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Throughout this century, brick plants have changed as emerging technology- from CAD software for brick design to computer controlled tunnel kilns- have further refined the methods of production. Such advancements increased the speed and quality of brick production. RAW MATERIALS: MINING AND PREPARATION Three types of clay are used to make brick: surface clays, shales, and fire clays. After it is mined in open pits, clay is moved to the plant for processing. The clay, along with inherent stones, soil, and sand, is then crushed into smaller pieces. Once crushed, the clay is then ground to a fine powder and conveyed to a pug mill. TEMPERING AND FORMING To create a homogenous, plastic material, the powdered clay is mixed with water in a pug mill, a mixing chamber with one or more revolving shafts with blades. The amount of water added depends on which of three methods will be used to form the brick. The most common method is stiff-mud extrusion. The clay is forced by auger through a steel die in a continuous extrusion or column of the desired size and shape. Then a large wire cutter automatically cuts the column of clay into bricks of equal size. DRYING AND GLAZING Green clay units can contain as much as 30% free moisture that must be removed before they are burned in the kiln. Modern plants use dryer kilns supplied with waste heat from the exhaust of the firing kilns. After drying but before burning, the brick might be glazed. Various chemical compounds are sprayed on the units before burning and then subjected to normal firing temperatures so the glaze fuses to the brick. FIRING Firing causes ceramic fusion of the clay particles and thus hardens the brick. Most modern plants use a continuous tunnel kiln, built of brick and lined with firebrick or pourable refractory. Dried brick are stacked on flat cars that move very slowly through this long, narrow, computer controlled kiln. By exposing each brick to the same heat treatment, tunnel kiln produce a more uniform product.