If you reheat plaster as a stone it would crumble into water, but if you grind it before reheating it, it will not crumble into water, on the contrary it will harden in water, if it is good plaster like the one from Paris or from Spain which is as hard as a stone, and which is found through poor and dry earths, and which looks like white salt. German people use this plaster to make statues for their fountains, even if the statues are varnished they will not be damaged with water. On the contrary harden this plaster with water. When this sand is unmixed it sets much sooner. Oil very lightly your shapes of metal or of other materials otherwise you will not be able to strip plaster from the mold. And dip your mold into hot water in order to open it, if it does not open in hot water, it could open into boiling water.