Blackening Reaction Between Wood and Metal

Wood tannins react with iron to form black-staining iron tannate; oxalic acid can remove it. November 18, 2006

One of our customers purchased black cherry lumber from us to be planed into flooring, but we got reports from the job site that some of the cherry turned dark in the areas where it had been in touch with metal. For example, where a can of finish had been sitting for a while, a dark ring remained. Reportedly it was not due to any leaking finish.

Does anyone know of chemical reactions between black cherry and any metals? If so, which ones? I haven't seen any pictures yet myself, so I can't offer any more information.

Forum Responses
(WOODnetWORK Forum)
From Gene Wengert, forum technical advisor:
Many woods have tanic acid that reactions with iron to form iron tannate. Iron tannate is black in color. It can be removed instantly by putting a little oxalic acid (see a drug store pharmacist) on it. Unfortunately, with cherry, the acid may turn the wood a deep red.