SOME CLAYS OF COMMERCIAL IMPORTANCE
Although clays occur in deposits of almost all geological periods, many
of them are of little or no commercial value. This may be due to their
situation or to their composition and other characteristics. Thus, a
Coal Measure clay is ordinarily quite inaccessible, and to sink a shaft
specially to obtain it may be an unprofitable undertaking; if, however,
a shaft is sunk for coal the clays in the neighbourhood of the coal
seams are rendered accessible and, usually, a certain amount of such
clays is brought to the surface in order to remove it out of the way of
the coal miners.
Again, a clay deposit may be so far removed from human habitations as to
make it practically valueless, but if, for any reason, the population of
the district in which the clay is situated grows sufficiently, the clay
may become of considerable value. It not infrequently happens,
therefore, that the commercial importance of a clay deposit is one which
fluctuates considerably, yet, in spite of this fact, there are certain
kinds of clay which are nearly always of some commercial value. The most
important of these are the kaolins (china clays), the pottery and
stoneware clays, the refractory clays (fireclays), the brick and
terra-cotta clays and shales, and the clays used in the manufacture of
Portland cement. The origin and manner in which these clays have been
accumulated have been described in the previous chapters; it now remains
to indicate their characteristics from the point of view of their
commercial value.
=Commercial china clays and kaolins= in the United Kingdom are not
simple natural products but, in the state in which they are sold
commercially, have all been subjected to a careful treatment with water,
followed by a process of sedimentation whereby the bulk of the
impurities have been removed. According to the extent to which this
treatment has been carried out, they will contain 10 per cent. or more
mica and quartz, with little or no tourmaline, felspar and undecomposed
granite. In some parts of Europe and America, kaolins are found in a
state of sufficient purity to need no treatment of this kind unless they
are to be used for the very highest class of wares.
[Illustration: _Magnified 220 Diameters_
_Magnified 920 Diameters_
_Magnified 220 Diameters_
_Magnified 220 Diameters_
Crystals of Kaolinite
_Magnified 920 Diameters_
Crystals of Secondary Muscovite.
Fig. 17. Kaolinite and Mica. (_After G. Hickling_ (36).)]
Mica is usually the chief impurity as its particles are so small and
their density resembles that of the purified china clay more closely
than do the other minerals. In commerce the term _china clay_ is almost
invariably used to denote the washed material obtained from the 'china
clay rock,' but at the pits the word 'clay' is used indiscriminately for
the carclazite (p. 78) and for the material obtained from it. As the
term 'kaolin' is used indifferently abroad for the crude 'deposit' and
for the purified commercial article, it should be understood that the
following information relates solely to the substance as usually sold
and not to the crude material.
Commercial china clay or kaolin is a soft white or faintly yellowish
substance, easily reduced to an extremely fine powder, which when mixed
with twice its weight of water will pass completely through a No. 200
sieve. Its specific gravity is 2·65, but the minuteness and nature of
its smallest particles and their character are such that it will remain
in suspension in water for several days; it thus appears to possess
colloidal properties, at any rate so far as the smaller particles are
concerned. It is almost infusible, but shows signs of softening at 1880°
C. (Seger Cone 39) or at a somewhat lower temperature, according to the
proportion of impurities present. When heated with silica or with
various metallic oxides it fuses more readily owing to the formation of
silicates.
China clays and kaolins are not appreciably affected by dilute acids,
but some specimens are partially decomposed by boiling concentrated
hydrochloric acid (26) and all are decomposed by boiling sulphuric
acid, the alumina being dissolved and the silica liberated in a form
easily soluble in solutions of caustic soda or potash. This has led to
the conclusion that some kaolins may have been produced by weathering,
as the bulk of true kaolinitic clays (such as Cornish china clay) is not
affected by boiling hydrochloric acid (p. 81).
Owing to the exceptional minuteness of its particles, it is extremely
difficult to ascertain whether pure china clay or kaolin is crystalline
or amorphous. Johnson and Blake (21) found that all the specimens they
examined 'consisted largely of hexagonal plates' and that in most
kaolins 'these plates are abundant--evidently constituting the bulk of
the substance.' This observation is contrary to the experience of most
investigators, the majority of whom have found the bulk of the material
to be amorphous and sponge-like, but a small portion of it to consist of
hexagonal or rhombic crystals.
