Science Fair Project Encyclopedia
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 150°C, 2(CaSO4 · 2H2O) → (CaSO4)2 · H2O + 3 H2O (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 drying into a solid. The structure is made up 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 dries into a hard surface. Unlike those materials plaster remains quite soft after drying, and can be easily manipulated with metal tools or even sandpaper.
Use in room interiors
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. 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 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, known as a cast.
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.
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 sculptural arts
Plaster may be cast directly into a damp clay mold. In creating this mold the sculpture will be working directly "in the negative". This method requires substantial skill and experience but is quite fast and is suitable for producing shallow relief decorations.
Plaster is also often used as an intermediate stage in the production of large cast sculptures (typically of cast bronze) or in the creation of carved stone, particularly for building decoration. The original work is usually first modeled in wet clay over a supporting structure called an armature. From this either 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 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.
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.
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