The Wood Shoppe #8 - Chessboards: Equilibrium Moisture Content in Hardwoods
In the world of solid hardwood chess boards, the idea of Equilibrium Moisture Content is ludicrous. The very description suggests that our wooden board will live happily in a steady dimensional state. Not. Nonetheless, it is wise to understand the concept of Equilibrium Moisture Content so that we can minimize the effects of wood movement.
As we've learned, even after harvesting and curing, hardwood is capable of moisture absorption and expands and contracts accordingly in anisotropic fashion. The designer must understand the nature of wood movement and its potential impact to a given design. One way to control wood movement is to understand where the product will be used in its final form. If for example, the product will be shipped into a region of high humidity, the lumber can be pre-conditioned for that environment by holding the lumber at a preset level of humidity for a period of time prior to construction. During this holding period, the wood comes into equilibrium with its controlled environment and expands or contracts accordingly. The product may then be fabricated, constructed, and shipped knowing that it has been preconditioned for its new environment.
Accordingly, the builder is well advised to precondition lumber to average destination humidity levels prior to starting construction. This is the best way to minimize future effects of cross-grain expansion, but as we know, trying to predict humidity levels is akin to predicting the weather. Further, some people condition the air in their homes with air conditioning, dehumidifiers, humidifiers, wood stoves, forced hot air heating systems and so forth. Preconditioning the lumber, while a good idea, is not always practical. Instead, most woodworkers will choose a ‘statistical’ approach to conditioning the lumber. If you start with the lumber somewhere near mid-range moisture content levels, you can’t be more than 50% off the mark. If therefore it is predicted that a given piece can expand as much as 3/16” over the full range of absorption, we've just cut that to a max of 3/32”. Beyond that, most people that live in extremely wet or extremely dry climates, condition the air in their homes to comfortable levels, so it’s reasonable to assume that the maximum growth may only be in the realm of 1/16” or less.
While the reader might take some level of comfort in knowing that properly constructed wood products attempt to minimize the effects of water absorption (or expulsion), any changes in dimension no matter how small spell big trouble unless the designer understands the potential impact of the dimensional change and negates it through good design practice.
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