CLAY SUBSTANCE: THEORETICAL AND ACTUAL
Having indicated the origin, modes of accumulation and general
characteristics of the numerous materials known as 'clay,' it now
remains to ascertain what substance, if any, is contained in all of them
and may be regarded as their essential constituent, to which their
properties are largely due. Just as the value of an ore is dependent to
a very large extent on the proportion of the desired metal which it
contains, and just as coal is largely, though not entirely, esteemed in
proportion to the percentage of carbon and hydrogen in it, so there may
be an essential substance in clays to which they owe the most important
of their characteristics.
The proportion of metal in an ore or of hydrocarbon in a coal can be
ascertained without serious difficulty by some means of analysis, but
with clay the difficulties are so great that, to some extent at least,
they must be regarded as being, for the present, insurmountable. This is
in no small measure due to the general recognition of all minerals or
rocks which become plastic when kneaded with water as 'clays' without
much regard being paid to their composition. Consequently materials of
the most diverse nature in other respects are termed clays if they are
known to become plastic under certain conditions.
There is, in fact, at the present time, no generally accepted definition
of clay which distinguishes it from mixtures of clay and sand or other
fine mineral particles. The usual geological definitions are so broad as
to include many mixtures containing considerably less than half their
weight of true clay or they avoid the composition of the material
altogether and describe it as a finely divided product of the
decomposition of rocks.
Many attempts have been made to avoid this unfortunate position, which
is alike unsatisfactory to the geologist, the mineralogist and the
chemist as well as to the large number of people engaged in the purchase
and use of various clays; and, whilst the end sought has not been
reached as completely as is desirable, great progress has been made and
much has been accomplished during the last twenty years.
One of the earliest attempts to ascertain whether there is an essential
constituent of all clays was made by Seger (7) who used two methods of
separating some of the ingredients of natural clays from the remaining
constituents. The first of these methods consists in an application of
the investigations of Schulze, Schloesing and Schoene on soils, viz.
the removal of the finest particles by elutriation; the second is an
extension of the method of Forschammer and Fresenius, viz. the treatment
of the material with sulphuric acid.
To the product containing the clay when either of these methods is used
Seger gave the name _clay substance_, but the material so separated is
by no means pure clay. The term clay substance must, therefore, be
confined to the crude product containing the clay together with such
other impurities as are in the form of extremely small particles or are
soluble in sulphuric acid.
It has not yet been found possible to isolate pure clay from ordinary
clays, so that in investigating the nature of what Seger was
endeavouring to produce when he obtained the crude clay substance,
indirect methods are necessary.
It has long been known that if a sample of 'clay'--using this word in
the broadest sense--is rubbed in a considerable quantity of water so as
to form a thin slip or slurry, it may readily be divided into a number
of fractions each of which will consist of grains of different sizes.
This separation may be effected by means of a series of sieves through
which the slurry is poured, or the slurry may be caused to flow at a
series of different speeds, the material left behind at each rate of
speed being kept separate; or, finally, the slurry may be allowed to
stand for a few seconds and may then be carefully decanted into another
vessel in which it may remain at rest for a somewhat longer period,
these times of resting and decantation, if repeated, providing a series
of fractions the materials in which are more or less different in their
nature.
'Clays' containing a considerable proportion of coarse material are most
conveniently separated into a series of fractions by means of sieves,
whereby they are divided into (i) stones, (ii) gravel, (iii) coarse
sand, (iv) medium sand, (v) fine sand and (vi) a slurry consisting of
such small particles that they can no longer be separated by sifting. If
the residues on the sieves are carefully washed free from any adhering
fine material and are then dried, they will be found on examination to
be quite distinct from anything definable as clay. They may consist of a
considerable variety of minerals or may be almost entirely composed of
quartz, but with the possible exception of some shales of great
hardness, they are undoubtedly not clay. This simple process therefore
serves to remove a proportion of material which in the case of some
'clays' is very large but in others is insignificant; thus 40 per cent.
of sand-like material may be removed from some brick-clays whilst a ball
clay used for the manufacture of stoneware or pottery may pass
completely through a sieve having 200 meshes per linear inch.
The material which passes through the finest sieve employed will contain
all the true clay in the material; that is to say, the coarser portion
will, as already mentioned, be devoid of the ordinary characteristics of
clay. At the same time, this very fine material will seldom consist
exclusively of clay, but will usually contain a considerable proportion
of silt, extremely fine mineral particles and, in the case of calcareous
clays, a notable proportion of calcium carbonate in the form of chalk or
limestone particles. Only in the case of the purest clays will the
material now under consideration consist entirely of clay, so that it
must be again separated into its constituents. This is best
accomplished, as first suggested by Schoene, by exposing the material to
the action of a stream of water of definite speed. H. Seger (7)
investigated this method very thoroughly and his recommendations as to
the manner in which this separation by elutriation should be carried out
remain in use at the present time. Briefly, all material sufficiently
fine to be carried away by a stream of water flowing at the rate of 0·43
in. per minute was found by Seger to include the whole of the clay in
the samples he examined, but, as was later pointed out by Bischof, it is
not correct to term the whole of this material 'clay substance,' as when
examined under the microscope, it contains material which is clearly not
clay.
Processes of decantation of the finest material obtained after
elutriation still fail to separate all the non-clay material, and Vogt
has found that when the material has been allowed to stand in suspension
for nine days some particles of mica are still associated with the clay.
It would thus appear that no process of mechanical separation will serve
for a complete purification of a clay; indeed, there are good reasons
for supposing that extremely fine particles of quartz and mica render
physical characteristics an uncertain means of accurately distinguishing
clays from other rock dust.
When chemical methods of investigation are employed the problem is not
materially altered, nor is its solution fully attained. It is, of
course, obvious that any chemical method should be applied to the
product obtained by treating the raw material mechanically as above
described, for to do otherwise is to create needless confusion. Yet by
far the greater number of published analyses of 'clays' report the
ultimate composition of the whole material, no attempt being made to
show how much of the various constituents is in the form of sand, stones
or other coarse particles of an entirely non-argillaceous character.
If the particles of a 'clay' which are sufficiently small to be carried
away by a stream of water with a velocity of only 0·43 in. per minute
are analysed, it will be found that their composition will vary
according to the origin of the clay and the subsequent treatment to
which it has been subjected during its transport and deposition. If the
clay is fairly free from calcareous material and is of a white-burning
nature it may be found to have a composition like china clays.
Red-burning clays, on the contrary, will vary greatly in composition, so
that it becomes difficult to find any close analogy between these kinds
of clay. This difference is partly due to the extremely fine state of
division in which ferric oxide occurs in clays, the particles of this
material corresponding in minuteness to those of the purest clays and so
being inseparable by any mechanical process.
