A Look at MOTHER’s Solar Wood-Drying Kiln: Part One

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The south face of the wood-drying kiln is covered with recycled tempered glass.
The south face of the wood-drying kiln is covered with recycled tempered glass.
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Roof vents allow moisture-laden air to escape. A 12
Roof vents allow moisture-laden air to escape. A 12" fan, driven by one 35-watt photovoltaic panel, provides airflow of 350-cubic-foot-per-minute through the distribution plenum.
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A Lignomat H 30 lignometer provides accurate moisture content readings.
A Lignomat H 30 lignometer provides accurate moisture content readings.
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The end-sealed lumber is stacked on the piers, with air spaces between each piece; stickers separate the layers, and a polyethylene jacket is stapled around each stack to assure consistent air distribution without leakage. Perforated air delivery tubes lie between the stack piers.
The end-sealed lumber is stacked on the piers, with air spaces between each piece; stickers separate the layers, and a polyethylene jacket is stapled around each stack to assure consistent air distribution without leakage. Perforated air delivery tubes lie between the stack piers.

When you pay a visit to your local lumberyard, the wood you take home will have more than likely been dried in a gas-powered wood-drying kiln. These sophisticated cookers can process commercial lumber by the tens of thousands of board feet at a clip. However, Mother Nature offers an excellent source of energy that ultimately can accomplish the same task and will allow you to produce your own lumber, from forest to finished board, at a cost your local dealer couldn’t come close to.

On the other hand, drying is tricky; it’s easy to get wood to shed moisture, but it’s another thing entirely to control the process so that the resulting lumber is usable. The last thing you’d want to do is simply lay your green boards in the sun to bake.

Why? Because the dampness in wood exists in two forms: bound water, which is captured in the cell walls, and free water, which is held in the cell cavities. The goal in seasoning is to bring the wood to a moisture content (MC) — designated, by percentage, as the ratio of the total weight of water in a given amount of wood to the weight of the sample when it’s been completely oven-dried — compatible with the dampness of its environment. This is known as the equilibrium moisture content (EMC), and it varies with the surrounding air’s relative humidity.

Simple air drying removes the free water, which accounts for the wood’s moisture content above 30%, or so. Below that fiber saturation point, natural evaporation occurs more slowly, since the wood must then give up its bound water. And the release of this cell wall moisture can give woodsmiths fits; it causes the cells to shrink, resulting in stresses that can warp or damage the finished product.

Now, shrinkage always accompanies drying, but uneven shrinkage creates problems. Wood, as you might suspect, dries from its surface inward. Hence, an imbalance is created between the high-moisture core and the lower-moisture exterior, which causes the water to move toward the surface, where it evaporates.

  • Published on Sep 1, 1984
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