Oven-Drying Wood for Moisture Content Testing

      Information on the correct methods for oven-drying wood samples to determine moisture content. September 27, 2008

In trying to come up with a very accurate method for determining MC in 5/16" sitka spruce quartersawn soundboard stock, I performed the following oven drying procedure on 3 different samples on different days.

From a glued up approx 10" panel, I cut a 1.5 inch x 10" strip, weighed the strip to .00g, measured its pre-oven length (.00"). One was dried at 250-300 deg and the other two at 200-175 deg until weight stopped dropping.

The hotter oven showed the sample to be at approx 7% MC, while the cooler oven showed the sample at approx 6% MC. I am assuming the hot oven to be vaporizing more than just H2O, so the moisture number is questionable.

As I observe the samples as they pick up moisture and come to equilibrium, despite stable temp/rh pre- and post-oven drying,

1- None of them recovered their original weight (read moisture %),
2- Post oven, after the samples reached equilibrium, they were 1% shy of the original MC (temp/rh was stable both pre/post oven drying). This 1% shy of the original MC reading of 6% is in closer agreement with the emc/temp/rh charts of what the sample's MC "should" have been.
3- None of them recovered their original length.

Does oven drying destroy the cell structure to some extent? Is there a way to be sure you are not evaporating solvents in the wood besides the expected H2O during the oven drying?

Forum Responses
(Sawing and Drying Forum)
From contributor P:
I have had trouble doing oven tests on Spanish cedar and was also wondering if oven testing at 225 degrees F removes some of the oils, thus producing a lower MC than it really is. I have tried lowering the temp on my sample oven and have had different readings from the same area of a test board. I would guess that it does remove some of the oily residue and give a false reading, but I am not sure.

From Professor Gene Wengert, forum technical advisor:
The definition of oven-drying for wood requires 215 to 217 F. Use this temperature at all times. Hotter will evaporate more than water for some species such as pine, but not for most non-resinous woods. Cooler will not achieve 0% MC so the calculated MC will be lower than true.

Spanish cedar does not have many oils that evaporate at 215 F. The fact that the wood did not return to its original MC and original size is due to the hysteresis effect. In addition, heating does cause some loss of hygroscopicity, so heated wood will have lower MCs than unheated wood.

From the original questioner:
Thanks Gene. What is the hysteresis effect?

From Professor Gene Wengert, forum technical advisor:
Hysteresis derives from an ancient Greek word meaning "deficiency," or "lagging behind." What it means, in part, is that if you approach a value or level from one direction, you will achieve one value or level, while if you approach this value from another direction, you will achieve a different value or level. For example, the EMC for 30% RH is 6% EMC. This value was obtained for initial drying, so you are approaching 6% MC from a higher MC value. Now, if the wood were dried to 3% MC and then exposed to 6% EMC or 30% RH, it will regain moisture to only about 5% MC.

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