Bonnie Dickerson, age 13, of Cumberland, Md., for her question:
HOW ARE BRICKS MADE?
Brick is usually made from clay or shale. It is a construction material that has been hardened into the shape of a block.
After clay for bricks has been dug, it is crushed. Manufacturers then grind the clay thoroughly in a rotating pan with heavy rollers. This machine is called a dry pan.
The clay is then screened to remove the coarse material. Water is added to the clay and revolving knives chop and mix the clay into a plastic mass. This mass is molded into bricks in one of three ways: the stiff mud; the soft mud or the dry press process.
After the bricks are formed, they are dryad and then fired or burned.
Stiff mud process is used to make most building bricks. Water is added to the clay to make it a stiff mud. The brickmaking machines force the clay through an opening to form a long ribbon. The ribbon of clay is cut into brick sizes by a brick cutter, an apparatus with evenly spaced wires that slice through the stiff clay.
Sometimes air is removed from the clay by attaching a vacuum pump to the brickmaking machine. Removing air makes the clay stronger and easier to handle.
Soft mud process is used for all handmade bricks. Molds are dipped in sand or water to prevent the clay from sticking. Water and clay are mixed to make a soft paste, which is pressed into the molds.
To obtain almost perfect face bricks thar are used on exposed walls, the dry press process is used. Only enough water is added to the clay to make it damp. The clay is then pressed into molds.
After the bricks have been formed, they are stacked in drying rooms. Air heated to temperature from 100 to 300 degrees Fahrenheit circulates through these drying rooms.
As a final step, the bricks are fired in ovens called kilns.
Brick kilns are heated by burning coal, oil or gas. The temperature in the kiln is increased slowly until it reaches 1,600 to 2,000 degrees Fahrenheit, or higher, depending on the kind of clay used. The clay particles become partly melted and fuse together, making the brick hard and strong.
Vitrified bricks are so well burned that they will not absorb water when they are soaked. Medium burned bricks absorb some water and soft burned bricks absorb much water.
Clays that are high in iron compounds make red bricks. Clays with low iron content are used for yellow or cream colored brick. Variations in color can be obtained with some clays by flashing the brick at the end of burning. In flashing, the fires are made very smoky to make the iron in the clay darker.
Brick is the oldest manufactured building material. Sun dried brick, similar to adobe brick, was used in ancient Babylonia, Assyria and Egypt as early as 4,000 B.C.