There are two main types of clay used in ceramics: pottery clay and sculpture clay.
Pottery clay. There are two kinds - gray stoneware and terra cotta clay - which will produce a good plastic, workable pottery clay. If both can be procured, they will increase the variety of possibilities available in decoration and colors. The stoneware clay fires a light buff and permits the use of lighter and brighter colors in glazing.
The terra cotta clay fires a reddish-brown and consequently darkens all glaze colors and this should be taken into account when choosing clay for a project. It is especially suited to sculpture, and is best left unglazed as the warm color and mat surface produce an effective appearance. Fifty pounds of moist clay of either kind, or twenty-five pounds of each, will be ample for one person to begin work.
Sculpture clay. This clay is particularly desirable for making sculpture because it is stronger, more rigid, and can take more strain and stress than the other clays. It contains an addition of grog (burnt fireclay ground up). Sculpture clay can be bought prepared, or made by mixing grog - about 20 percent by weight - with either one of the pottery clays. Grog can be procured from a ceramic supply house in buff or terra cotta. Twenty-five pounds of moist sculpture clay is ample for one person to begin work.
Preparing the Clay
Whenever possible, it is simpler to buy clay in a moist state, but it can be purchased in a dry powder form and mixed as needed. This will require several days, depending on the atmosphere and temperature of the room in which the clay is mixed. To prepare clay from the dry powder: The powder should be spread in a sink, tub, or flat pan and mixed with water to the consistency of thick molasses. All lumps should be removed by stirring until it is smooth.
Allow water to evaporate until clay reaches a slushy state, and then spread it on plaster bats to dry. Turn and press the mass before a crust develops on either side. When it passes beyond the sticky state to a soft firmness, remove it from the bats, wedge, and store it. Clay is easier and better to use when it has "aged" in the damp state. This means leaving it in a damp place (box or crock) for a period of a week to two or three months, the longer, the better.
How to Care for Clay
Clay must be kept in a container with a lid. Stoneware jars (five to ten gallon size), garbage cans, and laundry tubs are adaptable. A wet cloth over the clay will help maintain the right consistency. Consistency varies according to the use to which the clay will be put. For pottery, the clay must be quite soft, not sticky, and firm, not shapeless. For sculpture, it should be stiffer so that it will not slump or sink or lose its shape.
While working on an object, the unused portion of the wedged clay should be kept under a damp cloth, or it will become too hard. If clay is too moist, it can be rolled or wedged on a dry table or dry plaster bat until it is of the right consistency.
If it is too stiff, it can be rolled or wedged on a damp surface (a table moistened with a wet cloth) until it is right. When clay is very stiff it may be reclaimed by leaving it on a moist plaster bat (a bat soaked in water for about five minutes), covered with a damp cloth. Clay that is bone dry must be broken and mashed, and worked like dry powder clay.
Now you have the clay in good order, it is time for you to try some pottery work!
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