what is plaster of paris

Plaster of Paris is a quick-setting gypsum plaster consisting of a fine white powder (calcium sulfate hemihydrate) that hardens when moistened and allowed to dry. It is named after the abundance of gypsum found near Paris. Plaster of Paris is commonly used to precast and hold parts of ornamental plasterwork placed on ceilings and cornices, and it does not generally shrink or crack when dry, making it an excellent medium for casting molds. It is also used in medicine to make plaster casts to immobilize broken bones while they heal, though many modern orthopedic casts are made of fiberglass or thermoplastics. Some sculptors work directly in plaster of Paris, as the speed at which the plaster sets gives the work a sense of immediacy and enables the sculptor to achieve the original idea quickly. Plaster of Paris is also used in the construction industry to create exquisite works of art for adorning and enhancing buildings and other structures, such as wood and stone imitations. It is also used for fireproofing and fire prevention systems in residential and commercial buildings, as it releases water vapor when ignited, slowing the spread of the fire.