Trancriber's Note
Text emphasis is displayed as _Italic_ and =Bold=.
Whole and fractional parts are displayed as 8-1/2.
Although in general, subscripts are denoted _{#}. The formulae for
the minerals are presented as K2OAl2O3·6SiO2 (Orthoclase) where the
number following the element would normally be subscripted.
The Cambridge Manuals of Science and Literature
THE NATURAL HISTORY OF CLAY
CAMBRIDGE UNIVERSITY PRESS
London: FETTER LANE, E.C.
C. F. CLAY, Manager
[Illustration]
Edinburgh: 100, PRINCES STREET
London: WILLIAM WESLEY AND SON, 28, ESSEX STREET, STRAND
Berlin: A. ASHER AND CO.
Leipzig: F. A. BROCKHAUS
New York: G. P. PUTNAM'S SONS
Bombay and Calcutta: MACMILLAN AND CO., Ltd.
_All rights reserved_
[Illustration]
THE NATURAL HISTORY OF CLAY
BY
ALFRED B. SEARLE
Cantor Lecturer on Brickmaking,
Author of _British Clays, Shales
and Sands_; _The Clayworker's
Handbook_, etc., etc.
Cambridge:
at the University Press
New York:
G. P. Putnam's Sons
1912
Cambridge:
PRINTED BY JOHN CLAY, M.A.
AT THE UNIVERSITY PRESS
_With the exception of the coat of arms at the foot, the design on the
title page is a reproduction of one used by the earliest known Cambridge
printer, John Siberch, 1521_
PREFACE
Both as raw materials and in the form of pottery, bricks, tiles,
terra-cotta and many other articles of use and ornament, clays are
amongst the most important rock products. Yet the origin of the
substances we know as 'clay,' the processes occurring in its formation
and the causes of some of the most important of its characteristics are
of such a nature that it is remarkable that its use should have become
so extended in the arts and sciences, while we know so little of its
properties when in a pure state.
In the following pages an attempt has been made to state in a simple
form an outline of our present knowledge of the subject and to indicate
the problems which still lie before us.
The experimental solution of these problems is rendered peculiarly
difficult by the inertness of the materials at ordinary temperatures and
the ease with which the clay molecule appears to break down into its
constituent oxides at temperatures approaching red heat or as soon as it
begins to react with alkaline or basic materials.
Another serious difficulty is the highly complex nature of that property
known as 'plasticity' to which many clays owe their chief value. For
many years this has been regarded as an elementary property such as
hardness, cohesion or colour, but it is now known to be of so elusive a
nature as almost to defy measurement with any degree of accuracy.
The thoroughness with which the methods of physical chemistry have been
applied to geological and mineralogical problems during recent years has
been of very great assistance to the student of clay problems, as will
be seen on studying some of the works mentioned in the short
bibliography at the end of the present volume. When the principles of
hydrolysis, ionization, mass reaction and reactional velocity have been
applied in still further detail to the study of clays, our knowledge of
their natural history will increase even more rapidly than it has done
during the past few years.
No industry exercises so great a fascination over those engaged in it as
do the various branches of clayworking; no other substance offers so
many problems of such absorbing interest to the artist, the craftsman,
the geologist, the chemist and the general student of nature, whilst the
differences in legal opinion as to the nature of clay could themselves
occupy a volume far larger than the present one.
A. B. S.
The White Building,
Sheffield.
_November 1911._
CONTENTS
CHAP. PAGE
Table of clay rocks viii
I Introduction. The chemical and physical properties of clays 1
II Clay and associated rocks 48
III The origins of clays 70
IV The modes of accumulation of clays 84
V Some clays of commercial importance 103
VI Clay-substance: theoretical and actual 135
Bibliography 168
Index 170
LIST OF ILLUSTRATIONS
FIG.
1 Quartz crystals 9
2 Pyrite 14
3 Marcasite 14
4 Illustrating the structure of a 'clay crumb' 24
5 Chart showing rates of drying 27
6 Seger Cones indicating a temperature of 1250° C. 34
7 Ludwig's Chart 36
8 Coal Measures sequence in North Staffordshire 55
9 Lias clay being worked for the manufacture of hand-made
sand-faced roofing tiles 58
10 Oxford clay near Peterborough 60
11 Cliffs of Boulder clay at Filey lying on Calcareous Crag 66
12 China clay pit belonging to the North Cornwall China Clay Co. 72
13 Orthoclase Felspar 75
14 Illustrating the successive deposition of different strata 90
15 Lacustrine clay at Skipsea 92
16 Clay at Nostel, showing Marine Band 94
17 Kaolinite and Mica 105
18 Mining best Potter's clay in Devonshire 111
THE CHIEF CLAY ROCKS (arranged geologically)
+--------------------------------------------------------+
{|Recent (_alluvial clay_, _silt_, _brick earths_, |
{| _boulder clay_) |
{|--------------------------------------------------------|
Tertiary {|Pliocene } |
{|Miocene } (_brick earths_, _ball clays_, |
{|Oligocene } _coarse pottery clays_) |
{|Eocene } |
|--------------------------------------------------------|
{|Cretaceous (_cement clays_, _brick clays_) |
{|--------------------------------------------------------|
Secondary {|Oolitic (_brick and tile clays_) |
{|--------------------------------------------------------|
{|Triassic (_brick, tile and terra-cotta clays_) |
|--------------------------------------------------------|
{|Permian (_brick, tile and flower-pot clays_) |
{|--------------------------------------------------------|
{|Carboniferous (_brick clays_, _fireclays_, _ganister_) |
{|--------------------------------------------------------|
Primary {|Devonian } |
{|Silurian } |
{|Ordovician } (_clay schists, slates and clay shales_) |
{|Cambrian } |
{|Pre-Cambrian } |
|--------------------------------------------------------|
|Igneous Rocks occur on several horizons (_china clays_ |
| _and kaolins_) |
+--------------------------------------------------------+
(In the above Table only the clay-bearing strata are mentioned. The
formations named consist chiefly of other rocks in which the clays form
strata of variable thickness.)
CHAPTER I
INTRODUCTION. THE CHEMICAL AND PHYSICAL PROPERTIES OF CLAY
The chief uses of clay have been recognized since the earliest periods
of civilization; the ancient Assyrian and Egyptian records contain
numerous references to the employment of clay for the manufacture of
bricks and for fulling or whitening cloth.
Clays are distributed so widely and in many cases are so readily
accessible that their existence and some of their characteristics are
known in entirely uncivilized regions. The use of certain white clays as
a food, or at any rate as a means of staving off hunger, is common among
some tribes of very primitive peoples. The more important uses of clays
for building and other purposes are naturally confined to the more
civilized nations.
The term _clay_ (A.S. _cloeg_; Welsh _clai_; Dutch _kley_) although used
in a scientific sense to include a variety of argillaceous earths (Fr.
_argile_ = clay) used in the manufacture of bricks, tiles, pottery and
ceramic products (Gr. _keramos_ = potter's earth) generally, is really a
word of popular origin and use. Consequently, it is necessary to bear in
mind, when considering geological or other problems of a scientific
nature, that this term has been incorporated into scientific terminology
and that its use in this connection not infrequently leads to confusion.
