Description Hydro-Stone is a gypsum cement that is hard and strong, especially suitable where high strength and resistance to water absorption are necessary. It cannot be worked under a template. Used in high-quality art novelty and statuary castings. Should not be cast in glue molds, but works well in plaster and other flexible molding compounds. It is mixed at a ratio of 32 parts of water to 100 parts of hydro-stone by weight. 50 lb Bag Physical Properties.