In 1876 H. Seger (7) published what he termed a method of 'rational
analysis,' which consisted in treating the clay with boiling sulphuric
acid followed by a treatment with caustic soda. He found that the purer
china clays (kaolins) and ball clays were made soluble by this means and
that felspar, mica and quartz were to a large extent unaffected. Later
investigators have found that this method is only applicable to a
limited extent and that its indications are only reliable when applied
to the clays just named, but the principle introduced by Seger has
proved invaluable in increasing our knowledge of the composition of
clays. By means of this so-called rational analysis Seger found that the
purer clays yielded results of remarkable similarity and uniformity,
the material entering into solution having a composition agreeing very
closely with the formula Al2O3·2SiO2·2H2O which is generally
recognized as that of the chief constituent or constituents of china
clay (kaolin) and the purer ball clays. This crude substance, obtainable
from a large number of clays by the treatment just described, was named
_clay substance_ by Seger, who regarded it as the essential constituent
of all clays.
Red-burning clays when similarly treated do not yield so uniform a
product, and the ferric oxide entering into solution makes the results
very discordant. Moreover, even with the china clays or kaolins a small
proportion of alkalies, lime and other oxides enter into solution and a
number of minerals analogous to clay, but quite distinct from it, are
also decomposed and dissolved. For these reasons the 'rational analysis'
has been found insufficient; it is now considered necessary to make an
analysis of the portion rendered soluble by treatment with sulphuric
acid in order to ascertain what other ingredients it may contain in
addition to the true clay present.
As the china clays (kaolins) and ball clays on very careful elutriation
all yield a product of the same ultimate composition, viz. 39 per cent.
of alumina, 46 per cent. of silica, 13 per cent. of water, and 2 per
cent. of other oxides, they are generally regarded as consisting of
practically pure clay with a variable amount of impurities. Many years
ago Fresenius suggested that these non-clayey constituents of clays
should be calculated into the minerals to which they appeared likely to
correspond so as to obtain a result similar to that obtained by Seger
without the disadvantages of the treatment with sulphuric acid and as
supplementary to such treatment in the case of red-burning and some
other clays. More recent investigators have found that if a careful
microscopic examination of the clay is made the results of estimating
the composition from the proportion of the different minerals
recognizable under the microscope and by calculation from the analysis
of the material agree very closely and are, as Bischof (28) and, more
recently, Mellor have pointed out, more reliable than the 'rational
analysis' in the case of impure clays. If care is taken to make a
microscopical examination identifying the chief impurities present the
calculation from the analysis may usually be accepted as sufficiently
accurate, but it is very unsatisfactory to assume, as some chemists do,
that the alkalies and lime in the clay are all in the form of felspar
and that the silica remaining in excess of that required to combine with
the alkalies, lime and alumina is free quartz. Some clays are almost
destitute of felspar but comparatively rich in mica, whilst others are
the reverse, so that some means of identifying the extraneous minerals
is essential. When this is not used, the curious result is obtained
that German chemists calculate the alkalies, etc. to felspar whilst the
French chemists, following Vogt, calculate them to mica; English ceramic
chemists appear undecided as to which course to follow, and some of them
occasionally report notable amounts of felspar in clays quite destitute
of this mineral!
A statement of the composition of a 'clay' based on a mechanical
separation of the coarser ingredients followed by an analysis of the
finer ones and a calculation of the probable constituents of the latter,
as already described, is known as a _proximate analysis_ in order to
distinguish it from an _ultimate analysis_ which states the composition
of the whole material in terms of its ultimate oxides. A proximate
analysis therefore shows the various materials entering into the
composition of the clay in the following or similar terms:
Stones per cent.
Gravel "
Coarse sand "
Medium sand "
Fine sand "
Silt "
Felspar or mica dust "
Silica dust "
'True clay[12]' "
Moisture "
Carbon "
Other volatile matter "
[Footnote 12: In analytical reports a note should be appended stating
that the figure under this term shows the proportion of the nearest
approximation to true clay at present attainable.]
For some purposes it is necessary to show the proportion of calcium,
iron and other compounds as in an ordinary ultimate analysis.
A comparison of the foregoing with an ultimate or 'ordinary' analysis of
a clay (p. 16) will show at once the advantage of the former in
increasing our knowledge of the essential constituent of all clays, if
such a substance really exists. Its absolute existence is by no means
proved, for, as will have been noticed, its composition is largely based
on assumption even in the most thorough investigations, particularly of
the admittedly less pure clays.
In the purer clays the problem is much simpler and in their case an
answer of at least approximate accuracy can be given to the question
'What is clay?'
Even with these purer clays it is not sufficient to study an analysis
showing the total amount of the silica, alumina and other oxides
present; it is still necessary to effect some kind of separation into
the various minerals of which they are composed. When, however, the
accessory minerals do not exceed 5 per cent. of the total ingredients
their influence is less important and the nature and characteristics of
the 'clay substance' itself can be more accurately studied. By careful
treatment of well selected china clays, for example, it is possible to
obtain a material corresponding to the formula Al2O3·2SiO2·2H2O
within a total error of 1 per cent., the small amount of impurity
being, as far as can be ascertained, composed of mica. So pure a
specimen of clay is found on microscopical examination to consist of
minute irregular grains of no definite form, together with a few
crystals of the same composition and identifiable as the mineral
'kaolinite' (p. 107). This 'amorphous' material, which appears to be the
chief constituent of all china clays and kaolins, has been termed
_clayite_ by Mellor (22).
Johnson and Blake, Aron and other observers have stated that the
majority of the particles in china clays and kaolins are crystalline in
form. Owing to their extreme smallness it is exceedingly difficult to
prove that they are not so, though for all ordinary purposes they may be
regarded as amorphous, the proportion of obviously crystalline matter
present in British china clay of the highest qualities being so small as
to be negligible.
Hickling (36), using an exceptionally powerful microscope, claims to
have identified this 'amorphous' substance in china clay as 'worn and
fragmental crystals of kaolinite,' and recently Mellor and Holdcroft and
Rieke have shown that the apparently amorphous material shows the same
endo- and exothermal reactions as crystalline kaolinite.
So far as china clays or kaolins are concerned, kaolinite or an
amorphous substance of the same composition appears to be identical with
the 'ideal clay' or 'true clay' whose characters have so long been
sought.
This term--clayite--is very convenient when confined to china clays and
kaolins, but it is scarcely legitimate to apply it, as has been
suggested, to material in other clays until it has been isolated in a
sufficiently pure form to enable its properties to be accurately
studied. This restriction is the more necessary as in one very important
respect clayite obtained from china clay and some kaolins differs
noticeably from the nearest approach to it obtainable from the more
plastic clays: namely, in its very low plasticity. This may be explained
by the fact that it is only obtainable in a reasonably pure form in
clays of a primary character, whilst the plastic clays have usually been
transported over considerable areas and have been subjected to a variety
of treatments which have had a marked effect on their physical
character. Moreover, the fact that the purest 'clay' which can be
isolated from plastic clays appears to be amorphous and to some extent
colloidal greatly increases the difficulty of obtaining it in a pure
state, especially as no liquid is known which will dissolve it without
decomposing it. The fact that it is not an elementary substance, but a
complex compound of silica, alumina and the elements of water, also
increases the intricacy of the problem, for these substances occur in
other combinations in a variety of other minerals which are clearly
distinct from clay.
