Into the mould left by the saint's body liquid plaster of Paris was run, and a perfect model obtained, showing the features of the youth, the cords which bound him, and even the texture of his clothing.
To produce plaster of Paris European practice consists in baking the mineral in ovens, and in America in heating it in kettles.
The tools used are extremely primitive - hollow iron blowing-rods, solid rods for holding vessels during manipulation, spring tools, resembling sugar-tongs in shape, with steel or wooden blades for fashioning the viscous glass, callipers, measure-sticks, and a variety of moulds of wood, carbon, cast iron, gun-metal and plaster of Paris (figs.
This may be almost chemically pure, when it is generally used for Keene's cement; or it may contain smaller or greater quantities of impurities, in which case it is suitable for the preparation of cements of the plaster of Paris class.
The principles which govern the preparation and setting of the other class of calcium sulphate cements, that is, cements of the Keene class, are not fully understood, but there is a fair amount of knowledge on the subject, both empirical and scientific. The essential difference between the setting of Keene's cement and that of plaster of Paris is that the former takes place much more slowly, occupying hours instead of minutes, and the considerable heating and expansion which characterize the setting of plaster of Paris are much less marked.
Calcium sulphate, CaSO 4, constitutes the minerals anhydrite (q.v.), and, in the hydrated form, selenite, gypsum (q.v.), alabaster (q.v.), and also the adhesive plaster of Paris (see Cement).