Mellor (22) has proposed the name _clayite_ for this amorphous material,
the crystalline portion being termed _kaolinite_ as suggested by Johnson
and Blake.
Both kaolinite (crystalline) and clayite (amorphous) yield
the same results on analysis and correspond very closely to
the formula H4Al2Si2O9 or Al2O3·2SiO2·2H2O, so that it is most
probable that they are the same substance in different physical states.
According to Hickling (36) the general impression that the particles of
china clay are amorphous is due to the use of microscopes of
insufficient power. With an improved instrument, Hickling claims to have
identified the 'amorphous' portion of china clay with crystalline
kaolinite, the clay particles (fig. 17) being in the form of irregular,
curved, hexagonal prisms or in isolated plates. The former show strong
transverse cleavages. The index of refraction and that of double
refraction agree with those of Anglesea kaolinite crystals, as does the
specific gravity.
In spite of their great purity, commercial china clays and kaolins are
almost devoid of plasticity, nor can this property be greatly increased
by any artificial treatment. This has led to the conclusion that
plasticity is not an essential characteristic of the clayite or
kaolinite molecules, but is due to physical causes not shown by any
investigation of the chemical composition of the material.
In addition to the specially purified kaolins just described, alkaline
kaolins, siliceous kaolins and ferruginous kaolins are obtained from
less pure rocks and do not undergo so thorough a treatment with water.
Some of these varieties are not improbably derived from transported
kaolins, as they occur in Tertiary strata, and so bear some resemblance
to the white fireclays on the Carboniferous limestone of Staffordshire,
Derbyshire and North Wales, though the latter are far more plastic.
To be of value, a china clay or kaolin must be as white as possible and
must be free from more than an insignificant percentage of metallic
oxides which will produce a colour when the clay is heated to bright
redness. If the material is to be used in the manufacture of paper,
paint or ultra-marine, these colour-producing oxides are of less
importance providing that the clay is sufficiently white in its
commercial state.
The manufacturer of china-ware and porcelain requires china clay or
kaolin which, in addition to the foregoing characteristics, shall be
highly refractory. It must, therefore, be free from more than about 2
per cent. of lime, magnesia, soda, potash, titanic acid and other
fluxes.
It is a mistake to suppose that all white clays of slight plasticity are
china clays or kaolins. Some _pipe clays_ have these characteristics,
but they contain so large a proportion of impurities as to be useless
for the purposes for which china clay is employed and are consequently
of small value.
Users of china clays and kaolins generally find it necessary to carry
out a lengthy series of tests before accepting material from a new
source, as such a material may possess characteristics not readily shown
by ordinary methods of analysis, but which are sufficiently active to
make it useless for certain purposes (see p. 143).
=Pottery clays= are, as their name implies, those used in the
manufacture of pottery, and comprise the china clays already mentioned
(p. 104), the ball clays and the less pure clays used in the manufacture
of coarse red ware, flower pots, etc.
The _china clays_ (p. 104) are not used alone in pottery manufacture as
they lack plasticity and cohesion. In the production of china-ware or
porcelain they are mixed with a fluxing material such as Cornish stone,
pegmatite, or felspar, together with quartz or bone ash. Thus, English
china ware is produced from a mixture of approximately equal parts of
bone ash, china clay and Cornish stone, whilst felspathic or hard
porcelain is made from a mixture of kaolin, felspar and quartz, a little
chalk being sometimes added.
The _ball clays_ (p. 64) form the basis of most ordinary pottery, though
some china clay is usually added in order to produce a whiter ware.
Flint is added to reduce the shrinkage--which would otherwise be
inconveniently great--and the strength of the finished ware is
increased, its texture is rendered closer and its capability of emitting
a ringing sound when struck are produced by the inclusion of Cornish
stone or felspar in the mixture. Small quantities of cobalt oxide are
also added to improve the whiteness in the better classes of ware.
[Illustration: Fig. 18. Mining best Potter's clay in Devonshire. (_Photo
by Mr G. Bishop._)]
The ball clays are characterised by their remarkably high plasticity,
their fine texture and their freedom from grit. They are by no means so
pure as the china clays, and unless carefully selected can only be used
for common ware.