In short, whilst almost every dictionary includes one or more
definitions of clay, and most text-books on geology, mineralogy, and
allied sciences either attempt a definition or assume the reader's
knowledge of one, there is no entirely satisfactory limitation in regard
to the substances which may or may not be included under the term.
_Clay_ is a popular term for a variety of substances of very varied
origins, of great dissimilarity in their composition and in many of
their chemical and physical properties, and differing greatly in almost
every conceivable respect. It is commonly supposed that all clays are
plastic, but some of the purest china clays are almost devoid of this
property and some of the most impure earths used for brickmaking possess
it in a striking degree. Shales, on the one hand--whilst clearly a
variety of clay--are hard and rock-like, requiring to be reduced to
powder and very thoroughly mixed with water before they become plastic;
many impure surface deposits, on the other hand, are so highly plastic
as to necessitate the addition of other (sandy) materials before they
can be used for the manufacture of bricks and tiles.
Attempts have been made to include in the term clay 'all minerals
capable of becoming plastic when moistened or mixed with a suitable
quantity of water,' but this definition is so wide as to be almost
impracticable, and leads to the inclusion of many substances which have
no real connection with clays. The limitation of the use of the word
'clay' to the plastic or potentially plastic materials of any single
geological epoch is also impracticable, for clays appear to have been
deposited in almost every geological period, though there is some
difference of opinion as to the time of the formation of certain clays
known as _kaolins_.
Clay is not infrequently termed a _mineral_, but this does not apply at
all accurately to the many varieties of earths known as 'common clays,'
which, together with the 'boulder clays,' contain many minerals and so
cannot, as a whole, be included under this term.
Whatever may be the legal significance of the term 'mineral'--which has
an important economic bearing on account of minerals being taxed or
'reserved' in some instances where non-minerals (including brick clay)
are exempt--there can be no doubt that, scientifically, clay is _not a
mineral but a rock_. Whatever mineral (if any) may give the chief
characteristic property to the clays as a class must be designated by a
special title, for the general term 'clay' will not serve for this
purpose. Geologically, the clays are sedimentary rocks, some being
unaltered, whilst others--the slates--are notably metamorphosed and can
seldom be used for the purposes for which clays are employed.
Most clays may be regarded as a mixture of quartz grains, undecomposed
rock débris and various decomposition products of rocks; if the
last-named consists chiefly of certain hydrous alumino-silicates, they
may be termed 'clay substance' (see Chapter VI. The imperfections of
this statement as a definition are obvious when it is remembered that it
may include a mixture of fine sand and clay containing only 30 per cent.
of the latter substance.
It is, at the present time, quite impossible to construct an accurate
definition of the term 'clay.' The most satisfactory hitherto
published defines 'clay' as 'a solid rock composed mainly of
hydro-alumino-silicates or alumino-silicic acids, but often containing
large proportions of other materials; the whole possessing the property
of becoming plastic when treated with water, and of hardening to a
stone-like mass when heated to redness.'
From what has already been written, it will be understood that there is
no such entity as a standard clay, for the varieties are almost endless,
and the differences between them are sometimes so slight as to be
scarcely distinguishable.
A further consideration of this branch of the subject may, however,
conveniently be deferred to a subsequent chapter.
The best-known clays are the surface clays, loams and marls, the shales
and other sub-surface clays, and the pottery and china clays. The values
of these different materials vary enormously, some being almost
worthless whilst others are highly valued.
The _surface clays_ are chiefly used for the manufacture of bricks and
tiles (though some are quite unsuitable for this purpose) and form the
soil employed in agriculture in many districts.
The _sub-surface clays_ and _shales_ are harder, and usually require
mechanical treatment before they can be used for brick and terra-cotta
manufacture, or for the production of refractory and sanitary articles.
The _pottery and china clays_ are usually more free from accessory
constituents, and are regarded as the 'purest' clays on the market,
though a considerable amount of latitude must be allowed in interpreting
the term 'pure.' China clays are by no means pure in the state in which
they occur, and require careful treatment before they can be sold.
Further information with regard to the characteristics of certain clays
will be found in Chapter V.
The Chemical Properties of Clay.
The chief constituents of all clays are alumina and silica, the latter
being always in excess of the former. These two oxides are, apparently,
combined to form a hydro-alumino-silicate or alumino-silicic acid
corresponding to the formula H4Al2Si2O9[1], but many clays
contain a much larger proportion of silica than is required to form this
compound, and other alumino-silicates also occur in them in varying
proportions (see Chapters V and VI).
All clays may, apparently, be regarded as consisting of a mixture of one
or more hydrous alumino-silicates with free silica and other non-plastic
minerals or rock granules, and their chemical properties are largely
dependent on the nature and proportion of these accessory ingredients.
The purest forms of clay (china clays and ball clays) approximate to the
formula above-mentioned, but others differ widely from it, as will be
seen from the analyses on p. 16. The chemical properties of pure clay
are described more fully in Chapter VI.
[Footnote 1: This formula is commonly written Al2O33·2SiO2·2H2O,
but although this is a convenient arrangement, it must not be understood
to mean that clays contain water in a state of combination similar to
that in such substances as washing soda--Na2CO3·24H2O, or zinc
sulphate crystals--ZnSO4·7H2O (see Chapter VI).]
Taking china clay, which has been carefully purified by levigation, as
representative of the composition of a 'pure' clay, it will be found
that the chief impurities in clays are (_a_) stones, gravel and
sand--removable by washing or sifting; (_b_) felspar, mica and other
silicates and free silica--which cannot be completely removed without
affecting the clay and (_c_) lime, magnesia, iron, potash and soda
compounds, together with minute quantities of other oxides, all of which
appear to be so closely connected with the clay as to be incapable of
removal from it by any mechanical methods of purification.
To give a detailed description of the effect of each of the impurities
just referred to would necessitate a much larger volume than the
present, but a few brief notes on the more important ones are essential
to a further consideration of the natural history of clay.
_Stones_, _gravel_ and _sand_ are most noticeable in the boulder clays,
but they occur in clays of most geological ages, though in very varying
proportions. Sometimes the stones are so large that they may be readily
picked out by hand; in any case the stones, gravel and most of the sand
may be removed by mixing the material with a sufficient quantity of
water and passing the 'slip' through a fine sieve, or by allowing it to
remain stationary for a few moments and then allowing the supernatant
liquid to run off into a settling tank. Some clays contain sand grains
which are so fine that they cannot be removed in this manner and the
clay must then be washed out by a stream of water with a velocity not
exceeding 2 ft. per hour. Even then, the clay so removed may be found to
contain minute grains of silt, much of which may be removed by a series
of sedimentations for various periods, though a material perfectly free
from non-plastic granules may be unattainable.
Most of the sand found associated with clays is in the form of fragments
of _quartz_ crystals (fig. 1), though it may be composed of irregular
particles of other minerals or of amorphous silica.
_Felspar_, _mica_ and other adventitious silicates occur in many natural
clays in so fine a state of division that their removal would be
unremunerative. In addition to this they act as fluxes when the clays
are heated in kilns, binding the less fusible particles together and
forming a far stronger mass than would otherwise be produced.
