Furnace is an apparatus for producing heat. The term is general and may refer to any heat-producing apparatus, from a small, gas-fired metallurgical furnace to a towering blast furnace used in reducing iron ore. Furnaces may use any type of fuel suited to their design, including electricity. In common usage a fur­nace is a house-warming unit. Goal, coke, oil, gas, or electricity may be used as fuel.
   While boilers, or steam generators, are fired by coal, coke, oil, or gas and may be fitted with grates, stokers, or burners, the term furnace is replaced by firebox or combustion chamber in referring to this type of apparatus for producing heat. Blast furnaces are cylindrical steel structures, up to 100 feet in height, used to reduce iron ore to pig iron or ingot iron. See blast furnace.
   In the metal industry, furnaces are used to refine and to produce the various types of steel, steel alloys, and nonferrous alloys. Coke, oil, or gas may be used as fuel.
   Electric furnaces use electricity to furnish the heat for melting and alloying metals. The heat is obtained by induction or by the electric arc.