Editing Firebox (steam engine) (section)

===Brick arch===
[[File:6989 Wightwick Hall firebox.jpg|thumb|Firebox of a [[GWR 6959 Class]] steam locomotive, showing the underside of the brick arch, constructed from specially-shaped firebricks.]]
[[File:Liesel 07-05-2011 Feuerbüchse einer Baureihe 52.jpg|thumb|Empty firebox of a Baureihe 52, showing the brick arch]]
There is a large [[brick arch]] (made from [[fire brick]]) attached to the front wall (boiler throat plate) of the firebox immediately beneath the firetubes. This extends backwards over the front third to the half of the firebed. It is supported on arch tubes, thermic syphons, or circulators. The brick arch directs heat, flames, and smoke back over the fire towards the rear of the firebox. Visible smoke contains unburned combustible carbon particles and combustible gasses. The purpose of this redirection is to cause more complete combustion of these particles and gasses which make the locomotive more efficient and causes less visible smoke to be emitted from the stack. Without the arch, flames and visible smoke would be sucked straight into the firetubes without having been fully burned, causing visible smoke to be emitted at the stack. The invention of the brick arch allowed locomotives to burn cheaper coal (which contains volatiles) instead of coke, it having been a legal requirement from the outset for engines to "consume their own smoke".  The brick arch and its supports (arch tubes, thermic syphons, and circulators) require periodic replacement due to the extreme heat they endure. The brick arch also deflects "cold" air being drawn in through the firebox door from directly hitting the rear tube plate of the boiler and cooling it down when the boiler is hot.