Slag

A by-product tapped from the blast furnace.

A Blast furnace taps roughly one unit of slag for every six of iron it recovers, and a Sand Casting Bed freezes a slag heat into Slag Brick just as readily as an iron one, no second station needed. Slag itself lays straight into Slag Path with plain gravel, four slag and a gravel making two path tiles; the slab takes two slag and the stairs four, each for one block.