Plaster
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Plaster of Paris, or simply plaster, is a type of building material based on calcium sulfate hemihydrate (gypsum), nominally CaSO4 · 0.5H2O. It is created by heating gypsum to about 150ºC.
- CaSO4 · 2H2O → CaSO4 · 0.5H2O + 1.5H2O (released as steam).
A large gypsum deposit at Montmartre in Paris is the source of the name.[1] 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 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.
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[edit] 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.
[edit] Use in theatrical and movie sets
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.
[edit] 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.
[edit] Use in sculptural arts
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 cotta building decorations, or these may be used to create cast concrete sculptures. 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.
[edit] 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 (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 frescoes. Pigments, diluted in water, are applied to the still wet plaster. The pigments bind with the plaster as it sets.
[edit] Earthen plaster
Earthen plaster is a mixture of clay, sand and fibre. One of the most common fibres used is straw, although animal hair or dung has been a common additive over the centuries. Earthen plaster has seen a recent resurgence in its use in conjunction with strawbale building. Earthen plaster can be scuplted and colored by the type of clay used or other mineral additives. Other common additives and finishes to earthen plaster include cactus juice, casein, and linseed oil. See also earthen floors and cob.
[edit] Cement plaster
Cement plaster is a mixture of suitable plaster sand, portland cement and 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 fireproofing products, the world over. These usually use 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.
[edit] Passive fire protection
Plasters have been in use in passive fire protection, as fireproofing products, for many decades. They are subject to stringent bounding. Early versions of these plasters have used asbestos fibres, 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 wool)
- 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 intumescent and endothermic products, simply on technical merit. Trade jurisdiction on unionised construction sites in North America remains 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 firestop mortars. 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 concrete addmixtures, that enable easier tooling than common mortars.
[edit] Trade jurisdiction
In unionised construction sites in North America, plaster is installed by signatories to the OPCNIA. Unionised plasterers are represented by the International Union of Bricklayers and Allied Craftworkers (BAC).
[edit] Safety issues
The chemical reaction that occurs when plaster sets is exothermic in 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 art project in Lincolnshire, England. She subsequently had her her thumbs and most of her fingers amputated..[2]
[edit] See also
- Building construction
- Cast Courts (Victoria and Albert Museum)
- Endothermic
- Fireproofing
- Firestop
- International Union of Bricklayers and Allied Craftworkers
- Intumescent
- Italian plaster
- Joint compound
- limestone
- Passive fire protection
- Ready-mix lightweight joint compound
- stucco
- Venetian plaster
- Wattle and daub