Ever since the publication of Seger's memorable papers (7), and to a
small extent before that time, it has been generally understood that
china clay or kaolin represented the true essential constituent of
clays, but several investigators have been so imbued with the idea that
all true clay substance must have a crystalline form that they have
frequently used the term 'kaolinite' to include the 'amorphous'
substance in plastic clays. This is unfortunate as it is by no means
proved that the latter is identical with kaolinite, and a distinctive
term would be of value in preventing confusion. Other investigators have
used the word 'kaolin' with equal freeness, so that whilst it originally
referred to material from a particular hill or ridge in China[13] it has
now entered into general use for all clays whose composition
approximates to that of china clay (p. 16) in which the plasticity is
not well developed. Thus, in spite of the difference in origin between
many German and French kaolins and the china clays of Cornwall, it is
the custom in Europe generally to term all these materials 'kaolin.' Yet
they are very different in many respects from the material originally
imported from China.
[Footnote 13: _Kao-ling_ is Chinese for a high ridge or hill.]
As the essential clay substance has not yet been isolated in a pure form
from the most widely spread plastic clays, but is largely hypothetical
as far as they are concerned, the author prefers the term
_pelinite_[14] when referring to that portion of any plastic clays or
mixtures of clays with other minerals which may be regarded as being the
constituent to which the argillaceous portion of the material owes its
chief properties. In china clay and kaolin the 'true clay' is identical
with clayite--or even with kaolinite (p. 108)--and there is great
probability that this identity also holds in the case of the more
plastic clays of other geological formations, but until it is
established it appears wisest to distinguish the hypothetical or ideal
clay common to all clays (if there is such a substance) by different
terms according to the extent to which its composition and characters of
the materials most closely resembling it are experimentally known.
[Footnote 14: From the [Greek: pêlinos] = made of clay.]
The substances most resembling this 'ideal clay' which have, up to the
present been isolated, are:
(_a_) _Kaolinite._ Found in a crystalline form in china clays and
kaolins (p. 107).
(_b_) _Clayite._ A material of the same chemical composition as
kaolinite, but whose crystalline nature (if it be crystalline) has not
been identified--chiefly obtained from china clays and kaolins.
(_c_) _Pelinite._ A material similar to clayite, but differing from it
in being highly plastic and, to some extent, of a colloidal
nature--obtained from plastic clays.
(_d_) _Laterite._ A material resembling clayite in physical appearance,
but containing free alumina and free silica (p. 80).
(_e_) _Clay Substance._ A general term indicating any of the foregoing
or a mixture of them; it is also applied (unwisely) to the material
obtained when a natural clay is freed from its coarser impurities by
elutriation (p. 7).
The Chief Characteristics of 'True Clay' from Different Sources.
In so far as it can be isolated _true clay_ appears to be an amorphous,
or practically amorphous, material which may under suitable conditions
crystallize into rhombic plates of kaolinite. The particles of which it
is composed are extremely small, being always less than 0·0004 in. in
diameter. They adsorb dyes from solutions and show other properties
characteristic of colloid substances though in a very variable degree,
some clays appearing to contain a much larger proportion of colloidal
matter than do others. To some extent the power of adsorption of salts
and colouring matters appears to be connected with the plasticity (p.
41) of the material, but this latter property varies so greatly in
clayite or pelinite from different sources as to make any generalization
impossible.
True clay substance appears to be quite white, any colour present being
almost invariably traceable to ferric compounds or to carbonaceous
matter. The latter is of small importance to potters as it burns away in
the kiln. The specific gravity of clay substance is 2·65 according to
Hecht, the lower figures sometimes reported being too low. Its hardness
is usually less than that of talc--the softest substance on Mohs'
scale--but some shales are so indurated as to scratch quartz. It is
quite insoluble in water and in dilute solutions of acids or alkalies,
but is decomposed by hydrofluoric acid and by concentrated sulphuric
acid when heated, alumina entering into solution and silica being
precipitated in a colloidal condition.
It absorbs water easily until a definite state of saturation has been
reached, after which it becomes impervious unless the proportion of
water is so large and the time of exposure so great that the material
falls to an irregular mass which may be converted into a slurry of
uniform consistency by gently stirring it. With a moderate amount of
water, pelinite develops sufficient plasticity to enable it to be
modelled with facility, but clayite and some specimens of pelinite are
somewhat deficient in this respect. The pelinitic particles usually
possess the capacity to retain their plasticity after being mixed with
considerable proportions of sand or other non-plastic material and are
then said to possess a high binding power (p. 28).
If a large proportion of water is added to a sample of clayite or
pelinite and the mixture is stirred into a slurry it will be found to
remain turbid for a considerable time and will not become perfectly
clear even after the lapse of several days. Its power of remaining in
suspension is much influenced by the presence of even small amounts of
soluble salts in either the water or the clay substance, its
precipitation being hastened by the addition of such salts as cause a
partial coagulation of the colloidal matter present. Some specimens of
clayite and pelinite retain their suspensibility even in the presence of
salts, but this is only true of a very limited proportion of the
substance. In most cases the presence of soluble salts causes the larger
particles to sink somewhat rapidly and to carry the finer particles with
them.
The rate at which a slip or 'cream' made of elutriated clay and water
will flow through a small orifice is dependent on the viscosity of the
liquid and this in turn depends on the amount of colloidal material
present, _i.e._ on how much of the clay (pelinite) is in a colloidal
form. Its viscosity is greatly affected by the addition or presence of
small quantities of acid or alkali or of acidic or basic salts. Acids
increase the viscosity; alkalies and basic salts, on the contrary, make
the slip more fluid. Neutral salts behave in different ways according to
the concentration of the solution and to the amount of clay (pelinite)
present in the slip. If the slip contains so little water as to be in
the form of a thin paste, neutral salts usually have but a small action,
but when the slip contains only a small proportion of clay (pelinite)
the presence of neutral salts will tend to cause the precipitation of
the clay. In this way salts act in two quite different directions
according to the concentration of the slip.
On drying a paste made of clay and water the volume gradually diminishes
until the greater part of the water has been removed; after this the
remainder of the water may be driven off without any further reduction
in volume of the material. This is another characteristic common to
colloidal substances such as gelatin. The material when drying attains a
leathery consistency which is at a maximum at the moment when the
shrinkage is about to cease; on further drying the material becomes
harder and more closely resembles stone.
Providing that wet clay is not heated to a temperature higher than that
of boiling water it appears to undergo no chemical change and on cooling
it will again take up water[15] and be restored to its original
condition except in so far as its colloidal nature may have been
affected by the heating. If, however, the temperature is raised to about
500° C. a decomposition of the material commences and water is evolved.
