oven drying method

[Florida boy]

hi. I have some wood that's been drying for 6 months or so. 4/4 cypress mostly with a few other species inclouding oak and pine.
I've seen where you can do the ovenext test to get mc but don't really have an oven I can tie up for 24 hours. can the test be done in a microwave? what I've read said the temp needs to be low around 215 so as to only take the water and not cook the wood. so can it be dome in a microwave where there is not any temp control?
   I'm sure someone has done it. can I get a little guidance on this. I really don't want to spend a lot of money on a meter yet. from my understanding the oven test is the only true test. thanks for any input on the subject.

[Ianab]

Yes you can use a microwave. Trick is to keep the power down and do it in short  bursts, weighing it frequently on a digital scale. The wood quickly gets hot and steaming as the water is driven off. Don't let it get so hot that it starts to smoke!! Eventually the weight stops decreasing, and it's then basically "oven dry". Might take maybe 10 mins of zapping and weighing.

Many woods will stink up the microwave, so don't use the wife's good one.

[WLC]

Never dried a piece of lumber in the microwave, but have tried drying really thin turned wood bowls in it though.  Be careful to not get the wood so hot that it chars or starts smoking.  Short run times and if needed let it cool off between.

I've actually got a sample board in the oven drying right now.  I had cut some birch lumber this past fall and was curious as to how dry it was able to get during our frigid winter.  The oven has a standing pilot light and the oven stays about 125° all the time.  When we need to use the oven I take the board out and when we are through I put it back in.  Takes longer than just turning the oven to 200° and leaving it for 24-36 hours, but works for me.

[Florida boy]

excellent. thanks for the replys. I will go home and try it this afternoon.

[GeneWengert-WoodDoc]

You can indeed use a microwave and get perfect results.  You must use med-low power and have a rotating tray.  Samples cannot touch each other.  Weigh the samples before, and during drying, starting at 20 minutes and then every minute or so until weight loss stops.  If you smell smoke, the test will no longer work.  Start again with new samples and lower the power.  If you have a small mirror, put a sample on the mirror briefly and if you see condensate, then it is not dry yet.  I never broke a mirror, but it might happen so be careful.  The microwave technique is in Drying Hardwood Lumber.

[Florida boy]

does the size of the sample matter?  just whatever is manageable I guess?

[Ianab]

Needs to be big enough to get an accurate weight on your scales. My scales measure to the gram, so for example the wood weighs 220 grams before, and 200 after, then it must have started at ~10%, +/- 0.5%. That's basically accurate enough. Doesn't really matter if it's 10.2 or 9.8, for any practical purpose.

[Glenn1]

I cut 1" strips that usually average 60 - 70 grams. I run my cycles for 10 minutes at a time at 20% power. Make sure to use a carousel as the Doc has stated.  Weigh the pieces after each cycle.  Make sure to wait 20-30 minutes between cycles so the wood can cool. Otherwise, it could smoke.  After 5-6 times, you shouldn't see the weight going down anymore. 

[GeneWengert-WoodDoc]

If you have a 110 gram sample (technical term is Moisture Section) and after microwave drying it weighs 100 grams, the calculated moisture content is 10%.  However, the MS could have actually weighed 110.49 grams, which would read 110 grams.  The oven dry weight could actually be 99.51 which would read 100.  If you use the numbers 110.49 and 99.51 and get 11.0% MC.  So, with not enough digits, as in this case, you could be exactly correct or anywhere between 9.0 to 11.0% MC.  You just do not know without an extra digit.

Another way to look at this example.  Let's say you dry the wood before you cut the MS.  So, now the 110 gram sample will weigh 109 grams.  The OD weight is still 100.  But the moisture is now 9%.  You cannot get any values except for the whole numbers...the calculated values will jump in 1% increments.  If you dried the wood a bit more, the next value you will get is 8% MC.

Of course, it is possible that the hot-air oven is not 215 F (microwave is not an issue in this respect) or that a few slivers dropped off in the oven, or the saw used to cut the MS heated and drove off some moisture, or the sample was not fully dry when removed from the oven.  All these can create errors too.

[YellowHammer]

Many woods will stink up the microwave, so don't use the wife's good one.

I learned this lesson the hard way, and the stink will last a good long time. 

