Orthopedic cast - From Wikipedia, the free encyclopedia

A cast is a shell, frequently made from plaster, encasing a limb (or, in some cases, large portions of the body) to hold a broken bone (or bones) in place until it has healed.

Plaster bandages consist of a cotton bandage that has been impregnated with plaster of paris, which hardens after it has been made wet. Plaster of Paris is calcined Gypsum, ground to a fine powder by milling. When water is added, the more soluble form of calcium sulphate returns to the relatively insoluble form, and heat is produced.

2(CaSO4.½ H2O) + 3H2O -> 2(CaSO4.2H2O) + Heat[1]

The setting of unmodified plaster starts about 10 minutes after mixing and is complete in about 45 minutes, the cast is not fully dry though for 72 hours.[2]

Nowadays fiberglass bandages are often used. These consist of a knitted fiberglass bandage impregnated with polyurethane. These are lighter and dry much faster than plaster bandages. However, plaster can be more easily moulded to make a snug and therefore more comfortable fit. In addition, plaster is much smoother and does not snag clothing or abrade the skin. Plaster casts are generally made available only to patients who insist on them, because they take more time to apply, or when the cost of the fiberglass material is a consideration.

Full article: http://en.wikipedia.org/wiki/Orthopedic_cast