This water--which is commonly termed 'combined water'--is apparently an
essential part of the clay-molecule and when once it has been removed
the most important characteristics of the clay are destroyed and cannot
be restored. The reactions which occur when clay is heated are complex
and are rendered still more difficult to study by the apparent
polymerization of the alumina formed. Mellor and Holdcroft (29) have
recently investigated the heat reactions of the purest china clay
obtainable and confirm Le Chatelier's view (10) that on heating to
temperatures above 500° C. clay substance decomposes into free silica,
free alumina and water, the two former undergoing a partial
re-combination with formation of sillimanite (Al2O3SiO2) if a
temperature of 1200° C. is reached. Mellor and Holdcroft point out that
there is no critical point of decomposition for clay substance obtained
from china clay, as it appears to lose water at all temperatures, though
its decomposition proceeds at so slow a rate below 400° C. as to be
scarcely appreciable.
[Footnote 15: Some clays are highly hygroscopic and absorb moisture
readily from the atmosphere. According to Seger (7) this hygroscopicity
distinguishes true clay from silt and dust.]
After the whole of the 'combined water' has been driven off, if the
temperature continues to rise, it is found that at a temperature of 900°
C. an evolution of heat occurs. This exothermal point, together with the
endothermal one occurring at the temperature at which the decomposition
of the clay seems to be most rapid, has been found by Le Chatelier,
confirmed by Mellor and Holdcroft, to be characteristic of clay
substance derived from kaolin and china clay, and the two last-named
investigators state that it serves as a means of distinguishing
kaolinite or clayite from other alumino-silicates of similar
composition. These thermal reactions have not, as yet, been fully
studied in connection with plastic clays; with china clay, as already
noted, they probably indicate a polymerization of the alumina set free
by the decomposition of the clay substance, as pure alumina from a
variety of sources has been found by Mellor and Holdcroft to behave
similarly.
On still further raising the temperature of pure clay (pelinite or
clayite) no further reactions of importance occur, the material being
practically infusible. If, however, any silica, lime, magnesia,
alkalies, iron oxide or other material capable of combining with the
alumina and silica is present as impurities in the clay substance,
combination begins at temperatures above 900° C. This causes a reduction
of the heat-resisting power of the material; the silicates and
alumino-silicates produced fuse and begin to react on the remaining
silica and alumina, first forming an impermeable mass in place of the
porous one produced with pure clay substance, and gradually, as the
material loses its shape, producing a molten slag if the 'clay' is
sufficiently impure. As ordinary clays are never quite free from
metallic compounds other than alumina, this formation of a fused
portion--technically known as _vitrification_ (p. 37)--occurs at
temperatures depending on the nature of the materials present, so that a
wide range of products is obtained, the series commencing with the
entirely unfused pure clay (china clay), passing through the slightly
vitrified fireclays, the more completely vitrified ball clays to the
vitrifiable stoneware clays and ending with materials so rich in easily
fusible matter as scarcely to be worthy of the name of clays.
The constitution of the clay molecule is a subject which has attracted
the attention of many investigators and is being closely studied at the
present time. It is a subject of peculiar difficulty owing to the
inertness of clay substance at all but high temperatures, and to the
complexity of reactions which take place as soon as any reagent is
brought into active connection with it.
Without entering into details regarding the various graphic formulae
which have been suggested, it is sufficient to state that the one which
is most probably correct, as far as present knowledge goes, is Mellor's
and Holdcroft's re-arrangement of Groth's formula (30)
HO\ /OSiO\
\Al2/ \O
/ \ /
HO/ /\ \OSiO/
/ \
HO OH
which on removal of the hydroxyl groups might be expected to give the
anhydride
O\\ /OSiO\
\\Al2/ \
// \ /
O// \OSiO/
though in practice this substance--if formed at all--appears to be
instantly split up into Al2O3 and SiO2.
By regarding the aluminium as a nucleus, as above, and some aluminium
silicates as hypothetical alumino-silicic acids, as suggested by
Ulffers, Scharizer, Morozewicz (29) and others, clay substance may be
conveniently considered, along with analogous substances, as forming a
special group quite distinct from the ordinary silicates. In this way
Mellor and Holdcroft (29) consider that clay substance is not a hydrated
aluminium silicate--as is usually stated in the text-books--but an
alumino-silicic acid, the salts of which are the zeolites and related
compounds. From this hypothesis it naturally follows that clay substance
is analogous to colloidal silica which has been formed by the
decomposition of a silicate by means of water and an acid.
If this view be correct, pure clay substance or true
clay is a tetra-basic alumino-silicic acid H4Al2SiO9 or
Al2Si2O5(0H4). That its acid properties are not readily
recognizable at ordinary temperatures is due to its inertness; at higher
temperatures its power of combination with lime, soda potash and other
bases is well recognized, though the reactions which occur are often
complicated by decompositions and molecular re-arrangements which occur
in consequence of the elevated temperature.
There are a number of minerals which closely resemble clayite
or pure clay substance in composition, the chief difference
being in the proportion of water they evolve on being heated.
Thus _Rectorite_ H2Al2Si2O8, _Kaolinite_ H4Al2Si2O9,
_Halloysite_ H6Al2Si2O10 and _Newtonite_ H10Al2Si2O12. In the
crystalline form these minerals may be distinguished from each other by
means of the microscope, but as the chief materials of which clays are
composed appears to be amorphous it is impossible to ascertain with
certainty whether a given specimen of clay substance is composed of a
mixture of these analogous minerals in an amorphous form or whether it
consists entirely of clayite, _i.e._ the clay substance obtained from
china clay. As already stated, the thermal reactions which occur on
heating clayite appear to be characteristic of kaolinite whilst
halloysite is completely decomposed at a temperature somewhat below 200°
C.; but the not improbable presence of two or more of these
alumino-silicic acids in clays of secondary or multary origin makes it
almost impossible to determine whether clayite is an essential
constituent of all clays or whether the purest clay substance (pelinite)
obtained from some of the more plastic clays does not possess a
different chemical composition as well as different physical properties.
The view that clays may be regarded as impure varieties of clayite is
considered erroneous by several investigators for various reasons. For
instance, felspar is rarely found in china clays, but is a common
constituent of secondary (plastic) clays. J. M. van Bemmelen (26), who
has found that the alumina-silica ratio of clays produced by weathering
is always higher than that in clays of the china clay type produced by
hypogenic action. In a number of clays examined he found that a portion
was soluble in boiling hydrochloric acid whereas clayite is scarcely
affected by this treatment. He also found a varying proportion of
alumino-silicate insoluble in hydrochloric acid but dissolved on
treatment with boiling sulphuric acid and subsequently with caustic soda
solution; this latter he considers to be true clayite. Unfortunately,
his results were obtained by treating the crude clay with acid, instead
of first removing such non-plastic materials as can be separated by
washing, so that all that they show is that some clays contain
alumino-silicates of a nature distinct from clayite in addition to any
clayite which may be found in them.
The fact that all clays when heated to 700 or 800° C. readily react with
lime-water to form the same calcium silicates and aluminates indicates
so close a resemblance between the clay substance obtainable from
different sources as to constitute strong evidence of the identity of
this substance with clayite or with materials so analogous to it as to
be indistinguishable from it under present conditions.
In all probability, the plastic clays have been derived from a somewhat
greater variety of minerals than the primary clays (p. 71) and under
conditions of decomposition which differ in details, though broadly of
the same nature as those producing china clays. The presence of
colloidal matter suggests a more vigorous action--or even a
precipitation from solution--instead of the slower reactions which
result in the formation of the kaolinite crystals.
