Early 19th Century plasterer at work - painting by John Cranch (1751–1821).
Plaster or render is a building material used for coating walls and ceilings; in English "plaster" usually means a material used for the interiors of buildings, while stucco is used for the very similar material formulated for applying to exteriors, which normally requires a protective coating of paint that is regularly maintained. "Stucco" is also often used for plasterwork that is worked in some way to produce relief decoration, rather than flat surfaces.
The most common types of plaster mainly contain either gypsum, lime, or cement, but all work in a similar way. The plaster is manufactured as a dry powder and is mixed with water to form a stiff but workable paste immediately before it is applied to the surface. The reaction with water liberates heat through crystallization and the hydrated plaster then hardens.
Plaster can be relatively easily worked with metal tools or even sandpaper, and can be moulded, either on site or to make pre-formed sections in advance, which are put in place with adhesive. Plaster is not a strong material; it is suitable for finishing, rather than load-bearing, and when thickly applied for decoration may require a hidden supporting framework, usually in metal.
Forms of plaster have several other uses, for example in medicine plaster orthopedic casts are still often used for supporting broken bones. Various types of models and moulds are made with plaster. In art, lime plaster is the traditional matrix for fresco painting; the pigments are applied to a thin wet top layer of plaster and fuse with it, so that the painting is actually in coloured plaster. In the ancient world, as well as the sort of ornamental designs in plaster relief that are still used, plaster was also widely used to create large figurative reliefs for walls, though few of these have survived.