Seller: smokeyquartzrobert (520) 100%, Location: Colorado Springs, Colorado, Ships to: US, Item: 401235363201 There is a large brick arch (made from fire brick) the front third to half of the firebox. 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 brick arch and its supports (arch tubes, thermic syphons, and circulators) require periodic replacement due to the extreme heat they endure. measures 10 x 9 x 4.5, can see melt on it, can see where tube where. More numbers on side but I can't read them all. Very heavy, est. 20 - 25 pounds Condition: SHOWS SIGNS OF BURNING, MELT, VERY HEAVY, Color: Beige, Maker: American Arch Co.