The much smaller particles present in plastic clays also indicate a more
complete grinding during the transportation of the material or some form
of precipitation. If, as Hickling suggests, all clays are direct
products of the decomposition of _mica_, the fact that several varieties
of mica are known and that the conditions under which these decompose
must vary considerably, afford a good, if incomplete, explanation of
some of the widely diverse characteristics observed in different clays.
Notwithstanding the great complexities of the whole subject and the
apparently contradictory evidence concerning some clays, there is a
wide-spread feeling that whatever may be the mineral from which a given
clay has been derived, the _true clay substance_, which is its essential
constituent, would (if it could be isolated in a pure state) prove to be
of the same composition as kaolinite obtainable from china clay of
exceptional purity. The purest clay substances (pelinite) yet obtained
from some of the most plastic clays are, however, so impure as to make
any detailed investigation of their composition by present methods
abortive. The methods of synthesis which have proved so successful in
organic chemistry have hitherto yielded few intelligible results with
clays, on account of the complexity of the accessory reactions which
occur.
The Difference between Pure Clay Substance and Ordinary Clays.
The properties and characteristics of _true clay_ are very seriously
modified by other materials which may be associated with it. This may be
perceived by comparing the properties of clays mentioned in Chapter I
with those of various forms of true clay just given. Moreover, as true
clay never occurs in a perfectly pure state in nature, the properties of
clays must be largely dependent on the accessory ingredients.
Silica, for example, when alone is a highly refractory material, but in
the presence of true clay it reduces the refractoriness of the latter.
Lime has a similar effect though its chemical action on the clay is
entirely different. A very small proportion of some substances--notably
the oxides of sodium and potassium--will greatly alter the behaviour of
true clay when heated and will produce an impervious mass in place of a
porous one.
For these reasons, it is necessary in studying clays to pay attention to
both their physical and chemical properties and to separate the material
into fractions so that each of these may be studied separately and their
individual as well as their collective characteristics ascertained.
Failure to do this has been the cause of much obscurity and confusion in
investigations on certain clays composed of a considerable proportion of
non-argillaceous material which ought to have been separated before any
attempt was made to study the true clay present.
There is, therefore, a considerable difference between a natural clay
and the pure clay substance theoretically obtainable from it; this
difference being most marked in the case of low-grade brick clays of
glacial origin, which may contain 50 per cent. or more of adventitious
materials. If used in a natural state they would be found to be
valueless on account of their impurities giving them characteristics of
a highly undesirable character, whereas the true clay in them is
found--in so far as it can be separated--to bear a close resemblance to
that obtained from a high grade, plastic, pottery clay. Unfortunately,
it is, at present, impossible to isolate this clay substance in anything
approaching a pure form, and many clays are without commercial value
because of comparatively small proportions of impurities which cannot be
separated from the clay substance without destroying the latter.
Classification of Clays.
Owing to the widely differing substances from which clays can,
apparently, be formed and the peculiar difficulties which are
experienced in investigating the nature of clay substance from different
sources, it is by no means easy to devise a scheme of classification of
clays, though many of these have been attempted by different scientists.
The classification adopted by geologists is based on the fossil remains
and on the stratigraphical position of clays relative to other rocks, as
described in Chapter II. This is of great value for some purposes, but
the composition of the substances termed 'clay' by geologists differs so
greatly, even when only one formation is considered, as to make their
classification of little or no use where the value or worthlessness of
the material depends upon its composition. Thus the so-called Oxford
clay ranges from a hard silicious shale to a comparatively pure clay;
some portions of it are so contaminated with calcareous and ferruginous
matter as to make the material quite useless for the potter or
clayworker. A geological classification of clays is chiefly of value as
indicating probable origins, impurities and certain physical properties;
but the limits of composition and general characteristics are so wide as
to make it of very limited usefulness.
The classification of clays on a basis of chemical composition is
rendered of comparatively little value by the large number of clays
which occupy ill-defined borders between the more clearly marked
classes. Moreover, attempts to predict the value and uses of clays from
their chemical composition are generally so misleading as to be worse
than useless, unless a knowledge of some of the physical characters of
the clays is available. It is, of course, possible to differentiate some
clays from others by their composition, but not with sufficient accuracy
to permit of definite and accurate classification.
A classification based exclusively on the composition of clays is
equally unsatisfactory for other reasons, the chief of which is the
placing together of clays of widely differing physical character, and
the separation of clays capable of being used for a particular purpose.
To some extent the latter objection may be disregarded, though it is of
great importance in considering the commercial value of a clay.
Classification based on the uses of clays of different kinds has been
suggested by several eminent ceramists, but is obviously unsatisfactory,
particularly as it is by no means uncommon to use mixtures of clays and
other minerals for some purposes. Thus stoneware clays must be
vitrifiable under conditions which may be defined with sufficient
accuracy, but many manufacturers of stoneware do not use clays which are
naturally vitrifiable; they employ a mixture of refractory clay and
other minerals to obtain the material they require.
A classification based on the origin of clays regarded from the
petrological point of view offers some advantages, but is too cumbersome
for ordinary purposes and suffers from the disadvantage that the origin
of some important clays is by no means clearly known.
The author prefers a modification of Grimsley's and Grout's
classification (31) as follows:
I. Primary clays.
(_a_) Clays produced by 'weathering' silicates--as some
kaolins.
(_b_) Clays produced by lateritic action--very rich in
alumina, some of which is apparently in a free state.
(_c_) Clays produced by telluric water containing active gases
(hypogenically formed clays)--as Cornish china clay.
II. Secondary clays.
(_d_) Refractory[16] secondary clays--as fireclays and some
pipe clays.
(_e_) Pale-burning non-refractory clays--as pottery clays,
ball clays and some shales.
(_f_) Vitrifiable clays--as stoneware clays, paving brick
clays.
(_g_) Red-burning and non-refractory clays--as brick and
terra-cotta clays and shales.
(_h_) Calcareous clays or marls, including all clays
containing more than 5 per cent. of calcium carbonate.
III. Residual clays.
(_i_) Clays which have been formed by one of the foregoing
actions and have been deposited along with calcareous or
other matter but, on the latter being removed by subsequent
solution, the clay has remained behind--as the white clays
of the Derbyshire hills.
[Footnote 16: A refractory clay is one which does not soften
sufficiently to commence losing its shape at any temperature below that
needed to bend Seger Cone 26 (approximately 1600°C.) (see p. 116).]
Some further sub-division is necessary for special purposes,
particularly in sections _e_, _f_ and _h_, but to include further
details would only obscure the general scheme. Some clays will,
apparently, be capable of classification in more than one section, thus
a vitrifiable clay may owe its characteristic to a high proportion of
calcium carbonate and so be capable of inclusion as a calcareous clay.
Broadly speaking, however, if the clay is tested as to its inclusion in
each section of the scheme in turn it will be found that its highest
value will be in the section which is nearest to the first in which the
clay can legitimately be placed.
