by Professor Gene Wengert
I am interested in drying woods used in the construction of musical instruments.
I understand that concientious luthiers air-dry their tonewoods from 1-4 years in order to maintain the highest tone quality.
I have heard that in conventional kiln drying they try to achieve certain temperatures in order to "set" the pitch in softwoods for example. Even higher temperatures are required to release the "bound" water in green material.
My question is what takes place with the bound water and pitch in the air drying technique. I am curious about the effects of low temperature drying approaches such as Vacuum, or RF Vacuum.
Drying Methods--Air-Drying vs. Kiln Drying
We know that very slow drying of lumber enhances the musical properties. Drying at warm temperatures or hotter has a definite effect on wood properties. Therefore, air drying for a year and then continued drying in a home or office to achieve the correct final MC is essential. Air-drying alone will reach only 12% MC in most of North America; 7% MC is the typical final MC required for interior uses. We also know that rain on the lumber enhances certain other properties. For example, white oak lumber is normally quite acidic in character. But if you air dry it for 2 years, there is a vanilla odor that becomes very obvious and enhances the flavor of wine and whiskey in barrels made of such material! In short, air drying cannot be replaced for musical instruments--even low temperature systems do not do as good a job.
Bound and Free Water
Now, let's discuss "bound water." In the living tree, the wood cells, which are like skinny soda straws, are about 3 to 5 mm long; the diameter is 1/100 of their length. The center of the cell is hollow. Water that is in the hollow space is called free water. It could be removed by blowing it out of the cell. (Actually, we call the liquid "water," but the water contains many other chemicals--just ask anyone who likes maple syrup on their pancakes what the water in wood tastes like!) The cell wall itself (which is actually 1.5 times heavier than water, so wood actually doesn't float--the air in the hollow spaces makes wood float) also can absorb water. The wall can absorb up to 30% of its weight in water. This absorbed water is called bound water, as it is held in the cell wall by hydrogen bonding.
When drying wood, the free water leaves first. It requires less energy to evaporate than the bound water. At about 30% MC, all the free water is gone and just bound water remains. At 99.9% RH, the MC of the wood will stay at about 30%. But, if the RH is lower, the bound water will evaporate--for example, 80% RH is 16% MC, 50% RH is 9% MC, and 30% RH is 6% MC.
Setting the Pitch
Professor Gene Wengert is Extension Specialist in Wood Processing at the Department of Forestry, University of Wisconsin-Madison.
Click on Wood Doctor Archives to peruse past answers.