Setting of Plasters

When plaster is made, all of the water gets taken out of it. Mixing the plaster of water replaces this and new crystals start to grow. The crystals bind together to form a solid material. To slow down the setting, retarders are added during the manufacturing process in order to give the plasterer time to apply the material to the background.
Plaster can get fairly warm when setting due to the chemical reactions taking place. When the heat from the plaster disappears, the plaster is set.