| Bricks may be made from clay, shale, soft slate, | | | | The advantage of the BTK design is a much greater |
| calcium silicate, concrete, or shaped from quarried | | | | energy efficiency compared with clamp or scove |
| stone. | | | | kilns. Sheet metal or boards are used to route the |
| Clay is the most common material, with modern clay | | | | airflow through the brick lattice in such a way that |
| bricks formed in one of three processes - soft mud, | | | | fresh air flows first through the recently burned |
| dry press, or wire cut. | | | | bricks, thus heating the air, then through the active |
| Mud bricks | | | | burning zone. The air continues through the green |
| The soft mud method is the most common, as it is | | | | brick zone (pre-heating and drying them), and finally |
| the most economical. It starts with the raw clay | | | | out to the chimney exhaust where the rising gases |
| preferably in a mix with 25-30% sand to reduce | | | | create the suction which pulls the air through the |
| shrinkage. The clay is first ground and mixed with | | | | whole system. The reuse of heated air results in a |
| water to the desired consistency for forming in a | | | | considerable savings in fuel cost. |
| mould. The clay is pressed into steel moulds with a | | | | As with the rail process above, the BTK process is a |
| hydraulic press. The shaped clay is then fired | | | | continuous one. A half dozen laborers working around |
| ("burned") at 900-1000°C to achieve strength. | | | | the clock can fire approximately 15,000-25,000 bricks |
| Rail kilns | | | | a day. However, unlike the rail process, in the BTK |
| In modern brickworks, this is usually done in a | | | | process the bricks themselves do not move. Instead |
| continuously fired tunnel kiln, in which the bricks move | | | | the locations at which the bricks are loaded, fired, |
| slowly through the kiln on conveyors, rails, or kiln cars | | | | and unloaded gradually rotate through the trench. |
| to achieve consistent physical characteristics for all | | | | Dry pressed bricks |
| bricks. The bricks often have added lime, ash, and | | | | The dry press method is similar to mud brick but |
| organic matter to speed the burning. | | | | starts with a much thicker clay mix, so it forms more |
| Bull's Trench Kilns | | | | accurate, sharper-edged bricks. The greater force in |
| In Pakistan and India, brick making is still typically a | | | | pressing and the longer burn make this method more |
| manual process. The most common type of brick kiln | | | | expensive. |
| in use there are Bull's Trench Kiln (BTK), based on a | | | | Wire cut bricks |
| design developed by British engineer W. Bull in the | | | | In wire cut the clay mix is 20-25% water, this is |
| late 1800s. | | | | forced through a die to create a long cable of |
| An oval or circular trench, 6-9 meters wide, 2-2.5 | | | | material of the demanded width and depth. This cable |
| meters deep, and 100-150 meters in circumference, is | | | | is then cut into bricks of the desired length by a wall |
| dug in a suitable location. A tall exhaust chimney is | | | | of wires. The majority of structural bricks are made |
| constructed in the center. Half or more of the trench | | | | by this method as hard dense bricks are the result |
| is filled with "green" (unfired) bricks which are stacked | | | | and any needed holes or other perforations can be |
| in an open lattice pattern to allow airflow. The lattice | | | | introduced by the die. The introduction of holes |
| is capped with a roofing layer of finished brick. | | | | reduces the needed volume of clay through the |
| In operation, new green bricks, along with roofing | | | | whole process, with the consequent reduction in |
| bricks, are stacked at one end of the brick pile while | | | | costs per brick. The bricks are also lighter and so |
| cooled finished bricks are removed from the other | | | | easier to handle and have different thermal |
| end for transport. In the middle the brickworkers | | | | properties compared to solid bricks. The cut bricks |
| create a firing zone by dropping fuel (coal, wood, oil, | | | | are hardened by drying for between 20 and 40 hours |
| debris, etc) through access holes in the roof above | | | | at 50-150°C before being fired. The heat for the |
| the trench. | | | | drying is often the waste heat from the kiln. |