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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

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