Plaster is most commonly used to coat the surface of the interior walls of buildings. Also known as plaster of paris, it is a mixture of solids and water that sets to a hard solid.

Plastering is one of the oldest building techniques. Early plaster was mostly mud, used to strengthen buildings against weather and animals. Over time, other materials came to replace mud. The Great Pyramids of Egypt contain plasterwork that originated over 4,000 years ago and is still durable, made of gypsum and identical to plaster of paris. Even the plastering procedures used by the Egyptians were similar to those used today.

Plaster consists of water, sand, and usually gypsum. Heating the calcium-based mineral gypsum to temperatures up to 482°F (250°C) produces a fine, white powder. When mixed with a material like sand (to provide bonding), moistened and allowed to dry it produces the hard material with which we are familiar. (Gypsum near Paris, France was early used to make such plaster, hence the name "plaster of paris.")

Such plaster has been applied to the interior of building walls, where it provides structure, a base for decoration and painting, soundproofing, is easily cleaned, and also works as a fire retardant. The ornamental ceilings of the English monarchs Henry VIII, Elizabeth I, and James I are still admired for their artistry. When applied to the outside of buildings plaster is known as "stucco," a architectural technique first used by the Greeks. Plaster is also used in casts and molds--once poured, it hardens and expands slightly, and has been used for ornamental work on ceilings and walls, sculpture, and casts for broken arms and legs.