Grog (clay)

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Grog (also called firesand or chamotte) is a ceramic raw material. It has high percentage of silica and alumina. It can be produced by firing selected fire clays to high temperature before grinding and screening to specific particle sizes. It can also be produced from pitchers. The particle size distribution is generally coarser in size than the other raw materials used to prepare clay bodies. It tends to be porous and have low density. It is available as a powder, mortar, or in the form of fire bricks.

Grog is used in pottery and sculpture to add a gritty, rustic texture called "tooth"; it also reduces shrinkage and aids even drying. This prevents defects such as cracking, crow feet patterning, and lamination. The coarse particles open the green clay body to allow gases to escape. It also adds structural strength to hand-built and thrown pottery during shaping although it can diminish fired strength.

It is also used in the production of terra cotta pottery. In some cases, new terra cotta is created using grog made from recycled terra cotta.

Other uses for grog is processing ZnO Metal Oxide Varistor (MOV). It is put at the base of the "blocks" on the topside of the bottom of a refractory sagger to keep the blocks from sticking to the sagger. This is because ZnO MOV has a polymer binder that upon drying acts as a glue. The grog at the base prevents gluing and acts as a vent for carbon gases while drying and sintering. The grog is a gritty form of ZnO to prevent contamination.