Wiktionary
iron bloom
n. A mixture of fine iron particles, unreacted iron oxide (ore), slag and charcoal residue.
Usage examples of "iron bloom".
They were all men who loved their craft, and by the end of the fifth day they were filled with excited anticipation as Cavallo brought a lump of iron bloom to his furnace and poured it molten into clay racks, finally turning out heavy bars of the metal onto a workbench for them to examine.