PLHYS

Hydrostone is a gypsum cement that sets to a dense, close-grained mass with a hard, fine surface. It holds very fine detail and stands up to being handled and worked against.

Hydrostone is mixed stiff and pours thick, and it sets hard, taking a fine surface that reproduces small detail and takes knocks and rubbing without breaking up. It goes into models, patterns and master shapes that will be worked against repeatedly, and into solid castings that have to be handled. The setting is a chemical reaction rather than a drying: the powder takes water back on and grows interlocking crystals through the mass. That low-water set is what leaves it dense with a hard face, and why it belongs in models and castings rather than in a working slip mould, which has to draw water through itself.