Plaster is a white powdered substance that is made from gypsum, a kind of stone. When water is added to plaster it stays as a wet paste for a little while but then it goes hard. As it goes hard it gets hot - plaster setting is an exothermic reaction. If you are going to put plaster on your skin you should do it slowly, in thin layers or you could get burnt. Today, plaster is most commonly used mixed with lime to line the inside walls of houses, or put into bandages to hold a broken bone in place as it heals.