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Mysterious black stains on wooden cutting board

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Large black splotches have been appearing soon after use; can't correlate with any specific kind of food. What's the cause and, more important, what's the cure? Can I sand the surface down by hand with very fine grade sandpaper?

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  1. moisture may have seeped in to the wood fibers and now their starting to grow mold.

    1 Reply
    1. re: rebs

      The last splotch appeared within minutes of some chopping followed by a rinse-off, inconsistent with something having grown.

    2. Yes you can sand it. I have an edge grain Boos block that is several years old and a couple of times now my DH has taken the sander to it to smooth out the surface. But I don't know if that will solve your problem, whatever it is, and I agree it doesn't sound like mold.

      1. I don't know the cause (if you ever find out, let us know!) but I would sand it gently, disinfect (and rinse well) and then oil it down really well to try and seal it.

        1 Reply
        1. re: leanneabe

          Are you chopping something starchy like potatoes? The starch residue can turn black if not rinsed off immediately.

        2. Sounds a bit like mold. Try using some bleach in water, then rinse and dry. Bleach should take care of the mold, and it dissapates quickly. I keep some bleach/water in a spray bottle in my kitchen.

          1. The stain was probably caused by water and iron staining the wood. What you can do is get some OXALIC ACID powder (be careful this is strong stuff). Make a strong solution, blot it onto the stain, and leave a cloth soaked in the solution on the stain for a few hours.

            The stains won't affect the quality of the board, or affect the food in any way.

            Rinse the board with a soluton of water and vinegar.

            1 Reply
            1. re: mymymichl

              Oxalic acid can be somewhat dangerous, do not get it near your eyes or inhale
              any of the powder. It's sold as "wood bleach" at your local well-stocked hardware
              store (but not, apparently, at Home Depot). There's a second substance sold
              labeled as "wood bleach" which is a 2-part peroxide mixture. That's not what
              you want here, costs much more (a lifetime supply of oxalic acid powder is
              about $5), and doesn't work nearly as well.

              Make sure you rinse the board very, very well after the acid treatment. It's
              water soluble so a good rinse will remove it all. I get these stains on one of my
              boards pretty regularly. I don't think it's rust because oxalic acid is completely
              ineffective against rust stains but it cleans these things off in seconds. The
              stains seem to be directly related to cutting potatoes.

              Sanding the board is always a good idea. Makes it all new again. Hit it with a
              sheet of 110 grit sandpaper wrapped around a sanding block followed by 220
              grit and you should have a nice rejuvinated surface. Expect to spent 15-20 minutes
              of reasonably hard work.

              After destaining and sanding, you should give it a light oiling. Either some mineral oil,
              'butcher block oil" (which is just mineral oil at 2x the price), or walnut oil (right
              from the grocery store) would work. Don't use other sorts of vegetable, olive, etc oils.
              These don't ever dry and can eventually get rancid. And *never* use non-food-grade
              wood finishes or oils; they often contain all sorts of scary drying agents and stuff.