Consequently, they are valuable constituents in clays used for the
manufacture of articles in which strength or imperviousness is
important. If these minerals are present in the form of particles which
are sufficiently large to be removed by elutriation in the manner
described on the previous page, the purification of the clay is not
difficult. Usually, however, the most careful treatment fails to remove
all these minerals; their presence may then be detected by
microscopical examination and by chemical analysis. For most of the
purposes for which clays are used, small proportions of these silicates
are unimportant, but where clays of a highly refractory nature are
required; and for most of the purposes for which china clays (kaolins)
are employed, they must not be present to the extent of more than 5 per
cent., smaller proportions being preferable.
[Illustration: Fig. 1. Quartz crystals, natural size. (From Miers'
Mineralogy _by permission of Macmillan & Co._)]
_Oxides_, _sulphides_, _sulphates_ and _carbonates_ of various metals
form the third class of impurities in clays. Of these, the most
important are calcium oxide (lime), calcium carbonate (chalk and
limestone), calcium sulphate (gypsum and selenite), the corresponding
magnesia, magnesium carbonate, and sulphate, the various iron oxides,
ferrous carbonate and iron sulphides (pyrite and marcasite) (p. 13).
Potash and soda compounds are commonly present as constituents of the
felspar, mica, or other silicates present, and need no further
description, though small proportions of _soluble salts_--chiefly
sodium, potassium, calcium and magnesium sulphates--occur in most clays
and may cause a white scum on bricks and terra-cotta made from them.
_Lime and magnesia compounds_ may occur as silicates (varieties of
felspar, mica, etc.), but their most important occurrence is as chalk or
limestone. _Chalk_ is a constant constituent of malms[2] and of many
marls, but the latter may contain limestone particles. _Limestone_
occurs in many marls and to a smaller extent in other clays. In the
boulder clays it frequently forms a large portion of the stony material.
If the grains are very small (as in chalk), the lime compounds act as a
flux, reducing the heat-resisting power of the clay and increasing the
amount of vitrification; they produce in extreme cases a slag-like mass
when the clay is intensely heated. If, on the contrary, the grains are
larger (as frequently occurs with limestone), they are converted into
lime or magnesia when the clay is 'burned' in a kiln, and the lime, on
exposure to weather, absorbs moisture (_i.e. slakes_), swells, and may
disintegrate the articles made from the clay. Limestone (except when in
a very finely divided state) is almost invariably objectionable in
clays, but chalk is frequently a valuable constituent.
[Footnote 2: A _malm_ is a natural mixture of clay and chalk (p. 68).]
Chalk is added to clay in the manufacture of malm-bricks to produce a
more pleasing colour than would be obtained from the clay alone, to
reduce the shrinkage of the clay to convenient limits and, less
frequently, to form a more vitrifiable material. Chalk, on heating,
combines with iron oxide and clay, forming a white silicate, so that
some clays which would, alone, form a red brick, will, if mixed with
chalk, form a white one.
Lime compounds have the serious objection of acting as very rapid and
powerful fluxes, so that when clays containing them are heated
sufficiently to start partial fusion, a very slight additional rise in
temperature may easily reduce the whole to a shapeless, slag-like mass.
Magnesia compounds act much more slowly in this respect and so are less
harmful.
_Gypsum_--a calcium sulphate--occurs naturally in many sub-surface
clays, often in well-defined crystalline masses. It reduces the
heat-resisting power of the clays containing it and may, under some
conditions, rise to the surface of the articles made from the clay, in
the form of a white efflorescence or scum, such as is seen on some brick
walls.
_Iron compounds_ are highly important because they exercise a powerful
influence on the colour of the burned clays. The red oxide (ferric
oxide) is the most useful form in burned clay, but in the raw material
ferrous oxide and ferrous carbonate may also occur, though they are
converted into the red oxide on heating. The red iron oxide, which is
closely related to 'iron rust,' occurs in so finely divided a state that
its particles appear to be almost as small as those of the finest clays.
Hence attempts to improve the colour of terra-cotta and bricks by the
addition of commercial 'iron oxide' are seldom satisfactory, the finest
material obtainable being far coarser than that occurring in clays.
It is a curious fact that red iron oxide does not appear to form any
compound with the other constituents of clay under ordinary conditions
of firing, and although a 'base' and capable of reducing the
heat-resisting power of clays, it does not appear to do so as long as
the conditions in the kiln are sufficiently oxidizing. It is this which
enables red bricks and other articles to be obtained with remarkable
uniformity of colour combined with great physical strength. In a
reducing atmosphere, on the contrary, ferrous oxide readily forms and
attacks the clay, forming a dark grey vitreous mass. If the iron
particles are separated from each other they will, on reduction, form
small slag-like spots, but if they are in an extremely fine state of
division and well distributed, the brick or other article will become
slightly glossy and of an uniform black-grey tint. The famous
Staffordshire 'blue' bricks owe their colour to this characteristic;
they are not really 'blue' in colour. The effect of chalk on the colour
of red-burning clays has already been mentioned.
_Iron pyrite_ (fig. 2) and _marcasite_ (fig. 3)--both of which are forms
of iron sulphide--occur in many clays, particularly those of the Coal
Measures. _Mundic_ is another form of pyrites which resembles roots or
twigs, but when broken show a brassy fracture. When in pieces of
observable size the pyrite may be readily distinguished by its
resemblance to polished brass and the marcasite by its tin-white
metallic lustre and both by their characteristic cubic, root-like and
spherical forms; the latter only show a brass-like sheen when broken.
Even when only a small proportion of mundic, pyrite or marcasite is
present, it is highly objectionable for several reasons. In the first
place, half the sulphur present is given off at a dark red heat and is
liable to cause troublesome defects on the goods. Secondly, because the
remaining sulphur and iron are not readily oxidized, so that there is a
great tendency to form slag-spots of ferrous silicate, owing to the iron
attacking the clay at the same moment as it parts with its remaining
sulphur. For this reason, clays containing any iron sulphide seldom burn
red, but form products of a buff colour with black spots scattered
irregularly over their surface and throughout the mass--an appearance
readily observable on most hard-fired firebricks. If chalcopyrite
(copper-iron sulphide) is present the spots may be bright green in
colour.
[Illustration: Fig. 2. Pyrite.]
[Illustration: Fig. 3. Marcasite.]
Slightly magnified.
(_From Miers'_ Mineralogy _by permission of Macmillan & Co._)
_Carbon_, either free or as hydrocarbons (chiefly vegetable matter) or
in other forms, is a constituent of most clays, though seldom reported
in analyses. Its presence exercises an important influence in several
respects. On heating the clay, with an ample supply of air, the
carbonaceous matter may distil off (as shale oil), but more usually it
decomposes and burns out leaving pores in the material. If the
air-supply is insufficient and the heating is so rapid and intense that
vitrification commences before the carbon is all burned away, the pores
become filled with the fused ingredients of the clay, air can no longer
reach the carbon particles and a black 'core' or heart is produced.
