Gypsum Plaster

Gypsum plaster is a cementitious finishing material made by heating gypsum rock to drive off most of its chemically bound water, producing a powder that, when mixed with water, sets quickly into a smooth, hard, fire-resistant surface for interior walls and ceilings. Known since antiquity, notably for the plaster of Paris named for the extensive gypsum deposits beneath that city, it became a standard interior finish material across much of the world because it can be applied over lath, masonry or modern gypsum board and worked into smooth or decorative ornamental surfaces such as cornices and ceiling roses. Its fire resistance comes from the water chemically bound in its crystal structure, which is driven off as steam when the material is exposed to flame, slowing heat transfer through the wall or ceiling assembly.