Geology Associated with Clay Masonry Units
Clay masonry units are formed of clay, a sedimentary mineral. Clay is found in the form of surface clay, shale (naturally compressed and hardened clay), or fire clay (deeper clays). In the United States, clay is found primarily in central Texas and the east coast, although small amounts are found in sedimentary deposits throughout the country.
Chemistry Associated with Clay Masonry Units
Clays and shales are about 65 percent silicon oxide and 20 percent aluminum oxide. They may also contain varying amounts of other metallic oxides (manganese, phosphorus, calcium, magnesium, sodium, potassium, and vanadium). These metallic oxides give a fired clay units a distinctive color, decrease the units vitrification temperature, and also affect its appearance and durability. For example, small amounts of chromite, added to lightcolored (buff) clay, give it a gray color; small amounts of manganese, added to buff clay, give it a brown color.
Manufacturing of Clay Masonry Units
Three processes are in use today for manufacturing clay masonry units:
1. Soft mud process: Clay containing 20 to 30 percent water by weight is molded. This process is used occasionally in the United States, but more often in Europe.
2. Stiff mud process: Clay containing 12 to 15 percent water by weight is mixed, forced through a die, and cut with wire. This is the most common process in the United States.
3. Dry press process: Mixed clay containing 7 to 10 percent water by weight, form in hydraulic press. This process is rare. It is used, for example, to make fire brick. After forming, various surface textures can be imparted to the unit: wire-cut, rug (heavy scratches), matte (light scratches), or sand finished.
The clay units are then placed on specially insulated railway cars, and subjected to the firing process. This involves six basic steps.
The units then move into a tunnel kiln, which is kept relatively cool at the entrance, hot in the middle, and cooler again at the exit. The heat comes from burning fuel within the kiln itself. Over a period of 12 h to as long as 3 days, the units pass from the entrance to the hottest section, and then to the cooler exit. Temperatures in the different sections are regulated to produce different results. The units pass through the following steps:
1. Preheating: The green units are dried at about 350°F, in drying ovens heated by exhaust gases from the kiln. During this process, the units shrink.
2. Dehydration: The units continue to dry at temperatures from 300 to about 800°F.
3. Oxidation: At temperatures from about 800 to 1800°F, organic material burns.
4. Vitrification (or incipient vitrification): At temperatures of 1600 to 2400°F, the clay begins to vitrify. Silicates in the clay begin to fuse, binding the unvitrified clay particles together. This point is termed incipient fusion. The temperature used depends on the type of clay. Most clays will undergo incipient fusion at about 2000°F. The purest clays, which are used for refractory brick, are fired at temperatures up to 2400°F.
5. Control of Oxygen: The color of metallic oxides can be changed by feeding additional air into the kiln at this point to promote an oxidizing environment, or by intentionally withholding air to produce a reducing environment. The latter is termed reduction firing, or flashing.
6. Cooling: The units are then slowly cooled.
