Q: I have been inundated for the past six months with questions about wood (and products) received in North America from countries across the Pacific Ocean. The typical complaint is that there is some warp, shrinkage, surface or end cracks, and/or open joints.
I have been inundated for the past six months (the winter heating season) with questions (letters and e-mails) about wood and wood products received in North America from countries across the Pacific Ocean. The typical complaint is that there is some warp, shrinkage, surface or end cracks, and/or open joints.
There are a couple of basic principles that are not being followed at the point of manufacturing.
First and foremost, wood does not shrink or swell, and it does not change its size or shape unless its moisture content (MC) changes. (The only exception to this rule is that warp seen immediately when machining the wood. This is caused by stress or casehardening.)
Second , moisture readings taken after the problem has developed will always be very close to perfect. Understand that if the wood was originally wet and then dried out and cracked or warped, readings taken after the wood has warped or cracked will be low MC values, as the wood has dried out.
Third , the typical MC in use for most of North America during the wintertime heating season is no higher than 7.0 percent MC (equivalent to roughly 37 percent RH) and is often 6.0 percent MC or below (30 percent RH or less). A few warm coastal locations may have higher values.
Fourth , many so called 'off-shore' locations are at more humid locations, so the wood they process is often closer to 10 percent to 12 percent MC. This is true even if the wood is initially dried to a low value but then exposed to the normal in-plant, humid environment.
Fifth , most wood products can tolerate a 2 percent MC change if the change is done slowly. Fast changes or greater changes often result in warp, cracks, open joints, and so on.
Sixth , wood in a tight container or a sealed plastic bag will not gain any moisture during an ocean voyage, as no significant amount of moisture can get into or leave the enclosure. Temperature is not a factor. In other words, high MCs seen when the wood arrives in North America were high values when the wood was packaged and shipped.
Seventh , moisture tests using meters with bare needles will measure the wettest spot along their length. So, a slight increase in the surface moisture will make the piece seem wetter than it really is. Pin-less meters on lumber with a wet surface also will read too high. If the surface is dry, but the core is wetter, both meters will do well.
Eighth , the core moisture content (usually determined with a meter using insulated needles) reflects quite closely the moisture content of the lumber when it left the dry kiln. Core moisture changes very slowly compared to surface values.
The bottom line with all the problems that have been presented to me is that the wood or wood product is too wet for our North American environment.
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