A structure in which material can be heated to high temperatures. In general, the term furnace applies to a structure with a refractory superstructure which restricts air access to the charge. Thus, allowing the operator to control the reducing condition of the atmosphere the charge experiences. This is in contrast to a hearth which is open to the air. This is in contrast to a hearth which is open to the air. However, the term hearth is used for parts of furnaces in which air and fuel first react.
The most important distinguishing feature for furnaces is whether the charge is separated from the source of combustion or not. The introduction of the reverberatory and muffle furnaces, in which the fuel is burnt separate from the charge, marked major technological changes as this prevents contamination of metal by elements such as sulphur from the fuel.