Plaster of Paris is a calcium sulfate hemi-hydrate : (CaSO4, �½ H2O) derived from gypsum, a calcium sulfate dihydrate (CaSO4 , 2 H2O), by firing this mineral at relatively low temperature of 150o C and then reducing it to powder. In ancient times, in Paris, all the walls of wooden houses were covered with plaster as protection against fire. Since then the plaster was named as â??Plaster of Parisâ??. POP powder is mixed with water to form a paste which releases heat and then hardens once dried under normal temperature. Unlike mortar and cement, plaster remains quite soft after drying, and can be easily rubbed or scratched with metal tools or even sandpaper