Advantages

A solar kiln will give you the opportunity to "get to know your wood" better than you have ever known it before! The average "wood weight" of 1 BF (1" x 12" x 12") of red oak I have purchased in Southeast Texas has been 4.1 pounds (#). The average "water weight" has been 3.075# (or 75% mc). To achieve a 7% mc in a 1,000 BF load, the dryer must exhaust 2,738 pounds of water from the wood I have used as an example here. During a 60 day drying cycle that means 45.6# of water per day (1.67% of the moisture content) needs to be exhausted from the dryer. That is well within the tolerances that red oak can stand without inviting degrade.

If you don't quite understand how to measure the moisture content of wood here is a short explanation. Do the math and you will find the green board in the above example weighs 7 1/8 pounds per board foot. All that means is that a 10' long board is heavy. Mc is determined by comparing the weight of water in the wood to the absolute dry weight of the wood itself.

• If the absolute dry weight = 4.1 pounds per board foot then...

• 3.075 pounds of water = 75% of the absolute dry weight of the wood = 75% mc.

• One board foot @ 7% mc = 4.387 pounds.

It is during the first 1/3 of the drying cycle that the dryer will restrict the release of water from the wood due to 100% saturation of the air. As the mc of the wood drops, the dryer will begin to 'extract' the water from the middle portions of the wood. This is because there is less water (from the wood) available to readily saturate the air inside the dryer. Thus more dry air remains available to soak up the remaining water during the last 2/3 of the cycle.

A solar kiln is an enclosed 'natural environment' that allows you to control of the amount of heat (dry air) that is introduced. Do that by adjusting the exhaust baffles during the drying cycle. The rest is between the wood itself and the natural environment surrounding your dryer.

• 1 cubic foot of 70-degree air, at 100% relative humidity, heated to 140 degrees becomes 2 cubic feet of air containing 50% humidity. This means that 50% of the fresh air introduced into the dryer is dry air. (Air expands; water does not).

• Except in extreme conditions, the temperature inside your kiln will not reach 140 degrees. I would still suggest daily monitoring during the "heat of the day" so adjustments can be made as necessary.

• The performance of your kiln will depend on your geographic location. Learn the weight of the wood you choose to dry on the first day of the drying cycle and monitor daily. You will gain confidence with each load you dry and use.

Conclusion

Solar kilns are ineffective for high production shops, but they certainly have their place with serious hobbyists and individual furniture and cabinetmakers. You will receive as much gratification designing your solar kiln to fit your geographical location and your own needs as you do working the wood you produce with it.

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