Q: We have been working with red oak for many years, but just this past month we have had several problems with our furniture that are new to us. Specifically, we had one job that we finished with a white-colored paint finish. Several of the individual strips of wood developed a dark, dingy, dirty hue along almost the entire strip about a month after they were put into use. The back sides of the pieces were unfinished and had some mineral in the wood. Do you think that a recent switch to No. 2 Common lumber is involved?
A: The general darkening that you report could be from several different sources, but I will suggest a common cause related to the wood itself. Because you indicate the presence of mineral on the unfinished back side of the pieces, I think we might eliminate some of the common finishing problems. So, I am suggesting two possibilities. The term "mineral" is not well defined within the industry today. Therefore, the dark streaks and spots that are commonly called mineral can be fungal, bacterial, or even just a concentration of dark-colored chemicals. Some of these chemicals can be soluble in the finishing materials (the vehicle or solvent) that you are using, which will result in staining of light-colored finishes. Perhaps this is the case, especially if the off color was evident soon after finishing.
But there is another, more likely cause. Some trees we harvest today have a bacterial infection in them. Although the bacteria do not kill the tree, they do affect the wood - it dries more slowly, and often has a foul odor, ring separations, and machines poorly. Also, the by-products of the bacteria will migrate through a finish with time and darken the finish. On the other hand, the bacteria doesn't affect humans.
Here is how you can test for bacteria. Try wetting the back of the pieces and then see if you can detect a foul odor in a few minutes. The unpleasant odor in the wood characteristic of bacterial infection usually becomes obvious when the wood is in a humid environment.
You might have your rough mill people be especially observant for pieces or parts with shake (a crack going with the rings rather than across the rings, when you look at the end grain), as shake is probably a 99 percent indicator of bacterial infection. We find that the bottom log in older trees that are growing in a wetter site will be more likely to have the infection. The bacteria does not jump from board to board, so there is nothing we can do or need to do when bacterial infection is noted, other than to eliminate such wood from production and change the location where we purchase our wood.