I was sewing a batch of leather Plague Doctor masks ('cause that's what I do) when finally solved the mystery of strange blue-black stains we occasionally get in the leather. The tannins in the leather (from tanning the leather.... get it??? tannins, tanning...) react with the iron in my sewing needles when I leave the leather in water with the needles still threaded. The stains are the exact same color as my homemade powdered iron gall ink so I had an "Aha" moment.
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