All copper/jewellery enamelling is done in a kiln which is not pre-loaded. Only when it is red hot do you place the item inside the chamber, which just has an aperture or else a sort of door if its a bigger size so as not to lose too much heat. You need to be able to see the work to judge he moment to lift it out. If it overfires, it goes black. The eye of experience is much better than a thermometer or guage.
As soon as you see the enamel has melted you lift it out to cool, which can be rapid if he enamel layer is thin, or you may cool it more slowly (annealing).
Metal for enamel may be sheet copper, silver, gold, or even iron or steel.

My association with the term "Furnace" is a heating chamber for more industrial firing processes.

Stefany