Plaster of Paris. A fine white powder, basically gypsum or calcium sulphate,

(CaSO4)2 ∙H2O), which has been dehydrated in an autoclave. When combined with water it forms a paste which rapidly sets and is used for casts or molds that can be further carved, shaped, coated, or processed in other ways including serving as the pattern for galvanos, coin and medal dies, or three-dimensional study copies. Plaster of Paris is more often called simply plaster, objects made of plaster are often called chalk in the antiques field. Outside of France and America it is sometimes called Italian plaster.

Such plaster has been used since ancient times. It is named plaster of Paris for the large gypsum deposits in and around Paris, these were developed commercially and shipped around the world. It was the French who developed the use of plasterwork for interior decoration, on ceilings, cornices and moldings. It was also widely used in medicine and dentistry. Artists have used it because of its immediacy, it can be carved or molded, and ideal for pattern work.

Types of plaster. Because plaster of Paris has so many uses there are many kinds, from hard wall plaster to dental and medical to uses in the arts. For the sculptor the kind of plaster employed is a matter of personal taste or need for the task at hand, as a hard plaster is excellent for sharp, crisp detail, while a soft plaster is best if the artist wishes to carve the model, particularly in the negative. The caster in the coin and medal field wants a super fine plaster in a hardness to suit his purpose.

Plaster of Paris sets fairly quickly, generally six to thirty minutes. Hydrocal,

sets 20 to 30 minutes, high strength, hard plaster, excellent for detail, comes white

or grey. Densite, sets in 20 to 30 minutes, is harder than Hydrocal. Number 1 Casting Plaster sets in 25 to 30 minutes, mixes quickly but is more difficult to carve. White Art Plaster, sets 25 to 30 minutes, softer and a little more porous. Number 1 Molding Plaster, sets 25 to 35 minutes, softest of all, porous, needs a sealer. Other plasters and plaster products have a wide range of uses and tradenames: Quickstone, Microstone, Silky Rock, Die-Rock, Graystone, Crea-Stone, and Pariscraft (a crinoline impregnated with plaster, unnecessary in medallic model work).