayite is scarcely +affected by this treatment. He also found a varying proportion of +alumino-silicate insoluble in hydrochloric acid but dissolved on +treatment with boiling sulphuric acid and subsequently with caustic soda +solution; this latter he considers to be true clayite. Unfortunately, +his results were obtained by treating the crude clay with acid, instead +of first removing such non-plastic materials as can be separated by +washing, so that all that they show is that some clays contain +alumino-silicates of a nature distinct from clayite in addition to any +clayite which may be found in them. + +The fact that all clays when heated to 700 or 800° C. readily react with +lime-water to form the same calcium silicates and aluminates indicates +so close a resemblance between the clay substance obtainable from +different sources as to constitute strong evidence of the identity of +this substance with clayite or with materials so analogous to it as to +be indistinguishable from it under present conditions. + +In all probability, the plastic clays have been derived from a somewhat +greater variety of minerals than the primary clays (p. 71) and under +conditions of decomposition which differ in details, though broadly of +the same nature as those producing china clays. The presence of +colloidal matter suggests a more vigorous action--or even a +precipitation from solution--instead of the slower reactions which +result in the formation of the kaolinite crystals. + +The much smaller particles present in plastic clays also indicate a more +complete grinding during the transportation of the material or some form +of precipitation. If, as Hickling suggests, all clays are direct +products of the decomposition of _mica_, the fact that several varieties +of mica are known and that the conditions under which these decompose +must vary considerably, afford a good, if incomplete, explanation of +some of the widely diverse characteristics observed in different clays. + +Notwithstanding the great complexities of the whole subject and the +apparently contradictory evidence concerning some clays, there is a +wide-spread feeling that whatever may be the mineral from which a given +clay has been derived, the _true clay substance_, which is its essential +constituent, would (if it could be isolated in a pure state) prove to be +of the same composition as kaolinite obtainable from china clay of +exceptional purity. The purest clay substances (pelinite) yet obtained +from some of the most plastic clays are, however, so impure as to make +any detailed investigation of their composition by present methods +abortive. The methods of synthesis which have proved so successful in +organic chemistry have hitherto yielded few intelligible results with +clays, on account of the complexity of the accessory reactions which +occur. + + +The Difference between Pure Clay Substance and Ordinary Clays. + +The properties and characteristics of _true clay_ are very seriously +modified by other materials which may be associated with it. This may be +perceived by comparing the properties of clays mentioned in Chapter I +with those of various forms of true clay just given. Moreover, as true +clay never occurs in a perfectly pure state in nature, the properties of +clays must be largely dependent on the accessory ingredients. + +Silica, for example, when alone is a highly