A brownish-red or reddish-brown powder that is a form of iron oxide, used historically as a polishing agent and pigment.
From Arabic 'qulqul-tār' or possibly Persian origins referring to an iron oxide residue. The term entered European languages through trade and appeared in alchemical texts and early chemistry.
Colcothar sounds like it belongs in a fantasy book, but it's actually a real chemical waste product from making sulfuric acid—and people figured out it was perfect for polishing metals! Waste into treasure through chemistry!
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