Plaster of Paris is made by heating gypsum (calcium sulfate dihydrate, CaSO₄·2H₂O) to drive off some of the water, creating calcium sulfate hemihydrate (CaSO₄·½H₂O). This hemihydrate powder is what we know as plaster of Paris. When mixed with water, it rehydrates and re-forms gypsum, hardening into a solid.