Black residue after soaking walnuts

hardlikearmour is a trusted home cook.

added 8 months ago

I can't say for certain, but my suspicion is the tannin from the walnuts is reacting with iron ions in the metal to create iron tannate. You can actually make a dark brown to black ink or dye using walnut hulls (higher in tannin than the nut itself) and rusted nails (or the like).

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