The name plaster of Paris came from the fact that it was first of all made by heating gypsum which was mainly found in Paris. The compound plaster of paris is prepared by heating gypsum at 120oC.
2CaSO4.2H2O → (CaSO4)2.H2O + 3H2O
The chemical formula for the plaster of paris is (CaSO4)2.H2O.
The plaster of Paris used for the preparation of surgical dressings should be of the same quality as that which the dental surgeons employ in taking casts for teeth that are, the extra- calcined variety.