Grog is fired clay ground into sand-like particles and mixed into a clay body. It gives the clay tooth and strength, helps it dry and fire more evenly, and reduces shrinkage, cracking and warping.
What Is Grog?
Because grog has already been fired, it doesn’t shrink again. Mixed into a body, it forms a skeleton that holds the clay in shape and opens tiny channels that let water escape as the piece dries. That is why grogged clays are favoured for large pieces, sculpture, slab work and beginner wheel throwing.
Grog comes in grades from fine, almost like flour, to coarse, like gritty sand. Fine grog barely changes the feel of a throwing clay, while coarse grog gives a rough surface that is great for sculpture but hard on your hands at the wheel. Clay data sheets list the grog size and percentage.
The trade-off is surface. Grogged clay can feel scratchy, show texture through thin glazes, and the particles can trap air and cause pinholes. Many potters smooth the surface with a rib at leather hard or use a fine slip over grogged clay where they want a smooth finish, such as the inside of a mug.
This pottery glossary entry from Le Studio Art Space is general educational content. Always follow the clay, glaze and kiln makers’ data for your own materials and equipment.