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How Bricks and Tiles Are Made
Bricks are the oldest man-made building material. Early bricks were made of mud, using straw as a binder, and dried in the sun. They were not fired in a kiln. Bricks of this sort were used to build houses in Mesopotamia more than 6,000 years ago. The ancient Sumerians, Babylonians, and Egyptians used mud bricks. These peoples used the bricks for the construction of some public and religious buildings, as well as private dwellings.
Fired bricks, about the size of modern bricks, were first made about 3000 B.C. in the region that is now Iraq. Like the earlier mud bricks, they were shaped by hand in rectangular wooden molds. About 1800 B.C. the size of bricks fired in kilns became fairly standardized at about 12 inches square by 3 inches thick (30 x 30 x 7.5 cm). This size of brick was used in later Babylonian and Egyptian brickwork.
The Greeks used sun-dried mud bricks until about the middle of the 4th century B.C., when they first began to fire bricks in kilns. It was the Romans, however, who refined the art of masonry construction and perfected the techniques used in building walls, arches, floors, roofs, aqueducts, and sewers with bricks. The Romans also used tiles for surfacing floors, roofs, and walls and for building heating and plumbing systems.
Most of the brickworking skill of the Romans was lost after the fall of the Roman Empire in the 5th century A.D., and the production and use of bricks ceased almost entirely. In the 12th and 13th centuries the art was revived in Holland, Flanders, Germany, and Italy, and bricks regained their importance as a building material. By the middle of the 17th century, bricks were being produced and used in colonial America as well as in nearly every country in Europe.
How Bricks and Tiles Are Made
Bricks and tiles are made from finely ground clay or shale in three steps: forming, drying, and firing.
Forming. Forming prepares the clay or shale for molding and produces soft, unfinished brick and tile of the desired size and shape. There are three different methods of forming: the stiff-mud, or wire-cut, process, the soft-mud process, and the dry-press process.
In the stiff-mud process, heavy cast-iron rollers pulverize clay or shale in a large cast-iron pan 7 feet to 9 feet (2-2.7 meters) in diameter. The ground clay is forced through holes in the bottom of the pan into a circular mixing device called a pug mill. In the, pug mill, rotating knives cut the clay and blend it with water, forming a plastic mass containing about 20 percent water. This mass is forced out through a rectangular hole and emerges as a continuous ribbon of clay with the cross section of a brick. This ribbon is carried by rollers to a cutting table where a rotating wire "knife" cuts it into individual bricks ready for drying.
In the soft-mud process, a soft, easily molded mass containing more than 20 percent water is formed in the pug mill. The wet clay is shaped in molds that have been coated with sand or water to prevent the clay from sticking. The bricks are then removed from the molds and dried.
In the dry-press process, as little water as possible is added to the clay. The prepared clay, which contains no more than 10 percent water, is then poured into a strong steel mold. The top and bottom of the mold are powerful plungers that compress and form the clay under pressures from 500 to 1,500 pounds per square inch.
Drying. Slow drying, the second step in making brick and tile, evaporates most of the water from the brick and tile and helps prevent cracking and warping during firing. Unfired, or green, bricks are dried in one of several ways. Sometimes the wet bricks are stacked in chambers called humidity dryers. There they are heated to a temperature of 250° to 300° F. (170° C. to 200° C.), in air that is saturated with water vapor. Controlled currents of dry air heated to temperatures up to 390° F. (250° C.) are then blown through the chamber, drying the bricks quickly without warping or cracking them. Drying is also done in covered sheds where hot steam is used to dry the bricks. It usually takes about three days to dry bricks by either method.
Tunnel dryers are used to dry bricks made from clays that can be dried in 24 to 48 hours. The bricks are stacked on cars and wheeled into the tunnel entrance. As they are carried through the tunnel, which is usually over 100 feet (30 meters) long, air drawn in at the tunnel exit is heated and blown over them.
Firing. The hardening of brick and tile is effected by burning or firing. This process reduces the tendency of the finished product to absorb water and brings out the color of the clay.
Firing of dried, or green, bricks is done in ovens called kilns. During firing, which may take several days, the bricks are baked at 1600° to 3000° F. (800° C. to 1140° C.). The exact temperature depends on the kind of clay in the bricks and the use for which they are intended. The bricks are first preheated to a red heat to drive out any water remaining in them, then fired until hard, and finally cooled.
Today most bricks are fired in coal, oil, or gas-fueled kilns that operate continuously. In the United States, continuous-process tunnel kilns similar to tunnel dryers are widely used. Green bricks are wheeled into one end of the tunnel and preheated by the exhaust gases drawn from the central section of the tunnel, where the bricks are actually fired. The bricks continue to move through the tunnel, and after being fired in the central section, they are wheeled to the far end of the tunnel where the fresh air drawn into the tunnel cools them. The Hoffman kiln, a continuous-process kiln consisting of a closed loop of connected chambers, is widely used in Europe.