The better qualities burn to a vitrified mass of a light brownish tint,
but when mixed with the other materials used in earthenware manufacture
they should produce a perfectly white ware. The inferior qualities are
used for stoneware, drain pipes, etc. It should be noted that the term
'ball clay' is used for clays of widely differing characteristics though
all obtained from one geological formation; when ordering it is
necessary to state the purpose for which the clay is required or an
entirely unsuitable material may be supplied. For the same reason, great
care is needed in any endeavour to sell a ball clay from an hitherto
unworked deposit.
_Coarse pottery clays_[11] are usually found near the surface and whilst
they may be derived from any geological formation, those most used in
England are of Triassic or Permian origin, though some small potteries
use material of other periods, including alluvial or surface clays.
These clays are closely allied to those used for brickmaking, but are
somewhat finer in texture and more plastic. In some cases they are
prepared from brick clays by treating the latter in a wash-mill, the
coarser particles being then removed, whilst the finer ones, in the
state of a slip or slurry, are run into a settling tank and are there
deposited.
[Footnote 11: Coarse pottery has been defined as that made from natural
clay without the addition of any material other than sand and water.]
The presence of a considerable proportion of iron oxide results in the
formation of red ware, which is necessarily of a porous nature, as the
fluxes in the clay are such that they will not permit of its being
heated to complete vitrification without loss of shape. To render it
impervious the ware is covered with a glaze, usually producing red,
brown or black ware (Rockingham ware).
The _stoneware_ or _drain-pipe clays_, are the most important of the
_vitrifiable clays_ and owe their value to the fact that they can be
readily used for the manufacture of impervious ware without the
necessity of employing a glaze. They are, therefore, used in the
manufacture of vessels for holding corrosive liquids such as acids and
other chemicals, for sanitary appliances, sewerage pipes and in many
other instances where an impervious material is required.
Owing to the lime, magnesia, potash and soda they contain, the stoneware
clays undergo partial fusion at a much lower temperature than is
required by some of the purer clays. The fused portion fills the pores
or interstices of the material, making--when cold--a ware of great
strength and impermeability.
The chief difficulty experienced in the manufacture of stoneware is the
liability of the articles to twist and warp when heated. For this reason
it is necessary to burn them very carefully and to select the clays with
circumspection. Some clays are quite unsuitable for this branch of
pottery manufacture because of the practical impossibility of producing
ware which is correct in shape and is free from warping.
What is required are clays in which the partial fusion will commence at
a moderate temperature and will continue until all the pores are filled
with the fused material without the remaining ingredients being attacked
or corroded sufficiently to cause the ware to lose its shape. As the
temperature inside a potter's kiln is continually rising, the great
tendency is for the production of fused material to take place at an
ever-increasing rate, so that the danger of warping becomes greater as
the firing nears completion. Some clays commence to vitrify at a
moderate temperature and can be heated through a long range of
temperature before an appreciable amount of warping occurs; such clays
are said to possess a 'long range of vitrification' (p. 38). In other
clays the difference between the temperature at which vitrification
commences and that at which loss of shape occurs is only a few degrees;
such clays are useless for the manufacture of stoneware, as their
vitrification range is too short. It is therefore essential that, for
the manufacture of stoneware, a clay should contain a large proportion
of refractory material which will form a 'skeleton,' the interstices of
which will be filled by the more fusible silicates produced by the
firing.
It is generally found that of all the fluxes present in vitrifiable
clays, soda and potash compounds--the so-called 'alkalies'--and all lime
compounds are the most detrimental, as in association with clay they
form a material with a very short range of vitrification. Magnesia, on
the contrary, accompanies a long vitrification range.
The clays used in Great Britain for the manufacture of the best
stoneware are the Devonshire and Dorset ball clays, the upper portions
of these deposits being used for this purpose as they are somewhat less
pure than the lower portions used in the manufacture of white ware. For
coarser grades of stoneware, clays of other geological formations are
employed, especially where the finished ware may be coloured, as the
purity of the clay is of less importance. Providing a clay has a
sufficiently long vitrification range, a suitable colour when burned,
and that it is capable of being readily formed into the desired shapes,
its composition and origin are of small importance to the stoneware
manufacturer. In actual practice, however, the number of sources of good
stoneware clay is distinctly limited, and many manufacturers are thus
compelled to add suitable fluxes to refractory clays in order to meet
some of their customers' requirements. For this purpose a mixture of
fireclay with finely powdered felspar or Cornish stone is used.