From a consideration of a classification such as the foregoing, together
with a detailed study of the physical and chemical properties of the
material as a whole, and also of the various portions into which it may
be divided--particularly that which has been isolated by mechanical
methods of purification and separation--it is not difficult to gain a
fairly accurate idea of the nature of any clay. Although the present
state of knowledge does not permit them to be classified with such
detail as is the case with plants, animals, or simple chemical
compounds, the study of clays and the allied materials has a fascination
peculiarly its own, not the least interesting features of which are
those properties of the clay after it has been made into articles of use
or ornament. These are, however, beyond the scope of what is commonly
understood by the term 'the natural history of clay.'
BIBLIOGRAPHY
A complete bibliography of clay would occupy several volumes. The
following list only includes the more accessible of the works quoted in
the text.
1. "Second Report of the Committee on Technical Investigation--Rôle
of Iron in Burning Clays." Orton and Griffith. Indianapolis.
1905.
2. "British Clays, Shales and Sands." Alfred B. Searle. Charles
Griffin and Co. Ltd. London. 1911.
3. "Transactions of the English Ceramic Society." v. p. 72. Hughes
and Harber. Longton, Staffs. 1905.
4. "Royal Agricultural Society's Journal." XI.
5. "Die Tone." P. Rohland. Hartleben's Verlag. Vienna. 1909.
6. "Clays: their Occurrence, Properties and Uses." H. Ries. Chapman
and Hall. London. 1908.
7. "Gesammelte Schriften." H. Seger. Tonindustrie Zeitung Verlag.
Berlin. 1908.
8. "Tonindustrie Zeitung." 1902. p. 1064.
9. "Tonindustrie Zeitung." 1904. p. 773.
10. "Treatise on Ceramic Industries." E. Bourry (Revised translation
by A. B. Searle). Scott, Greenwood and Son. London. 1911.
11. "The Colloid Matter of Clay." H. E. Ashley. U.S.A. Geological
Survey Bulletin 388. Washington. 1909.
12. "Sprechsaal." 1905. p. 123.
13. "Action of Heat on Refractory Materials." J. W. Mellor and F. J.
Austen. Trans. Eng. Cer. Soc. VI. Hughes and Harber. Longton,
Staffs. 1906.
14. "Wiedermann's Annalen." VII. p. 337.
15. "Geological Contemporaneity." 1862.
16. "Geological Magazine." IV. pp. 241, 299.
17. "La Céramique industrielle." A. Granger. Gauthier Frères. Paris.
1905.
18. "American Journal of Science." 1871. p. 180.
19. "The Hensbarrow District." J. H. Collins. Geological Survey.
1878.
20. "Monographs of the U.S.A. Geological Survey." XXVIII. C. R. van
Hise. 1897.
21. "On Kaolinite and Pholerite." American Journal of Science.
XLIII. 1867.
22. "The Nomenclature of Clays." J. W. Mellor. Eng. Cer. Soc. VIII.
Hughes and Harber. Longton, Staffs. 1908.
23. "On the present distribution of Coal Balls." M. C. Stopes and D.
M. S. Watson. Phil. Trans. Royal Society. B. Vol. CC. 1908.
24. "Natural History of Coal." E. A. N. Arber. Cambridge University
Press. 1911.
25. "Modern Brickmaking." A. B. Searle. Scott, Greenwood and Son.
London. 1911.
26. "Die verschiedene Arten der Verwitterung." J. M. van Bemmelen.
Zeits. angewandte Chemie. LXVI. Leopold Voss Verlag. Hamburg.
1910.
27. "Pyrometrische Beleuchtung." Carl Bischof. Tonindustrie Zeitung.
1877.
28. "Die feuerfeste Tone." Carl Bischof. Quandt and Haendler.
Leipzig. 1904.
29. "The Chemical Constitution of the Kaolinite Molecule." Trans.
Eng. Cer. Soc. X. Hughes and Harber. Longton, Staffs. 1911.
30. "Tabellarische Uebersicht der Mineralien." P. Groth. Brunswick.
1898.
31. "West Virginia Geological Survey." III. 1906.
32. "Memoirs of the Geological Survey." London.
33. "The Publications of Stanford's Geographical Institute." London.
34. "Handbuch der gesam. Tonwarenindustrie." B. Kerl. Verlag der
Tonindustrie Zeitung. 1910.
35. "Causal Geology." E. H. L. Schwarz. Blackie and Sons, Ltd. 1910.
36. "China Clay: its nature and origin." G. Hickling. Trans. Inst.
Mining Engineers. 1908.
INDEX
Absorption, 40, 151
Absorptive power of clays, 40
Accumulation of clays, 84
Acid-proof ware, 113
Acids, effect of, 106, 151, 152
Adsorption, 40, 150
Agriculture, clays in, 5, 56, 57, 59, 61, 62, 63, 67
Air, 43, 85
Alkalies in clay, 38, 115, 133, 142, 143, 155
Alluvial deposits, 68, 87, 112, 132
Alum clays and shales, 57, 123, 124
Alum manufacture, clays for, 124
Alumina, 6
Alumina, free, 80, 82, 154
Alumina-silica ratio, 133, 159
Alumino-silicic acid, 6, 76, 81, 118, 155, 157
Aluminous clays, 82, 117
'Amorphous' clay, 107, 146
Analyses of clays, 16, 141, 144
Anauxite, 134
Architectural ware, 129, 130
Argillaceous earths, 1
Argillaceous limestone, 88, 132
Associated rocks, 48
Bagshot clays and sands, 64, 125
Ball clays, 6, 19, 28, 62, 64, 82, 110, 115, 119, 125, 138, 141, 156, 166
Bending of clay, 33
Bibliography, 168
Binding power, 28, 151
Binds, 53
Bituminous shales, 57, 59
Black spots, 14, 128
Black ware, 113
Bleaching oil, 134
Blue bricks, 13, 56
Bone-ash, 110
Boulder clays, 3, 7, 10, 65, 101
Bovey Tracey clay, 62
Brick clays, earths and shales, 1, 2, 5, 10, 11, 12, 13, 31, 37, 40, 46,
56, 57, 59, 61, 63, 65, 67, 68, 91, 100, 101, 104, 112, 117, 123, 125,
129, 138, 162, 166
Brittleness, 46
Brown ware, 113
Buff bricks, 128
Burned clay, 28, 31, 41, 119, 121
