kiln (kĭl, kĭln) [key], furnace for firing pottery and enamels, for making brick, charcoal, lime, and cement, for roasting ores, and for drying various substances (e.g., lumber, chemicals). Kilns may be updraft or downdraft; round, conical, annular, or rectangular; arranged for intermittent or continuous firing; and of the muffle (double-wall) or direct-contact type, as required. Rotary kilns are much used in continuous processes, including cement manufacturing and the drying of granular materials. They consist of long tubes lying almost horizontally that are rotated slowly as heat is applied to the material being treated inside the tubes. The fuel used may be electricity, oil, gas, or coal. The temperature of firing and the length of time required depend on the design of the kiln and the type of material being fired.