Smelt Dark Iron

Smelt Dark Iron allows a miner to smelt 8 Dark Iron Ore into 1 Dark Iron Bar.

Source
You must give a tribute to, a ghost in the Summoners' Tomb in the Blackrock Depths before he will teach this recipe. After speaking to him, a bowl will appear at his feet. Touching the bowl will offer the quest The Spectral Chalice. Speak to him again after completing the tribute and he will teach you to smelt.

Completing the quest requires 20 Gold Bar, 10 Truesilver Bar and 2 Star Ruby.

It is important to talk to Gloom'rel before any other dwarf since Doom'rel will begin the event to open the door which requires you to then kill Gloom'rel. Dead dwarves make poor teachers.

Dark Iron can only be smelted at the Black Forge deep in the instance. Unlike most smelting normal ore, it requires eight pieces of Dark Iron ore to create one bar of Dark Iron. You should take this into account, as some of the smithing recipes require as much as eighteen Dark Iron bars. That translates into at least one hundred and forty-four Dark Iron ore.

Also, Gloom'rel will remember that you paid him so if you ever decide to temporary drop mining as a skill then you can relearn the ability to smelt it again at a later time by simply paying him a visit.