<P> When the dry plaster powder is mixed with water, it re-forms into gypsum . The setting of unmodified plaster starts about 10 minutes after mixing and is complete in about 45 minutes; but not fully set for 72 hours . If plaster or gypsum is heated above 266 ° F (130 ° C), hemihydrate is formed, which will also re-form as gypsum if mixed with water . </P> <P> On heating to 180 ° C, the nearly water - free form, called γ - anhydrite (CaSO nH O where n = 0 to 0.05) is produced . γ - Anhydrite reacts slowly with water to return to the dihydrate state, a property exploited in some commercial desiccants . On heating above 250 ° C, the completely anhydrous form called β - anhydrite or dead burned plaster is formed . </P> <P> A large gypsum deposit at Montmartre in Paris led "calcined gypsum" (roasted gypsum or gypsum plaster) to be commonly known as "plaster of Paris". </P> <P> Plasterers often use gypsum to simulate the appearance of surfaces of wood, stone, or metal, on movie and theatrical sets for example . Nowadays, theatrical plasterers often use expanded polystyrene, although the job title remains unchanged . </P>

How did plaster of paris get its name
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