Board Thread:Questions and Answers/@comment-74.92.168.165-20130302185923/@comment-5297911-20130504082306

honestly, using a slag furnace is not only cheap to build which is up to only 22 cobblestone and not only does it take only the same fuel a regular furnace takes, it also gives you more than what you bargained for including smooth stone when its an ore.

The down side is when I was smelting percisly 26 Iron ore which smelted in a regular furnace would give me the exact number of ingots I need to make a full set of Iron armor, but I got more in the slag furnace of 8 ingots extra and 5 smooth stone. It did take more than 3 coal which is normaly how much you would need to smelt down 26 Iron ore without leftover fuel.