Alan Hawkins, age 11, of Des Moines, Iowa, for his question:
HOW ARE BRICKS MADE?
Bricks are the oldest manufactured building material used by man. They can be traced back to the Egyptians, Assyrians and Babylonians to about 4000 B.C. The Israelites had brick making as their chief occupation when they lived in Egypt. Clay soil, taken from the banks of the Nile River, was pressed by the Israelites into bricks and then dried under the hot desert sun.
Bricks are made from either clay or shale. After being dug, the material is crushed thoroughly in rotating pans under heavy rollers. The machine is called a dry pan.
Before water is added to the ground clay, it is screened carefully to remove any coarse material. The amount of water depends on the brick manufacturer's formula. He can use the stiff-mud or soft-mud process or the dry-press process.
A stiff-mud brick making process is used to make most building bricks. Not too much water is used with this method, making the material quite dry and stiff in its early stages. The brick making machine forces the clay through an opening and it comes out in the form of a long ribbon. Cutting is accomplished by a special machine with evenly spaced wires that slices through the clay.
While still being mixed, some brick making manufacturers remove extra air from the wet material by using a special vacuum pump. Removing air seems to make the clay easier to handle and the bricks stronger.
The soft-mud process is used for all expensive handmade bricks. Molds are dipped in sand or water at first to prevent the clay from sticking. Clay and water make a soft paste which is then pressed into molds.
The third type of brick making is called dry-press. Only enough water to make the clay damp is added. When the clay is pressed into molds, an almost-perfect face is produced. This type of brick is used on exposed walls.
After bricks have been made by pressing the clay into molds, they are stacked and placed in drying rooms. Air temperatures range between 100 and 300 degrees F. Bricks are dried slowly so there won't be cracking or shrinking. The process can take up to six weeks.
From the drying room bricks go to ovens called kilns. The heat is increased slowly until it reaches between 1,600 to 2,000 degrees F., depending on the type of clay used. The kilns are heated by burning gas, oil or coal.
Baking or firing of bricks actually melts and fuses together the particles in the clay. The result is a strong, hard-finished product.
Clays high in iron compounds make red bricks while those containing small amounts of iron produce cream-colored or yellow bricks. A process called flashing, where the fires in the kilns are made to smoke, produces variations of color in the finish bricks.