Used in a Smelter to melt ores and create alloys.
The recipe shows up once you have done any one of these:
Read the quest's page
The Blacksmith needs a Smelter to craft better equipment. To craft a Smelter, a Crucible is needed.
He suspects the Scavengers have his old Crucible at their mine in the Revelwood.
The Crucible was found. It needs to be crafted into a Smelter with the Blacksmith in order to unlock more crafting recipes.