What drywall does now, cover interior structures like beams and joists and provide a flat surface for papering or painting, plaster used to do. Plaster, however, took many days to apply and required long drying times. In 1916, The United States Gypsum Company (USGC) invented Sheetrock, plaster dried into the form of boards, as an alternative to labor-intensive plastering. The need to quickly build many homes after World War II created a demand for Sheetrock, also called plasterboard or drywall. Drywall made construction much faster and more efficient.
Drywall, made primarily of gypsum — the abundant mineral that constitutes plaster, is fairly inexpensive to produce. After being calcined, or cooked, gypsum makes a powder that can be mixed with water to form a paste. When the paste dries, it regains the consistency of rock. The resulting plaster is dense enough to support its own weight, but not so hard that it cannot be cut with a knife.
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The drywall manufacturer adds bits of paper, cotton, or fiberglass to the gypsum powder. Drywall made with these fibers is stronger and less liable to crack than is pure plaster. Starch, which helps the paper linings stick to the plaster, goes in the mix as well. Once the powder is combined with water, the addition of a foaming agent introduces air, which will make the boards lighter and easier to use. Drywall made with vermiculite, a natural mineral, is more fire resistant than other types of drywall. Other additives include chemicals that are aimed at reducing mildew.
When the plaster paste is mixed, it is ready to pour on the paper. Drywall, made to be painted or otherwise covered, doesn’t have to look pretty, and the paper that lines the drywall comes from recycled newspaper. It comes in two varieties, a light-colored paper for the front of the board, and a gray paper for the back. On a wide conveyor belt, a sheet of paper is rolled out and receives the plaster from above. Another sheet rolls over the layer of plaster, and the whole plaster-paper sandwich is then pressed to create the desired thickness of board. Drywall made today comes in several standard thicknesses, each aimed at a particular application.
As the conveyor belt continues to move along, this very long board is cut into panels of standard lengths. These panels then roll into a room-sized kiln to be dried. Here the boards bake under gradually decreasing temperatures until the gypsum core is set. When the boards emerge from the drying chamber, they are ready to use.
Drywall made such a difference in the way buildings are put together that it is used in the construction of many homes. The usefulness of drywall makes it abundant, and its abundance is very apparent on the sites of buildings recently demolished. Drywall made unstable through fracture or dampness can’t be repaired, but it can be recycled for use in new drywall. The gypsum within it also has applications as a component in cement, fertilizer, and as a soil enhancer.