Plaster of Paris is named after the hill beneath Montmartre
The white powder in a plaster cast gets its name from a big gypsum deposit under Montmartre, the hill in northern Paris. Heat that gypsum in a kiln, grind it, add water, and it sets hard again, a trick builders have used for at least 5,000 years.
Whether based on gypsum, lime or cement, plaster behaves the same way. It is sold as a dry powder, mixed with water into a stiff paste just before use, and gives off heat as it crystallises and hardens. It is easy to shape while wet, which makes it ideal for smooth interior walls, ceilings and decorative mouldings, though it is not meant to bear loads and thick ornaments may need a hidden frame. In English, plaster usually means the indoor material; render covers outside walls, and stucco is a different mixture again.
It was first used for building and decoration in the Middle East. Egyptians burned gypsum in open fires, powdered it and mixed it with water to make mortar between pyramid blocks and a smooth facing on walls. In ancient Jericho, human skulls were coated in plaster and painted to look lifelike. The Romans spread plaster-work through Europe.
Chemically, plaster of Paris is calcium sulfate hemihydrate, made by heating gypsum to around 120 to 180 °C; gypsum itself holds twice as much water in its crystals. Beyond walls, plaster sets broken bones in orthopaedic casts, forms dental models poured into impressions, and supplies the wet lime layer of fresco painting, where pigments fuse into the surface so the picture is literally coloured plaster.
Humbler recipes used clay, sand, water and plant fibres. American colonists plastered their houses with clay; one Massachusetts contract from about 1675 called for lime and hair over clay. At Old Economy Village, an early 19th-century utopian settlement in Pennsylvania, builders split oak lath by hand and mixed animal hair, manure and straw into clay plaster. Manure added fibre and a protein glue, though moisture weakens it. Clay lost out to lime and then gypsum as kilns improved, yet some of it still clings to lath on rusty square nails centuries later.
Source: Plaster
