for: Adhesive bandage

The term plaster can refer to plaster of Paris, lime plaster, or cement plaster. This article deals mainly with plaster of Paris.

Plaster of Paris is a type of building material based on calcium sulfate hemihydrate, nominally CaSO₄·0.5H₂O. It is created by heating gypsum to about 150 °C.

2 CaSO₄·2H₂O → 2 CaSO₄·0.5H₂O + 3 H₂O (released as steam).

A large gypsum deposit at Montmartre in Paris is the source of the name. When the dry plaster powder is mixed with water, it re-forms into gypsum. Plaster is used as a building material similar to mortar or cement. Like those materials plaster starts as a dry powder that is mixed with water to form a paste which liberates heat and then hardens. Unlike mortar and cement, plaster remains quite soft after drying, and can be easily manipulated with metal tools or even sandpaper. These characteristics make plaster suitable for a finishing, rather than a load-bearing material.

Plaster was a common building material for wall surfaces in a process known as lath and plaster, whereby a series of wooden strips are covered with a semi-dry plaster and then hardened into surface. The plaster used in most lath-and-plaster construction was mainly lime plaster. Lime plaster cure time is about a month. To stabilize the lime plaster during curing, small amounts of Plaster of Paris were mixed into the putty. Because Plaster of Paris sets quickly, "retardants" were used to slow setting time enough to allow workers to mix large working quantities of lime putty plaster. A modern form of this method uses expanded metal mesh over wood or metal structures, which allows a great freedom of design as it is adaptable to both simple and compound curves. Today this building method has been partly replaced with drywall, also composed mostly of gypsum plaster. In both these methods a primary advantage of the material is that it is resistant to a fire within a room and so can assist in reducing or eliminating structural damage or destruction provided the fire is promptly extinguished.

One of the skills used in movie and theatrical sets is that of "plasterer", and the material is often used to simulate the appearance of surfaces of wood, stone, or metal. Nowadays, plasterers are just as likely to use expanded polystyrene, although the job title remains unchanged.

Use in architecture

Use in the arts

Many of the greatest paintings in Europe, like Michelangelo's Sistine Chapel ceiling are executed in Fresco, meaning they are painted on a thin layer of wet plaster, called intonaco (in fact the general term for plaster in Italian); the pigments sink into this layer so that the plaster itself becomes the medium holding them, which accounts for the excellent durability of fresco. Additional work may be added a secco on top of the dry plaster, though this is generally less durable. Fact: date=October 2008