Gypsum has been used throughout the world for
thousands of years as an interior finish material for walls and
ceilings. Plaster made from gypsum was used to finish the walls
inside the Pyramids of Giza, in Egypt 4500 years ago.
Gypsum is a very common mineral, with thick and extensive evaporite beds in association with sedimentary rocks. Gypsum is deposited in lake and sea water, as well as in hot springs, from volcanic vapors, and sulfate solutions in veins.
Because the gypsum from the quarries of the Montmartre district of Paris has long furnished burnt gypsum used for various purposes, it is commonly known as plaster of Paris.
Heating gypsum to between 100°C and 150°C (302°F) partially dehydrates the mineral by driving off exactly 75% of the water contained in its chemical structure. . The partially dehydrated mineral is called calcium sulfate hemihydrate or calcined gypsum.
In contrast to most minerals, which when rehydrated simply form liquid or semi-liquid pastes, or remain powdery, calcined gypsum has an unusual property: when mixed with water at normal temperatures, it quickly reverts chemically to the preferred dihydrate form, while physically "setting" to form a rigid and relatively strong gypsum crystal lattice: This phenomenon is responsible for the ease with which gypsum can be cast into various shapes including sheets (for drywall), sticks (for blackboard chalk), and molds (to immobilize broken bones, or for metal casting).
Various forms of gypsum plaster are
commercially available and can be mixed with different pigments and
sands to create an unlimited selection of interior finishes
and can be applied over any solid substrate.