Under peculiarly disadvantageous conditions the material may also swell
greatly. This is a serious defect in many classes of clay used for
brickmaking, and its causes and prevention have been exhaustively
studied by Orton and Griffiths (1)[3] but, beyond the brief summary
given above, these are beyond the scope of the present work.
_Water_ is an essential constituent of all unburned clays, though the
proportion in which it occurs varies within such wide limits that no
definite standard can be stated. This water is found in two conditions:
(_a_) as moisture or mechanically mixed with the clay particles and
(_b_) in a state of chemical combination.
[Footnote 3: References to original papers, etc. will be found in the
appendix.]
ANALYSES OF TYPICAL CLAYS
_The samples were all dried at 105° C._
+-------------------+--------+-------+---------+--------+-------+-------+
| Clay | China | Ball |Fireclay | Brick |Boulder| Marl |
| | Clay | Clay | | Clay | Clay | |
+-------------------+--------+-------+---------+--------+-------+-------+
| Locality |Cornwall| Dorset|Yorkshire|Midlands| Lancs.|Suffolk|
+-------------------+--------+-------+---------+--------+-------+-------+
|Ultimate Analysis: | | | | | | |
| Silica | 47·1 | 49·1 | 68·9 | 57·7 | 63·7 | 43·7 |
| Alumina | 39·1 | 33·7 | 19·3 | 24·3 | 20·4 | 15·5 |
| Ferric oxide | ·6 | 1·2 | 1·0 | 5·0 | 3·0 | 5·2 |
| Titanium oxide | -- | ·2 | 1·8 | ·1 | ·2 | -- |
| Lime | ·4 | ·8 | ·9 | 3·7 | 4·3 | 16·3 |
| Magnesia | ·2 | ·3 | ·3 | 2·5 | 2·7 | 2·1 |
| Potash and Soda | ·3 | 2·5 | ·9 | 2·8 | 2·9 | ·7 |
| Carbon | 2·6 | 4·3 | 1·8 | 1·6 | ·4 | 1·6 |
| Water | 9·3 | 7·7 | 4·8 | 2·0 | 2·2 | 2·4 |
| Other Matter | ·4 | ·2 | ·3 | ·3 | ·2 | 12·5 |
+-------------------+--------+-------+---------+--------+-------+-------+
| Total | 100·0 | 100·0 | 100·0 | 100·0 | 100·0 | 100·0 |
+-------------------+--------+-------+---------+--------+-------+-------+
|Proximate Analysis:| | | | | | |
| Gravel and Sand | -- | 8·4 | 4·6 | 22·1 | 23·1 | 9·2 |
| Silt | -- | 4·8 | 9·0 | 3·1 | 8·4 | 16·0 |
| Felspar- and mica-| | | | | | |
| dust | 5·2 | 15·4 | 10·3 | 24·3 | 18·5 | 8·9 |
| Silica-dust | 3·1 | 4·0 | 38·0 | 3·1 | 12·6 | 2·0 |
| Free calcium | | | | | | |
| carbonate | -- | -- | -- | 2·1 | ·2 | 28·4 |
| Free iron oxide | | | | | | |
| and pyrites | ·4 | ·9 | ·7 | 4·2 | 1·6 | 3·9 |
| 'True clay' | 91·3 | 66·5 | 37·4 | 41·1 | 35·6 | 31·6 |
+-------------------+--------+-------+---------+--------+-------+-------+
| Total | 100·0 | 100·0 | 100·0 | 100·0 | 100·0 | 100·0 |
+-------------------+--------+-------+---------+--------+-------+-------+
For other analyses the books in the Bibliography at the end of the
present volume should be consulted, particularly No. 2, _i.e._ _British
Clays, Shales and Sands_.
The amount of mechanically mixed water will naturally vary with the
conditions to which the clay has been subjected; it will be greatest in
wet situations and will diminish as the clay is allowed to dry.
The 'combined water,' on the contrary, appears to be a function of the
true clay present in the material, and reaches its highest proportions
in the china clays and kaolins, which contain approximately 13 per cent.
On heating a clay to 105° C. the moisture or mechanically mixed water is
evaporated, but the combined water remains unaffected[4] until the
temperature is raised to more than 600° C., when it is driven off and
the clay is converted into a hard stone-like mass with properties
entirely different from those it previously possessed (see Chapter VI).
[Footnote 4: Strictly, there is a slight loss at lower temperatures, but
it is too small to be important.]
The Physical Characters of Clays.
The physical characters of clays are of far more interest and importance
than their chemical ones, though the two are naturally connected in many
ways, and just as the chemical composition of clays is a subject of
extreme complexity so is a study of many of their physical properties.
Hence only a few of the more important characteristics can be mentioned
here: for further details the reader must consult a larger treatise (2).
Clays are moderately soft, solid bodies, particularly when moistened,
and can usually be cut with a knife, though some indurated clays and
shales are almost as hard as felspar. Their apparent specific gravity
varies greatly, some clays being much more porous than others, but the
true specific gravity is usually between 2·5 and 2·65; it is similar to
that of quartz and slightly lower than that of felspar and mica. Many
clays appear to be devoid of structure, but those obtained from a
considerable depth below the surface are frequently laminated and have a
structure not unlike that of mica. This will be discussed later.
Examined under a microscope, clays are seen to consist of grains of a
variety of sizes, the largest of which will usually be found to be
composed of adventitious materials such as sand, quartz, felspar, mica,
chalk and limestone. The smallest particles--to which clays owe their
chief characteristics--are so minute as to make any examination of their
shape very difficult, but they are usually composed of minute
crystalline plates together with a much larger proportion of apparently
amorphous material. The exact nature of both the crystals and the
amorphous material is still unknown in spite of many investigations; in
the purer clays both forms of substance appear to have the same chemical
composition, viz. that of _kaolinite_ (H4Al2Si2O9), which the
crystalline portion closely resembles.
Clays emit a characteristic yet indefinable odour when moist; the cause
of this is very imperfectly understood, though it is not improbably due
to decomposing organic matter, as this occurs in most clays.
The colours of freshly-dug clays are extremely varied and range from an
almost pure white through all shades of yellow, red and brown to black.
The predominating colours are grey or greyish brown and a peculiar
yellow characteristic of some surface clays. The natural colour of a
clay is no criterion as to its purity, for some of the darkest ball
clays produce perfectly white ware on burning, whilst some of the paler
clays are useless to the potter on account of the intensity of their
colour when they come out of the kiln. The colour of raw clays is
largely due to the carbonaceous matter they contain, and as this burns
away in the kiln, the final colour of the ware bears no relation
whatever to that of the original clay.
The colour of burned ware depends upon the iron compounds in the
clay--these producing buff, red, brown or black (usually termed 'blue')
articles--on the presence of finely divided calcium carbonate (chalk)
which can destroy the colouring power of iron compounds and produce
white ware, and on the treatment the clay has received in the kiln. A
clay which is white when underfired will usually darken in colour if
heated to vitrification, and one which burns red in an oxidizing
atmosphere may turn blue-grey or black under reducing conditions. The
extent to which the carbonaceous matter is burned out also determines
the colour of the fired ware.
The presence of adventitious minerals in the clay may also affect its
colour, particularly when fired.