Calcareous clays, 38, 61, 68, 88, 133, 139, 166
Calcareous sands, 88
Calcium, see _Lime compounds_
Cambrian clays, 51
Carbon in clay, 15, 119, 144
Carbonates in clay, 10, 82
Carboniferous clays and shales, 52, 124
Carboniferous limestone, 52, 108
Carclazite, 78, 106
Cellulose in clays, 27
Cement clays, 57, 104, 131
Chalcopyrite, 14
Chalk, 10, 11, 61, 67, 68, 88, 116, 127, 128, 132, 134, 139
Chamotte, 121
Chemical properties of clay, 6
China clay rock, 78, 106, 116
China clays, 2, 5, 6, 7, 9, 22, 27, 40, 49, 64, 71, 75, 78, 82, 84, 104,
110, 116, 141, 146, 147, 148, 156, 165
China-ware, 109, 110
Chinese clay, 73
Classification of clays, 163
Clay molecule, 156
Clay-shales, 122
Clay substance, 135 _et seq._
Clay substance, defined, 150
Clayite, 83, 107, 147, 149
Clinker, 132
Clunches, 118
Coagulated clays, 97
Coagulation, 43, 152
Coal Measure clays and shales, 53, 96, 103, 117, 124, 130
Coarse pottery, 112
Cobalt, 110
Colloid theory, 97
Colloidal properties of clay, 25, 81, 82, 97, 106, 147, 150, 152
Colloidal silica, 81, 134, 157
Colloids, 24, 41, 43, 76, 160
Colluvial clays, 99
Colours of burned ware, 19, 123, 131
Colours of clays and shales, 19, 59, 115, 119, 124, 126
Combined water, 45, 154
Common clays, 3
Composition of clays, 4, 6, 16, 23, 35, 44, 107, 117, 118, 133, 134,
144, 156, 164
Composition of clays (burned), 46
Cornish stone, 110, 116
Cracked ware, 46, 127, 130, 131
'Cream,' 39, 43, 152
Cretaceous clays, 61
'Crumb' of clay, 24
Crushing clay, 45
'Crystalline' clay, 107, 146, 148
Crystals in clay-ware, 46
Decantation, 139
Decomposition of clay, 154
Definitions of clay, 2-5, 120, 135, 149, 150
De-greasing wool, 134
Deposition of clays, 49, 51, 90, 99
Devonian clays, 51
Diluvial clays, 99
Dinas rock, 54
Disintegration, 102
Distribution of clays, 1
Drain-pipe clays, 112, 113
Drift, 65, 101
Drift clays, 101
Drying clays, 27, 127, 153
Durability, 131
Dyes, 41, 150
Earth movements, 85, 96
Earthenware, 37, 112
Earths for bricks, see _Brick clays_
Electrolytes, 43
Elutriation, 8, 137, 140
Eocene clays, 63
Epigenic clays, 82
Erosion, 89, 99, 100
Estuarine clays, 90, 93, 118
Etruria marls, 55, 130
Expansion, 32
Exposure, 43
Faience, 129
Farewell Rock, 54
Fat clays, 29
Felspar, 7, 8, 41, 74, 104, 110, 116, 141, 144, 159
Ferric and Ferrous compounds, 12, 121, see _Iron_
Fine clays, 112
Fineness, see _Texture_
Firebricks, 14, 54, 61, 116
Fireclay, 33, 35, 52, 54, 82, 104, 108, 116, 123, 156, 166
Fissile clays, 117
Flint, 110, 116
Flint clays, 117
Floods, 85, 87, 99
Flower-pot clays, 57, 110
Fluoric vapours, 75, 77, 165
Fluviatile clays, 88, 92
Fluxes, 8, 11, 38, 39, 115, 116, 131
Food-clays, 1
Formation of clays, 48, 70
Formula of clay, 156
Free alumina, 80, 154
Free silica, 7, 80, 154, 161
Frost, 43, 86
Fuller's earth, 59, 133
Fulling cloth, 1, 133
Fusibility, 32, 58, 113, 116
Fusible clays, 116
Fusing point, 32
Fusion, 47, 113, 120, 132, 155
Ganister, 52, 54, 118
Gault, 61, 132
Geological classification, 163
Geological nature of clay, 4, 50
Glacial clays, 65, 100, 162
Glaciers, 85, 89, 100
Glass, 116
Glassy structure, 47
Glazed bricks, 119
Glazed pottery, 129
Glazed terra-cotta, 56, 129
Grades of fireclay, 120
Gravel, 7, 62, 65, 89, 100, 101, 102, 138, 144
Green colour, 14
Greensand, 133
Grinding, 80, 121
Grit, 112, see also _Millstone Grit_
Grog, 28, 31, 41, 119, 121
Growan, 78
Gypsum, 10, 12, 62
Halloysite, 118, 158
Hardness, 45
Heat, effects of, 28, 37, 39, 45, 80, 122, 146, 153, 154, 158, 159
Hydrargillite, 80
Hydro-alumino-silicates, 6
Hydrocarbons in clay, 15
Hydrolysis, 78, 97
Hygroscopic clays, 153
Hypogenic clays, 165
Ice-action, 85, 100
Ideal clay, 146
Impermeability, 40, 113
Impervious articles, 113, 155
Impurities in clays, 7, 49, 82, 102, 104, 109, 121, 126, 142, 143, 155,
162, 163
Ions, 43
Indurated clays, 18
Infusibility, 106, 119, see _Refractoriness_
Iron compounds, 7, 10, 12, 13, 20, 62, 112, 119, 121, 128, 133, 141,
145, 164
Ironstone, 62
Irregularity in shape, 131
Jurassic clays and shales, 57, 124
Kao-ling, 148
Kaolinite, 19, 80, 105, 107, 146, 149, 158
Kaolinization, 76, 77, 79
Kaolins, 9, 21, 49, 64, 71, 73, 76, 79, 82, 84, 104, 116, 141, 146,
147, 148, 165
Keele series, 55
Kellaways clay, 59, 61
Keuper marls, 57
Kiln shrinkage, 30
Kimeridge clays, 59
Knotts, 124
Lacustrine clays, 90, 91
Lake-deposited clays, 85, 88, 91
Lakes, 85, 88
Laminated clays, 53, 117, 122
Laterite, 80, 149
Lateritic action, 80, 165
Lateritic clays, 82
Lean clays, 29
Liassic clays and shales, 57, 125, 132
Lime, 7, 10, 102, 159
Lime compounds, 10, 11, 38, 41, 47, 113, 115, 116, 121, 127, 139, 142,
143, 145, 155, 157, 162, 164
Limestone, 10, 11, 52, 59, 61, 62, 88, 102, 117, 127, 132, 139
Lime troubles, 11
Loam, 57, 67, 88
London clay, 62, 63, 125
Ludwig's chart, 35
Magnesium compounds, 7, 10, 11, 41, 47, 113, 115, 116, 121, 133, 155
Malm-bricks, 11
Malms, 10, 68
Marcasite, 10, 13
Marine clays, 61, 93
Marls, 10, 51, 54, 55, 57, 67, 68, 88, 130, 132, 166
Mechanical analysis, 137
Medway mud, 132
Melting point, 31, 32
Mica, 7, 8, 76, 104, 105, 116, 140, 141, 144, 160
Microscopical examination, 18, 105, 143, 158
Millstone grit, 54, 55, 117
Mineral nature of clay, 3
Minerals resembling clay, 158
Mining ball clay, 111
