CLAYS AND PASTES
"It is the Art which gives the value, and not the material."
--DRESSER.
Clay being the chief material used by the potter, it demands a description
which, without being too technical, will give from the start a clear idea
of the nature of clays or pastes in general use.
It would be a needless complication to enter here into a discussion of the
chemical analysis of bodies and materials. Whilst the chemist can and does
determine with exactitude the relative quantities of each component, he
cannot yet, let us perhaps be thankful, lay down with the same certainty
the structural and molecular changes all these compounds will undergo in
the fire. The old potters' rule "of thumb" or, rather, common sense and
experience, still count for something.
Clay is the word generally applied to the natural article when used
without preparation, or after picking and washing. Paste is the term used
for all composite bodies that have been through a complicated process of
washing, grinding, mixing, and sieving, or even fritting, according to
the desired quality of the ware for which it is required. Natural clays
range from the pure white and very infusible kaolin, containing only
alumina and silica with a very small percentage of alkalies, to the impure
grey, red, or brown clays, containing, along with alumina and silica,
magnesia, potash, soda, iron, lime, and carbon. Kaolin is used with China
stone (a combination of felspar and quartz) to make porcelain, the finest
and hardest paste known to potters. It has a very hard white translucent
body, only slightly vitreous at the highest fire (around 1700°
Centigrade).
From this, the highest grade, we have almost insensible gradations to
common earthenware. Old English and French porcelain were compounded of
clay, sand, and alkalies ground together to make a frit, re-ground and
mixed with a stiffening material (in English porcelain, bone-ash), to
support the vitreous matter in the intense heat. The finest earthenware
does not differ greatly in its formula from soft porcelain, but it is not
so hard or transparent. From this the scale descends to where the presence
of lime or iron in the body colour it and render it easily fusible, so
that at any great heat it turns black and collapses to a slag.
Clay on being dug up is usually weathered in the open, and dried and
broken up and the greater impurities picked out. It is then thoroughly
mixed with water in a blunger and passed through a succession of sieves
until all foreign matter and impurities are left behind and it is the
consistency of cream. This was formerly done by hand, the clay being raked
into a thick "slub" and washed through a series of tanks until all
impurities had settled, leaving only the fine clay in suspension. It is at
this stage that any additions are made to form a paste. The modifying
ingredients, ground and sieved to the requisite degree, are thoroughly
incorporated with the slip, which is allowed to settle. The clear water on
top is siphoned off and the paste dried sufficiently to handle. The modern
method of preparation is to force the slip through a series of straining
bags which remove most of the water and leave it stiff enough to work.
Some of the hard pastes are so stiff or short that they require soap water
to give plasticity, but usually after a thorough wedging it is now ready
for the thrower.
Generally speaking, kaolin, China clay, ball clay, pipe clay, China stone,
felspar, flint, quartz, sand, lime, chalk, and calcined bone are the
ingredients of most modern pastes. These supply the alumina, silica, lime,
potash, and soda, with traces of iron and magnesia, that are found in all
clays when analyzed. Carbon is only present in impure bodies fired at a
low heat.
Of these materials the clays rich in alumina and silica, such as kaolin
or China clay, form the body-giving substance. The felspar or China stone
furnish the fluxing ingredients for fusing and binding. The flint or bone
supplies the stiffening matter for supporting and retaining the shape of
the object in the fire.
Porcelain, though differing so widely in appearance and texture from the
coarse mediæval earthenware or the pottery of the ancients, is found to
have a distinct relationship when all these bodies are submitted to
analysis. Much of the difference in bodies, apart from the impurities,
lies in the temperature of the fire to which it has been submitted. At a
low temperature such constituents as lime and iron are not much affected,
but at a greater heat they act as fluxing agents.
To generalize upon a complex and difficult subject one might say that
porcelain, both hard and soft (pâte dure and pâte tendre), is
characterized by its pure white colour and by extreme hardness of body and
glaze with transparency; fine stoneware by a very hard, opaque, and heavy
body which may be white, buff, or grey, and salt-glazed or with a fine
hard transparent glaze. Earthenware is softer and mostly opaque. It may
range from something a little softer than soft porcelain to the coarse
"Majolica" with a tin glaze, differing widely in colour of body and
hardness of glaze.
With porcelain and the finest high-fired wares a purity of materials and
uniformity of mass is absolutely necessary. It is here that one may well
call in the aid of the chemist and manufacturer. In any case it is
advisable to call in the chemist and the manufacturer when working on a
large scale. With a small output, as with all good craftsmen, the
fabrication of a good, reliable stoneware or earthenware paste is only a
matter of patience and hard work. Before commencing to produce finished
work on any scale, repeated experiments with different clays should be
carried out. Notes of all trials, with and without glaze, are invaluable
to the potter.
Rich clays can be stiffened, short clays enriched, and colour modified
without a mass of expensive machinery. Rich, easily fusible clays tend to
_stunt_ or buckle at a high fire. Hard refractory clays often remain
porous and are a fruitful source of crazing and breaking. The addition of
flint or fine washed sand, finely powdered _grog_, or _pitchers_, or even
refractory China clays, in quantities varying from about 5 per cent to 20
per cent, but settled only by repeated trials, will stiffen up or open out
rich clays inclined to warp or burst. Rich fusible clays added to hard
clays may stop the crazing, or the fusing point may be lowered by the
addition of spar. Stiff gravelly clay will require finer sieving or
repeated washing to rid it of some of the grit or sand. Rich greasy clays
are better when not too finely sieved, but this point is of course
dependent on the class of work to be undertaken. Slip can be settled in
tubs, the water siphoned off, and then put to dry on plaster bats, or dry
clay, powdered and sieved, may be stirred in until the mass is stiff
enough to wedge thoroughly by hand.
In mixing or modifying without machinery it is sometimes advisable to do
it in the dry state, otherwise some of the heavier materials are likely to
sink and are thus not thoroughly incorporated with the body. The dry mass
when well mixed is wetted enough to be wedged. When the body is coloured
and a white ground is indispensable, an "engobe," or dip, of white clay
slip must be resorted to.
These processes are certainly tedious, but that will not deter a craftsman
searching for the right clay in which best to shape his ideas. To the
craftsman working alone it is the only way by which he can accommodate his
clays to the various necessities of throwing, casting, or modelling. Each
process will require a slightly different nature in the clay if the finest
results are to be obtained.
Although in this craft book we shall not approach porcelain, a good, hard,
true-ringing body with a tough well-fitting glaze should be a _sine qua
non_ with all craftsmen; and it is only in the fire that any true idea can
be gained of the important influences clays and pastes have on the
finished work. Here we have tried to indicate the chief characteristics of
clays and to make clear the inevitable tendency of all potters who seek an
imperishable medium for their craft towards a purer body and a higher
fire.