Plaster of Paris is a type of plaster which can be used in art, architecture, fireproofing, and medical applications. When people think of “plaster,” they are often thinking specifically of plaster of Paris, although there are a number of different types of plaster on the market including lime plaster and cement plaster. Many art and construction supply stores sell plaster of Paris, and it can also be ordered through specialty companies.
This plaster is made by calcining gypsum, a process which involves exposing the gypsum to very high temperatures to create calcium sulfate and then grinding it into a fine white powder. When water is added to the powder to make a slurry, the slurry can be molded in a variety of ways, and as it sets, a firm matrix is created, creating a solid shape which is also very smooth. One advantage to plaster of Paris is that there is no volume loss, so casts made with this plaster are true to the size of the mold.
History seems to indicate that, despite the name, plaster of Paris was invented by the Egyptians. It was used as an artistic decoration in many Egyptian tombs, and the Greeks picked up the technique, using plaster in their own homes, temples, and works of art. Paris became synonymous with this type of plaster in the 1600s, thanks to a large deposit of gypsum which made it easy to produce plaster of Paris. The substance was also used extensively in fireproofing, giving Parisian homes a distinctive appearance.
In art, plaster of Paris can be used to make sculptures, and test molds for bronze and other metal castings. Plaster can also be used to make molds which will be very dependable while withstanding high temperatures. Plaster can also be used as an architectural feature, as for example in the case of plaster moldings mounted on doorways and window frames.
As a building material, plaster of Paris can be used to increase the fire resistance of a structure. It is often applied as a coating over other construction materials like metal and wood. Doctors have historically used plaster of Paris to make smooth, hard casts for broken limbs. This material can also be utilized in criminal investigations, with plaster of Paris being used to make molds of footprints, tire treads, and other markings. Because the plaster will not shrink as it dries, it can be used to create a court-admissible cast as well as a reference which can be used in an investigation.