Plaster of Paris, quick-setting gypsum plaster consisting of a fine white powder (calcium sulfate hemihydrate), which hardens when moistened and allowed to dry. Known since ancient times, plaster of Paris is so called because of its preparation from the abundant gypsum found near Paris. Plaster of Paris is prepared by heating calcium sulfate dehydrate, or gypsum, to 120–180 °C (248–356 °F). With an additive to retard the set, it is called wall, or hard wall, plaster, which can provide passive fire protection for interior surfaces. Because Plaster of Paris sets quickly, “retardants” were used to slow setting time enough to allow workers to mix large working quantities of wall putty. A modern form of this method uses expanded metal mesh over wood or metal structures, which allows a great freedom of design as it is adaptable to both simple and compound curves. Commercial uses include mixing plaster of Paris with wall putty to give strength. Can also be used to level and fill cracks in wall. Other uses include being used for molding and casting decorative items, giving aesthetic finishing touch to buildings, art and medical uses.