Chalk--which is a cheaper and more powerful flux--or powdered glass
cannot be used as the range of vitrification of the mixture would be too
short.
Some manufacturers take the opposite course and add fireclay, flint, or
other refractory material to a readily fusible clay. This is
satisfactory if the latter clay is relatively low in lime and owes its
fusibility to potash, soda or magnesia in the form of mica or felspar.
The mica and felspar grains enter so slowly into combination with the
clay that a long range of vitrification occurs, whereas with lime, or
with some other soda and potash compounds, the combination occurs with
great rapidity and the shape of the ware is spoiled.
The =refractory clays= are commonly known as _fireclays_ on account of
their resistance to heat. The china clays and kaolins are also
refractory, but are too expensive and are not sufficiently plastic to be
used commercially in the same manner as fireclays, except to a very
limited extent, though bricks have been made for many years from the
inferior portions of china clay rock at Tregoning Hill in Cornwall.
The geological occurrence of the fireclays of the Coal Measures has
already been described on p. 53. In addition, there are the refractory
clays occurring in pockets or depressions in the Mountain Limestone of
North Wales, Staffordshire, Derbyshire and Ireland, which consist of
siliceous clays and sands, the insoluble residue of the local
dissolution of the limestone, intermixed with the débris of the
overlying Millstone Grit (see p. 54). These clays and sands can be mixed
to produce bricks of remarkably low shrinkage, but the pockets are only
large enough to enable comparatively small works to be erected and the
clays are so irregular both in composition and distribution as to render
their use somewhat speculative.
A third type of refractory clay--termed _flint clay_--is used in large
quantities in the United States, but is seldom found in Great Britain.
When moistened, flint clays do not soften, but remain hard and
flint-like with a smooth shell-like fracture. For use they are ground
extremely fine, but even then they develop little plasticity. They are
considered by Ries (6) to have been formed by solution and
re-precipitation of the clay subsequent to its primary formation, in a
manner similar to flint. They are somewhat rich in alumina and many
contain crystals of pholerite (Al2O3·2SiO2·3H2O).
The Coal Measure fireclays (p. 53)--which are by far the most
important--are divided into two sections by the coal seams, those above
the coal being shaly and fissile in structure whilst those below
(_underclays_) are without any distinct lamination. Both these clays
may be equally refractory, but the underclays are those to which the
term fireclay is usually applied. The lowest portions are usually more
silicious and in some areas are so rich in silica as to be more
appropriately termed silica rock or _ganister_. Fireclays may, in fact,
be looked upon as a special term for the grey clays of the Coal
Measures, interstratified with and generally in close proximity to the
seams of coal. They are known locally as _clunches_ and _underclays_ and
were at one time supposed to represent the soil that produced the
vegetation from which the coal was formed, but are now considered by
many authorities to be of estuarine origin.
It is important to notice that whilst the coals almost invariably occur
in association with underclays, some fireclays are found at a
considerable distance from coal.
The fireclays of the Coal Measures have a composition varying within
comparatively wide limits even in contiguous strata; those chiefly used
having an average of 20 to 30 per cent. of alumina and 50 to 70 per
cent. of silica. They appear to consist of a mixture of clay and quartz
with a small proportion of other minerals, but in some of them a portion
of the clay is replaced by halloysite--another hydro-alumino-silicate
with the formula
H6Al2Si2O10 or Al2O3.2SiO2.3H2O.
Their grey colour is largely due to vegetable (carbonaceous) matter and
to iron compounds. The latter--usually in the form of pyrites--is
detrimental to the quality of the goods as it forms a readily fusible
slag. Unlike the iron in red-burning clays it can seldom be completely
oxidized and so rendered harmless. The fireclays must therefore be
carefully selected by the miners.
On the Continent, and to a much smaller extent in Great Britain,
refractory articles are made from mixtures of grog or burned fireclay
with just sufficient raw clay to form a mass of the required strength.
For this purpose a highly plastic, refractory clay is required and the
Tertiary ball clays of Devon and Dorset (p. 64) are particularly
suitable.
The most important characteristics of a fireclay are that it shall be
able to resist any temperature to which it may be exposed and that the
articles into which it is made shall not be affected by rapid changes in
temperature. Other characteristics of importance in some industries are
the resistance to corrosive action of slags and vapours, to cutting and
abrasion by dust in flue-gases or by the implements used in cleaning the
fires. For those purposes it is necessary that a fireclay should possess
high infusibility (p. 32), a low burning shrinkage (p. 29) and a high
degree of refractoriness (p. 34), and before it is used these
characteristics should be ascertained by means of definite tests, as
they cannot be determined by inspection of a sample or from a study of
its chemical analysis.
