In the case of hearth furnaces the height is the same or less than the diameter. They can also be worked with forced draught. Here too there is direct contact between fuel and ore and the process can be guided towards an oxidising or reducing reaction.  These furnaces comprise liquation hearths, finery fires, etc. They are used for the production of lead from galena, for the production of wrought iron directly from iron ore (Catalan forge, etc.) and as a smith's hearth.

The second category of furnaces, in which the products of combustion heat the ore or metal are called reverbatory furnaces.  They are quite modern apparatus and entirely unknown in Antiquity. They are now used for many roasting, calcining and melting processes.

The third family of furnaces protects the ore or metal from contact either with the fuel or with the products of its combustion.  They are all derived from the crucible.  In the case of the crucible furnace, the heating chamber (which may be a simple crucible!) is movable.  Such a process is used in Antiquity for refining gold, for producing "wootz" steel, etc.  In other, so-called muffle furnaces, the heating chamber is a fixed part of the structure, which may for instance be used to store crucibles during the heating period.  Small models are used for testing ores and metals in the laboratory since the sixteenth century A.D.  Some kinds of furnaces are built to volatilize and condense certain compounds or metals recovered from the ore. These retort furnaces are especially used in modern zinc manufacture.

In most furnaces there is a certain zone in which the essential reactions take place or where the temperature is such that corrosive compounds are formed (which may again be destroyed or bound in other parts of the furnace).  Such conditions determine the life of the furnace land many of the most primitive furnaces served only once. That is why we often find the remains of thousands of ancient furnaces on these smelting sites.  Once, however, the furnace is built up of stone (on the inside at least) its life is considerably longer and the way is paved for the evolution of furnaces which can produce continuously. The type of stone used in building these furnaces should be carefully selected and adjusted to the peculiarities of the ore treated. We distinguish acid materials such as flint, ganister, sand and fireclays, neutral materials such as graphite and chromite and basic materials such as limestone, dolomite, magnesite and bauxite.

The choice of these materials depends on the gangue of the ore and it will aid the flux in destroying the effect of the gangue on the process. In the case of acid, siliceous gangue a basic lining of the furnace will be chosen and renewed after the furnace has worked for some time.

When we turn to ancient furnaces we find that this truth has been realized quite early and that their construction ranges from the very simple open-hearth-furnaces to the precursor of the blast-furnace, the Stückofen. (pg. 124-125).