refractory material, but in +the presence of true clay it reduces the refractoriness of the latter. +Lime has a similar effect though its chemical action on the clay is +entirely different. A very small proportion of some substances--notably +the oxides of sodium and potassium--will greatly alter the behaviour of +true clay when heated and will produce an impervious mass in place of a +porous one. + +For these reasons, it is necessary in studying clays to pay attention to +both their physical and chemical properties and to separate the material +into fractions so that each of these may be studied separately and their +individual as well as their collective characteristics ascertained. +Failure to do this has been the cause of much obscurity and confusion in +investigations on certain clays composed of a considerable proportion of +non-argillaceous material which ought to have been separated before any +attempt was made to study the true clay present. + +There is, therefore, a considerable difference between a natural clay +and the pure clay substance theoretically obtainable from it; this +difference being most marked in the case of low-grade brick clays of +glacial origin, which may contain 50 per cent. or more of adventitious +materials. If used in a natural state they would be found to be +valueless on account of their impurities giving them characteristics of +a highly undesirable character, whereas the true clay in them is +found--in so far as it can be separated--to bear a close resemblance to +that obtained from a high grade, plastic, pottery clay. Unfortunately, +it is, at present, impossible to isolate this clay substance in anything +approaching a pure form, and many clays are without commercial value +because of comparatively small proportions of impurities which cannot be +separated from the clay substance without destroying the latter. + + +Classification of Clays. + +Owing to the widely differing substances from which clays can, +apparently, be formed and the peculiar difficulties which are +experienced in investigating the nature of clay substance from different +sources, it is by no means easy to devise a scheme of classification of +clays, though many of these have been attempted by different scientists. + +The classification adopted by geologists is based on the fossil remains +and on the stratigraphical position of clays relative to other rocks, as +described in Chapter II. This is of great value for some purposes, but +the composition of the substances termed 'clay' by geologists differs so +greatly, even when only one formation is considered, as to make their +classification of little or no use where the value or worthlessness of +the material depends upon its composition. Thus the so-called Oxford +clay ranges from a hard silicious shale to a comparatively pure clay; +some portions of it are so contaminated with calcareous and ferruginous +matter as to make the material quite useless for the potter or +clayworker. A geological classification of clays is chiefly of value as +indicating probable origins, impurities and certain physical properties; +but the limits of composition and general characteristics are so wide as +to make it of very limited usefulness. + +The classification of clays on a basis of chemical composition is +rendered of comparatively little value by the large number of clays +which occupy