The most obvious feature in a piece of moist clay is its _plasticity_[5]
or ability to alter its shape when kneaded or put under slight pressure
and to retain its new shape after the pressure has been removed. It is
this property which enables the production of ornaments, vessels of
various kinds, and the many other articles which are the result of the
application of modelling tools, of moulding or of the action of a
potter's wheel. So long as clay contains a suitable proportion of
moisture it is plastic and may be made into articles of any desired
shape, but if the amount of moisture in it is reduced or removed
completely, the material is no longer plastic. It may become so,
however, on adding a further suitable quantity of water and mixing,
provided that it has not been excessively heated. If, in the removal of
the moisture, the clay has been heated to 600° C. or more, it loses its
power of becoming plastic and is converted into a material more closely
resembling stone.
[Footnote 5: A plastic substance is one with the characteristics of 'a
fluid of so great a viscosity that it does not lose its shape under the
influence of gravitation.']
The causes of plasticity appear to be somewhat numerous, though there is
no generally accepted explanation of this remarkable quality which
distinguishes clays from most other substances. It is true that wet
sand, soap, wax, lead and some other materials possess a certain amount
of plasticity, but not to anything like the same extent as clay.
So far as clays are concerned, their plasticity appears to be connected
with the presence of combined water as well as of mechanically mixed
water, for if either of these are removed, plasticity--both actual and
potential--is destroyed. The part played by water is not, however,
completely known, for the many theories which have been advanced only
cover some of the conditions and facts.
A number of observers agree that the molecular constitution of clay is
peculiar and that it is to this that plasticity is due. Yet the curious
fact that the purest clays--the kaolins--are remarkably deficient in
plasticity shows that molecular constitution is not, alone, sufficient.
Others hold that the remarkably small size of clay particles enables
them to pack together more closely than do particles of other materials
and to retain around them a film of water which acts partly as a
lubricant, facilitating the change of shape of the mass when under
pressure, and partly as an adhesive, causing the particles to adhere to
each other when the pressure is removed.
Zschokke has laid much emphasis on the importance of molecular
attraction between clay and water as a cause of plasticity, and has
suggested that the absorption of the water effects a change in the
surfaces of the clay particles, giving them a gelatinous nature and
enabling them to change their form and yet keep in close contact.
The fact that mica, fluorspar and quartz, when in a sufficiently finely
divided state, are also slightly plastic, appears to be opposed to the
molecular constitution theory. Smallness of grain undoubtedly has an
influence on the plasticity of clay, coarse-grained clays being notably
less plastic than others.
Daubrée pointed out that felspar, when ground with water, develops
plasticity to a small extent, and Olschewsky carried this observation
further and has suggested that clays owe their plasticity to prolonged
contact with water during their removal from their place of formation
and previous to or during their deposition. A further confirmation of
this theory is due to Mellor (3) who showed that on heating china clay
with water under very considerable pressure its plasticity was increased
and that felspar and some other non-plastic materials developed
plasticity under these conditions.
Johnson and Blake (21) supposed that plasticity is due to the clay being
composed of extremely minute plates 'bunched together,' a view which was
also held by Biedermann and Herzfield, Le Chatelier and others.
Olschewsky enlarged this theory by suggesting that the plasticity of
certain clays is dependent on the large surface and the interlocking of
irregular particles with the plates just mentioned. These theories of
interlocking are, however, incomplete, because the tensile strength of
clays should accurately represent the plasticity if interlocking were
the sole cause. Zschokke has shown that tensile strength is only one
factor which must be determined in any attempt to measure plasticity.
E. H. L. Schwarz (35) has suggested that many clays are composed of
small globular masses of plates so arranged as to form an open network
(fig. 4) which is sufficiently strong not to be destroyed by pressure.
In the presence of water and much rubbing the plates are separated and
are made to lie flat on each other, thereby giving a plastic and
impermeable mass. If this is really the case it would explain the
porosity and large surface of some clays and might account for their
adsorptive power.
[Illustration: Fig. 4. Illustrating the structure of a 'clay crumb.'
(_After Schwarz._)]
A theory which was first promulgated in 1850 by Way (4), but which has
only received detailed attention during the last few years, attributes
plasticity to the presence of colloid substances in clay or to the fact
that clay particles possess physical characters analogous to those of
glue and other colloids. These colloid substances have a submicroscopic
or micellian structure; they are web-like, porous and absorb water
eagerly. This water may be removed by drying, only to be re-absorbed on
cooling, but if the heating temperature is excessive the structure of
the colloids is destroyed. This colloid theory explains many of the
facts noted by earlier investigators such as Aron, Bischof, Seger,
Olschewsky, etc., but it is not entirely satisfactory, though Rohland
(5)--to whom the present prominence of this theory in Europe is largely
due--persistently maintains the contrary. One great objection is the
fact that no characteristic _inorganic_ colloid substance has been
isolated from pure clay. It is possible that some of the so-called
'colloidal' properties of clay may be due to the smallness of its
particles and to their great porosity, as suggested by Olschewsky.
Despite the present impossibility of producing a plastic material from
artificially prepared colloidal hydro-alumino-silicates of the same
ultimate composition as clay, and the fact that the addition of
colloidal substances does not necessarily increase the true plasticity
of clay, it cannot be denied that the presence of colloids has an
important influence on it. The addition of starches, glue, gums and
similar substances whilst apparently increasing the plasticity of clay
does not do so in reality. The addition of 1 per cent. of tannin, on the
contrary, has been found by Ries (6) to increase both plasticity and
binding power.
Plasticity appears to be composed of a number of characteristics so that
it is scarcely likely that any single cause can be assigned to it. On
the contrary, a study of the binding power, tensile strength,
extensibility, adsorption, texture and molecular constitution of clays
suggests very strongly that all these properties are involved in the
production of plasticity and that it is due to the chemical as well as
the physical nature of clay. No clay is entirely colloidal--or it would
be elastic and not plastic--but all appear to contain both colloidal and
non-colloidal (including plate-like) particles, and it is not improbable
that materials in both these states are required, the colloidal matter
acting as a cement. Ries (6) has, in fact, pointed out that colloids
alone lack cohesiveness and solidity, and a fine mineral aggregate is
necessary to change them into a plastic mass resembling clay. The
relative proportions of the colloidal material and the sizes of the
non-plastic grains will exercise an important influence on all the
physical characteristics mentioned above, and therefore on the
plasticity.
The manner in which slightly plastic clays become highly plastic in
nature is by no means certainly known. It has long been understood that
the increase of plasticity is due to changes undergone by the clay
during transportation. The most illuminating suggestion is that made by
Acheson in 1902, who concluded that it is due to impurities in the water
used in transporting the clay or remaining in contact with it during and
after its deposition. These impurities may be considered as derived from
the washings of forests, and after many experiments with plant extracts
Acheson believed the most important substance in this connection to be
tannin or gallo-tannic acid, a dilute solution of which he found
increased the plasticity of china clay by 300 per cent. From this he
further argued that the use of chopped straw by the Israelites in Egypt
in the manufacture of bricks was unconsciously based on the tannin
content of the straw increasing the plasticity of the material.
