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

We've come up with this design for a solar-powered wood-drying kiln with the aim of creating seasoned lumber at a fraction of the cost of commercial facilities.


| September/October 1984



wood drying kiln - front exterior of kiln building

The south face of the wood-drying kiln is covered with recycled tempered glass.


Photo by MOTHER EARTH NEWS Staff

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.

Too rapid or uncontrolled moisture removal shrinks the cells at the surface, preventing the interior moisture from escaping properly through the outer shell. The stress created can cause a variety of defects, including honeycombing (internal collapse), case hardening (simultaneous compression and tension in the same slab), warping, checking, and splitting.

michael hilliker
7/10/2009 9:19:07 PM

As an old mother from the 70's, I remember when this article was first published. I used rough cut lumber to build my home. I build a solar lumber dyer using old window and doors. Much of the dryer was built using the description I read in the magazine. It worked well and the house was built in 1984 and there was no shrinkage was ever found.






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