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Originally Posted by apolaki
Is the black smudge a result of zinc coating on the blade? I saw a video where vinegar turns zinc coated screws black. So did i essentially strip the zinc off the blade?
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I don't think zinc is involved to any great degree here. The blade is iron/steel with a number of impurities and has likely been heat treated. Heat treatment can make those areas darker when etched, owing to different crystalline structures and inclusions relative to the untreated areas. We have professional bladesmiths here who can probably give you a more complete answer.
Ian.