Grog, also known as firesand and 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.
Contents
It is normally available as a powder or chippings, and is an important ingredient in Coade stone.
Properties
Grog is composed of: 40% minimum alumina, 30% minimum silica, 4% maximum Iron(III) oxide, 2% maximum of calcium oxide and magnesium oxide combined.
Its melting point is approximately 1,780 °C (3,240 °F). Its boiling point is over 9,000 °C (16,230 °F). Its water absorption is 7% maximum. Its thermal expansion coefficient is 5.2 mm/m and thermal conductivity is 0.8 W/(m·K) at 100 °C and 1.0 W/(m·K) at 1000 °C. It is also not easily wetted by steel.
Applications
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.
The finer the grog particles are, the closer the clay bond, and the denser and stronger the resulting 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."
In archaeology
In archaeological terminology, 'grog' is crushed fired pottery of any type that is added as a temper to unfired clay. Several pottery types from the European Bronze Age are typologised on the basis of their grog inclusions.