Modelling clays, 130
Moisture, 15, 144
Molecular attraction, 22
Molecular constitution of clay, 21, 156
Montmorillonite, 134
Mundic, 13
Muscovite, 105
Newtonite, 158
Nodules, 121, 127
Non-plastic material, 43, 121, 151
Non-refractory clays, 166
Occurrence of clays, 48, 116
Ocean currents, action of, 89
Odour of clay, 19
Oil, bleaching, 134
Oil shales, 61, 122, 123
Oolite clays, 59, 134
Ooze, 95, 99
Organic matter, 19, 119
Origins of clays, 48, 71, 160, 165
Oxford clay, 59, 95, 164
Oxides in clay, 10, 82
Paint, clays for, 109
Paper, clays for, 109
Particles, nature of, 18, 31, 106, 107, 150
Paving brick clays, 166
Pelagic ooze, 95, 99
Pelinite, 83, 148, 149
Permian clays and shales, 57, 112, 124
Pholerite, 117
Physical characters of clays, 17
Picking clay, 121
Pipe clays, 64, 65, 82, 109, 166
Plant-extracts in clays, 26
Plastic clays, 2, 43, 65, 67, 82, 88, 102, 112, 123, 147, 148, 160
Plasticity, 20-27, 41, 46, 97, 98, 99, 108, 109, 112, 117, 123, 125,
127, 151, 160
Pleistocene clays, 67
Pockets, 65, 85, 101, 116, 166
Porcelain, 37, 46, 73, 109, 110, 125, 129
Pores in clay, 30, 114
Porosity, 30, 39, 121, 131, 155
Portland cement, 131
Potash compounds, 7, 10, 113, 115, 116, 121, 157, see _Alkalies_
Pottery clays, 1, 5, 31, 46, 66, 100, 101, 104, 110, 112, 114, 125,
129, 162, 166
Precambrian clays, 51
Precipitated clays, 97, 152
Primary clays, 70, 71, 84, 165
Proximate analysis, 16, 144
Purbeck clays, 59
Pure clays, 5, 6, 7, 142, 155, 156
Purification of clay, 7, 66, 78, 104, 113, 128, 140
Pyrites, 10, 13, 44, 56, 57, 119, 124, 128
Quartz, 8, 104, 110, 118, 140, 141, 143
Rain, 44, 85, 86
Rational analysis, 141
Reading clays, 63
Recent clays, 67
Rectorite, 158
Re-deposited clays, 98
Red bricks, 12
Red burning clays, 141, 142, 166
Red iron oxide, 12
Red ware, 113
Reduction in volume, 30
Refractoriness, 34, 119, 120, 123, 155
Refractory articles, 5, 119
Refractory clays, 9, 32, 33, 35, 38, 52, 65, 82, 104, 116, 165, 166
Residual clays, 70, 84, 166
Resistance to abrasion, 119
Resistance to corrosion, 119
Resistance to crushing, 46
Resistance to cutting, 119
Resistance to temperature, see _Refractoriness_
Resistance to weathering, 76
Ringing sound, 110
River-deposited clays, 88
Rivers, 85, 87
Rock binds, 53
Rockingham, 113
Rock-like clays, 2
Rocks associated with clay, 48
Roman cements, 133
Roofing tiles, 57, 63, 126, 128
Sagger marls, 54
Sand, 7, 31, 41, 62, 82, 89, 100, 101, 117, 133, 138, 144
Sandstones, 53
Sandy clays, 68
Sandy loams, 88
Sandy marls, 88
Sanitary articles, 5, 113
Sawdust, 40
Scum, 10
Sea, action of, 85, 89, 99
Sea-deposited clays, 93
Secondary clays, 70, 82, 83, 166
Sedimentary rocks, 48
Sedimentation of clay, 43, 88, 90, 104
Seger cones, 33, 34
Selection of clay, 122
Selenite, 10
Separation of clays, 90, 145
Settling, 43
Sewerage pipes, 113
Shale oil, 15, 61
Shale tar, 123
Shales, 2, 5, 51, 52, 53, 57, 61, 96, 104, 122, 130, 132, 138, 151,
162, 166
Shrinkage, 11, 29, 58, 68, 102, 110, 117, 119, 121, 127, 131, 153
Sifting, 7, 138
Silica, 6, 7, 80, 154, 155, 161
Silica rock, 118
Silicates, 8, 82
Siliceous clays, 117
Sillimanite, 46, 154
Silt, 90, 91, 99, 139, 144
Silurian clays and shales, 51, 124
Sintering, 38
Size of particles, 18, 21, 31, 106, 107, 150
'Skeleton,' 115
'Skin' on ware, 131
Slag in bricks, etc., 11, 13, 119, 155
Slates, 51
Slurry, 39, 43, 152
Snow, 85
Soda compounds, 7, 10, 113, 115, 116, 121, 157, see _Alkalies_
Softening point, 33
Soil, see _Agriculture_
Solubility of clay, 151, 159
Soluble salts, 10
Sorting, 90
Sources of clays, 85
Specification of fire clays, 120
Specific gravity, 18, 106, 151
Staffordshire bricks, 13, 56
Standard clay, 4
Stone, Cornish, 110, 116
Stoneware clays, 104, 112, 113, 156, 165, 166
Stones, 7, 65, 100, 102, 128, 138, 144
Streams, 85, 86
Strength, 23, 45, 113
Sub-surface clays, 5
Sulphates in clay, 10, 12, 82
Sulphides in clay, 10, 82
Sulphuric acid, 124
Sunlight, 45
Surface clays, 2, 5, 52, 112
Suspension of clay, 43, 90, 140, 152
Swelling, 15, 102
Tannin in clay, 25, 26, 41
Telluric water, 165
Temperature, resistance to, 119, 120
Tensile strength, 23, 45
Terra-cotta clays and shales, 5, 10, 12, 31, 46, 56, 63, 91, 104, 123,
124, 129, 166
Tertiary clays, 62
Texture, 112, 130
Thermal reactions, 146, 154, 158
Tiles, 1, 5, 57, 91, 101, 129
Titanium compounds, 121
Tourmaline, 76, 104
Transportation of clays, 49, 86, 98, 99, 100
Transported clays, 70
Triassic clays, 57, 112, 130
True clay, 144, 146, 149, 150, 160
Twisted ware, 114, 129, 131
Types of clay, 82
Ultimate analysis, 16, 144
Ultra-marine, clays for, 109
Underclays, 53, 117, 118
Uses of clay, 1, 165
Valuation of clay, 103, 109, 123, 126, 162, 165
Vegetable matter, 15, 119
Veins, 85
Viscosity, 152
Verifiable clays, 113, 156, 166
Vitrification, 15, 20, 37, 112, 113, 114, 156
Vitrification range, 38, 114, 115, 116, 129
Volcanoes, 85
Warp, 99
Warped ware, 114, 129, 131
Washing, 7, 79
Water, effect of, 74, 76, 81, 85, 86, 151
Water in clays, 15, 17, 29, 39, 45, 154
Wealden clay, 62
Weathering, 44, 74, 76, 79, 80, 97, 107, 165
White bricks, 68, 128
White clays, 70, 166
Wind, 86
Zeolites, 157
CAMBRIDGE: PRINTED BY JOHN CLAY, M.A. AT THE UNIVERSITY PRESS
End of Project Gutenberg's The Natural History of Clay, by Alfred B. Searle