ill-defined borders between the more clearly marked +classes. Moreover, attempts to predict the value and uses of clays from +their chemical composition are generally so misleading as to be worse +than useless, unless a knowledge of some of the physical characters of +the clays is available. It is, of course, possible to differentiate some +clays from others by their composition, but not with sufficient accuracy +to permit of definite and accurate classification. + +A classification based exclusively on the composition of clays is +equally unsatisfactory for other reasons, the chief of which is the +placing together of clays of widely differing physical character, and +the separation of clays capable of being used for a particular purpose. +To some extent the latter objection may be disregarded, though it is of +great importance in considering the commercial value of a clay. + +Classification based on the uses of clays of different kinds has been +suggested by several eminent ceramists, but is obviously unsatisfactory, +particularly as it is by no means uncommon to use mixtures of clays and +other minerals for some purposes. Thus stoneware clays must be +vitrifiable under conditions which may be defined with sufficient +accuracy, but many manufacturers of stoneware do not use clays which are +naturally vitrifiable; they employ a mixture of refractory clay and +other minerals to obtain the material they require. + +A classification based on the origin of clays regarded from the +petrological point of view offers some advantages, but is too cumbersome +for ordinary purposes and suffers from the disadvantage that the origin +of some important clays is by no means clearly known. + +The author prefers a modification of Grimsley's and Grout's +classification (31) as follows: + + I. Primary clays. + + (_a_) Clays produced by 'weathering' silicates--as some + kaolins. + + (_b_) Clays produced by lateritic action--very rich in + alumina, some of which is apparently in a free state. + + (_c_) Clays produced by telluric water containing active gases + (hypogenically formed clays)--as Cornish china clay. + + II. Secondary clays. + + (_d_) Refractory[16] secondary clays--as fireclays and some + pipe clays. + + (_e_) Pale-burning non-refractory clays--as pottery clays, + ball clays and some shales. + + (_f_) Vitrifiable clays--as stoneware clays, paving brick + clays. + + (_g_) Red-burning and non-refractory clays--as brick and + terra-cotta clays and shales. + + (_h_) Calcareous clays or marls, including all clays + containing more than 5 per cent. of calcium carbonate. + + III. Residual clays. + + (_i_) Clays which have been formed by one of the foregoing + actions and have been deposited along with calcareous or + other matter but, on the latter being removed by subsequent + solution, the clay has remained behind--as the white clays + of the Derbyshire hills. + +[Footnote 16: A refractory clay is one which does not soften +sufficiently to commence losing its shape at any temperature below that +needed to bend Seger Cone 26 (approximately 1600°C.) (see p. 116).] + +Some further sub-division is necessary for special purposes, +particularly in sections _e_, _f_ and _h_, but to include further +details would only obscure the general scheme. Some clays will, +apparently, be capable of classification in more than one section, thus +a vitrifiable