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 sulfatehemihydrate, nominally CaSO4·0.5H2O. It is created by heating gypsumto about 150 °C.
:2 CaSO4·2H2O → 2 CaSO4·0.5H2O + 3 H2O (released as steam).
A large gypsum deposit at
Montmartrein Parisis the source of the name. [ [http://www.bartleby.com/61/63/P0356300.html plaster of Paris. The American Heritage® Dictionary of the English Language: Fourth Edition. 2000 ] ] 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
constructionwas 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
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.
Use in the arts
Many of the greatest paintings in Europe, like Michelangelo's
Sistine Chapel ceilingare 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
Plaster may be cast directly into a damp clay mold. In creating this "moldmolds" (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
terra cottabuilding decorations, or these may be used to create cast concretesculptures. If a plaster positive was desired this would be constructed or cast to form a durable image artwork. As a model for stonecutters this would be sufficient. If intended for producing a bronze casting the plaster positive could be further worked to produce smooth surfaces. 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. 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. Fact|date=October 2008 Plaster is often used in
Faux Finishingto create textures for wall and furniture surfaces, as in Venetian Plasterand also in stenciling for raised details. For these processes, artists use limestone based plasters or new user friendly acrylic based plaster. Fact|date=October 2008
Use 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 immobilisation 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 immobilisation shell. Fact|date=October 2008
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. Fact|date=October 2008
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). Whitewashis 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
frescoes. Pigments, diluted in water, are applied to the still wet plaster.
Cement plaster is a mixture of suitable plaster, sand,
portland cementand water which is normally applied to masonry interiors and exteriors to achieve a smooth surface. Interior surfaces sometimes receive a final layer of gypsum plaster. Walls constructed with stock bricks are normally plastered while face brick walls are not plastered. Various cement-based plasters are also used as proprietary spray fireproofingproducts, the world over. These usually use vermiculiteas lightweight aggregate. Heavy versions of such plasters are also in use for exterior fireproofing, to protect LPG vessels, pipe bridges and vessel skirts.
Passive fire protection
Plasters have been in use in
passive fire protection, as fireproofingproducts, 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 insulation to retard
heatflow into structural steelelements, that would otherwise lose their strength and collapse in a fire. Early versions of these plasters have used asbestosfibres, which have by now been outlawed in industrialised nations and have caused significant removal and re-coating work. More modern plasters fall into the following categories:
* fibrous (including
mineral wooland glass fiber)
* cement mixtures either with mineral wool or with
* gypsum plasters, leavened with
polystyrenebeads, 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
intumescentand endothermicproducts, simply on technical merit. Trade jurisdiction on unionised construction sites in North Americaremains with the plasterers, regardless of whether the plaster is decorative in nature or is used in passive fire protection. Cementitious and gypsum based plasters tend to be endothermic. Fireproofing plasters are closely related to firestopmortars. In fact, most firestop mortars can be sprayed and tooled very well, due to the fine detail work that is required of firestopping, which leads their mix designers to utilise concreteaddmixtures, that enable easier tooling than common mortars.
In unionized construction sites in North America, plaster is installed by contractors signatory to the Operative Plasterers’ and Cement Masons’ International Association (OPCMIA), which represents unionized
The chemical reaction that occurs when plaster is mixed with water is
exothermicin nature. The danger of this was illustrated in January 2007, when a sixteen-year-old girl suffered third-degree burns after encasing her hands in plaster as part of a school artproject in Lincolnshire, England. She subsequently had her thumbs and most of her fingers amputated. [ [http://news.bbc.co.uk/1/hi/education/6485481.stm BBC NEWS | Education | Amputation after art class burns ] ] For this reason only thin layers of plaster should be used, with time to cool between layers, or strips of cloth in plaster laid-up in the method used by the medical field. In place of plaster, alginatecan safely be used for casting body parts.
Some variations of plaster that contain powdered
silicaor asbestosmay present health hazards if inhaled. Asbestos is a known carcinogenwhen inhaled in powder form, especially in people who smoke, and inhalation can also cause asbestosis. Inhaled silica can cause silicosisand (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.)
Special cleanup methods should be used with of plaster products, as they can interefere with the flow of plumbing systems downstream of the disposal area. The residue of these products will often solidify underwater and plug up drains, stain gutters and sidewalks and spoil planting areas.
Plaster of Paris has been used illegally by some professional
boxers in the past, such as Luis Resto. It makes a boxer's taped hands harder.fact|date=August 2008
bassaniteis the naturally occurring compound of the formula CaSO4.0.5H2O. It may be found, for example, in some lavas.fact|date=August 2008
Cast Courts (Victoria and Albert Museum)
International Union of Bricklayers and Allied Craftworkers
Passive fire protection
Ready-mix lightweight joint compound
Wattle and daub
* [http://www.bacweb.org/ International Union of Bricklayers and Allied Craftworkers ]
* [http://www.opcmia.org/ Plasterers Union ]
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