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ОглавлениеHOW TO MAKE POTTERY
CHAPTER I
THE CLAYS AND TOOLS
Clay is what might be called the drift of the rocks of ages past. Most stone consists mainly of silica and alumina. In some bygone age, the potash and other alkalies, under the action of heat, fused these materials into rock. Potash is affected by the atmosphere, so it gradually left the rock, which decomposed, and the many particles, the fine refuse, after various changes became clay. Perfectly pure clay, or hydrated silicate of alumina, is found where some granites and other felspar-bearing rocks have decomposed. In the course of years, the clays, especially those on the surface, gathered impurities. Black clay, for example, which is found at the top of the bed, contains a large per cent. of carbon, formed by decayed leaves and twigs—like coal or peat. Strange to say, this burns whiter than any other pottery clay. It contains more potash than the other clays, and fires at a lower heat. The different proportions in which certain minerals are found in clays determine their plastic or non-plastic, fusible or refractory character. Potters have not been able to agree as to the reason for the plasticity of clay. Some believe that the silica gives it plasticity; others that it is due to the water inherent in the clay (which is called the water of combination), and give as their reason for the belief the fact that burned clay loses its plasticity, never to get it again. Yet water alone does not make every clay plastic: other ingredients are sometimes necessary. The early potter used the clay just as it came from the ground, but when he began to bake it in the fire he found that tempering materials were necessary. Sand was often added to make it easier to handle, and coarse, sharp pieces of shell or rock helped to prevent the clay from cracking, by lessening the shrinkage.
Later, as the art advanced, the value of certain materials in regulating the fusible and refractory qualities of the natural clay were recognised. There is in clays the play of melting and non-melting substances. Flint and alumina are hard, and uphold the clay in the fierce fire of the kiln; potash and soda make it soft and fusible. It is believed that the value of ground flint in pottery was not discovered until late in the seventeenth century. The story is that a certain potter travelling on horseback was delayed on account of an inflammation of his horse’s eyes. An ostler, wishing to help him out of his difficulty, took a flint from the road, and, after putting it in the fire until it was red-hot, threw it into cold water. It broke into pieces, pure white, which were easily powdered and applied to the animal’s eyes. Delighted as the potter was with the cure of his horse, the discovery of a white, infusible material for his pottery pleased him still more.
The combination of iron and lime in a clay forms a natural flux. For example: Fire-clay, which is quite refractory but which contains a small per cent. of iron, will, by the addition of a proportion of whiting (lime), become more fusible. Iron and lime in larger quantities are found in the clay which is made into red ware—flower-pots, red bricks, and other common articles. The large per cent. of iron gives it its red colour. It does not make as strong a ware as other clays. This red ware, which fires at the lowest heat, we will classify as Group I. To Group II. belongs the yellow or Rockingham ware. This fires at a light-orange heat, from 1,800 to 2,000 degrees Fahrenheit. It is often finished with a dark-brown glaze. Mixing-bowls and other kitchen ware come under this group. Group III. contains the buff or cream ware, usually made of fire-clays. This hardens at an intense heat, from 2,200 to 2,500 degrees Fahrenheit. In this group is the fine pottery. Art pottery comes under this head. Group IV. is the buff or cream ware, fired at white heat and beyond. This is the stoneware. On account of the great heat at which it is fired, it is possible to use only certain colours in the glazes. They are limited to gray, brown, green and blue. Ox-blood has been obtained in this ware, but it is uncertain. Claret-and vinegar-jugs and butter-crocks come in this group. Group V. is the white or tableware, and Group VI. the translucent ware or porcelain, made of kaolin, the purest grade of clay. Felspar makes it meltable and translucent, and, to give it plasticity, a small proportion of ball or fire-clay is added.
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.
Tools
Fig. 1
Fig. 2
Fig. 3
Fig. 4
Fig. 5
Boxwood modelling tools are useful for the hand-moulded pottery and for decoration. One
pointed tool, like a sharpened pencil (see Fig. 1), and another double-ended one, with points that are more flattened and rounded (see Fig. 2), will be enough to start with. One should also have a double-ended steel tool with pointed knife-ends (see Fig. 3), for incising and other decorative work. A steel tool with narrow, flattened ends (see Fig. 4) is necessary for cutting away the background to leave the design in low relief.
Fig. 6
Fig. 7
The tools for use with the potter’s wheel are mostly of thin steel, and may easily be cut from a sheet of steel which one can buy of a dealer in hardware. There should be an oval shape, two and a quarter by four inches, and another of the same size and shape with the edges cut in saw-teeth (see Fig. 5). There should also be a tool with one straight side and the other curved (see Fig. 6), and a rectangular one about two by four inches. A hoe-shaped tool of steel (see Fig. 7) is indispensable for working on the wheel, as is a slender point of steel, an inch and a quarter long, set in a wooden handle (see Fig. 8), and a piece of stout wire. A rubber polisher, the size and shape of the oval tool of sheet steel, will also be found useful. These may be bought of dealers in potters’ tools. A sponge, a ground-glass slab about a foot square, a rolling-pin, and sandpaper are also essential.
Fig. 8
Round and square slabs of plaster of paris should be provided upon which to put the pieces of pottery while they are being formed. These may be bought or cast in moulds, according to the directions in Chapter VII.
