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Plaster is a building material used for coating walls and ceilings. Plaster starts as a dry powder similar to sandpaper. These characteristics make plaster suitable for a finishing, rather than a load-bearing material.
The term plaster can refer to cement plaster.
 Gypsum plaster (plaster of Paris)
Gypsum plaster, or plaster of Paris, is produced by heating 
- CaSO4·2H2O + Heat → CaSO4·½H2O + 1½ H2O (released as steam).
When the dry plaster powder is mixed with water, it re-forms into gypsum. The setting of unmodified plaster starts about 10 minutes after mixing and is complete in about 45 minutes; but not fully set for 72 hours.
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 composite material.
 Lime plaster
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 (Whitewash is based on the same chemistry.
To make lime plaster, slaked lime (calcium hydroxide), which is sold as a wet putty or 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 calcium carbonate, causing the plaster to set.
Lime plaster was a common building material for wall surfaces in a process known as 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.
Lime plaster is used for true Pigments, diluted in water, are applied to the still wet plaster.
 Cement plaster
Cement plaster is a mixture of suitable plaster, sand, vermiculite as lightweight aggregate. Heavy versions of such plasters are also in use for exterior fireproofing, to protect LPG vessels, pipe bridges and vessel skirts.
 In architecture
Plaster may also be used to create complex detailing for use in room interiors. These may be geometric (simulating wood or stone) or naturalistic (simulating leaves, vines, and flowers) These are also often used to simulate wood or stone detailing found in more substantial buildings.
 In art
Many of the greatest mural paintings in Europe, like Michelangelo's 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.
Plaster may be cast directly into a damp clay mold. In creating this piece molds (molds designed for making multiple copies) or waste molds (for single use) would be made of plaster. This "negative" image, if properly designed, may be used to produce clay productions, which when fired in a kiln become lost wax casting, a far more expensive process. In lieu of producing a bronze image suitable for outdoor use the plaster image may be painted to resemble a metal image; such sculptures are suitable only for presentation in a weather-protected environment.
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, a process raised details. For these processes, limestone or acrylic based plaster may be employed.
 In medicine
Plaster is 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 an orthopedic cast; however, this is slowly being replaced by a fibreglass variety.
Plaster is also used within radiotherapy when making immobilization casts for patients. Plaster bandages are used when constructing an impression of the patients head and neck, and liquid plaster is used to fill the impression and produce a plaster bust. Perspex is then vacuum formed over this bust creating an immobilization shell.
In dentistry, plaster is used for mounting casts or models of oral tissues. These diagnostic and working models are usually made from dental stone, a stronger, harder and denser derivative of plaster which is manufactured from gypsum under pressure. Plaster is also used to invest or flask wax dentures, the wax being subsequently removed and replaced with the final denture base material which is cured in the plaster mold.
 In fire protection
Plasters have been in use in fireproofing products, for many decades.
The finished plaster releases water vapor when exposed to flame, acting to slow the spread of the fire, for as much as an hour or two depending on thickness. It also provides some removal and re-coating work. More modern plasters fall into the following categories:
- fibrous (including mineral wool and glass fiber)
- cement mixtures either with mineral wool or with vermiculite
- gypsum plasters, leavened with polystyrene beads, as well as chemical expansion agents to decrease the density of the finished product
One differentiates between interior and exterior fireproofing. Interior products are typically less substantial, with lower densities and lower cost. Exterior products have to withstand more extreme fire and other environmental conditions. Exterior products are also more likely to be attractively tooled, whereas their interior cousins are usually merely sprayed in place. A rough surface is typically forgiven inside of buildings as dropped ceilings often hide them. Exterior fireproofing plasters are losing ground to more costly concrete admixtures, that enable easier tooling than common mortars.
 In dentistry
dental impression using a soft, pliable material that can be removed from around the teeth and gums without loss of fidelity and using the impression to creating a wax model of the teeth and gums. The model is used to create a plaster mold (which is heated so the wax melts and flows out) and the denture materials are injected into the mold. After a curing period, the mold is opened and the dentures are cleaned up and polished.
 Safety issues
The chemical reaction that occurs when plaster is mixed with water is alginate can safely be used for casting body parts.
Some variations of plaster that contain powdered silica or asbestos may present health hazards if inhaled. Asbestos is a known irritant when inhaled in powder form can cause cancer, especially in people who smoke, and inhalation can also cause asbestosis. Inhaled silica can cause silicosis and (in very rare cases) can encourage the development of cancer. Persons working regularly with plaster containing these additives should take precautions to avoid inhaling powdered plaster, cured or uncured. (Note that asbestos is rarely used in modern plaster formulations because of its carcinogenic effects.)
 See also
|Look up plaster in Wiktionary, the free dictionary.|
- Cast Courts (Victoria and Albert Museum)
- International Union of Bricklayers and Allied Craftworkers
- Joint compound
- Passive fire protection
- Polished plaster
- Ready-mix lightweight joint compound
- Wattle and daub
- Staff. "CaSO4 , ½ H2O". LaFargePrestia. http://www.lafargeprestia.com/caso4___h2o.html. Retrieved 27 November 2008.
- Schmidt, V.E.; Somerset, J.H.; Porter, R.E. (1973). "Mechanical Properties of Orthopeadic Plaster Bandages". Journal of Biomechanics (Elsevier) 6: p173-185.
- Deer, Howie, & Zussman. "An Introduction to the Rock Forming Minerals." Pearson Education Limited, England, 2nd Edition, 1992, Page 614. ISBN 0-582-30094-0
- plaster of Paris definition. Webster's New World College Dictionary at YourDictionary.com.
- "Amputation after art class burns". BBC News. 2007-03-23. http://news.bbc.co.uk/1/hi/education/6485481.stm. Retrieved 2010-05-22.
- "Fine as art pupil loses fingers". BBC News. 2009-10-12. http://news.bbc.co.uk/1/hi/england/lincolnshire/8303246.stm. Retrieved 2010-05-22.
- Britten, Nick (2009-10-13). "Schoolgirl, 16, lost eight fingers in plaster of Paris accident during art lesson". The Daily Telegraph (London). http://www.telegraph.co.uk/news/newstopics/politics/lawandorder/6311398/Schoolgirl-16-lost-eight-fingers-in-plaster-of-Paris-accident-during-art-lesson.html. Retrieved 2010-05-22.
This article uses material from the Wikipedia article Plaster, which is released under the Creative Commons Attribution-Share-Alike License 3.0.
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