Pottery clay differs from modelling clay in that it has more stiffening, so to speak. It is not so plastic, but, on the other hand, it will stand the heat of the kiln, which the other will not. The addition of whiting or felspar to modelling clay will give the needed stiffness. A pottery clay suitable for the work one can produce in one’s kiln must be so arranged as to harden at the degree of heat at which the kiln fires. Practically, we shall have little to do with any but the blue or ball clay and fire-clay. These are the best for pottery which is to be beautiful as well as useful. It is possible, of course, to use the flower-pot clay, and, perhaps, if one can easily obtain this clay, and is near enough to a pottery where the red ware is fired—so that one’s pieces can conveniently be sent there—it might be wiser to use it than to buy finer clays that come from a great distance, and then be obliged to send one’s pottery to a far-away kiln for firing.
The flower-pot ware is not strong, however, and fine glazes cannot be used upon it, so that, if possible, clay that will fire at a greater heat should be chosen. A mixture of fire-clay and ball or blue clay will give good results, and the pieces made with it may be sent to the nearest art pottery or to a kiln where stoneware is fired. Having once found a good clay, potters rarely change. In fact, this feeling is so strong with some that it amounts almost to a superstition. A potter who has obtained a clay that is satisfactory in a certain State will always send to the same State and bed for that clay, no matter how far he may be from it, nor how nearly the clay in his own State may come to the analysis of that which he has used.
The clay is, of course, brought from the clay beds in caked masses. It should be kept out of doors, for the wind and weather are good for it. In preparing it, the great lumps are pounded up fine and put through a sieve in which there are thirty to forty meshes to the inch. It is now ready for soaking. A common mistake is to pour water upon the clay in its rock state and then employ a boy or spend one’s own time kneading it into condition. The more simple and practical process is to sift it as already described, and then add the powdered clay to the water. In this way, each particle is wet at once, and in much less time the clay is ready for use. If it proves that too much water has been provided, pour off some after the clay is well mixed, and, if it is still too moist, spread it upon plaster slabs in the air; which will soon dry it sufficiently—the plaster itself absorbing much of the surplus moisture.
It is often possible, at slight expense, to obtain the clay already sieved and mixed from a pottery, and this is, of course, an advantage.
Keep your clay in a large earthern crock in which a little water has first been poured. Make deep holes with the thumb and pour water into them. Cover with a damp cloth and then with the earthen top. When the clay dries out so much that it is difficult to mould, let it dry thoroughly and pound it to a powder on a strong bench or table with an old flat-iron. It may then be re-mixed, as already described.
When ready to begin work, take a good-sized lump of clay, say about ten pounds; pound and knead it upon a table. Then draw a strong wire through it at the middle, dividing it in halves. Press the two outer surfaces together, kneading out the air-holes, which will be found on the inner surfaces. This process repeated a number of times will finally expel all the air-bubbles and leave the clay in good condition for moulding. Clay that is to be used on the wheel will need to be much more thoroughly kneaded than that for hand-moulded or built pottery.
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