This section is from the book "Cyclopedia Of Architecture, Carpentry, And Building", by James C. et al. Also available from Amazon: Cyclopedia Of Architecture, Carpentry And Building.
12 to 18 inches. In cast iron furnaces of the better class the fire pot is made very heavy to insure durability and to render it less likely to become red hot. The fire pot is sometimes made in two pieces to reduce the liability of cracking. The heating surface is sometimes increased by corrugations, pins or ribs.
A fire brick lining is necessary in a wrought iron or steel furnace to protect the thin shell from the intense heat of the fire.
Since brick lined fire pots are much less effective than cast iron in transmitting heat, such furnaces depend to a great extent for their efficiency on the heating surface in the dome and radiator, and this as a rule is much greater than in those of cast iron.
Cast iron furnaces have the advantage when coal is first put on, (and the drop flues and radiator are cut out by the direct damper) of still giving off heat from the fire pot, while in the case of brick linings very little heat is given off in this way and the rooms are likely to become somewhat cooled before the fresh coal becomes thoroughly ignited.