Plaster of Paris was first named for a gypsum deposit that was located near Paris in France. While gypsum is a hydrated salt, the plaster is the result of reformed gypsum. It was eventually discovered that when mixed with water, the plaster becomes hard, durable and resistant to temperature changes and water. Today, it is used for art, construction, casts and pottery.
When gypsum powder is mixed with water, it creates a plaster. This plaster can be developed in softer and harder forms and is used today for setting casts on bone fractures. To achieve better strength, the plaster must be allowed to dry fully before being manipulated. The crystal structure of the plaster determines how strong it will be. Temperatures up to 100 degrees C (212F), can be used to increase the rate at which the plaster hardens. Additionally, the final strength is dependent on how thick the plaster bandage is on the cast area. Plaster of Paris is beneficial because its strength helps protect a fractured bone from additional pressure that may occur from daily activities. It also gives stability to keep the bone in place so that it will heal properly.
The water resistant nature of plaster of Paris makes it ideal for pottery and medical casts. For pottery that will be used -- such as bowls, cups, mugs or plates -- the water resistance ensures that liquids will not seep into the pores of the dish and cause it to crack. Water resistance also makes the dish microwave and dishwasher safe, as well as washable. For medical casts, this water resistance prevents moisture from getting in between the cast and the skin, which can lead to infections. The cast would lose firmness and strength if the plaster absorbed water.
Softer forms of plaster of Paris are used in construction. Once the joists are built and plasterboard is installed, the plaster is smoothed on the wall to level holes from nails and to seal joints. The softness of plaster of Paris makes it easily spreadable onto the wall. Once it has dried, workers use sandpaper to smooth the surface before painting. This creates the flush walls that we see inside houses. Plaster of Paris stops moisture from paint getting into nail holes and joints, which can lead to softening and deterioration.