diff options
Diffstat (limited to '43297.txt')
| -rw-r--r-- | 43297.txt | 5067 |
1 files changed, 0 insertions, 5067 deletions
diff --git a/43297.txt b/43297.txt deleted file mode 100644 index 50a8beb..0000000 --- a/43297.txt +++ /dev/null @@ -1,5067 +0,0 @@ -Project Gutenberg's The Natural History of Clay, by Alfred B. Searle - -This eBook is for the use of anyone anywhere at no cost and with -almost no restrictions whatsoever. You may copy it, give it away or -re-use it under the terms of the Project Gutenberg License included -with this eBook or online at www.gutenberg.org - - -Title: The Natural History of Clay - -Author: Alfred B. Searle - -Release Date: July 25, 2013 [EBook #43297] - -Language: English - -Character set encoding: ASCII - -*** START OF THIS PROJECT GUTENBERG EBOOK THE NATURAL HISTORY OF CLAY *** - - - - -Produced by Chris Curnow, Tom Cosmas and the Online -Distributed Proofreading Team at http://www.pgdp.net (This -file was produced from images generously made available -by The Internet Archive) - - - - - - - - - -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 deg. 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 debris 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 deg. 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 deg. 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 deg. 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 deg. 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. - -Daubree 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 deg. C. it cannot be -completely restored. Moreover, a clay which has once been heated to a -temperature above 100 deg. 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 deg., -250 deg., 300 deg., 350 deg. and 400 deg. 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 deg. 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 deg. C., but -it may be heated for some hours at 1800 deg. 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 deg. C.) and a really good one will have a softening point of cone 34 -or 35 (1750 deg. to 1800 deg. C.). - -[Illustration: Fig. 6. Seger Cones indicating a temperature of 1250 deg. 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 deg. 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 deg. 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 deg. -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 debris 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 deg. C. or Cone -30 at the rate of 10 deg. C. per minute, and as No. 2 grade fireclay those -which show no signs of fusion when similarly heated to 1580 deg. 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 deg. 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 debris 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: pelinos] = 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 deg. 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 deg. 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 deg. 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 deg. 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 deg. -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 deg. 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 deg. -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 deg. 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 deg.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--Role - 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 Ceramique industrielle." A. Granger. Gauthier Freres. 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 THIS PROJECT GUTENBERG EBOOK THE NATURAL HISTORY OF CLAY *** - -***** This file should be named 43297.txt or 43297.zip ***** -This and all associated files of various formats will be found in: - http://www.gutenberg.org/4/3/2/9/43297/ - -Produced by Chris Curnow, Tom Cosmas and the Online -Distributed Proofreading Team at http://www.pgdp.net (This -file was produced from images generously made available -by The Internet Archive) - - -Updated editions will replace the previous one--the old editions -will be renamed. - -Creating the works from public domain print editions means that no -one owns a United States copyright in these works, so the Foundation -(and you!) can copy and distribute it in the United States without -permission and without paying copyright royalties. Special rules, -set forth in the General Terms of Use part of this license, apply to -copying and distributing Project Gutenberg-tm electronic works to -protect the PROJECT GUTENBERG-tm concept and trademark. Project -Gutenberg is a registered trademark, and may not be used if you -charge for the eBooks, unless you receive specific permission. If you -do not charge anything for copies of this eBook, complying with the -rules is very easy. You may use this eBook for nearly any purpose -such as creation of derivative works, reports, performances and -research. They may be modified and printed and given away--you may do -practically ANYTHING with public domain eBooks. Redistribution is -subject to the trademark license, especially commercial -redistribution. - - - -*** START: FULL LICENSE *** - -THE FULL PROJECT GUTENBERG LICENSE -PLEASE READ THIS BEFORE YOU DISTRIBUTE OR USE THIS WORK - -To protect the Project Gutenberg-tm mission of promoting the free -distribution of electronic works, by using or distributing this work -(or any other work associated in any way with the phrase "Project -Gutenberg"), you agree to comply with all the terms of the Full Project -Gutenberg-tm License available with this file or online at - www.gutenberg.org/license. - - -Section 1. General Terms