Temper refers to solid components that a potter deliberately added to the clay (as opposed to inclusions, which are presumed to have existed naturally in the clay). A temper is added to clay to prevent shrinkage and cracking during drying and firing of vessels. Tempers may include:

    Sand
    Grit
    Chalk
    Grog (chamotte - crushed shards)
    Crushed stone (sandstone, limestone, igneous rocks, feldspar, mica, schist)

    Crushed mollusc shells (including fossilized)

    Bone
    Chaff
    Charcoal
    Wood ash
    Plant fiber
    Horse manure (dried and sifted)
    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.

Clay color and evidence of the temper used can identify the source of ancient pottery and aid in authentication.