Solidified Iron
Iron that cooled and hardened before it could be tapped or poured.
A pair of these blocks is what an extinguished furnace leaves behind: when the hearth goes out, whatever molten iron it was holding sets in place as two Solidified Iron blocks rather than being lost. Mine them back out for one iron bit per 5 units the reservoir held; the slag the furnace was also holding is not recovered this way. See Ironmaking for what actually puts a furnace out.
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