[Illustration: Fig. 5. Chart showing rates of drying. (_After
Bleininger._)]
Beadle has stated that 2 per cent. of dissolved cellulose will increase
the plasticity of china clay and make it equal to that of ordinary
clay.
Plasticity is diminished by heating clays, and whilst much of it may be
recovered if the temperature has not risen above 400° C. it cannot be
completely restored. Moreover, a clay which has once been heated to a
temperature above 100° C. dries in a somewhat different manner to a raw
clay. This is well shown in fig. 5 in which are summarized the results
obtained by A. V. Bleininger on a sample of ball clay from Dorset before
heating and after portions of it had been heated for 16 hours to 200°,
250°, 300°, 350° and 400° C. respectively. It is not impossible that if
subjected to the influence of water for a sufficiently long time the
whole of the plasticity of a heated clay may be restored, providing that
the temperature has not been sufficient to cause a destruction of the
clay molecule, but as this resumption requires a certain amount of time,
Bleininger has proposed to use the reduction in plasticity effected by
the heating to enable excessively plastic clays to be worked without the
necessity of adding non-plastic material to them. If any destruction of
the clay-molecules has occurred, the plasticity of that portion of the
clay can never be restored.
The _binding power_ of clays is a characteristic closely connected with
plasticity and occasionally confused with it. All plastic clays have the
power of remaining plastic when mixed with materials such as sand,
brick-dust ('grog') and other materials which are quite devoid of
plasticity. The extent to which a clay can thus bind other materials
together into a plastic mass depends, apparently, on the plasticity of
the clay itself and on the size and nature of the particles of the added
material; the more plastic the clay the larger will be the amount of
material it can thus 'bind,' and the finer the latter the more easily
will it form a strong material when mixed with a plastic clay.
Rohland (5) has shown that the binding power of clay is not alone due to
its cohesion, but that it is closely associated with the colloidal
nature of plastic clays: 'fat' clays being those which are highly
colloidal, highly plastic and possessing great binding power, whilst
'lean' clays are those deficient in these characteristics. The fact
that, as a general rule, the dark coloured clays possess the most
binding power, confirms this suggestion, as the dark colour is largely
due to organic materials, probably in a colloidal state.
The _shrinkage_ which all clays undergo on drying and when heated is
another important characteristic. It is due to the fact that as water is
removed the solid particles approach closer to each other, the volume of
the whole mass being thereby reduced. In a wet piece of clay each
particle is surrounded by a film of water, the thickness of which
depends on the nature of the clay. As this water evaporates from the
surface of the clay its place is taken by water from the interior which
rises to the surface by capillary attraction. So long as there is any
water between the particles of clay there will be shrinkage when this
water is removed, but a stage is eventually reached when the particles
of clay are in contact with each other and no more shrinkage can occur.
That this cessation of shrinkage may take place before all the water has
been removed from the clay is easily understood when it is remembered
that whilst the clay particles may be in contact, yet there are still
places (pores) where the contact is incomplete, and in these pores water
may be retained. The amount of shrinkage clays undergo on drying depends
partly on the proportion of water added to them and partly on the sizes
of the different particles of clay, sand, etc. present. An average
reduction in volume of 12 to 38 per cent. may be regarded as normal, but
coarse loams may shrink only 1 per cent. and very finely ground, highly
plastic ball clays may shrink as much as 50 per cent., though this is
unusual.
As all coagulated colloids, which have absorbed water, shrink on drying,
this behaviour of clay appears to confirm the view as to its partially
colloidal nature held by some investigators.
When a piece of dry clay is heated sufficiently a further shrinkage
(technically known as _kiln shrinkage_) occurs. This begins somewhat
below a red heat and increases in rough proportion to the temperature
and the duration of the heating. Prolonged heating at a lower
temperature will effect the same amount of shrinkage as a short exposure
to a higher temperature, but though the greater part of the shrinkage
occurs in a comparatively short time, continued heating will be
accompanied by a further reduction in volume.
This is due to the fact that clays have no definite melting point, but
undergo partial fusion at all temperatures above 950° C. or, in some
cases, at even lower ones. As a portion of the material fuses, it fills
up the pores in the mass and attacks the unfused material, this process
being continued until either the heating is stopped or the whole
material is reduced to a viscous slag.
The reduction in the volume of commercial articles made of clay and
placed in kilns varies greatly. With bricks, terra-cotta and pottery it
must not, usually, exceed 40 per cent. or the warping and cracking which
occur will be so great as to make the articles useless. The fineness of
the particles exercises an important influence on the kiln shrinkage of
a clay, and the latter is frequently reduced in commercial clayworking
by adding burned clay ground to a coarse powder to the plastic clay
before it is used. Sand is sometimes added for the same purpose, though
its more frequent use is to reduce the shrinkage in drying.
Quartz and other forms of free silica expand on heating, so that clays
containing them in large quantities shrink very slightly or may even
expand.
As clays shrink equally in all directions it is usual to state the
contraction in linear instead of volume form. Thus instead of stating
that a certain clay when moulded into bricks, dried and burned, shrinks
18 per cent. by volume, it is customary to state that it shrinks 3/4 in.
per (linear) foot. For many purposes, it is sufficient to regard the
linear shrinkage as one-third the volume-shrinkage, but this is not
strictly accurate.
The _fusibility_ of clays is a characteristic which has been very
imperfectly studied. Most clayworkers and investigators employ the term
'fusibility' in a special sense which is apt to be misleading. Owing to
the extremely high temperatures to which refractory clays can be heated
without even losing their shape, it is almost impossible to fuse them
completely. In addition to this, clays are not perfectly homogeneous
materials and some of their constituents melt at lower temperatures than
others. For this reason a clay may show signs of fusion at 1100° C., but
it may be heated for some hours at 1800° C. and yet not be completely
melted! Consequently no single 'fusing point' can be stated.
In practice, a suggestion made many years ago by Seger (7) is used; the
clay to be tested is made into a small tetrahedron (fig. 6), heated
slowly until it bends over and the point of the test-piece is almost on
a level with the base. The temperature at which this occurs is termed
the 'fusing point' though it really only indicates the heat-treatment
which is sufficient to soften the material sufficiently to cause it to
bend in the manner described. In spite of the apparent crudeness of the
test this 'softening point' appears to be fairly constant for most
refractory clays.
The bending of a test-piece in this manner is the result of the action
of all fluxes[6] in the clay, and as this depends on the size of grain
and the duration of the heating above incipient fusion and does not give
a direct measure of temperature, nor is the softening effect under one
rate of rise in temperature the same as that at another rate.
Nevertheless a study of the behaviour of various clays heated
simultaneously is valuable and the method forms a convenient means of
comparing different materials.
[Footnote 6: For fluxing materials see p. 8.]
The temperature may be measured by means of a pyrometer, but for the
reason just stated it is more convenient and in some respects more
accurate to use standard mixtures known as Seger Cones (fig. 6), and to
state the softening point in terms of the 'cone' which behaves like the
clay being tested. A medium fireclay will not soften below Seger Cone 26
(1650° C.) and a really good one will have a softening point of cone 34
or 35 (1750° to 1800° C.).
