Originally posted by Raven:

nah just the regular furnace, is it not just stones slapped together, and then you have a furnace? dont see the need for those cubes other than a time blocker for people to get things?

I'm not going to attempt to get inside the minds of these awesome Devs, but I think the "idea" is that... while you are out there trudging along in the Dark Forest gathering ores, you might as well pop into a dungeon or two and grab some Cores. In all their uses (so far for me) they seem to bring the "fire" element to whatever they're being used for. So in that respect, it makes sense to me they would be needed for the Smelter, the Kiln, and the Blast Furnace (and other things as well).