Q: We have just encountered a few pieces of 4/4 red oak lumber, all from one supplier. After we plane the lumber and let it sit for a few days, we will have small (1/4 inch along the grain and 1/16 inch wide) black spots that look like there is tar inside the wood. The black tar seems to bleed out a little bit after a week or so. The lumber has been kiln dried at 160 F maximum. If we do not find these pieces, the black will bleed through in small streaks through the light colored finishes.

A: Once a year, I receive a call about this problem. Your description is excellent, so I am certain about what the defect is without examining a sample. Thank goodness this stain is so rare, as it does indeed cause appearance problems in the final product, often months after finishing and shipping.

The spots are a result of the tree picking up some chemicals from the soil during the tree's growth. In fact, it is not unusual to see this problem in many of the trees from the same small geographic area. Usually the stain is confined to the butt log. As soon as that area is logged and the logger moves to a new area, the problem will be gone. So, this is why you notice that the problem is coming from one supplier. By the time I received your letter, the problem had already disappeared, as you had moved on to logs from other sources.

Kiln drying does not eliminate the staining. Cleaning the wood with a solvent will clean the surface, but the stain will reappear with time. Sanding will also eliminate the staining temporarily. If such wood ends up in a product, replacement of the defective pieces is the only long-term solution. Therefore, eliminating such wood in manufacturing is the best solution. Have your people who handle the wood before planing and/or sanding keep their eyes out for this stain, as immediately after planing or sanding, it is not visible.