Sourcing the Gypsum Gypsum is a naturally occurring mineral, and millions of tons of it are mined each year; CertainTeed Corp. alone has six mines in North America. But most wall-board plants built in the past 20 years are located not near mines, but near coal-fired power plants, which provide a ready supply of synthetic gypsum—derived from sulfur dioxide filtered from the power plants’ flue exhaust.
Calcining and Second Grind Raw gypsum rock is crushed before it arrives at the wall-board plant. Once there, it is heated to 350 F in a process called calcining, which removes 75 percent of the molecularly bonded water. With this water removed, a second grind of the gypsum (which occurs during or after calcining) results in a superfine powder called stucco.
Slurry and Placing on Face Paper Stucco is mixed with solid additives such as potash, starch, boric acid, and glass fiber (the latter is for fire-rated wall board) and liquid additives such as retarders, foaming agents, silicone, and—most importantly—water, which rehydrates the mixture into a slurry. This slurry is then pumped onto a conveyor belt that is lined with wall-board face paper.
Forming and Drying The slurry-covered face paper goes through a series of conveyors to level the material, and a second layer of paper is applied to the top surface. The material is then pushed through a forming plate that determines the thickness of the finished gypsum board. This is followed by another trip on a conveyor covered with a forming belt that ensures a smooth surface for the board’s face. The still-wet contiguous slurry-and-paper sandwich is cut into nominal lengths before being inverted and fed face-up into a gas-fired board dryer. The boards stay in the dryer for up to 50 minutes.
Cutting and Bundling The dry boards are stacked face to face (in a process called "booking") to preserve the smoothness of the face paper, and thus the eventual wall surface. The stacked pieces then go through a finish saw to trim them to the appropriate length, and tape is applied to the edges to seal the finished wall board. The boards are then bundled and stacked in a warehouse while awaiting shipment.