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