CALCIUM CARBONATE - Chalk and Whiting
Calcium Carbonate is the main source of calcium in glazes and also a flux at high temperatures. Contributes hardness and durability and in large quantities produces a matt effect. It can enhance the finish of salt glaze ware by developing a thicker finish.
Calcium Carbonate is a chemical compound with the chemical formula CaCO3. It is a common substance found in rocks all over the world, and is the main component of shells of marine organisms, snails, pearls, and eggshells. Calcium carbonate is the active ingredient in agricultural lime, and is usually the principal cause of hard water. It is commonly used medically as a calcium supplement or as an anti-acid, but excessive consumption can be hazardous.
The main use of calcium carbonate is in the construction industry, either as a building material in its own right (e.g. marble) or limestone aggregate for road building or as an ingredient of cement or as the starting material for the preparation of builder’s lime by burning in a kiln.
It is the main source of calcium in glazes and also a flux in all types of glazes, it reduces crazing and increases hardness and durability and in large quantities produces a matt effect.
Link: Wikipedia/Calcium carbonate