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The Wood Shoppe - #5 Chessboards: More about wood movement: Water Content

Jan 20, 2011, 11:15 AM 3

Hardwoods are initially harvested with very high levels of water within.  The accepted method of measuring water content within a given wood sample is to record the weight of a sample piece of green lumber (as harvested), then dividing by the weight of the same piece in an oven dry condition.  The quotient is referred to as 'percent moisture content' and generally ranges between 55% and 130% above the 'oven dry (completely dry)' weight of the hardwood in question.  By way of comparison, percent moisture content in softwoods typically ranges between 98% and 250%.

After harvest, based on environmental conditions, the water within the wood evaporates into the atmosphere.  As previously discussed, this is a gradient type process that starts at the outermost fibers and works toward the inner fibers.  For discussions sake, let's harvest a tree and cut it up into rough sawn planks.  From those planks, let's take a sample piece to focus on.  We will take our sample piece under roof for a period of time so as it continues to dry out, it will come to a point where the moisture content stabilizes with respect to the surrounding atmosphere.  During this process, something incredible happens.  As the moisture content in the wood goes lower and lower, a critical point is reached called the 'fiber saturation point'.  When this critical point is reached, the cell cavities are completely emptied of water, but the cell walls are still fully saturated.* This is the point where wood begins to change dimensionally.  Typically, this occurs in the vicinity of 29% moisture content but depends on the type of hardwood in question.

As we continue to remove moisture from the specimen, the anisotropic characteristics previously discussed take over, and we note that the percentage dimensional shift will be different in longitudinal vs. radial vs. tangential vs. transverse axes and planes.  The implications are profound and very real.

Now, life would be easy if once all water was expelled, the specimen would retain its final physical form.  Unfortunately, this is not the case.  Hardwoods (and softwoods) will expand and contract constantly, slave to changes in relative humidity, and to a lesser degree, temperature.

*Hoadley, R. Bruce 1980.  Understanding Wood, 256 pp.  Newtown, CT: The Taunton Press, Inc.

In our next discussion, we'll talk about proper curing practices, and why they're so important.

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