of Use and Redistributing Project Gutenberg-tm -electronic works - -1.A. By reading or using any part of this Project Gutenberg-tm -electronic work, you indicate that you have read, understand, agree to -and accept all the terms of this license and intellectual property -(trademark/copyright) agreement. If you do not agree to abide by all -the terms of this agreement, you must cease using and return or destroy -all copies of Project Gutenberg-tm electronic works in your possession. -If you paid a fee for obtaining a copy of or access to a Project -Gutenberg-tm electronic work and you do not agree to be bound by the -terms of this agreement, you may obtain a refund from the person or -entity to whom you paid the fee as set forth in paragraph 1.E.8. - -1.B. "Project Gutenberg" is a registered trademark. It may only be -used on or associated in any way with an electronic work by people who -agree to be bound by the terms of this agreement. There are a few -things that you can do with most Project Gutenberg-tm electronic works -even without complying with the full terms of this agreement. See -paragraph 1.C below. There are a lot of things you can do with Project -Gutenberg-tm electronic works if you follow the terms of this agreement -and help preserve free future access to Project Gutenberg-tm electronic -works. See paragraph 1.E below. - -1.C. The Project Gutenberg Literary Archive Foundation ("the Foundation" -or PGLAF), owns a compilation copyright in the collection of Project -Gutenberg-tm electronic works. Nearly all the individual works in the -collection are in the public domain in the United States. If an -individual work is in the public domain in the United States and you are -located in the United States, we do not claim a right to prevent you from -copying, distributing, performing, displaying or creating derivative -works based on the work as long as all references to Project Gutenberg -are removed. Of course, we hope that you will support the Project -Gutenberg-tm mission of promoting free access to electronic works by -freely sharing Project Gutenberg-tm works in compliance with the terms of -this agreement for keeping the Project Gutenberg-tm name associated with -the work. You can easily comply with the terms of this agreement by -keeping this work in the same format with its attached full Project -Gutenberg-tm License when you share it without charge with others. - -1.D. The copyright laws of the place where you are located also govern -what you can do with this work. Copyright laws in most countries are in -a constant state of change. If you are outside the United States, check -the laws of your country in addition to the terms of this agreement -before downloading, copying, displaying, performing, distributing or -creating derivative works based on this work or any other Project -Gutenberg-tm work. The Foundation makes no representations concerning -the copyright status of any work in any country outside the United -States. - -1.E. Unless you have removed all references to Project Gutenberg: - -1.E.1. The following sentence, with active links to, or other immediate -access to, the full Project Gutenberg-tm License must appear prominently -whenever any copy of a Project Gutenberg-tm work (any work on which the -phrase "Project Gutenberg" appears, or with which the phrase "Project -Gutenberg" is associated) is accessed, displayed, performed, viewed, -copied or distributed: - -This eBook is for the use of anyone anywhere at no cost and with -almost no restrictions whatsoever. You may copy it, give it away or -re-use it under the terms of the Project Gutenberg License included -with this eBook or online at www.gutenberg.org - -1.E.2. If an individual Project Gutenberg-tm electronic work is derived -from the public domain (does not contain a notice indicating that it is -posted with permission of the copyright holder), the work can be copied -and distributed to anyone in the United States without paying any fees -or charges. If you are redistributing or providing access to a work -with the phrase "Project Gutenberg" associated with or appearing on the -work, you must comply either with the requirements of paragraphs 1.E.1 -through 1.E.7 or obtain permission for the use of the work and the -Project Gutenberg-tm trademark as set forth in paragraphs 1.E.8 or -1.E.9. - -1.E.3. If an individual Project Gutenberg-tm electronic work is posted -with the permission of the copyright holder, your use and distribution -must comply with both paragraphs 1.E.1 through 1.E.7 and any additional -terms imposed by the copyright holder. Additional terms will be linked -to the Project Gutenberg-tm License for all works posted with the -permission of the copyright holder found at the beginning of this work. - -1.E.4. Do not unlink or detach or remove the full Project Gutenberg-tm -License terms from this work, or any files containing a part of this -work or any other work associated with Project Gutenberg-tm. - -1.E.5. Do not copy, display, perform, distribute or redistribute this -electronic work, or any part of this electronic work, without -prominently displaying the sentence set forth in paragraph 1.E.1 with -active links or immediate access to the full terms of the Project -Gutenberg-tm License. - -1.E.6. You may convert to and distribute this work in any binary, -compressed, marked up, nonproprietary or proprietary form, including any -word processing or hypertext form. However, if you provide access to or -distribute copies of a Project Gutenberg-tm work in a format other than -"Plain Vanilla ASCII" or other format used