The term plaster can refer to plaster of Paris, lime plaster, or cement plaster. This article deals mainly with plaster of Paris. Plaster of Paris's a type of building material based on calcium sulfate hemihydrate, nominally CaSO4·H2O. It's created by heating gypsum to about 150 °C. » 2 CaSO4·2H2O → 2 CaSO4·0.5H2O + 3 H2O (released as steam). A large gypsum deposit at Montmartre in Paris's the source of the name. When the dry plaster powder's mixed with water, it re-fo… (More on Plaster)