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