clay may owe its characteristic to a high proportion of +calcium carbonate and so be capable of inclusion as a calcareous clay. +Broadly speaking, however, if the clay is tested as to its inclusion in +each section of the scheme in turn it will be found that its highest +value will be in the section which is nearest to the first in which the +clay can legitimately be placed. + +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. Although the present +state of knowledge does not permit them to be classified with such +detail as is the case with plants, animals, or simple chemical +compounds, the study of clays and the allied materials has a fascination +peculiarly its own, not the least interesting features of which are +those properties of the clay after it has been made into articles of use +or ornament. These are, however, beyond the scope of what is commonly +understood by the term 'the natural history of clay.' + + + + +BIBLIOGRAPHY + + +A complete bibliography of clay would occupy several volumes. The +following list only includes the more accessible of the works quoted in +the text. + + 1. "Second Report of the Committee on Technical Investigation--Rôle + of Iron in Burning Clays." Orton and Griffith. Indianapolis. + 1905. + + 2. "British Clays, Shales and Sands." Alfred B. Searle. Charles + Griffin and Co. Ltd. London. 1911. + + 3. "Transactions of the English Ceramic Society." v. p. 72. Hughes + and Harber. Longton, Staffs. 1905. + + 4. "Royal Agricultural Society's Journal." XI. + + 5. "Die Tone." P. Rohland. Hartleben's Verlag. Vienna. 1909. + + 6. "Clays: their Occurrence, Properties and Uses." H. Ries. Chapman + and Hall. London. 1908. + + 7. "Gesammelte Schriften." H. Seger. Tonindustrie Zeitung Verlag. + Berlin. 1908. + + 8. "Tonindustrie Zeitung." 1902. p. 1064. + + 9. "Tonindustrie Zeitung." 1904. p. 773. + + 10. "Treatise on Ceramic Industries." E. Bourry (Revised translation + by A. B. Searle). Scott, Greenwood and Son. London. 1911. + + 11. "The Colloid Matter of Clay." H. E. Ashley. U.S.A. Geological + Survey Bulletin 388. Washington. 1909. + + 12. "Sprechsaal." 1905. p. 123. + + 13. "Action of Heat on Refractory Materials." J. W. Mellor and F. J. + Austen. Trans. Eng. Cer. Soc. VI. Hughes and Harber. Longton, + Staffs. 1906. + + 14. "Wiedermann's Annalen." VII. p. 337. + + 15. "Geological Contemporaneity." 1862. + + 16. "Geological Magazine." IV. pp. 241, 299. + + 17. "La Céramique industrielle." A. Granger. Gauthier Frères. Paris. + 1905. + + 18. "American Journal of Science." 1871. p. 180. + + 19. "The Hensbarrow District." J. H. Collins. Geological Survey. + 1878. + + 20. "Monographs of the U.S.A. Geological Survey." XXVIII. C. R. van + Hise. 1897. + + 21. "On Kaolinite and Pholerite." American Journal of Science. + XLIII. 1867. + + 22. "The Nomenclature of Clays." J. W. Mellor. Eng. Cer. Soc. VIII. + Hughes and Harber. Longton, Staffs. 1908. + + 23. "On the present distribution of Coal Balls." M. C. Stopes and D. + M. S. Watson. Phil. Trans. Royal Society. B. Vol. CC. 1908. + + 24. "Natural History of Coal." E. A. N. Arber. Cambridge University + Press. 1911. + + 25. "Modern Brickmaking." A. B. Searle. Scott, Greenwood and Son. + London. 1911. + + 26. "Die verschiedene Arten der Verwitterung." J. M. van Bemmelen. + Zeits. angewandte Chemie. LXVI. Leopold Voss Verlag. Hamburg. + 1910. + + 27. "Pyrometrische Beleuchtung." Carl Bischof. Tonindustrie Zeitung. + 1877. + + 28. "Die feuerfeste Tone." Carl Bischof. Quandt and Haendler. + Leipzig. 