[Illustration: Fig. 6. Seger Cones indicating a temperature of 1250° C.]
The _refractoriness_ of a clay, or its resistance to high temperatures,
is an important requirement in bricks required for furnace linings, in
crucibles, gas retorts and other articles used in the metallurgical and
other industries. The term is much abused and is frequently understood
to mean resistance to the cutting action of flue gases and flame, the
corrosive action of slags, and the strains set up by the repeated
changes in temperature. This is unfortunate, for the term refractoriness
has a perfectly definite meaning and should be employed exclusively to
denote that a given clay is capable of retaining its shape at a given
temperature or under given conditions when heated alone and without
being subjected to any pressure. In Great Britain there is no officially
recognized standard of minimum refractoriness[7], but where one is
required the suggested minimum of Seger Cone 26 (1650° C.) made by E.
Cramer (8) is usually employed. This is the recognized minimum in
Germany for fireclays, and though objections may be urged against the
use of Seger Cones as a standard, equally forcible ones may be brought
against making a temperature-scale the basis of measurement. Under
present circumstances, however, it is necessary to adopt one or other of
these.
[Footnote 7: See _Refractory Clays_, Chapter V.]
Various attempts have been made to ascertain the relationship (if any)
between the refractoriness of clays and their chemical composition. If
attention is confined strictly to the more refractory clays, some kind
of relationship does appear to exist. Thus Richter found that the
refractoriness of clay is influenced by certain oxides in the following
order: magnesia, lime, ferrous oxide, soda and potash, but this only
applies to clays containing less than 3 per cent. of all these oxides.
Cramer, in 1895, found that free silica also interfered with the action
of these oxides and more recently Ludwig (9) has devised a chart (fig.
7), on the upright sides of which are plotted the equivalents of the
lime, magnesia and alkalies, whilst the silica equivalents are plotted
on the horizontal base. In each case the 'molecular formula' of the clay
is calculated from its percentage composition, and this 'formula' is
reduced so as to have one 'molecule' of alumina, thereby fixing the
alumina as a constant and reducing the number of variables to two--the
metallic oxides and the silica. Unfortunately Ludwig's chart is only
applicable to the more refractory clays and cannot be relied upon even
for these, though it is extremely useful for comparing clays from
identical or similar geological formations.
[Illustration: Fig. 7. Ludwig's Chart.]
Attempts to express the refractoriness of clays by means of formulae
proving abortive, there only remains the direct test of heating a clay
under definite conditions in the manner previously described.
_Vitrification_ is closely connected with the fusibility and
refractoriness of clays, and, as a term, indicates the amount of fusion
which has occurred under certain conditions of heating. As already
mentioned, all clays, on being subjected to a high temperature, undergo
partial fusion, the more powerful bases attacking the finest particles
of clay and silica, forming molten silicates, and then slowly attacking
the more refractory portion; this slow fusion and solution continues
until the whole of the material is melted. If the heating is stopped
before the fusion has begun, the clay will be porous and comparatively
soft, but as more and more material fuses, the mass (on cooling) becomes
harder and less porous, as the fused material occupies the pores and
sets to a dense, firm glassy mass. The amount of vitrification, or
partial fusion, which occurs is, therefore, of great importance in some
industries, as by stopping it at an appropriate stage articles of any
desired degree of porosity, translucency or strength may be obtained.
Thus for common bricks, only sufficient vitrification is permitted to
bind the particles firmly together, but in engineering bricks--where
much greater strength is required--the vitrification is more complete.
Porcelain and earthenware may be similarly distinguished.
The extent to which a given clay will vitrify depends on the amount of
fluxing material (metallic compounds, and oxides other than ferric oxide
and alumina) it contains, on the smallness of its particles and on the
duration and intensity of the heating. Clays containing alkalies and
lime compounds vitrify with great rapidity when once the necessary
temperature has been reached, so that unless great care is exercised the
action will proceed too far and the goods will be warped and twisted or
may even form a rough slag. Refractory clays, on the contrary, vitrify
more slowly and at much higher temperatures so that accidental
overheatings of them are far less common.
The difference between the temperature at which sintering
or vitrification occurs and that at which the clay melts
completely--usually termed the 'vitrification range'--varies with the
nature of the clay. In some cases the clay melts as soon as
vitrification becomes noticeable, in others the vitrification occurs at
a dull red heat, but the material does not lose its shape until after a
prolonged heating at the highest temperature of a firebrick kiln or
testing furnace.
Calcareous clays have the melting and sintering points close together,
so that it is almost impossible to produce vitrified and impervious ware
from them, as they lose their shape too readily. If, however, the
difference between the sintering and fusing temperatures can be
enlarged--that is, if the vitrification range can be extended--more
impervious ware can be made. The easiest means of extending the
vitrification range consists in regulating the proportion of large and
small particles. The former increase and the latter diminish the range.
Basic compounds and fluxes cause a lowering of the melting-point and a
shortening of the vitrification range.
The _porosity_ of raw clay is usually of small importance, but the
porosity of fired clay or ware is often a serious factor in determining
the suitability of certain articles for their intended purposes. In its
natural state, clay does not readily absorb much water; on the contrary
it becomes pasty and impervious unless it is disturbed and its texture
destroyed, when it may be mixed with water to form a paste or, with more
water, a thin 'cream' or 'slurry.'
When heated moderately, clay forms a porous material and, unless the
heating is excessive, it will absorb about one-eighth of its weight of
water. Further heating at a higher temperature reduces its porosity--the
more easily fused material filling some of the pores--until a stage is
reached when the material is completely vitrified and is no longer
porous.
Porosity may thus be regarded as the opposite of vitrification; porous
goods being relatively light and soft whilst vitrified ones are dense
and hard. For some purposes, porosity is an important characteristic:
for example, building bricks which are moderately porous are preferable
to those which are vitrified. The manufacture of porous blocks for the
construction of light, sound-proof partitions, etc. has increased
rapidly of late. They are made by adding sawdust or other combustible
material to the clay. The added substances burn out on firing the goods
in a kiln.
Clays which are porous can be dried more readily and with less risk of
cracking than those which are more dense. For this reason, some
clayworkers mix non-plastic material such as sand or burned clay with
their raw material.
The _impermeability_ of plastic clay to water is a characteristic which
is important for many purposes.
The _absorptive power_ of clays is closely related to their porosity so
far as pure water is concerned, but if the water contains certain salts
in solution a selective absorption occurs, the bases being retained by
the clay in such a manner that they cannot be removed by washing. The
selective action is known as _adsorption_ and is most noticeable in
highly plastic clays. Bourry (10) has shown that the slightly plastic
china clays only exercise a small power of adsorbing calcium carbonate
from solution, but highly plastic clays may adsorb 20 per cent. of it.
The alkaline chlorides and sulphates do not appear to be adsorbed in
this manner, but the carbonates are readily removed from solution. All
calcium and magnesium compounds appear to be adsorbed, though in
variable quantities, the reaction being complicated when several soluble
salts are present. Ries (6) has found that gallo-tannic acid is adsorbed
readily and increases the plasticity of clay.
