As Dorothy, Scarecrow and Tin Man walked the yellow brick road and came to the forest, their minds were filled with fearful images of the potential dangers that lay ahead – “Lions, tigers and bears – Oh, my!” For those individuals who contemplate storing their precious, heirloom furniture in a storage unit, the concern instead may well be the potential for lines and cracks due to moisture loss in the wood surface.
These concerns are not unfounded, as they were for Dorothy and her friends: Wood is hygroscopic, meaning, when exposed to air, wood will lose or gain moisture until it is in equilibrium with the humidity and temperature of the surrounding air. Protective coatings (such as a wood stain or a polyurethane type finish coating) cannot prevent wood from gaining or losing moisture; they merely slow the process. As a rule of thumb, a relative humidity of 25 percent will result in an average moisture content of 5 percent, and a relative humidity of 75 percent equal to that of about 14 percent. According to the National Wood Flooring Association, the ideal environment for finished wood (flooring, furniture, etc.) is a relative humidity range of 30 to 50 percent and a temperature range 60 to 80 degrees Fahrenheit. Fortunately for you and I, this is also the range of humidity and temperature that the human body is most comfortable, and also why the temperature and humidity levels in a full-service, climate controlled storage facility are maintained within this range.
Here is an informative diagram from The Workshop Companion, a website with a wealth of information for all things wood. The link to the picture, and the extremely helpful article it was attached to, can be found here.
Following are some quick points about wood shrinkage and swelling, as spelled out in a technical publication prepared by the National Wood Flooring Association (publication No. A100):
• Shrinkage usually begins at 25 to 30 percent moisture content, the fiber saturation point. Shrinkage continues to zero percent moisture content, an oven-dry state.
• Swelling occurs as wood gains moisture. Different woods exhibit different moisture stability factors, but they always shrink and swell the most in the direction of the annual growth rings (tangentially), about half as much across the rings (radially) and only in miniscule amounts along the grain (longitudinally).This means that wood will tend to shrink and swell in width, but for the most part, will not shrink or swell measurably in length.
• Generally, wood is expected to shrink in dry environments and expand in wetter environments.
• Wider boards tend to move more than narrower boards. Movement in a 5-inch-wide plank is more dramatic than in a 2 1⁄4 inch strip.
Do you have plans to store wooden furniture in the future? Keep these known issues with regards to wood shrinkage and swelling in mind – it could save you heartache down the road. The knowledge in advance that excessive humidity swings are the bane of wooden items in storage, and that this can be mitigated by storing your items in a climate controlled environment, will help to keep your own yellow brick road free of lions, tigers and bears. Remember what Dorothy found out: the lion was no more than a scary face. Her knowledge of this helped her to face her fears head-on, and make decisions accordingly.