Excluding moisture exchange from wood

August 7, 2017

Wood is hygroscopic meaning it wants to absorb or release moisture based on the humidity of the environment. Wood reaches an equilibrium moisture content based on whatever the conditions are. In a humid Iowa summer any wood can get close to 12% moisture content (MC) in a non air conditioned home. In winter the same wood will pretty quickly drop to close to 6% MC in a normally heated home. When wood absorbs or loses moisture it shrinks or swells across the grain but not along the grain. It continues to do this as long as it exists as a piece of wood. Different species swell or shrink different amounts, but all move to some degree. We have a coffee table that I built of 150 year old reclaimed walnut from central Illinois. The top is 2 ft. wide, and it will swell 3/8″ from winter to summer, then go back when winter comes again. This is quite a lot, and if a wood worker doesn’t plan how to construct something based on this knowledge, it can lead to trouble.

Even plywood is affected to a degree, but since it is layers with the grain going alternating directions, it is less affected.

In order to build a hammered dulcimer that is completely tuning stable, we learned that moisture exchange has to be excluded from the wood. It is the shrinking and swelling that throws a dulcimer out of tune, and it can happen fast if a dulcimer that is used to one environment is taken to another that is much different. So we did a lot of research and experiments to find an appropriate coating. Shellac doesn’t work, nor does lacquer or varnish or polyurethane. Surprisingly, according to the Forest Products Lab the most effective coating is melted parrafin wax! It does work, but is quite a mess! Boat builders epoxy also works, but needs to be at least 3 coats to be effective. Other than that epoxy paint also works, but again needs to be at least 3 coats.

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