You can remove some of the coarse foreign matter natural to secondary clays, but coarse matter can also be used in varying quantities to achieve particular effects. A certain amount of coarse grain in the clay helps the vessel retain its shape in firing, and potters using fine-grained clays often temper the clay by adding coarser materials such as sand, fine stones, ground shells, or grog (fired and pulverized clay) before kneading the clay into a workable condition. The plasticity of clay allows pottery to be shaped in several traditional ways. The clay can be flattened and then shaped by being pressed against the inside or outside of a molda stone or basket, or a clay or plaster form. Liquid clay can be poured into plaster molds. A pot can be coil built: Clay is rolled between the palms of the hands and extended into long coils, a coil is formed into a ring, and the pot is built up by superimposing rings. Also, a ball of clay can be pinched into the desired shape. The most sophisticated pottery-making technique is wheel throwing.

The potter's wheel is a flat disk that revolves horizontally on a pivot. Both handsone on the inside and the other on the outside of the clayare free to shape the pot upward from a ball of clay that is thrown and centered on the rotating wheel head. Some wheels are set in motion by a stick that fits into a notch in the wheel (often activated by an assistant); called a handwheel.

To fire without breaking, the clay must first be air dried. If the clay is thoroughly dry, porous and relatively soft, the pottery can be baked directly in an open fire at temperatures of 650-750 C (1202-1382 F); Various effects are achieved by oxidizing the flames (giving them adequate ventilation, to produce a great flame) or by reducing the oxygen through partially obstructing the entrance of air into the kiln. For example, a clay high in iron will typically burn red in an oxidizing fire, whereas in a reducing fire it will turn gray or black.

A pot can be decorated before or after firing. When the clay is partially dry and somewhat stiffened (leather hard), bits of clay can be pressed into the pot; the body of the vessel can be incised, stamped, or pressed with lines and other patterns; or clay can be cut out and the body pierced. The vessel walls can be smoothed by burnishing, or polishing, so that rough particles are driven inward and the clay particles are aligned in such a way that the vessel surface is shiny and smooth. (Some clays can be polished after firing.) Slip (liquefied clay strained of coarse particles) may be used: The bone-dry or partially dry pot can be dipped into slip of creamy consistency (to which color is sometimes added). Designs can be drawn with a pointed tool that scratches through the slip to reveal the body.

Glaze is a form of glass, consisting basically of glass-forming minerals (silica or boron) combined with stiffeners (such as clay and fluxes) and melting agents (such as lead)). In raw form, glaze can be applied either to the unfired pot or after an initial unglazed, or biscuit, firing. The pot is then glaze fired; the glaze ingredients must melt and become glasslike at a temperature that is compatible to that required for the clay. Many kinds of glazes are used. Some heighten the color of the body; others mask it. Alkaline glazes, popular in the Middle East, are shiny and frequently transparent. Lead glazes are transparent, with traditional types made of sand fused with sulfide or oxide of lead.

Credits:

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