Grog is used in
pottery and
sculpture to add a gritty, rustic texture called "tooth"; it reduces shrinkage and aids even drying. This prevents defects such as cracking, crows feet, patterning, and
lamination. The coarse particles open the green clay body to allow gases to escape. Grog adds structural strength to hand-built and thrown pottery during shaping, although it can diminish fired strength. The finer the particles, the closer the clay bond, and the denser and stronger the fired product. "The strength in the dry state increases with grog down as fine as that passing the 100-
mesh sieve, but decreases with material passing the 200-mesh sieve." About 20% grog is added to crude clay (in the dry form) before mixing with water. 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. Substitutes for grog used in pottery are dried and sifted horse manure, or sand collected from dry riverbeds (which has been sifted through a screen), or finely ground
schist. Others make use of volcanic ash. Some natural clays already contain an admixture of some "natural temper," for which cause the potters who make use of the clay do not add any temper of their own. 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. A typical example of domestic use is a
pizza stone made from chamotte. Because the stone can absorb heat, you can bake pizza or bread on the stone in a regular domestic oven. The advantage is supposed to be a more even heat. A normal commercial domestic oven cools down when the door is opened. The stone however remains hot, creating a more even bake. Another advantage is the fact that the stone can absorb some moisture making for a drier bake. ==Archaeology==