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 pulverized 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 oxidizes. The hardness of the brick increases and the color 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.