Just discovered lately that Black furnaces cannot make copper bars (didn't try the others), to me it seems odd that it could possibly work on one material (Black metal) & not what's below it in progression, since they are basically Smelters.

I'm not sure if they looked up the idea of making them more efficient or have higher capacity , but I think they are odd to do what smelters can, wish the developers look it up or if it is intended, explaining it.