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Oven-Drying Wood for Moisture Content Testing
Question
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 %),
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
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. Would you like to add information to this article? Interested in writing or submitting an article? Have a question about this article? Have you reviewed the related Knowledge Base areas below? 
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