Terminology: Furnace, Forge, Kiln

Just to keep myself from going crazy with these terms, I looked them up to find out which one meant what. According to Merriam-Webster...

A furnace is the most generic term:

 : an enclosed structure in which heat is produced (as for heating a house or for reducing ore)

A kiln narrows the uses to which the heat is put:

 : an oven, furnace, or heated enclosure used for processing a substance by burning, firing, or drying

A forge narrows the thing being heated to a metal:

: a furnace or a shop with its furnace where metal is heated and wrought

So while most of the things I discuss here could use any of these terms,  I'll try to use the term furnace generally. Of course the dictionary only records the current meaning of things, so older texts may contradict.