Most iron in the ancient world was smelted in a bloomery. The bloomery burns charcoal, which gets hot enough to separate the iron from the other impurities, but not enough to melt the iron. The impurities melt off as “slag” or “bloom”. The iron “sponge” is then heated and beaten to remove the remaining slag, then forged into whatever is needed.
About 300BC, blacksmiths in China figured out if you mixed the iron and charcoal, it dramatically lowers the melting point. Cast iron is much more brittle than forged iron due to the high carbon content, but can be cast like bronze and gold, which they were already using.
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