1904. + + 29. "The Chemical Constitution of the Kaolinite Molecule." Trans. + Eng. Cer. Soc. X. Hughes and Harber. Longton, Staffs. 1911. + + 30. "Tabellarische Uebersicht der Mineralien." P. Groth. Brunswick. + 1898. + + 31. "West Virginia Geological Survey." III. 1906. + + 32. "Memoirs of the Geological Survey." London. + + 33. "The Publications of Stanford's Geographical Institute." London. + + 34. "Handbuch der gesam. Tonwarenindustrie." B. Kerl. Verlag der + Tonindustrie Zeitung. 1910. + + 35. "Causal Geology." E. H. L. Schwarz. Blackie and Sons, Ltd. 1910. + + 36. "China Clay: its nature and origin." G. Hickling. Trans. Inst. + Mining Engineers. 1908. + + + + +INDEX + + + Absorption, 40, 151 + Absorptive power of clays, 40 + Accumulation of clays, 84 + Acid-proof ware, 113 + Acids, effect of, 106, 151, 152 + Adsorption, 40, 150 + Agriculture, clays in, 5, 56, 57, 59, 61, 62, 63, 67 + Air, 43, 85 + Alkalies in clay, 38, 115, 133, 142, 143, 155 + Alluvial deposits, 68, 87, 112, 132 + Alum clays and shales, 57, 123, 124 + Alum manufacture, clays for, 124 + Alumina, 6 + Alumina, free, 80, 82, 154 + Alumina-silica ratio, 133, 159 + Alumino-silicic acid, 6, 76, 81, 118, 155, 157 + Aluminous clays, 82, 117 + 'Amorphous' clay, 107, 146 + Analyses of clays, 16, 141, 144 + Anauxite, 134 + Architectural ware, 129, 130 + Argillaceous earths, 1 + Argillaceous limestone, 88, 132 + Associated rocks, 48 + + + Bagshot clays and sands, 64, 125 + Ball clays, 6, 19, 28, 62, 64, 82, 110, 115, 119, 125, 138, 141, 156, 166 + Bending of clay, 33 + Bibliography, 168 + Binding power, 28, 151 + Binds, 53 + Bituminous shales, 57, 59 + Black spots, 14, 128 + Black ware, 113 + Bleaching oil, 134 + Blue bricks, 13, 56 + Bone-ash, 110 + Boulder clays, 3, 7, 10, 65, 101 + Bovey Tracey clay, 62 + Brick clays, earths and shales, 1, 2, 5, 10, 11, 12, 13, 31, 37, 40, 46, + 56, 57, 59, 61, 63, 65, 67, 68, 91, 100, 101, 104, 112, 117, 123, 125, + 129, 138, 162, 166 + Brittleness, 46 + Brown ware, 113 + Buff bricks, 128 + Burned clay, 28, 31, 41, 119, 121 + + + Calcareous clays, 38, 61, 68, 88, 133, 139, 166 + Calcareous sands, 88 + Calcium, see _Lime compounds_ + Cambrian clays, 51 + Carbon in clay, 15, 119, 144 + Carbonates in clay, 10, 82 + Carboniferous clays and shales, 52, 124 + Carboniferous limestone, 52, 108 + Carclazite, 78, 106 + Cellulose in clays, 27 + Cement clays, 57, 104, 131 + Chalcopyrite, 14 + Chalk, 10, 11, 61, 67, 68, 88, 116, 127, 128, 132, 134, 139 + Chamotte, 121 + Chemical properties of clay, 6 + China clay rock, 78, 106, 116 + China clays, 2, 5, 6, 7, 9, 22, 27, 40, 49, 64, 71, 75, 78, 82, 84, 104, + 110, 116, 141, 146, 147, 148, 156, 165 + China-ware, 109, 110 + Chinese clay, 73 + Classification of clays, 163 + Clay molecule, 156 + Clay-shales, 122 + Clay substance, 135 _et seq._ + Clay substance, defined, 150 + Clayite, 83, 107, 147, 149 + Clinker, 132 + Clunches, 118 + Coagulated clays, 97 + Coagulation, 43, 152 + Coal Measure clays and shales, 53, 96, 103, 117, 124, 130 + Coarse pottery, 112 + Cobalt, 110 + Colloid theory, 97 + Colloidal properties of clay, 25, 81, 82, 97, 106, 147, 150, 152 + Colloidal silica, 81, 134, 157 + Colloids, 24, 41, 43, 76, 160 + Colluvial clays, 99 + Colours of burned