Microwave wood

[David Keller NC]

Keith,
If you buy kiln dried lumber, no problems. If you're cheap like me and buy maple that's 30 cents a BF, then you might want to see what your getting. I've never had much of a problem, but it can cause the project to come apart/crack if not dried correctly. I've taken the risk for 30 years with no moisture meter and did fine, however I'm now ventureing into more risky wood.

Josh - While I do buy expensive, kiln-dried wood like mahogany, I also fell and mill my own wood when the tree seems too good for firewood, or if it's boxwood or other small, desirable species.

I don't have a moisture meter, and I use the oven-dry method. You don't assume that the original weight is 100% - this is actually your "unkown" (and what you're trying to get at). I place a sample of the wood into a 200 degree oven for about 5 hours, weighing it on an elcheapo kitchen scale about every 30 minutes. After I get 3 consecutive readings that are the same, I consider that 0% moisture. Then, (initial weight-final weight)/final weight X 100% = initial moisture content.

For smaller pieces of rosewood, ebony, boxwood and other exotics, I just use the direct-weigh method. I plane off a face, weigh the board, write the weight and date on the planed face, wait a month, and repeat. After 3 consecutive identical weights, the piece is acclimated to my shop (and I really don't care what it's absolute moisture cotnent is).

[Josh Bowman]

After I get 3 consecutive readings that are the same, I consider that 0% moisture. Then, (initial weight-final weight)/final weight X 100% = initial moisture content.

I'm in so much trouble!!! Can I come and sleep at one of your houses tonight!!!!!
David,
I got a sample of the wood. Then I weighed it. That was 27.8 grams. I then microwaved it, thinking 30 minutes ought to do it. Well I came back to the computer and after about 8 minutes thought, I better go check on that maple sample. As I walked into the smoked filled kitchen, I found the wood was now very dry and very charred! My wife will kill me. I have all the fans going trying to clear the house. Mybe I need to start a fire in the wood stove and say some smoke leaked out.
Anyway, is the math right? I took the 27.8-the charred 15.1 and divided by charred 15.1 and came up with 84% moisture content. This wood has been in a barn for 8 years sticked and in the dry. I think I did some wrong....more than burning the wood.

[Josh Bowman]

Oh, I got it now. I forgot to measure the smoke. The second try, I put the microwave on 30% and am watching and weighing every few minutes. The wood is losing weight much slower now. Guess burning things gets rid of mass much quicker than just drying them.

[Nick Laeder]

Weight before drying = Wi

Weight after drying = Wf

Moisture content = (Wi-Wf)/Wf

There are all kinds of ways to show this in an equation.


I love this board. I'm nowhere near the kind of woodworker who needs to get to this level of geekery, but I have other hobbies where I love the minutia. The level of geekery on this board is awesome.

[Josh Bowman]

I love this board. I'm nowhere near the kind of woodworker who needs to get to this level of geekery, but I have other hobbies where I love the minutia. The level of geekery on this board is awesome.

Man, that hurts! I just smoked up the house, got the wife mad. Now you call me a geek!
Anyway, the numbers are in and worth it. Started out at 38.6 grams, ended at 33.5 grams, looks like 15% moisture content to me. My spike type cheap moisture meter called it 7%. Looks like the cheapo moisture meter is off and the wood needs to sit a while longer.
For the geeks that want to know. I used 30% power and it took about 1/2 hour run time on the microwave. I would turn it off some to let the wood cool.
The geeks down the street at the Lazy Boy factory use this method to determine buy or not buy wood from suppliers. They said it is the most accurate. Not so interested in accurate as how good is the cheapo moisture meter.

[Lucas G Hager]

David nailed it.
That is the method most large sawmills use. The sample is generally baked longer than 5 hrs in a small conventional oven to assure it is as close to 0% as possible. But, he's right. Once it quits losing, it's dry.
Microwaves are generally avoided because of the possibility of fire. It's generally only used when information is needed fast. A conventional oven generally doesn't need as much attention.
That's how the NHLA teaches to do it.

[David Keller NC]

One thought based on extensive experience cutting trees into lumber and drying it myself - the absolute MC just isn't nearly as important as that it's at equilibrium with your shop environment and is stable.

For this reason, I don't sweat whether the walnut I cut up 3 years ago is at 12%MC or 8%MC. Since almost all of the things I build have built-in compensation for wood movement, what's important to me is that the wood's a uniform MC from the center to the outside, and that it won't move like crazy once I machine it.

Depending on the species, air-dried wood can be ready to use in as little as about 3 months (for 1" thick eastern red cedar) to as long as 4 years (walnut and beech). The rule of thumb that it takes one year per inch of thickness is just that - a rule of thumb. In my experience, the actual result varies a whole lot from the rule of thumb.

[Josh Bowman]

One thought based on extensive experience cutting trees into lumber and drying it myself - the absolute MC just isn't nearly as important as that it's at equilibrium with your shop environment and is stable.

You sound like the person I want to ask this to. Based on what you said in using air dried wood and uniform equilibrium. What do you use to check for these conditions?