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?
(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.
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.