What is Gypsum? - Gypsum Association
Gypsum rock is mined or quarried, crushed and ground into a fine powder. In a process called calcining, the powder is heating to approximately 350 degrees F, driving off three fourths of the chemically combined water. The calcined gypsum, or hemihydrate, becomes the base for gypsum plaster, gypsum board and other gypsum products.