It would be nice to be able to configure the sulfur furnace to choose other ores for the limestone flux to work with (such as iron, as real life iron smelting uses limestone flux).

I know this is a bit of an older issue but figured i'll add to here instead of making a new one.

Would it be possible in the meantime to make lists ORES and INGOTS in ItemRegistry public, or a way to add to it, so an addon could register more ores/ingots to be smelted and doubled in the furnace?

That would be amazing, thanks!

That should be fairly simple, will look into it.
I'll probably add a list of flux item + ore item pairs such that certain ores can also use different items as flux.