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The term plaster can refer to gypsum plaster (also known as plaster of Paris), lime plaster, or cement plaster.
Plaster of Paris is a type of building material based on calcium sulfate hemihydrate, nominally CaSO40.5H2O. It is created by heating gypsum to about 150 °C.
2 CaSO42H2O 2 CaSO4·0.5H2O + 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.
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.
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. Various cement-based plasters are also used as proprietary spray fireproofing products. These usually use vermiculite as lightweight aggregate.
Usage of Plaster in Art
Many of the greatest paintings in Europe, like Michelangelo's Sistine Chapel ceiling are 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.
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