CLAY modeling is one of the world's oldest crafts. There is hardly a primitive people that have not fashioned useful and ornamental objects of the native material found at their feet. Fired bricks over ten-thousand years old have been found. The Egyptians made glazed pottery as early as three thousand b.c. It was practiced by the Chinese thousands of years ago. It is a craft that is relatively unaffected by material shortages and expensive equipment purchases, excluding a kiln.
Clay suited to modeling and potterymaking is found in nearly every locality. To make a simple test, take up an amount of the soil sufficient for making a brick 1x1x4 inches. Form the clay into a brick by patting it with the hands and a ruler. Make the measurements accurate. Note the shrinkage when the clay is completely dry. Fire in a kiln and make another measurement. The shrinkage is an important item in estimating the dimensions of the finished piece of pottery. If the brick does not crack in the drying and in the firing, it is suited for potterymaking and modeling. Ordinary dirt will crumble. Sometimes a clay has too much stone and vegetable matter in it. Then it can be cleaned by running the pulverized clay through a fine wire screen. Small pieces of pottery may be made instead of a brick in testing the clay for drying and firing. However, a brick is easier to measure.
PLATE XXII. Breaking clay.
PLATE XXIII. A homemade sieve.
Clay dug from hills and stream beds should be processed. The dry clay is first pulverized by breaking up the lumps with a hammer, Plate XXII, or placing it in a strong canvas bag and pounding it. A pottery or a school having large ceramics classes mill their own clay. The hard lumps of clay are poured into the large round mill and a heavy roller crushes the clay.
The powdered clay is then sifted into a tub of water. It should be allowed to stand overnight. The excess water is dipped off the top. The wet clay is stirred briskly with a wooden paddle. A tub equipped with mechanical paddles, called blungers, is used in mixing large quantities of clay. The clay is about the consistency of very thick whipping cream. This is known as slip.
To remove lumps from the slip it should be screened with a sieve no coarser than a fly screen. The sieve in Plate XXIII was made by tacking fly screen over a wooden frame. A fruit sieve or a colander can be used.
In order to dry the slip so that it can be used for pottery and modeling, it may be allowed to dehydrate in a large tub. If poured upon several thicknesses of newspaper the water will be absorbed. In a ceramics studio, the slip is poured upon a plaster-of-Paris slab and the moisture is absorbed by the plaster. When ready for handling, the clay should be stored in a zinc-lined bin or in covered stoneware crocks.
Ancient Chinese potters seasoned their clay by letting it stand for the next generation to use, always using what had been prepared for them and preparing new clay for the next potters in their family. We do not season our clay that long. However, the consistency is improved by allowing the clay to stand for a week or more.
To make dry chunks plastic again, they should be completely dried, pulverized, and soaked in water. Partially dry clay is difficult to rework.