Plaster
Plaster of Paris, or simply plaster, is a type of building material based on calcium sulfate hemihydrate, nominally CaSO4*0.5H2O. It is created by heating gypsum to about 150ºC. CaSO4 · 2H2O → CaSO4 · 0.5H2O + 1. more...
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5H2O (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, initially as a paste but eventually hardening into a solid. The structure consists of sheets of Ca²+ and SO4²- ions held together by hydrogen bonds in the water molecules. The grip between these sheets is easily broken, so plaster is fairly soft.
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 then hardens. Unlike those materials 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.
Use in room interiors
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 (see below). 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 almost completely 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.
Plaster expands while hardening, then contracts slightly just before hardening completely. This makes plaster excellent for use in molds, and it is often used as an artistic material for casting. Plaster is also commonly spread over an armature (form), usually made of wire, mesh or other materials. In medicine, it is also widely used as a support for broken bones; a bandage impregnated with plaster is moistened and then wrapped around the damaged limb, setting into a close-fitting yet easily removed tube, known as a cast.
Plaster moulding is made using a sliding jig that holds a die with a cross section of the moulding.
Read more at Wikipedia.org
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