Plaster of Paris is the most recognized name for casting plaster, but the two terms are used interchangeably. There are several different types of plaster that can be used for a variety of casting.
Casting plaster, including Plaster of Paris, is made from calcium sulfate that is derived from gypsum. Extreme heat is used to leach the water from the gypsum and create a fine powder that, when mixed with water, produces a cement-like material.
The name Plaster of Paris first came about in Paris, where there is a large deposit of gypsum. In the 1700s, the king of France ordered that every wooden building be coated in plaster to provide fire deterrent. Gypsum plaster was used as early as ancient Egypt, according to “A Brief History of Plaster and Gypsum.”
Different grades of casting plaster are manufactured for different purposes. Each grade requires a different ratio of water to plaster and will have different drying times.
Casting plaster and art plaster, a different grade of plaster, are best used in three-dimensional casting. Pottery plaster—again, a different grade—is commonly used to make molds for ceramics, according to an article in "Old-House Journal." Orthopedic casts, now made from fiberglass, used to use gypsum plaster.
Casting plaster is manufactured by many different companies and sold under a variety of names. The most common name is Plaster of Paris, but it can also be found as gypsum plaster, casting plaster and art plaster.