temper

A temper is a non-plastic material added to clay to prevent shrinkage and cracking during drying and firing of vessels made from the clay. Tempers may include: Bone; Chaff; Charcoal; Ground schist; Wood ash; Grit; Sand or crushed sandstone; Crushed limestone; Crushed igneous rocks, such as volcanic rock, feldspar, or mica; Grog; Plant fiber; Horse manure (dried and sifted); Crushed mollusc shells (including fossilized) (see Shell tempering in the Mississippian culture); and Freshwater sponge spicules. Some clays used to make pottery do not require the addition of tempers. Pure kaolin clay does not require tempering. Some clays are self-tempered, that is, naturally contain enough mica, sand, or sponge spicules that they do not require additional tempering. See also Ceramic#Archaeology