in the official version -posted on the official Project Gutenberg-tm web site (www.gutenberg.org), -you must, at no additional cost, fee or expense to the user, provide a -copy, a means of exporting a copy, or a means of obtaining a copy upon -request, of the work in its original "Plain Vanilla ASCII" or other -form. Any alternate format must include the full Project Gutenberg-tm -License as specified in paragraph 1.E.1. - -1.E.7. Do not charge a fee for access to, viewing, displaying, -performing, copying or distributing any Project Gutenberg-tm works -unless you comply with paragraph 1.E.8 or 1.E.9. - -1.E.8. You may charge a reasonable fee for copies of or providing -access to or distributing Project Gutenberg-tm electronic works provided -that - -- You pay a royalty fee of 20% of the gross profits you derive from - the use of Project Gutenberg-tm works calculated using the method - you already use to calculate your applicable taxes. The fee is - owed to the owner of the Project Gutenberg-tm trademark, but he - has agreed to donate royalties under this paragraph to the - Project Gutenberg Literary Archive Foundation. Royalty payments - must be paid within 60 days following each date on which you - prepare (or are legally required to prepare) your periodic tax - returns. Royalty payments should be clearly marked as such and - sent to the Project Gutenberg Literary Archive Foundation at the - address specified in Section 4, "Information about donations to - the Project Gutenberg Literary Archive Foundation." - -- You provide a full refund of any money paid by a user who notifies - you in writing (or by e-mail) within 30 days of receipt that s/he - does not agree to the terms of the full Project Gutenberg-tm - License. You must require such a user to return or - destroy all copies of the works possessed in a physical medium - and discontinue all use of and all access to other copies of - Project Gutenberg-tm works. - -- You provide, in accordance with paragraph 1.F.3, a full refund of any - money paid for a work or a replacement copy, if a defect in the - electronic work is discovered and reported to you within 90 days - of receipt of the work. - -- You comply with all other terms of this agreement for free - distribution of Project Gutenberg-tm works. - -1.E.9. If you wish to charge a fee or distribute a Project Gutenberg-tm -electronic work or group of works on different terms than are set -forth in this agreement, you must obtain permission in writing from -both the Project Gutenberg Literary Archive Foundation and Michael -Hart, the owner of the Project Gutenberg-tm trademark. Contact the -Foundation as set forth in Section 3 below. - -1.F. - -1.F.1. Project Gutenberg volunteers and employees expend considerable -effort to identify, do copyright research on, transcribe and proofread -public domain works in creating the Project Gutenberg-tm -collection. Despite these efforts, Project Gutenberg-tm electronic -works, and the medium on which they may be stored, may contain -"Defects," such as, but not limited to, incomplete, inaccurate or -corrupt data, transcription errors, a copyright or other intellectual -property infringement, a defective or damaged disk or other medium, a -computer virus, or computer codes that damage or cannot be read by -your equipment. - -1.F.2. LIMITED WARRANTY, DISCLAIMER OF DAMAGES - Except for the "Right -of Replacement or Refund" described in paragraph 1.F.3, the Project -Gutenberg Literary Archive Foundation, the owner of the Project -Gutenberg-tm trademark, and any other party distributing a Project -Gutenberg-tm electronic work under this agreement, disclaim all -liability to you for damages, costs and expenses, including legal -fees. YOU AGREE THAT YOU HAVE NO REMEDIES FOR NEGLIGENCE, STRICT -LIABILITY, BREACH OF WARRANTY OR BREACH OF CONTRACT EXCEPT THOSE -PROVIDED IN PARAGRAPH 1.F.3. YOU AGREE THAT THE FOUNDATION, THE -TRADEMARK OWNER, AND ANY DISTRIBUTOR UNDER THIS AGREEMENT WILL NOT BE -LIABLE TO YOU FOR ACTUAL, DIRECT, INDIRECT, CONSEQUENTIAL, PUNITIVE OR -INCIDENTAL DAMAGES EVEN IF YOU GIVE NOTICE OF THE POSSIBILITY OF SUCH -DAMAGE. - -1.F.3. LIMITED RIGHT OF REPLACEMENT OR REFUND - If you discover a -defect in this electronic work within 90 days of receiving it, you can -receive a refund of the money (if any) you paid for it by sending a -written explanation to the person you received the work from. If you -received the work on a physical medium, you must return the medium with -your written explanation. The person or entity that provided you with -the defective work may elect to provide a replacement copy in lieu of a -refund. If you received the work electronically, the person or entity -providing it to you may choose to give you a second opportunity to -receive the work electronically in lieu of a refund. If the second copy -is also defective, you may demand a refund in writing without further -opportunities to fix the problem. - -1.F.4. Except for the limited right of replacement or refund set forth -in paragraph 1.F.3, this work is provided to you 'AS-IS', WITH NO OTHER -WARRANTIES OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO -WARRANTIES OF MERCHANTABILITY OR FITNESS FOR ANY PURPOSE. - -1.F.5. Some states do not allow disclaimers of certain implied -warranties or the exclusion or limitation of certain types of damages. -If any disclaimer or limitation set forth in this agreement violates the -law of the state applicable to this agreement, the agreement shall be -interpreted to make the maximum disclaimer or limitation permitted by -the applicable state law. The