plaster of Paris

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Any of a group of gypsum cements, essentially hemihydrated calcium sulfate, CaSO₄· ¹/₂ H₂O, a white powder that forms a paste when it is mixed with water and then hardens into a solid, used in making casts, molds, and sculpture.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A hemihydrate of calcium sulfate, made by calcining gypsum, that hardens when moistened and allowed to dry; used to make casts, molds or sculpture