Wonder, Walls and Windows: History of Gypsum and Plaster of Paris
by Plaster of Paris is named both for the large gypsum deposits near Montmartre, a butte in the north end of Paris, and for its use to fireproof Paris homes after the London fire of 1666. It is used to make decorative architectural features such as crown molding, as a fireproof coating for houses, as a bonding agent in building with brick and stone, to make figurines and other home decor, and to prime artist’s canvasses in a form called gesso.
Plaster of Paris is a fine powder that has been in use since ancient times. Plaster is made from gypsum that has been heated to between 140 degrees Fahrenheit and 180 degrees. This removes 75% of its water content. Plaster can then be ground, mixed with water and a softening agent, then cast into molds or spread on walls. It has been used for everything from making death masks to setting bones.
Renders and Daubs
Fill cracks and seal walls with plaster. Between 9000 and 5000 year ago, plaster was used in renders and daubs to cover walls, fill and seal cracks, and to bond bricks and stones. The builders of the Great Pyramid of Cheops used plaster to bind the stones in place. Renders combine gypsum powder with sand and hair or other fiber to make a coarse, thick coating to cover a surface. Apply render over a frame of sticks, much like the lath that was used to hold plaster on walls before the invention of drywall. Daub is used to fill cracks and crevices. It is finer than render and contains more gypsum, no sand and very little fiber.
Windows of the Gods
Make windows. The Greeks used the clear form of gypsum, called selenite, to make temple windows. Selenite looks like cloudy glass. Selenites, in Greek, means “moonstone.” Selene is the goddess of the moon, also known as Artemis or Diana.
Gesso
Create fine art and decorate your home. Gesso, used extensively during the Renaissance in the 1500s, is a form of plaster that has been superhydrated until it will not set. Heat gypsum, grind it into powder, and mix it with water. Stir vigorously the entire time to prevent it from settling. After a day of being stirred, the water and gypsum powder must be left alone for an hour. Pour off the old water and continue the mixing process. Gesso is usually mixed over 30 days or until the water from the final settling runs clear. Use gesso to prime canvas, create a surface for gilding, and for sculpting. It can also be cast to make decorative moldings in place of wood carvings, or used to make the molds themselves.
Fireproofing
Fireproof your home. On September 2, 1666, the Great Fire of London swept through, destroying most of the city after burning for three days. Fearing a similar disaster, the King of France ordered that all Paris buildings be given a coating of plaster to serve as a fireproofing agent to prevent the same tragedy happening in France. Because plaster contains a great deal of water, it is difficult to burn. Until the water evaporates from the heat of the fire, gypsum board will only reach 212 degrees Fahrenheit, well below the temperature needed for combustion.
Cave of Swords
Stare in awe at the breathtaking beauty of an entire room of house-sized gypsum crystals. In 1910, selenite gypsum crystals were discovered growing in the Naica Mine in Chihuahua, Mexico, in the Cave of Swords. Some of the crystals in this cave are over 30 feet long. It is very challenging to work in the Cave of Swords in order to study the crystals. One mineworker died of dehydration while trying to steal some crystals. The heat in the cave roasted his body. A second, more impressive Cave of Crystals was discovered in 2000. The cave was closed to preserve the clarity of the crystals and prevent thefts.
Praying for Sheetrock
Read about it. Gypsum plaster is pressed between sheets of stiff paper, with a cardboard seam binding. This forms what is called drywall in the Northern and Eastern US, and sheetrock in the South and Southwestern US. In a 1991 nonfiction novel by Melissa Fay Greene, “Praying for Sheetrock,” Thurnell Alston protests against institutionalized racism and poverty in Georgia’s McIntosh County. The title came from the practice of taking salvaged materials from trucks and trains passing through the county. One of the characters in the book spoke of “praying for sheetrock” to be in one of the trucks so that he could fix the walls of his home before winter. References
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