Plaster of Paris is a building material that consists of a fine, white-colored powder, known as calcium sulfate hemihydrate. The term 'plaster' encompasses plaster of Paris, lime plaster, and cement plaster. It is believed to be used since ancient times; the oldest traces were found in Anatolia and Syria, and were estimated to be about 9000 years old. The Egyptians, Greeks, and Romans used it for sculpting their tombs and other monuments, as well as in buildings for fireproofing.
It is obtained by heating gypsum or calcium sulfate dihydrate to about 140°C - 180°C. When gypsum is heated to such a temperature, it loses three-quarters of its water content, leading to the formation of Plaster of Paris. The process of making it is reversible, i.e., if dry plaster powder is mixed with water, it re-forms gypsum. The particular name of this material is derived from the large deposits of gypsum found at Montmartre hill in Paris.
It is more or less similar to other building substances like mortar and cement; a paste can be made by adding water, which liberates heat and eventually hardens. When it is mixed with water molecules, there is hydrogen bonding between the calcium and sulfate ions. Hence, the resulting plaster is soft and smooth. While using this material for a specific purpose, its paste is molded into a desired shape before it hardens. This is a major advantage as there is no volume loss after drying, and hence the mold and cast are of the same size.
As this plaster remains soft (to some extent) even after drying, it can be easily manipulated by using sandpaper or metal. Hence, it is used for construction purposes, either by mixing with other substances, or as a finishing material. However, it is widely used in home décors; various designs can be made on the walls and ceilings. For many modern sculptors, it is a favorite sculpting material, mainly because of its easy occurrence and settling speed. In addition to arts and architecture, it is also used in orthopedics to make smooth casts for broken limbs. In forensic science, molds of footprints and other important markings are made by using plaster of Paris for further investigation.
Its properties can be studied with the help of its homemade version, which can be a good project for preschool and elementary students. You can try to prepare it at home by several methods. The easiest ones consist of making it either with white glue or flour. In the former case, prepare a paste of white glue and warm water. Make sure to add the water gently by stirring continuously; stop adding it when the mixture turns into a paste form. Your homemade plaster is ready, and you can create different designs out of it. In the flour method, mix white flour and water, so that a thick substance is formed, which is your own variety of this material. You now can make masks and bubble paint from such examples of homemade plaster of Paris.