"a large vat of plaster of Paris"

Classical figure in plaster of Paris
Creative Commons Attribution Share AlikeClassical figure in plaster of Paris - Credit: Afsar Naqvi
Gypsum plaster, or Plaster of Paris, is employed by mixing water with calcium sulfate hemihydrate, which is produced by heating gypsum to about 150 °C.  When the dry plaster powder is mixed with water, it re-forms into gypsum.

A large gypsum deposit at Montmartre in Paris led gypsum plaster to be commonly known as "plaster of Paris."