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Archaeologists have found bricks in the Middle East dating 10,000 years ago. Scientists suggest that these bricks were made from mud left after the rivers in that area flooded. The bricks were molded by hand and left in the sun to dry. Structures were built by layering the bricks using mud and tar as mortar. The ancient city of Ur (modern Iraq) was built with mud bricks around 4,000 B.C. The Bible (Exodus 1:14; 5:4-19) provides the earliest written documentation of brick production—the Israelites made bricks for their Egyptian rulers. These bricks were made of clay dug from the earth, mixed with straw, and baked in crude ovens or burned in a fire. Many ancient structures made of bricks, such as the Great Wall of China and remnants of Roman buildings, are still standing today. The Romans further developed kiln-baked bricks and spread the art of brickmaking throughout Europe.
The oldest type of brick in the Western Hemisphere is the adobe brick. Adobe bricks are made from adobe soil, comprised of clay, quartz, and other minerals, and baked in the sun. Adobe soil can be found in dry regions throughout the world, but most notably in Central America, Mexico, and the southwestern United States. The Pyramid of the Sun was built of adobe bricks by the Aztecs in the fifteenth century and is still standing. In North America, bricks were used as early as the seventeenth century. Bricks were used extensively for building new factories and homes during the Industrial Revolution. Until the nineteenth century, raw materials for bricks were mined and mixed, and bricks were formed, by manual labor. The first brickmaking machines were steam powered, and the bricks were fired with wood or coal as fuel. Modern brickmaking equipment is powered bygas and electricity. Some manufacturers still produce bricks by hand, but the majority are machine made.
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The manufacture of bricks entails several steps and starts with obtaining the raw materials. Clays are mined from open pits or underground mines. Storage areas are located at the mining site so that portions from various "digs" can be blended. The clay mixture goes through a process called primary crushing, where the clay is put through giant rollers that break the clay into small chunks. This mixture is transported to the manufacturing site, where the clay mixture is pulverised and screened to remove impurities. Further blending of materials may take place at this time.
There are three methods of forming bricks. The most common is the stiff-mud process where the clay blend is put into a machine called a pug mill that mixes the clay with water (12-15% by weight), kneads the mixture, removes trapped air, and transfers the mixture to an auger machine. The auger forces or extrudes the wet clay through a die that forms a continuous rectangle-shaped column. The column is cut with steel wires into desired lengths. The newly formed bricks are place on drying racks for a few days and then fired in a kiln. The soft-mud process is used when the mined clay is naturally too wet (20-30% by weight) to undergo the stiff-mud process. The clay is mixed, extruded, and placed in lubricated molds. Each mold makes six to eight bricks. The drying process takes more time than with stiff mud, but the firing procedure is the same. The third method is the dry-press process, which is most commonly used when making refractory bricks. The clay has minimal water content (up to 10% by weight) and is exposed to high pressure (in a hydraulic or mechanical press) while in the molds. The bricks are dried and fired. While still damp and moldable, textures, designs, or functional grooves can be pressed into the brick. Special glazes can be applied for decorative and for functional purposes.
Firing or burning the bricks takes two to five days. The most common type of kiln used to fire bricks is the tunnel kiln, where the bricks, stacked on cars, move slowly though a long chamber or tunnel. Many changes in the physical properties occur during the firing process. During firing, any residual water evaporates, some minerals melt, blend, and fuse, and organic matter oxidises. The hardness of the brick increases and the colour develops. The whole process of making bricks takes 10-12 days.
With handmade bricks, the clay is kneaded and put into molds. Excess clay is skimmed off the top of the mold, and the brick is then dumped out, dried, and fired. Handmade bricks are usually more expensive than machine-made bricks. They are often used in special projects, such as historical restoration. |
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