Ashley (11) has endeavoured to measure the plasticity of clays by
determining their adsorption capacity for various aniline dyes, but his
untimely decease prevented the investigation being completed. There is
reason to suppose that the relation between adsorption and plasticity is
extremely close in many clays and that the former may, to an important
extent, be used as a measure of the latter. In some clays, however, this
relationship does not exist.
Sand and burned clay only show faint adsorption phenomena; felspar shows
them to a slight and almost negligible extent and most of the other
non-plastic ingredients of clays are non-adsorptive.
Selective adsorption being an important characteristic of colloidal
substances, the possession of this power by plastic clays supports the
claim that plasticity is due, at least in part, to the presence of
colloids.
The addition of small quantities of a solution of certain substances to
a stiff clay paste usually reduces its stiffness, and in some cases
turns it into a liquid. The alkalies are particularly powerful in this
respect and their action may be strikingly illustrated by mixing a few
drops of caustic soda with a stiff clay paste. In a few moments the
mixture will be sufficiently liquid to pour readily, but it may be
rendered quite stiff again by adding sufficient acid to neutralize the
alkali previously used. Weber (12) has utilized this characteristic to
great advantage in the production of sanitary ware and crucibles for
glass-making by a process of casting which he has patented.
The effect of adding water to a dry clay is curious. At first the
particles in contact with the water become sticky and plastic, and if
the proportion of water added is suitable and the mixing is sufficiently
thorough a plastic mass will be produced, the characteristics of which
will depend on the nature of the clay used. This process of mixing clay
with a limited amount of water is known as 'tempering.' The proportion
of water required to make a paste of suitable consistency for modelling
appears to be constant for each clay. If, however, a larger proportion
of water is added the particles of clay will be separated so widely from
each other that they lose their cohesion, and instead of a plastic mass,
the material will form a liquid of cream-like consistency. If a piece of
stiff clay paste is suspended in a large volume of water without
stirring, disintegration will still occur (though a much longer time
will be required) and the clay will be deposited as a sediment at the
bottom of the vessel. The leaner the clay or the larger the proportion
of non-plastic material it contains, the more rapidly will this
disintegration take place. A highly plastic clay will become almost
impervious and will retain its shape indefinitely.
If a mixture of clay and water in the form of a cream or slurry be
allowed to rest, the larger and less plastic particles will settle, but
many of the particles of true clay will remain suspended for several
hours and some of them for several days. Some particles of clay are so
small that it is doubtful if they would ever settle completely unless
some coagulant were added, and as they readily pass through all ordinary
filtering media it is extremely difficult to collect them in a pure
state. These turbid suspensions of clay may be rapidly cleared by the
addition of sodium chloride which increases the surface tension of the
solution. The fine particles behave in the same way as colloidal
substances, _i.e._ as if they possessed an electrostatic charge. Hence
the addition of a salt (electrolyte), whose ions annul the opposite
charges of the electric double layer assumed by Helmholtz to be present,
enables the particles to coagulate in accordance with the ordinary laws
of surface tension (14).
_Exposure_ to the action of air and frost has a marked effect on many
clays. When freshly dug these may be hard and difficult to crush, but
after exposure they break up readily into small fragments. Clays differ
greatly in the extent to which they are affected by exposure; some are
completely disintegrated by standing 48 hours in the open air, whilst
others are scarcely affected by exposure in bleak places through several
years of storm, sunshine and frost. Usually, however, the effect of a
couple of nights exposure to hard frost will produce a marked
disintegration of the material.
This process of exposure is known as 'weathering' and its effects are so
important that it is employed whenever possible for clays requiring to
be crushed before use. All clays are rendered more workable by exposure,
but some of them are damaged by the oxidation of some impurities (_e.g._
pyrites) in them, though in other clays this very oxidation, if followed
by the leaching action of rain, effects an important purification of the
material.
Weathering appears to have no effect on the chemical composition of the
particles of true clay in the material, though it may decompose the
impurities present. On the clay itself its action is largely physical
and consists chiefly in separating the particles slightly from each
other, thereby enabling water to penetrate the material more readily and
facilitating the production of a plastic paste. The disintegrating
action of the weather on some 'clays' is so complete that they require
no crushing but can be converted into a homogeneous paste by simply
kneading them with a suitable proportion of water.
It is possible that on exposure to the heat of the sun's
rays--particularly in tropical climates--some chemical decomposition of
the clay may occur, but compared with the purely physical action of
weathering the amount of such chemical decomposition must be relatively
unimportant in most cases. It may, however, account for the presence of
free silica and free alumina in some clays.
The action of the weather on rocks, resulting in the formation of clays,
is described in Chapter III.
_Heat_ effects remarkable changes in the physical character of clays;
the most important of these have already been noted. At a gentle heat,
the clay is dried and retains most of its power of becoming plastic when
moistened; very little, if any, decomposition occurs. At a higher
temperature it loses its 'combined water,' the clay molecule apparently
dissociating, and a hard stony mass--consisting of particles of free
silica and free alumina cemented together by the more easily fusible
impurities present--is formed. If the heating is continued the hardness
of the material is increased owing to more molten silicate having been
produced from the impurities present, and on cooling, its tensile
strength and resistance to crushing will be found to be enormously
greater than those of the original clay. All potential plasticity is
destroyed by heating to 700° C. and no method of restoring it has yet
been devised. As clays are abundant, this is not a serious disadvantage
for the specially desired characteristics of bricks, terra-cotta,
pottery and porcelain are all such as to be incompatible with
plasticity. The latter is extremely valuable in the shaping of the wares
mentioned, but after the manufacture is completed, the destruction of
the plasticity is an essential feature of their usefulness.
If the heating is very prolonged or is repeated several times, clays
change other of their physical characters and become brittle and liable
to crack under sudden changes of temperature. This is partly due to the
further fusion (vitrification) which occurs and partly to the formation
of crystalline silicates, notably _Sillimanite_ (13).
The extent to which clays are ordinarily heated and the conditions under
which they are cooled do not usually induce the formation of crystals;
the object of the clayworker being to produce a homogeneous mass, the
particles of which are securely held together. The result is that burned
clay products are usually composed of amorphous particles cemented by a
glass-like material formed by the fusion of some of the mineral
ingredients of the original substance. The silicates formed are,
therefore, in a condition of solid, super-cooled solution in which the
tendency to crystallize is restrained by viscosity.
On raising the temperature of firing or on prolonging the heating at the
previous maximum temperature the viscosity of the fused portion is
diminished and crystallization may then occur. The facility with which
crystallization occurs varies greatly with the composition of the fused
material, those silicates which are rich in lime and magnesia
crystallizing more readily than those containing potash or soda. Vogt
has stated that small quantities of alumina promote the formation of a
glassy structure, and Morozewicz has shown that a large excess of this
substance must be present if crystallization is to occur.
The study of the reactions which occur when clays are heated is,
however, extremely complex, not only on account of the variety of
substances present, but also on account of the high temperatures at
which it is necessary to work, so that for a further consideration of it
the reader should consult special treatises on the fusion of silicates.
This subject has now become an important branch of physical chemistry.