Calcined gypsum caso. Ho is prepared by heating the naturally occurring gypsum Caso, 2Hzo. If mixed water, the hemihydrate rehydrates to the dihydrate, the reaction being accompanied by evolution of heat, expansion and a stiffening of the mass to rigidity. Plaster is used in making models and moulds, since it is cheap and can easily converted from a powder into a hard mass of the desired shape. The setting time of a plaster-water mixture can be lengthened or shortened by addition of various chemicals to the plaster. Where an original model of an article is to be made prior to making working mould, it is desirable thatthe setting time should be extended so that more time is available for modelling: t can be done making suitable retarder by additions to the plaster. in considering the dimensions of these master models, it is necessary to take into account the expansion of the plaster in setting (which can also be adjusted by chemical and the drying and firing addition). contractions of the body to be used.