What is plaster stuff?
Plaster is calcium sulphate hemihydrate. Gypsum is the dihydrate, and is roasted to extract the water of crystallization and ground. Certain plasters contain additives to improve texture, adhesion etc. Old plaster (even a couple of months) is best thrown away, as it either sets very fast, or not at all. Confusingly, in some parts of the world “plaster” is a term used to describe a sand/cement render.