Several grades of fireclay have long been recognized on the Continent
and in the United States of America, but the recent Specification of the
Institution of Gas Engineers is the only official recognition in Great
Britain of definite grades. This specification defines as No. 1 grade a
fireclay which shows no signs of fusion when heated to 1670° C. or Cone
30 at the rate of 10° C. per minute, and as No. 2 grade fireclay those
which show no signs of fusion when similarly heated to 1580° C. or Cone
26.
It is regarded as a sign of fusion if a test piece with sharp angles
loses its angularity after heating to a predetermined temperature (see
p. 35).
It is customary to regard as 'fireclay' all clays which, when formed
into the shape of a Seger Cone (fig. 6) do not bend on heating slowly
until a temperature of 1580° C. (Cone 26) is reached. Any clays
comprised within this definition and yet not sufficiently refractory to
be of the No. 2 grade just mentioned may be regarded as No. 3 grade
fireclays. Many of the last named are well suited for the manufacture of
blocks for domestic fireplaces, for glazed bricks and for firebricks not
intended to resist furnace temperatures.
To resist sudden changes in temperature the material must be very
porous--the article being capable of absorbing at least one-sixth of its
volume of water. For this reason it is customary to mix fireclays with a
large proportion of non-plastic material of a somewhat coarse texture,
the substance most generally employed being fireclay which has been
previously burned and then crushed. This material is known as _grog_ or
_chamotte_ and has the advantage over other substances of not affecting
the composition of the fireclay to which it is added, whilst greatly
increasing its technical usefulness. The addition of grog also reduces
the shrinkage of the clay during drying and ensures a sounder article
being produced.
The most serious impurities in refractory clays are lime, magnesia,
soda, potash and titanium and their compounds as they lower the
refractoriness of the material. Iron, in the state of ferric oxide is of
less importance, but pyrites and all ferrous compounds are particularly
objectionable. Pyritic and calcareous nodules may, to a large extent, be
removed by picking, and by throwing away lumps in which they are seen to
occur. There is, at present, no other means of removing them.
Fireclays may be ground directly they come from the mine, but it is
usually better to expose them to the action of the weather as this
effects various chemical and physical changes within the material,
which improves its quality as well as reduces the power required to
crush it.
To take full advantage of the refractory qualities of a clay it is
necessary to select it with skill, prepare and mould it with care, to
burn it slowly and steadily, to finish the heating at a sufficiently
high temperature and to cool the ware slowly.
Rapidly heated fireclay is seldom so resistant to heat under commercial
conditions as that which has been more steadily fired. Rapid or
irregular heating causes an irregular formation and distribution of the
fused material during the process of vitrification (p. 37) and so
produces goods which are too tender to be durable. It is, therefore,
necessary to exercise great care in the firing.
=Shales= are rocks which have been subjected to considerable pressure
subsequent to their deposition and are, consequently, laminated and more
readily split in one direction than in others. Some shales are almost
entirely composed of silica or calcareous matter, but many others are
rich in clay, the term referring to physical structure and not to
chemical composition. The clay-shales occur chiefly in the Silurian and
Carboniferous formations, the latter being more generally used by
clayworkers.
Clay-shales are valued according to (_a_) the proportion of oil which
can be distilled from them, those rich in this respect being termed _oil
shales_; (_b_) the colour when burned, as in _brickmaking and
terra-cotta shales_; (_c_) the refractoriness, as in _fireclay shales_
and (_d_) the facility with which they are decomposed on exposure or on
heating and form sulphuric acid as in _alum shales_.
_Oil shales_ contain so much carbonaceous matter that on distillation at
a low red heat they yield commercially remunerative quantities of a
crude oil termed _shale tar_. In composition they are intermediate
between cannel coal and a purely mineral shale. To be of value they
should not yield less than 30 gallons of crude oil per ton of shale,
with ammonia and illuminating gas as by-products. They are of Silurian,
Carboniferous or Oolitic origin, the Kimeridge shale associated with the
last-named being very valuable in this respect.
The most important oil shales occur in Scotland.
