Q: We have one supplier of red oak lumber that seems to have a lot of blue- to black-colored spots. They are just on the surface, but it makes the lumber look poor. Is this a defect?

A: When tannic acid (which is plentiful in red oak), water (which is plentiful in freshly sawn oak), and iron (from a saw, forklift forks, metal bands, etc.) are mixed, the result is iron tannate, a blue-black-colored chemical. You must eliminate the iron source.

The stain can be easily removed with a dilute solution of oxalic acid applied to the lumber. Commercial chemicals, often called lumber brighteners, work well.