A furnace is an enclosed space that is raised to high temperatures by the combustion of fuels or by electric heating. Most industrial furnaces are used in the extraction of metals or the making of alloys. These are usually lined with firebrick. Some furnaces heat ovens, boilers, and kilns. A domestic boiler, especially if it burns solid fuel, is sometimes called a furnace.

There are three main types of electric furnace. An arc furnace relies on the heat generated by an electric arc (spark), often between two large carbon electrodes, which are slowly consumed. A resistance furnace is heated by passing an electric current through a heating element or directly through metallic material. An induction furnace uses electromagnetic induction to cause a current to flow in a metallic charge. The resulting heat is sufficient to melt the metal.