The _fireclay shales_ have already been described on pages 53 and 116.
The _brickmaking shales_ are those which are sufficiently rich in clay
to form a plastic paste when ground and mixed with water. They can be
made into bricks of excellent colour and great strength, but for this
purpose require the use of powerful crushing and mixing machinery. They
are usually converted into a stiff paste of only moderate plasticity and
are then moulded by machinery in specially designed presses, though some
firebricks are made from crushed shale mixed into a soft paste with
water and afterwards moulded by hand. Some shales, such as the _knotts_
at Fletton near Peterborough are not made into a paste, the moist
powdered shale being pressed into bricks by very powerful machinery.
Brickmaking shales may be found in any of the older geological
formations, though they occur chiefly in the Silurian, Permian,
Carboniferous and Jurassic systems. The purer shales of the Coal
Measures burn to an agreeable cream or buff colour, the less pure ones
and those of the other formations mentioned produce articles of a
brick-red or blue-grey colour.
Where the shales are of exceptionally fine grain and their colour when
burned is very uniform and of a pleasing tint they are known as
_terra-cotta_ shales, the red terra-cottas being chiefly made from those
occurring in Wales and the buff ones from the lower grade fireclays of
the Coal Measures.
_Alum shales_ are characterised by a high proportion of pyrites, which,
on roasting, form ferrous sulphate and sulphuric acid. The latter
combines with the alumina in the shale and when the roasted ore is
extracted with water a solution of iron sulphate and aluminium sulphate
is obtained. From this solution (after partial evaporation) alum
crystals are obtained by the addition of potassium or ammonium sulphate.
The chief alum shales are those of the Silurian formation in Scotland
and Scandinavia. The Liassic shales of Whitby were at one time an
equally important source of alum.
During recent years a large amount of alum has been obtained from other
sources or has been made from the lower grade Dorset and Devonshire ball
clays by calcining them and then treating them with sulphuric acid.
These clays being almost free from iron compounds yield a much purer
alum at a lower cost.
=Brick clays= are those which are not suitable--either from nature or
situation--for the manufacture of pottery or porcelain and yet possess
sufficient plasticity to enable them to be made into bricks. The term is
used somewhat loosely, and geologists not infrequently apply it to clays
which are quite unsuitable for brickmaking on account of excessive
shrinkage and the absence of any suitable non-plastic medium. Large
portions of the 'London clay' are of this nature and can only be
regarded as of use to brick- and roofing-tile-manufacturers when the
associated Bagshot sands are readily accessible. Similarly, some of the
very tough surface clays of the Northern and Midland counties are
equally valueless, though designated 'brick clays' in numerous
geological and other reports. It is, therefore, necessary to remember
that, as ordinarily used, the term 'brick clay' merely indicates a
material which appears at first sight to be suitable for brickmaking,
but that more detailed investigations are necessary before it can be
ascertained whether a material so designated is actually suitable for
the purpose.
It is also important to observe that local industrial conditions may be
such that a valuable clay may be used for brickmaking because there is a
demand for bricks, but not for the other articles for which the clay is
equally suitable. For instance, a considerable number of houses in
Northumberland and Durham were built of firebricks at a time when it was
more profitable to sell these articles for domestic buildings than for
furnaces.
In many ways the bricks used for internal structural work form the
simplest and most easily manufactured of all articles made from clay.
The colour of the finished product is of minor importance and so long as
a brick of reasonably accurate shape and of sufficient strength is
produced at a cheap rate, little else is expected.
Impurities--unless in excessively large proportions--are of small
importance and, indeed, sand may almost be considered an essential
constituent of a material to be used for making ordinary bricks. It is,
therefore, possible to utilize for this purpose some materials
containing so little 'clay' as to make them scarcely fit to be included
in this term. So long as the adventitious materials consist chiefly of
silica and chalk and the mixture is sufficiently plastic to make strong
bricks, it may be used satisfactorily in spite of its low content of
clay, but if the so-called 'brick clay' contains limestone, either in
large grains or nodules, it will be liable to burst the bricks or to
produce unsightly 'blow-holes' on their surfaces. If too much sand or
other non-plastic material is present, the resulting bricks will be too
weak to be satisfactory.
No brick clay can be regarded as 'safe' if it contains nodules of
limestone--unless these can be removed during the preparation of the
material--or if the resulting bricks will not show a crushing strength
of at least 85 tons per square foot.
