Description
Gypsum plaster
Gypsum plaster, or plaster of Paris, is produced by heating gypsum
to about **0 °F (**0 °C):[*5]
CaSO4·2H2O + heat → CaSO4·0.5H2O + 1.5H2O (released as steam).
When the dry plaster powder is mixed with water, it re-forms into
gypsum. The setting of unmodified plaster starts about *0 minutes
after mixing and is complete in about *5 minutes; but not fully set
for *2 hours.[*6] If plaster or gypsum is heated above **6 °F (**0
°C), hemihydrate is formed, which will also re-form as gypsum if
mixed with water.
On heating to **0 °C, the nearly water-free form, called
γ-anhydrite (CaSO4·nH2O where n = 0 to 0.*5) is produced.
γ-Anhydrite reacts slowly with water to return to the dihydrate
state, a property exploited in some commercial desiccants. On
heating above **0 °C, the completely anhydrous form called
β-anhydrite or dead burned plaster is formed.
A large gypsum deposit at Montmartre in Paris led "calcined gypsum"
(roasted gypsum or gypsum plaster) to be commonly known as "plaster
of Paris".
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.
Plaster of Paris can be used to impregnate gauze bandages to make a
sculpting material called plaster bandages. 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 plaster orthopedic casts
to protect limbs with broken bones, the artistic use having been
partly inspired by the medical use (see orthopedic cast). Set
Modroc is an early example of a composite material. The hydration
of plaster of Paris relies on the reaction of water with the
dehydrated or partially hydrated calcium sulfate present in the
plaster.
