paxman

Oxalic acid (use ZUD cleaner) will remove those stains instantly, and oxalic acid bleach will remove acid stains on oak also. The tannins in the wood react with any mild acid, like vinegar, and iron (even in small amounts) to produce ferric tannate, used for centuries as a source for black ink and famously written about by George Franck in his wonderful books and articles about wood finishing, which I highly recommend. You can make your own ferric tannate to dye wood and other things grey (not quite black, for that your really need aniline dyes) by placing oak shavings in vinegar along with a wad of steel wool. Just let it stand for a few days, strain out the particulate, and there you have it.