The introduction of machinery in place of hand-moulding and of kilns
instead of clamps has greatly raised the standard of strength, accuracy
in shape and uniformity in colour in many districts, and many builders
in the Midlands now expect to sort out from the 'common bricks'
purchased, a sufficient number of superior quality to furnish all the
'facing bricks' they require. Apart from this, and in districts where
buildings are faced with stone or with bricks of a superior quality, the
'stock' or 'common brick' may be made from almost any clay which will
bear drying and heating to redness without shrinking excessively or
cracking. A linear shrinkage of 1 in. per foot (= 8-1/2 per cent.) may
be regarded as the maximum with most materials used for brickmaking.
Clays which shrink more than this must have a suitable quantity of grog,
sand, chalk, ashes or other suitable non-plastic material added.
If the clay contains much ferric oxide it will produce red or brown
bricks according to the temperature reached in the kiln, but if much
chalk is also present (or is added purposely) a combined
lime-iron-silicate is produced and the bricks will be white in colour.
If only a small percentage of ferric oxide is present a clay will
produce buff bricks, which will be spotted with minute black specks or
larger masses of a greyish black slag if pyrites are also present or if
ferrous silicate has been produced by the reduction of the iron
compounds and their subsequent combination with silica.
Further information on brick earths will be found on page 67.
A description of the processes used in the manufacture of bricks being
outside the scope of the present work, the reader requiring information
on this subject should consult _Modern Brickmaking_ (25) or some similar
treatise.
_Roofing tiles_ require clays of finer texture than those which may be
made into bricks. Stones, if present, must be removed by washing or
other treatment, as it is seldom that they can be crushed to a
sufficiently fine powder, unless only rough work is required. If
sufficiently fine, the clay used for roofing tiles may be precisely the
same as that used for bricks and is treated in a similar manner. It
must, however, be of such a nature that it will not warp or twist during
the burning; it must, therefore, have a long range of vitrification (p.
38).
_Terra-cotta_ is an Italian term signifying baked earth, but its meaning
is now limited to those articles made of clay which are not classed as
pottery, such as statues, large vases, pillars, etc., modelled work used
in architecture, or for external decoration. Although the distinction
cannot be rigidly maintained, articles made of clay may be roughly
divided into
(_a_) Pottery (_faience_) and porcelain (glazed),
(_b_) Terra-cotta (unglazed),
(_c_) Bricks and unglazed tiles devoid of decoration.
In this sense, terra-cotta occupies an intermediate position between
pottery and bricks, but no satisfactory definition has yet been found
for it. Thus, bricks with a modelled or moulded ornament are, strictly,
terra-cotta, yet are not so named, and some pottery is unglazed and yet
is never classed as terra-cotta, whilst glazed bricks are never regarded
as pottery. Again during the past few years, what is termed 'glazed
terra-cotta' has been largely used for architectural purposes, yet this
is really 'faience.'
Although this overlapping of terms may appear confusing to the reader,
it does not cause any appreciable amount of inconvenience to the
manufacturers or users, as it is not difficult for a practical
clayworker to decide in which of the three classes mentioned a given
article should be placed.
Partly on account of the lesser weight, but chiefly in order to reduce
the tendency to crack and to facilitate drying and burning, terra-cotta
articles are usually made hollow.
It is necessary that clays used in the manufacture of terra-cotta should
be of so fine a texture that the finest modelling can be executed. Such
clays occur naturally in several geological formations, and some may be
prepared from coarser materials by careful washing, whereby the larger
grains of sand, stones, etc., are removed. Some shales, when finely
ground, make excellent clays for architectural terra-cotta, portions of
all the better known fireclay deposits being used for this purpose. It
is, however, necessary to use only those shales which are naturally of
fine texture, as mechanical grinding cannot effect a sufficient
sub-division of the particles of some of the coarser shales.
The finer Triassic 'marls' are also admirable for terra-cotta work, the
most famous deposit being the Etruria Marl Series in the Upper Coal
Measures near Ruabon.
The most important characteristics required in terra-cotta clays are
(_a_) fine texture, or at any rate the ability to yield a fine, dense
surface, (_b_) small shrinkage with little tendency to twist, warp or
crack in firing, (_c_) pleasing and uniform colour when fired, and (_d_)
a sufficient proportion of fluxes to make it resistant to weather
without giving a glossy appearance to the finished product.
