Anytime you can get a pile of charcoal with a blower on it, it will melt something. Your furnace makes it more controlled and way more efficient.
Here's one I did a couple years ago with some junk from an old foundry. It's a refractory burner nozzle about 1 gallon is size. I put it on another tube like thing and filled the bottom with charcoal. Cheap stuff from the dollar store. Added a little squirrel cage blower and it put out plenty of heat for Aluminum and Brass. No insulation and not very efficient, but it worked.
This day I was melting down some Lite Salt to make some chunks for fluxing Aluminum.
