I would *guess* it came in with the use of coal in the high middle ages. The cruder the wrought iron the more "natural flux" it contains and charcoal ash is a flux for wrought iron as well; so as your iron gets better and you switch to a "non-fluxing" fuel a bit more flux might help.
As for accidental; how about dropping the piece on a sandy smithy floor and finding out after reheating that it worked better?
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Thomas
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