Gypsum Plaster of Paris

Gypsum plaster, or plaster of Paris, is produced by heatinggypsumto about 300 °F (150 °C).

Gypsum plaster, or plaster of Paris, is produced by heatinggypsumto about 300 °F (150 °C):^[2]

2CaSO₄·2H₂O +Heat→ 2CaSO₄·½H₂O + 1½H₂O (released as steam).

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.^[3]If plaster or gypsum is heated above 392°F (200°C),anhydriteis formed, which will also re-form as gypsum if mixed with water.^[4]

A large gypsum deposit atMontmartrein Paris led "calcined gypsum" (roasted gypsum or gypsum plaster) to be commonly known as "plaster of Paris".^[5]

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 expandedpolystyrene, although the job title remains unchanged.

Plaster of Paris can be used to impregnategauzebandages to make a sculpting material calledmodroc. It is used similarly to clay, as it is easily shaped when wet, yet sets into a resilient and lightweight structure. This is the material that was (and sometimes still is) used to make classic plasterorthopedic caststo protect limbs with broken bones, the medical use having been partly inspired by the artistic use (seeorthopedic cast). Set modroc is an early example of acomposite material