Arch

An arch is placed in the front third to half of the firebox. Prototype locomotives typically use brick for the arch, but live steam models most often use stainless steel.

The 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, and also more soot to accumulate in the firetubes. The arch and its supports require periodic replacement due to the extreme heat they endure.