ware, 19, 123, 131 + Colours of clays and shales, 19, 59, 115, 119, 124, 126 + Combined water, 45, 154 + Common clays, 3 + Composition of clays, 4, 6, 16, 23, 35, 44, 107, 117, 118, 133, 134, + 144, 156, 164 + Composition of clays (burned), 46 + Cornish stone, 110, 116 + Cracked ware, 46, 127, 130, 131 + 'Cream,' 39, 43, 152 + Cretaceous clays, 61 + 'Crumb' of clay, 24 + Crushing clay, 45 + 'Crystalline' clay, 107, 146, 148 + Crystals in clay-ware, 46 + + + Decantation, 139 + Decomposition of clay, 154 + Definitions of clay, 2-5, 120, 135, 149, 150 + De-greasing wool, 134 + Deposition of clays, 49, 51, 90, 99 + Devonian clays, 51 + Diluvial clays, 99 + Dinas rock, 54 + Disintegration, 102 + Distribution of clays, 1 + Drain-pipe clays, 112, 113 + Drift, 65, 101 + Drift clays, 101 + Drying clays, 27, 127, 153 + Durability, 131 + Dyes, 41, 150 + + + Earth movements, 85, 96 + Earthenware, 37, 112 + Earths for bricks, see _Brick clays_ + Electrolytes, 43 + Elutriation, 8, 137, 140 + Eocene clays, 63 + Epigenic clays, 82 + Erosion, 89, 99, 100 + Estuarine clays, 90, 93, 118 + Etruria marls, 55, 130 + Expansion, 32 + Exposure, 43 + + + Faience, 129 + Farewell Rock, 54 + Fat clays, 29 + Felspar, 7, 8, 41, 74, 104, 110, 116, 141, 144, 159 + Ferric and Ferrous compounds, 12, 121, see _Iron_ + Fine clays, 112 + Fineness, see _Texture_ + Firebricks, 14, 54, 61, 116 + Fireclay, 33, 35, 52, 54, 82, 104, 108, 116, 123, 156, 166 + Fissile clays, 117 + Flint, 110, 116 + Flint clays, 117 + Floods, 85, 87, 99 + Flower-pot clays, 57, 110 + Fluoric vapours, 75, 77, 165 + Fluviatile clays, 88, 92 + Fluxes, 8, 11, 38, 39, 115, 116, 131 + Food-clays, 1 + Formation of clays, 48, 70 + Formula of clay, 156 + Free alumina, 80, 154 + Free silica, 7, 80, 154, 161 + Frost, 43, 86 + Fuller's earth, 59, 133 + Fulling cloth, 1, 133 + Fusibility, 32, 58, 113, 116 + Fusible clays, 116 + Fusing point, 32 + Fusion, 47, 113, 120, 132, 155 + + + Ganister, 52, 54, 118 + Gault, 61, 132 + Geological classification, 163 + Geological nature of clay, 4, 50 + Glacial clays, 65, 100, 162 + Glaciers, 85, 89, 100 + Glass, 116 + Glassy structure, 47 + Glazed bricks, 119 + Glazed pottery, 129 + Glazed terra-cotta, 56, 129 + Grades of fireclay, 120 + Gravel, 7, 62, 65, 89, 100, 101, 102, 138, 144 + Green colour, 14 + Greensand, 133 + Grinding, 80, 121 + Grit, 112, see also _Millstone Grit_ + Grog, 28, 31, 41, 119, 121 + Growan, 78 + Gypsum, 10, 12, 62 + + + Halloysite, 118, 158 + Hardness, 45 + Heat, effects of, 28, 37, 39, 45, 80, 122, 146, 153, 154, 158, 159 + Hydrargillite, 80 + Hydro-alumino-silicates, 6 + Hydrocarbons in clay, 15 + Hydrolysis, 78, 97 + Hygroscopic clays, 153 + Hypogenic clays, 165 + + + Ice-action, 85, 100 + Ideal clay, 146 + Impermeability, 40, 113 + Impervious articles, 113, 155 + Impurities in clays, 7, 49, 82, 102, 104, 109, 121, 126, 142, 143, 155, + 162, 163 + Ions, 43 + Indurated clays, 18 + Infusibility, 106, 119, see _Refractoriness_ + Iron compounds, 7, 10, 12, 13, 20, 62, 112, 119, 121, 128, 133, 141, + 145, 164 + Ironstone, 62 + Irregularity in shape, 131 + + + Jurassic clays and shales, 57, 124 + + + Kao-ling, 148 + Kaolinite, 19, 80, 105, 107, 146, 149, 158 + Kaolinization, 76, 77, 79 + Kaolins, 9, 21, 49, 64, 71, 73, 76, 79, 82, 84, 104, 116, 141, 146, + 147, 148, 165 + Keele series, 55 + Kellaways clay, 59, 61 + Keuper marls, 57 + Kiln shrinkage, 