When gypsum is calcined to 128°C, it loses water to give the hemi-hydrate, the most commonly used form of plaster. When heated to
163°C, the hemi-hydrate in turn loses water, and anhydrous calcium sulphate is formed. Commercial practices may use slightly different temperatures, depending on what type of plaster is wanted, the type of kiln used ('kettle' or rotary) and the humidity of the atmosphere in the kiln. If gypsum is heated to significantly higher temperatures (above 700°C), it then becomes much slower to re-hydrate when suspended in water, and is known as dead-burned gypsum.