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Encyclopedia > Plaster of paris

This article is about the building material. For the adhesive medical dressing, see Sticking plaster.


Plaster of Paris, or simply plaster, is a type of building material based on calcium sulfate hemihydrate, nominally (CaSO4)2. H2O. It is created by heating gypsum to about 150C, 2(CaSO4 2H2O) → (CaSO<


Plaster expands while drying, 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.


Lime plaster is a mixture of calcium hydroxide and sand (or other inert fillers). Carbon dioxide in the atmosphere causes the plaster to set by transforming the calcium hydroxide into calcium carbonate (limestone). Whitewash is based on the same chemistry.


To make lime plaster, Limestone (calcium carbonate) is heated to produce quicklime (calcium oxide). Water is then added to produce slaked lime (calcium hydroxide), which is sold as a white powder. Additional water is added to form a paste prior to use. The paste may be stored in air tight containers. Once exposed to the atmosphere, the calcium hydroxide turns back into limestone, causing the plaster to set.


Lime Plaster is used for true frescos. Pigments, diluted in water, are applied to the still wet plaster. The pigments bind with the plaster as it sets.


See also


  Results from FactBites:
 
Plaster - Wikipedia, the free encyclopedia (874 words)
Plaster is used as a building material similar to mortar or cement.
Plaster was a common building material for wall surfaces in a process known as lath and plaster, in which a series of wooden strips were covered with a semi-dry plaster and then hardened into a flat surface.
An advantage of this plaster image is that it is relatively cheap; should a patron approve of the durable image and be willing to bear further expense, subsequent molds could be made for the creation of a wax image to be used in lost wax casting, a far more expensive process.
ArtLex on Plaster (998 words)
Plaster can be painted when dry, or mixed with pigments while still wet, as in fresco painting.
The term plaster is sometimes used as a synonym for stucco, which is the finest and whitest type of plaster used for modeling and molding.
In order to bond plaster to a flat surface such as a sheet of plywood, obtain a strong wire-screen material, and staple it to the wood, intentionally leaving gaps between some of the wood and the screen.
  More results at FactBites »

 
 

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