Exactly how iron was first smelted has long been a question for paleometallurgists. Copper smelting is pretty obvious: heat chalcopyrite (CuFeS2) and the sulfur burns off and sulfur dioxide leaves. The bonds holding copper and iron break, and the copper melts.
Cassiterite, the Cornwall tin ore, is SnO2; heat it and the tin oxygen bond breaks, giving molten tin.
Iron ore requires carbon in the fire because the heat that breaks the iron-oxygen bond can recreate the bond before the oxygen leaves; add carbon and it leaves as carbon dioxide, giving molten iron. But why would you just happen to make a very hot fire with carbon and iron ore? It has been speculated that the Egyptians accidentally smelted iron while extracting gold from magnetite sands (the black stuff the magnet picks up in the sandbox).
I have a simpler explanation. The slag left over from smelting chalcopyrite is mostly iron, some other impurities in the ore and likely some insufficiently melted copper. If you wanted the last bit of copper goodness out of the slag, what would you do? Heat up the slag in a very hot fire; maybe throw some charcoal in for extra heat. Along with copper dripping out, you might well get molten iron.
That's a good theory. I recall someone else wondering how iron took over from bronze as iron is much harder to make, and not that much harder than bronze, so it's not like an iron sword was breaking everyone's bronze sword.
ReplyDeleteYou might find this undergrad paper by me of interest: http://claytoncramer.com/unpublished/Iron2.pdf
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