The step of the process named in item (3) of the preceding paragraph is that of smelting, and is usually carried on in furnaces built of common silica brick for the outer layers, and high grade refractory brick for the inner layers. This brick work is either incased in a shell of iron plates, or is held together by iron bands and rods, which are wholly on the outside of the furnace and do not enter the fire space. There are several modifications of furnaces for smelting ores of different metals, but all smelting furnaces are included in the two general types of this class, namely, the blast furnace and the reverberatory furnace. These must be strongly built on permanent foundations, and must be able to stand the intense heat of the operation.