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Moisture content by oven-dry method

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Moisture content by oven-dry method

#1

Moisture content by oven-dry method

John K Jordan

I got some waxed 1" square Ebony blanks from Pete Kekel (Big Monk Lumber in KY) with unknown moisture content. Waxed wood blanks can be anywhere from green to dry and I find it difficult to guess with hard exotics. My pinless moisture meter needs a bigger contact area so I decided to use the oven-dry method which gives the absolute moisture content independent of the density of the wood. This method is always accurate and cheap if you already have the simple equipment on hand.

If not familiar with this, basically a wood sample is first weighed then gently dried in an oven and weighed repeatedly until the weight quits changing. Then subtract the dry weight from the wet weight to get the water weight and divide that by the dry weight to get the moisture content.

I cut away the surface wax and cut samples from one of the Ebony blanks. A toaster oven dried the wood. I put a thermocouple in the oven to monitor the temperature which needs to be between 212 and 220, ideally 217. The little digital scale is good to 0.1 grams (checked with my triple-beam balance).


Since I couldn't watch it all the time I put the toaster oven on the concrete floor in my shop office to keep it away from combustibles. I checked the temperature every few hours and quit when the weight was unchanged over two readings.

The moisture content of this Ebony was just over 13% which matched the expected equilibrium MC of air-dried lumber in both TN and KY. It's good to know I can turn these without further drying.

If interested in more detail: https://ir.library.oregonstate.edu/downloads/jm214q048

JKJ

Re: Moisture content by oven-dry method

#2

Re: Moisture content by oven-dry method

john lucas

John and I discusses this yesterday. I sometimes do the same thing but use a microwave. the downside of using the microwave is you can't just put the wood in there and leave it unattended. It can catch fire. I do many short cycles. I simply hit the button for 30 seconds and go do something else. come back when I think about it and do it again. If the wood doesn't feel too hot I go to 1 minute cycles. The advantage of a microwave is it dries the wood from the inside out which in theory reduces checking. I have done this a lot on box blanks. I have done an occasional bowl but don't do it often because it takes so long. John's method has the advantage of being able to stack a bunch of blanks in there. Mine is more of very few or even just one.

Re: Moisture content by oven-dry method

#3

Re: Moisture content by oven-dry method

John K Jordan

I don't know if I would use it to dry blanks for turning, I'd probably use the microwave method. If drying several at once, the oven would need plenty of ventilation to remove all the moisture released. The times I used the microwave was for just one piece and when I was impatient!

This method is more of a destructive way to measure the total moisture content of a small sample instead of a drying method. I understand it is sometimes used to sample a board or two in a kiln load to know when the entire load has reached the desired dryness.

I plan another test on a much larger blank with just Anchorseal on the end grain, maybe 6" square and long, sampling both from the center, the sides, and the ends. The wood books say this method is good for measuring the moisture gradient through a thicker block or slab, something which can trick a moisture meter, either pin type or pinless, since the surface may be much drier than the center. That might be good to know when picking a box blank from a lot all cut at the same time.

For this experiment I sacrificed a piece of one 1x1x12 blank to measure the probable MC of the entire lot. I didn't mention it but when since moisture the ends are likely to be drier than the middle the wood experts recommend cutting a sample out of the center of a board. I don't think that is as important for turning blanks that are heavily waxed on all surfaces.

JKJ

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