Determining moisture content of wood
Bill Tindall, E.Tn.
The National Harwood Lumber Association reference method for determining moisture content of hardwood lumber is to dry the test specimen to constant dry weight at approximately 210 F and express % moisture content based on the weight loss and the dry weight.
A method based on microwave drying is described here:
http://www.richardjonesfurniture.com/Articles/microwave-dry-wood/microwave-dry-wood.html
I will critique this method at the end of this discussion but to understand the critique one needs to understand how wood dries.
Understanding drying: Drying is often confused with boiling water. Heat the sample to the boiling point of water by oven or microwave and the water evaporates and leaves the test specimen at the 0% moisture content essential for the calculation of moisture content. This picture of the drying process is wrong and misleading.
Water exists in a freshly cut piece of wood in three environments. There is liquid water in the open spaces in the wood, as well as inside the wood cells. The cellulose and lignin contain about 10% water dissolved in these components. Dissolved means this water is present as dispersed water molecules, not liquid water. Air dried wood may have some water in cells and most dissolved in the cellulose and lignin. The wood we typically consider using will only contain this latter water.
To dry to 0% moisture content the water in all these places needs to be removed. Removing the last 20% (approximately) of the water has nothing to do with evaporation. The water is already molecularly dispersed, the process boiling enables. Rather, drying involves diffusing water molecules from inside the wood to the wood surface.
At the microscopic level wood consists of solid cellulose and lignin, and open spaces. A small molecule like water zips through open spaces hundreds or thousands of times faster than it diffuses through the solid parts. Water molecules dissolved in the center of a test board will pass from open spaces to solid to open spaces dozens of times before it reaches the surface and leaves. Some of the open spaces will be connected to the surface of the ends of the test piece. If the water molecule finds itself in an open space near the end it will leave by rapidly diffusing out the end rather than the much slower route to the surface. Research at Virginia Tech has shown that water leaves the ends of boards more than 10 times faster than through the surface. For this reason we coat the ends of lumber to keep it from checking as a result of the ends drying more rapidly than the center. Furthermore it makes sense that dense woods, like oak, dry slower than light woods. The woods are lighter because they have less cellulose and lignin, the barriers to diffusion.
This drying property is important in measuring moisture content of lumber by drying a test piece. A short test piece is going to dry substantially faster than a long one. For example, I dried two test pieces of oak at 210 F in an oven. One was 1” long, 1” thick and 5” wide. The other was 10” long, ¾” thick and 12” wide. The short piece reached 0% moisture content in one hour and it was down to 1% moisture content in 45 minutes. The test piece 10 times longer was still 5% moisture content after 45 minutes and it took 6 hours to reach 0 %.
The method proposed in the link uses microwave energy to dry the wood. Unlike boiling water where the microwave will dump energy into the liquid water to provide the heat of evaporation to hasten boiling, the only effect the microwave has on drying wood is to heat the wood, which it will do more rapidly than an oven. Diffusion is highly temperature dependent. But once the microwave heats the wood to 210, the maximum temperature shown to not cause wood decomposition, the diffusion of water out of the wood will be at the same rate as a test specimen heated to the same temperature by an oven. The only time gain of the microwave will be the reduced time to reach drying temperate.
Although the microwave method was not validated by comparing it to the oven reference method, I have no doubt that it will provide a reliable estimate of moisture content sufficient for woodworkers, and not risk damage to the microwave, provided the method is practiced as described. However, the rapid drying of the test piece demonstrated depends mostly on its short 1” length and little on the use of the microwave heat source.