invalidity or unenforceability of any -provision of this agreement shall not void the remaining provisions. - -1.F.6. INDEMNITY - You agree to indemnify and hold the Foundation, the -trademark owner, any agent or employee of the Foundation, anyone -providing copies of Project Gutenberg-tm electronic works in accordance -with this agreement, and any volunteers associated with the production, -promotion and distribution of Project Gutenberg-tm electronic works, -harmless from all liability, costs and expenses, including legal fees, -that arise directly or indirectly from any of the following which you do -or cause to occur: (a) distribution of this or any Project Gutenberg-tm -work, (b) alteration, modification, or additions or deletions to any -Project Gutenberg-tm work, and (c) any Defect you cause. - - -Section 2. Information about the Mission of Project Gutenberg-tm - -Project Gutenberg-tm is synonymous with the free distribution of -electronic works in formats readable by the widest variety of computers -including obsolete, old, middle-aged and new computers. It exists -because of the efforts of hundreds of volunteers and donations from -people in all walks of life. - -Volunteers and financial support to provide volunteers with the -assistance they need are critical to reaching Project Gutenberg-tm's -goals and ensuring that the Project Gutenberg-tm collection will -remain freely available for generations to come. In 2001, the Project -Gutenberg Literary Archive Foundation was created to provide a secure -and permanent future for Project Gutenberg-tm and future generations. -To learn more about the Project Gutenberg Literary Archive Foundation -and how your efforts and donations can help, see Sections 3 and 4 -and the Foundation information page at www.gutenberg.org - - -Section 3. Information about the Project Gutenberg Literary Archive -Foundation - -The Project Gutenberg Literary Archive Foundation is a non profit -501(c)(3) educational corporation organized under the laws of the -state of Mississippi and granted tax exempt status by the Internal -Revenue Service. The Foundation's EIN or federal tax identification -number is 64-6221541. Contributions to the Project Gutenberg -Literary Archive Foundation are tax deductible to the full extent -permitted by U.S. federal laws and your state's laws. - -The Foundation's principal office is located at 4557 Melan Dr. S. -Fairbanks, AK, 99712., but its volunteers and employees are scattered -throughout numerous locations. Its business office is located at 809 -North 1500 West, Salt Lake City, UT 84116, (801) 596-1887. Email -contact links and up to date contact information can be found at the -Foundation's web site and official page at www.gutenberg.org/contact - -For additional contact information: - Dr. Gregory B. Newby - Chief Executive and Director - gbnewby@pglaf.org - -Section 4. Information about Donations to the Project Gutenberg -Literary Archive Foundation - -Project Gutenberg-tm depends upon and cannot survive without wide -spread public support and donations to carry out its mission of -increasing the number of public domain and licensed works that can be -freely distributed in machine readable form accessible by the widest -array of equipment including outdated equipment. Many small donations -($1 to $5,000) are particularly important to maintaining tax exempt -status with the IRS. - -The Foundation is committed to complying with the laws regulating -charities and charitable donations in all 50 states of the United -States. Compliance requirements are not uniform and it takes a -considerable effort, much paperwork and many fees to meet and keep up -with these requirements. We do not solicit donations in locations -where we have not received written confirmation of compliance. To -SEND DONATIONS or determine the status of compliance for any -particular state visit www.gutenberg.org/donate - -While we cannot and do not solicit contributions from states where we -have not met the solicitation requirements, we know of no prohibition -against accepting unsolicited donations from donors in such states who -approach us with offers to donate. - -International donations are gratefully accepted, but we cannot make -any statements concerning tax treatment of donations received from -outside the United States. U.S. laws alone swamp our small staff. - -Please check the Project Gutenberg Web pages for current donation -methods and addresses. Donations are accepted in a number of other -ways including checks, online payments and credit card donations. -To donate, please visit: www.gutenberg.org/donate - - -Section 5. General Information About Project Gutenberg-tm electronic -works. - -Professor Michael S. Hart was the originator of the Project Gutenberg-tm -concept of a library of electronic works that could be freely shared -with anyone. For forty years, he produced and distributed Project -Gutenberg-tm eBooks with only a loose network of volunteer support. - -Project Gutenberg-tm eBooks are often created from several printed -editions, all of which are confirmed as Public Domain in the U.S. -unless a copyright notice is included. Thus, we do not necessarily -keep eBooks in compliance with any particular paper edition. - -Most people start at our Web site which has the main PG search facility: - - www.gutenberg.org - -This Web site includes information about Project Gutenberg-tm, -including how to make donations to the Project Gutenberg Literary -Archive Foundation, how to help produce our new eBooks, and how to -subscribe to our email newsletter to hear about new eBooks. |