Grog is clay which has been fired then ground up. Grog can come in many particle sizes, from fine to coarse. It is used to reduce shrinkage in clay bodies. The best size of grog for larger sculptuures, raku clays, or heavy thrown work is a 20 mesh.

This means that the grog is small enough to go through a screen with 20 openings per inch, but is to large to go through a screen with say 35 openings per inch. Commercially available grog almost always has some iron in it and may not be suitable if you want a pure white clay body. For throwing or hand building, grog in a clay body reduces the shrinkage and makes it less likely that a piece will crack during drying and firing.