30 + Kimeridge clays, 59 + Knotts, 124 + + + Lacustrine clays, 90, 91 + Lake-deposited clays, 85, 88, 91 + Lakes, 85, 88 + Laminated clays, 53, 117, 122 + Laterite, 80, 149 + Lateritic action, 80, 165 + Lateritic clays, 82 + Lean clays, 29 + Liassic clays and shales, 57, 125, 132 + Lime, 7, 10, 102, 159 + Lime compounds, 10, 11, 38, 41, 47, 113, 115, 116, 121, 127, 139, 142, + 143, 145, 155, 157, 162, 164 + Limestone, 10, 11, 52, 59, 61, 62, 88, 102, 117, 127, 132, 139 + Lime troubles, 11 + Loam, 57, 67, 88 + London clay, 62, 63, 125 + Ludwig's chart, 35 + + + Magnesium compounds, 7, 10, 11, 41, 47, 113, 115, 116, 121, 133, 155 + Malm-bricks, 11 + Malms, 10, 68 + Marcasite, 10, 13 + Marine clays, 61, 93 + Marls, 10, 51, 54, 55, 57, 67, 68, 88, 130, 132, 166 + Mechanical analysis, 137 + Medway mud, 132 + Melting point, 31, 32 + Mica, 7, 8, 76, 104, 105, 116, 140, 141, 144, 160 + Microscopical examination, 18, 105, 143, 158 + Millstone grit, 54, 55, 117 + Mineral nature of clay, 3 + Minerals resembling clay, 158 + Mining ball clay, 111 + Modelling clays, 130 + Moisture, 15, 144 + Molecular attraction, 22 + Molecular constitution of clay, 21, 156 + Montmorillonite, 134 + Mundic, 13 + Muscovite, 105 + + + Newtonite, 158 + Nodules, 121, 127 + Non-plastic material, 43, 121, 151 + Non-refractory clays, 166 + + + Occurrence of clays, 48, 116 + Ocean currents, action of, 89 + Odour of clay, 19 + Oil, bleaching, 134 + Oil shales, 61, 122, 123 + Oolite clays, 59, 134 + Ooze, 95, 99 + Organic matter, 19, 119 + Origins of clays, 48, 71, 160, 165 + Oxford clay, 59, 95, 164 + Oxides in clay, 10, 82 + + + Paint, clays for, 109 + Paper, clays for, 109 + Particles, nature of, 18, 31, 106, 107, 150 + Paving brick clays, 166 + Pelagic ooze, 95, 99 + Pelinite, 83, 148, 149 + Permian clays and shales, 57, 112, 124 + Pholerite, 117 + Physical characters of clays, 17 + Picking clay, 121 + Pipe clays, 64, 65, 82, 109, 166 + Plant-extracts in clays, 26 + Plastic clays, 2, 43, 65, 67, 82, 88, 102, 112, 123, 147, 148, 160 + Plasticity, 20-27, 41, 46, 97, 98, 99, 108, 109, 112, 117, 123, 125, + 127, 151, 160 + Pleistocene clays, 67 + Pockets, 65, 85, 101, 116, 166 + Porcelain, 37, 46, 73, 109, 110, 125, 129 + Pores in clay, 30, 114 + Porosity, 30, 39, 121, 131, 155 + Portland cement, 131 + Potash compounds, 7, 10, 113, 115, 116, 121, 157, see _Alkalies_ + Pottery clays, 1, 5, 31, 46, 66, 100, 101, 104, 110, 112, 114, 125, + 129, 162, 166 + Precambrian clays, 51 + Precipitated clays, 97, 152 + Primary clays, 70, 71, 84, 165 + Proximate analysis, 16, 144 + Purbeck clays, 59 + Pure clays, 5, 6, 7, 142, 155, 156 + Purification of clay, 7, 66, 78, 104, 113, 128, 140 + Pyrites, 10, 13, 44, 56, 57, 119, 124, 128 + + + Quartz, 8, 104, 110, 118, 140, 141, 143 + + + Rain, 44, 85, 86 + Rational analysis, 141 + Reading clays, 63 + Recent clays, 67 + Rectorite, 158 + Re-deposited clays, 98 + Red bricks, 12 + Red burning clays, 141, 142, 166 + Red iron oxide, 12 + Red ware, 113 + Reduction in volume, 30 + Refractoriness, 34, 119, 120, 123, 155 + Refractory articles, 5, 119 + Refractory clays, 9, 32, 33, 35, 38, 52, 65, 82, 104, 116, 165, 166 + Residual clays, 70, 84, 166 + Resistance to abrasion, 119 + Resistance to corrosion, 119 + Resistance to crushing, 46 + Resistance to cutting, 119 + Resistance to temperature, see _Refractoriness_ + Resistance to weathering, 76 + Ringing sound, 110 + River-deposited clays, 88 + Rivers, 85, 87 + Rock binds, 53 + Rockingham, 113 + Rock-like clays, 2 + Rocks associated with clay, 48 + Roman cements, 133 + Roofing tiles, 57, 63, 126, 128 + + + Sagger marls, 54 + Sand, 7, 31, 41, 62, 82, 89, 100, 101, 117, 133, 138, 144 + Sandstones, 53 + Sandy clays, 68 + Sandy loams, 88 + Sandy marls, 88 + Sanitary articles, 5, 113 + Sawdust, 40 + Scum, 10 + Sea, action of, 85, 89, 99 + Sea-deposited clays, 93 + Secondary clays, 70, 82, 83, 166 + Sedimentary rocks, 48 + Sedimentation of clay, 43, 88, 90, 104 + Seger cones, 33, 34 + Selection of clay, 122 + Selenite, 10 + Separation of clays, 90, 145 + Settling, 43 + Sewerage pipes, 113 + Shale oil, 15, 61 + Shale tar, 123 + Shales, 2, 5, 51, 52, 53, 57, 61, 96, 104, 122, 130, 132, 138, 151, + 162, 166 + Shrinkage, 11, 29, 58, 68, 102, 110, 117, 119, 121, 127, 131, 153 + Sifting, 7, 138 + Silica, 6, 7, 80, 154, 155, 161 + Silica rock, 118 + Silicates, 8, 82 + Siliceous clays, 117 + Sillimanite, 46, 154 + Silt, 90, 91, 99, 139, 144 + Silurian clays and shales, 51, 124 + Sintering, 38 + Size of particles, 18, 21, 31, 106, 107, 150 + 'Skeleton,' 115 + 'Skin' on ware, 131 + Slag in bricks, etc., 11, 13, 119, 155 + Slates, 51 + Slurry, 39, 43, 152 + Snow, 85 + Soda compounds, 7, 10, 113, 115, 116, 121, 157, see _Alkalies_ + Softening point, 33 + Soil, see _Agriculture_ + Solubility of clay, 151, 159 + Soluble salts, 10 + Sorting, 90 + Sources of clays, 85 + Specification of fire clays, 120 + Specific gravity, 18, 106, 151 + Staffordshire bricks, 13, 56 + Standard clay, 4 + Stone, Cornish, 110, 116 + Stoneware clays, 104, 112, 113, 156, 165, 166 + Stones, 7, 65, 100, 102, 128, 138, 144 + Streams, 85, 86 + Strength, 23, 45, 113 + Sub-surface clays, 5 + Sulphates in clay, 10, 12, 82 + Sulphides in clay, 10, 82 + Sulphuric acid, 124 + Sunlight, 45 + Surface clays, 2, 5, 52, 112 + Suspension of clay, 43, 90, 140, 152 + Swelling, 15, 102 + + + Tannin in clay, 25, 26, 41 + Telluric water, 165 + Temperature, resistance to, 119, 120 + Tensile strength, 23, 45 + Terra-cotta clays and shales, 5, 10, 12, 31, 46, 56, 63, 91, 104, 123, + 124, 129, 166 + Tertiary clays, 62 + Texture, 112, 130 + Thermal reactions, 146, 154, 158 + Tiles, 1, 5, 57, 91, 101, 129 + Titanium compounds, 121 + Tourmaline, 76, 104 + Transportation of clays, 49, 86, 98, 99, 100 + Transported clays, 70 + Triassic clays, 57, 112, 130 + True clay, 144, 146, 149, 150, 160 + Twisted ware, 114, 129, 131 + Types of clay, 82 + + + Ultimate analysis, 16, 144 + Ultra-marine, clays for, 109 + Underclays, 53, 117, 118 + Uses of clay, 1, 165 + + + Valuation of clay, 103, 109, 123, 126, 162, 165 + Vegetable matter, 15, 119 + Veins, 85 + Viscosity, 152 + Verifiable clays, 113, 156, 166 + Vitrification, 15, 20, 37, 112, 113, 114, 156 + Vitrification range, 38, 114, 115, 116, 129 + Volcanoes, 85 + + + Warp, 99 + Warped ware, 114, 129, 131 + Washing, 7, 79 + Water, effect of, 74, 76, 81, 85, 86, 151 + Water in clays, 15, 17, 29, 39, 45, 154 + Wealden clay, 62 + Weathering, 44, 74, 76, 79, 80, 97, 107, 165 + White bricks, 68, 128 + White clays, 70, 166 + Wind, 86 + + + Zeolites, 157 + + +CAMBRIDGE: PRINTED BY JOHN CLAY, M.A. AT THE UNIVERSITY PRESS + + + + + + + + + +End of Project Gutenberg's The Natural History of Clay, by Alfred B. Searle + +*** END OF THE PROJECT GUTENBERG EBOOK 43297 *** |