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It is rare that a clay as dug from the ground will suit our needs for forming. Typically we alter the clay by mixing it with other clays, silica, feldspar and grog or sand. We call the resulting mixture a clay body. Another clay might be added to increase the plasticity of a less plastic clay or to alter its color and texture. Silica, is added to control shrinkage and to increase the thermal expansion of the body to help make glazes "fit" the body. Feldspar or other fluxes are added to decrease the temperature at which the body will fuse (vitrify) to become water tight. Grog is added to control shrinkage and cracking as the clay dries.