In large pieces of terra-cotta some irregularity of shape is almost
unavoidable, but, if care is taken in the selection and manipulation of
the material, this need not be unsightly.
The durability of terra-cotta is largely dependent on the nature of the
surface. The most suitable clays, when fired, have a thin 'skin' of
vitrified material which is very resistant to climatic influences, and
so long as this remains intact the ware will continue in perfect
condition. If this 'skin' is removed, rain will penetrate the material
and under the influence of frost may cause rapid disintegration.
In the manufacture of very large pieces of terra-cotta a coarse, porous
clay is used for the foundation and interior, and this is covered with
the finer clay. By this means a greater resistance to changes in
temperature is secured, the drying and the burning of the material in
the kiln are facilitated and the risks of damage in manufacture are
materially reduced.
=Cement clays= are those used in the manufacture of Portland cement and
of so-called natural cements. They are largely of an alluvial character
and are of two chief classes: (_a_) those which contain chalk or
limestone dust and clay in proportions suitable for the manufacture of
cement and (_b_) those to which chalk or ground limestone must be added.
They vary in composition from argillaceous limestones containing only a
small proportion of clay to almost pure clays.
The manufacture of Portland cement has assumed a great importance and
owing to the large amount of investigations made in connection with it,
it may be said to represent the chief cement made from argillaceous
materials, the others being convenient though crude modifications of it.
The essential constituents are calcium carbonate (introduced in the form
of chalk or powdered limestone) and clay, the composition of the
naturally occurring materials being modified by the addition of a
suitable proportion of one or other of these ingredients. The material
is then heated until it undergoes partial fusion and a 'clinker' is
formed. This clinker, when ground, forms the cement.
In Kent, the Medway mud is mixed with chalk; in Sussex, a mixture of
gault clay and chalk is employed; in the Midlands and South Wales,
Liassic shales and limestone are used; in Northumberland a mixture of
Kentish chalk and a local clay is preferred, and in Cambridgeshire a
special marl lying between the Chalk and the Greensand is found to be
admirable for the purpose because it contains the ingredients in almost
exactly the required proportions.
For cement manufacture, clays should be as free as possible from
material which, in slip form, will not pass through a No. 100 sieve, as
coarse sand and other rock débris are practically inert. The proportion
of alumina and iron should be about one-third, but not more than
one-half, that of the silica, and in countries where the proportion of
magnesia in a cement is limited by standard specifications, it will be
found undesirable to use clays containing more than 3 per cent. of
magnesia and alkalies.
Whilst calcareous clays usually prove the most convenient in the
manufacture of cement, it is by no means essential to use them, and
where a clay almost free from lime occurs in convenient proximity to a
suitable chalk or limestone deposit an excellent cement may usually be
manufactured.
The 'clays' from which the so-called 'natural' or 'Roman cements' are
made by simple calcination and crushing, usually fuse at a lower
temperature than do the mixtures used for Portland cement, and unless
their composition is accurately adjusted they yield a product of such
variable quality as to be unsuitable for high class work.
=Fuller's earth= is a term used to indicate any earthy material which
can be employed for fulling or degreasing wool and bleaching oil. True
fuller's earth is obtained chiefly from the neighbourhood of Reigate,
Surrey, Woburn Sands, Bedfordshire and from below the Oolite formation
near Bath, but owing to the scarcity of the material and the
irregularity of its behaviour, china clay is now largely used for the
same purpose. True fuller's earth is much more fusible than the white
clays usually substituted for it, and when mixed with water it does not
form a plastic paste but falls to powder. As the chief requirement of
the fuller is the grease-absorbing power of the material there is no
objection to the substitution of other earths of equal efficiency.
Fuller's earth does not appear to be a true clay, though its
constitution and mineralogical composition are by no means
clearly known. T. J. Porter considers that it is chiefly
composed of montmorillonite (Al2O3·4SiO2H2O), anauxite,
(2Al2O3·9SiO2·6H2O), and chalk with some colloidal silica and a
little quartz. It therefore appears to resemble the less pure kaolins,
but to contain little or no true clay, though in many respects it
behaves in a manner similar to a kaolin of unusually low plasticity.
=Other clays= of commercial importance, with further details of the ones
just mentioned, are described in the author's _British Clays, Shales and
Sands_ (2).