Avoid acidic woods to protect a precious flag
I'm building a flag case for a recently deceased family member who was in the military. I read somewhere that you should avoid acidic woods to protect the flag. Would red alder or madrone be suitable?
—Bill Collins, Gold Hill, Ore.
The acidity of a species of wood depends primarily on its tannin content, Bill. As a rule of thumb, the lighter-colored the wood, the lower the tannin content. Oak, walnut, cherry, and mahogany, have higher tannin levels, while maple, birch, and aspen rank low in tannin content and acidity. Unfortunately, both madrone and red alder fall into the high-tannin category. In fact, madrone bark has been used to tan leather. (Tan, tannin. See the connection?)
The good news: Most film-forming finishes provide a barrier against acidity. Polyurethane adds the maximum protection against the moisture that dissolves tannins into tannic acid. Avoid catalyzed lacquer and other two-part finishes that use acid as one of their curing agents, because you could be adding to the problem.