I have been asked to make photo plaques from solid wood (approximately 5” square), to which photos would be applied directly. There is a concern that acid in the wood may affect the photo over time. Having done some research online, it is evident that there are a variety of acids occurring naturally in various species of wood, but I cannot find a comprehensive publication identifying the type and amount of acid. Is there a study that produces a chart showing the acid content of various species of wood? As an alternative, what finishing product (i.e., shellac) would provide an effective and permanent barrier to the acids? – Ralph Petersen
Tim Inman: Attaching a photo directly to any wood plaque would be a recipe for long-term damage to the photo. The wood and the paper, not to say anything about the glue used, move at different rates as humidity and temperature change. The glued-on photo will eventually give up, acid or not. So, I would suggest making copies of important photos to apply. Keep the originals safely in acid-free, archival storage files. Making copies of photos is so easy today, it hardly seems right to risk damage to any important picture or historical article.
That said, walnut, when wet, will bleed a dye that can damage the paper. Oaks are full of tannic acids. I would suggest consulting with the USDA Forest Products Research Station, at the University of Wisconsin, in Madison, Wisconsin. They have a great website http://www.fpl.fs.fed.us/, and would be the major reference resource for all things wood. Any good sealer would be recommended prior to attaching the photos. In my view, two or three coats of even the worst sealer would be superior to one coat of the best. Be sure the finish and the wood are completely dry before proceeding.