The scientific classification of plaster of paris is calcium sulfate hemihydrate, which has the notation CaSO4, ½ H2O. You can fire this substance at a low temperature and change its composition to powder. This powdered form can then be rehydrated with water.
Plaster of paris has roots that go back more than 9,000 years, where it is found on remnants in Syria and Anatolia. Archaeological finds in Egypt show that 5,000 years ago Egyptians placed gypsum into fires, which transformed it into a dry substance that could be crumbled into a fine powder. They used this form of plaster of paris to join the blocks in their monumental structures.
Plaster of paris gets its name from the deep deposits of gypsum that are located around Paris, France. These resources influenced local design, as builders coated wooden walls of Parisian homes in plaster as a fire retardant. By the 18th century, Paris was known as the capital of plaster.
Plaster of paris is versatile and used for many projects. You can paint dry plaster, or dye it while it's wet. Due to its porous surface upon drying, most artists seal it with linseed oil, wax or shellac. Builders use plaster of paris as a base with other substances to create stucco and a form of adobe. Architects mold and emboss plaster of paris to create moldings. Dentists and doctors use it to create crowns, bandage wounds and support broken bones.
Dry plaster of paris is a thick, heavy white powder. When you mix the powder with water, it creates a thick paste. This paste sets quickly but remains pliable. As plaster of paris dries, it expands, but when it completely dries, it shrinks again.
Airborne particles from plaster of paris are extremely hazardous. People use a dust mask and goggles to protect their faces, eyes, mucous membranes and lungs while working with it in dry form. Cleaning up after using plaster of paris requires special care, because it can solidify inside of pipes and drains.