Make a Ceramic Biscuit
Natural clay composes about half of a tile’s makeup with granular materials such as feldspar, grog, and rock making up the balance. The amount of recycled content can be as high as 70 percent. The ingredients are sifted, screened, and mixed together with a little water to form a tile body either by using a dry press, in which the mixture is dropped into molds and compacted at pressures of around 5,000 psi, or extruded through a die and then wire cut. The ceramic biscuits are dried at about 250 F to remove all moisture, a process that takes about 30 minutes for pressed tile and up to six hours for extruded tile, which uses a damper mixture.
Add a Splash of Color
The dried ceramic bodies pass under a stream of primer known as a bell, waterfall, or curtain application. Decorative glazes may then be applied using slingers or disc sprayers, glazing drums, or inkjet printers. A machine can glaze about 2,000 square feet per day. Hand applying the glaze, in a manner similar to silk screening, can generate about 50 pieces daily. The water-based glazes inherently are free of VOCs, but the minerals and ceramic ingredients that produce the color can be toxic; red, for example, traditionally requires lead and cadmium. New formulations without toxic ingredients are continually tested.
Light a Fire
The tiles are fired to achieve their hardness and final finish color. In a roller oven, similar to a commercial toaster, the tiles—packed densely together to maximize fuel consumption and minimize temperature differentials—are gradually heated to temperatures of around 2,000 F over the oven’s 125-meter-long course. Depending on the tiles’ thickness, the trip from start to finish can take between 50 minutes and two hours. A roller-oven kiln can fire about 4,000 square feet of tile per day, while a shuttle kiln, in which tiles bake on racks like bread loaves, can fire about 2,000 square feet daily.
Add the Final Touches
For a semi-glossy or glossy effect, the tiles pass through one or more polishing brushes, respectively. Only through-body tiles, in which the finish color is embedded in the tile body during the mixing process, are polished. Rectification, which trims and squares the edges of each tile to a consistent size, is done using a dry-cutting process and water jets. The tiles are then dried again to remove any remaining moisture, and then packaged for transport. About 10 square feet of tiles—totaling about 40 to 50 pounds—are packed together for easy handling.