Grog (clay)

Grog, also known as firesand and chamotte, is a raw material usually made from crushed and ground potsherds, reintroduced into crude clay to temper it before making ceramic ware.

[1] Grog is used in pottery and sculpture to add a gritty, rustic texture called "tooth"; it reduces shrinkage and aids even drying.

Adding grog to clay serves two primary functions: 1) It helps prevent cracking of the clay when the ceramic piece is being worked and when it dries, by reducing its plasticity; 2) it protects the ceramic piece from thermal shock while firing, particularly, at the sudden rise or lowering of temperature, and which, if not added, can cause breakage.

[4] In Middle and South Europe, grog is used to create fire-resistant chamotte type bricks and mortar for construction of fireplaces, old-style and industrial furnaces, and as component of high temperature application sealants and adhesives.

In archaeology, "grog" is crushed, fired pottery of any type that is added as a temper to unfired clay.

The practice of adding grog to clay as a temper was widespread throughout many cultures and is mentioned in the writings of Hai Gaon (939–1038), who wrote in his commentary on the Mishnah, compiled in 189 CE: "ḥarsit [= grog], that which they grind [of potsherds] and make therewith clay is called [in Hebrew] ḥarsit.