Ultracal 30 Plaster

Ultracal 30 changes size very little between the wet pour and the finished piece, and patternmaking depends on that. It is used for master models, block and case work, dies, and the rigid jackets that back a flexible mould. In all of those, any change during setting would be passed on to everything cast from the piece afterwards. It also stays workable for a good while after mixing, so a jacket or a case can be built up over a model without rush. Setting is a chemical reaction, not drying. When the powder takes water on it begins growing interlocking crystals, and the mix warms as that happens.