Editing Brick (section)

==== Kilns ====
{{See also|Hoffmann kiln}}[[File:Xhosa brickmaker at kiln near Ngcobo.jpg|thumb|left|[[Xhosa people|Xhosa]] brickmaker at kiln near [[Ngcobo]] in 2007]]
In many modern [[brickworks]], bricks are usually fired in a continuously fired tunnel [[kiln]], in which the bricks are fired as they move slowly through the kiln on [[conveyors]], rails, or kiln cars, which achieves a more consistent brick product. The bricks often have [[Lime (material)|lime]], ash, and organic matter added, which accelerates the burning process.

The other major kiln type is the Bull's Trench Kiln (BTK), based on a design developed by British engineer W. Bull in the late 19th century.

An oval or circular trench is dug, {{convert|6–9|metres}} wide, {{convert|2-2.5|metres}} deep, and {{convert|100–150|metres}} in circumference. A tall exhaust chimney is constructed in the centre. Half or more of the trench is filled with "green" (unfired) bricks which are stacked in an open lattice pattern to allow airflow. The lattice is capped with a roofing layer of finished brick.

In operation, new green bricks, along with roofing bricks, are stacked at one end of the brick pile. Historically, a stack of unfired bricks covered for protection from the weather was called a "hack".<ref>Connolly, Andrew. ''Life in the Victorian Brickyards of Flintshire and Denbigshire'', p34. 2003, Gwasg Carreg Gwalch.</ref> Cooled finished bricks are removed from the other end for transport to their destinations. In the middle, the brick workers create a firing zone by dropping fuel (coal, wood, oil, debris, etc.) through access holes in the roof above the trench. The constant source of fuel maybe grown on the [[woodlot]]s.<ref name=":0" />{{Rp|page=6}}

The advantage of the BTK design is a much greater energy efficiency compared with [[clamp kiln|clamp]] or [[scove kiln]]s. Sheet metal or boards are used to route the airflow through the brick lattice so that fresh air flows first through the recently burned bricks, heating the air, then through the active burning zone. The air continues through the green brick zone (pre-heating and drying the bricks), and finally out the chimney, where the rising gases create suction that pulls air through the system. The reuse of heated air yields savings in fuel cost.

As with the rail process, the BTK process is continuous. A half-dozen labourers working around the clock can fire approximately 15,000–25,000 bricks a day. Unlike the rail process, in the BTK process the bricks do not move. Instead, the locations at which the bricks are loaded, fired, and unloaded gradually rotate through the trench.<ref>Pakistan Environmental Protection Agency, [http://www.environment.gov.pk/EA-GLines/I1B-Brick%20Kilns.pdf Brick Kiln Units (PDF file)] {{Webarchive|url=https://web.archive.org/web/20070616090248/http://www.environment.gov.pk/EA-GLines/I1B-Brick%20Kilns.pdf |date=16 June 2007 }}</ref>