How Bricks Are Made
Three kinds of clay are used to make structural brick: surface or alluvial clay, clay shale, and fire clay. These clays, when wetted and kneaded, form a plastic mass that is shaped into brick by machinery or by hand molding. Surface clays, which collect in valleys, are obtained by digging. Clay shales, which are hard like stone, are obtained by blasting. Fire clays usually are obtained by mining. Brick production facilities generally are located near clay deposits.
Preparing the Materials
Several processing steps are necessary before extracted raw materials can be made into bricks. The end objective of these steps is to produce a plastic mass of such a consistency that it can readily be formed into bricks.
The first step, called the primary crushing operation, often is done at the mine site so that trucks or rail cars can be loaded fully for delivery of raw materials to the plant. Large and very powerful jaw-, roll-, or cone-type crushers, called primary crushers, break large chunks of clay, shale, or other minerals into smaller pieces about 2 to 4 inches (5 to' 10 cm) in diameter. Sometimes a piece of shale as large as a. dining table is fed into a typical primary crusher.
The next step in processing the clays is fine crushing and screening. In most cases, this is done at the manufacturing site. The clays go through one or more crushing and screening operations, depending on the size of the received material and the final size required for forming the brick. After crushing and then screening, the clay materials are stored until they are needed.
Most bricks are formed from raw materials that include two or more clays, sometimes with other materials added. Several different materials are needed to obtain a blend that is economical, sufficiently plastic to be formed and handled, and sufficiently fusible so that the brick can be fired at relatively low temperatures. Also, the blend must contain one or more materials for producing the desired color.
There are three basic methods for forming the clay mass into brick: the stiff-mud process, the soft-mud process, and the dry-press process. These methods vary according to the consistency of the clay mass that is to be formed into brick. The water content of clay material formed by the stiff-mud method is about 15%; in the soft-mud process it is about 25%; in the dry-press process it usually does not exceed 10%.
The stiff-mud process is the principal method for forming brick in the United States. In this process, the clay blend is fed to a pug mill (mixing machine) where it is made plastic and homogeneous by the addition of water and by kneading. The kneading is done by knives attached to a rotating shaft in the pug mill. The clay mass then is sent to a chamber where entrapped air is removed from the mass to make it denser and stronger.
The clay mass then falls through the chamber into an auger machine that forces it through a brick-forming die. The extruded clay is in the form of a continuous rectangular column. Steel wires are used to cut the column into six or more bricks at a time. The bricks then are set on a kiln car; moisture is removed from the bricks before they enter the kiln.
The traditional soft-mud process uses a machine that compresses the clay in a mold, shaping the soft clay into bricks. A single mold usually has six or eight cavities that are filled with clay. When the plunger of the machine compresses the clay, six or eight bricks are made at one stamping. The bricks then are placed on pallets, dried, and set on kiln cars.
A soft-mud process is very popular in Europe because many clays there are very wet as mined from river banks. In the United States, the European soft-mud process recently has been reintroduced for brickmaking at fully automated plants. In this method, the clay is extruded, cut into slugs, and placed in molds. The brick is dumped from the mold, set on a pallet until dried, and then put on a kiln car.
The dry-press process is seldom used to make face brick or common brick, but it is a major process for making refractory brick. In this process, a hydraulic or mechanical press compresses relatively dry clay into brick form. After a multiple-cavity mold is filled with clay, the press head comes down and applies a very high pressure on the clay. Four to six bricks are usually formed at one time.
The most important changes in the physical properties of brick occur when it is fired in a kiln. During firing the hardness of the brick is increased significantly and the color of the brick is developed. These changes in the physical properties of brick are the result of complex chemical changes. As the temperature in the kiln increases, the following changes occur: (1) water between the particles of the brick is driven off (by about 250° F, or 120° C); (2) fusible clay minerals start to decompose; (3) water formed as one product of decomposition of clay is given off in the form of steam (at about 1110° F, or 600° C); (4) organic matter in the brick is oxidized; and (5) more fusible clay minerals begin to melt and flow into spaces between the brick particles (1500° to 1800° F, or 815° to 980° C).
The fused material fills some of the spaces between the brick particles and unites them, forming a strong and durable general-purpose brick. A very durable special-purpose brick, called vitrified brick, is formed when all of the spaces are filled after prolonged heating at temperatures greater than 1800° F (980° C).
In the United States, brick is most often fired in tunnel (continuous) kilns, which are long chambers through which bricks are continuously moved on kiln cars. Firing is precisely controlled by the use of burners on the sides or top of the kiln; the temperature inside the kiln is adjusted automatically.
Periodic (intermittent) kilns are round, down-draft kilns that are loaded, sealed, fired, and emptied for each cycle. The shuttle kiln, which may use cars, is operated in cycles in the same manner as the periodic kiln.