[GeneWengert-WoodDoc]

In a drying class for beginners that I ran in Memphis a few years ago, two guys were in a hurry to get their oven-dry weights, so they increased the power above Med-Low.  Unfortunately, the smoke odor was throughout the building for a few days and also the fire department (smoke alarm went off and automatically called the FD) was not especially happy either...false alarm.

At the least, always stay with the microwave, just in case.  Remember also not to let the pieces touch each other.

[Den Socling]

I haven't bothered to do it but I would like to see what you get when 4 or 5 samples from one Black Walnut board. I wonder how much they would vary in MC.

Edit: Let's make it 8/4 and maybe one sample will be a perfect 7%. If that was the spot you chose to test, you might think the load was perfect. However, I have a feeling the other 3 or 4 samples could be higher or lower. If you had happened to pick a high spot, you might decide to run the load longer. My point to all of this is that oven testing is very accurate for the sample you cut. But I would use a meter to determine whether or not a kiln charge was dry.

[WLC]

Well drying my sample in the gas oven worked OK.  Took several days, but have had the same weight the last three days of weighing.  My green cut White Birch went down to 16% MC in six months of air-drying even in our frigid winter.  I honestly thought it would be higher as our temps have been below freezing for months now. As soon as I can get to sawing again I plan on seeing what happens to green lumber air dried over one of our summers.   

[Denny]

I haven't bothered to do it but I would like to see what you get when 4 or 5 samples from one Black Walnut board. I wonder how much they would vary in MC.

Edit: Let's make it 8/4 and maybe one sample will be a perfect 7%. If that was the spot you chose to test, you might think the load was perfect. However, I have a feeling the other 3 or 4 samples could be higher or lower. If you had happened to pick a high spot, you might decide to run the load longer. My point to all of this is that oven testing is very accurate for the sample you cut. But I would use a meter to determine whether or not a kiln charge was dry.

Well thats the whole point of the equalizing stage, to get all those high and low MC numbers to even out.  I too try to get the greenest sample boards and then, if possible, I run my meter over the board to ensure I got the wettest section as my sample. I always assume there's wetter lumber in the kiln than my samples, so my temp changes dont get changed until I'm solidly a few points under the recommended schedule change.

[Denny]

"or that a few slivers dropped off in the oven".

THIS !! New guys, when you cut your 1 inch moisture sections run your pocket knife over the cut to remove all the little slivers hanging on the cut moisture sections.  I've had a sample oven catch fire(edit; well I guess it just smoldered and smoked really bad) overnight because of accumulated slivers that fell off the moisture sections over the course of months.  I've had it happen to me once and I've seen happen to the previous kiln op before I took the job.

p.s.
I too am guilty of smokin' samples in the microwave. Don't walk away from microwave !! That bugger will look good one minute, turn you back on it and BAM your control room stinks like burnt red oak for the next few days.

[Den Socling]

We unloaded some 12/4 Black Walnut slabs today. My Wagner 612 read 7 to 11% on some pieces even though they were at 165' for 5 days in a vacuum. Was this a difference in density that caused error in in the meter or were the MC's really that varied. Of course you don't cut up a $2000 slab to get MC samples. I just wonder.

[WDH]

I am sure that it is due to density variances since the Wagner meter is calibrated for wood specific gravity (a measure of density).  The specific gravity of a wood is an average since the density will vary from the outside edge of a board (tree) where the growth rings are closer together that the center of the board (tree) where there is the juvenile wood and wider growth rings.   

[GeneWengert-WoodDoc]

We know that density affects the meter significantly, but density also affects drying rate in a vacuum.  So, it is likely both effects together.

[boardmaker]

does the size of the sample matter?  just whatever is manageable I guess?

If you're looking for a little more speed, instead of cutting a 1" sample cut 4 1/4" samples.  The thinner pieces will dry faster and easier in the microwave or oven.

We cut 3 pieces from each side of our kiln sample board.  Remember to cutoff about 2" of your kiln sample because it will be a little low.  Then you can cut your wafers for your microwave.

[GeneWengert-WoodDoc]

If you cut thinner pieces for moisture sections, use a sharp saw so that the heat of the saw does not dry them at all before you weigh them.  Also, keep and weigh the pieces from one end together, as most balances using grams weighing thinner pieces individually need two decimal places in order to get reliable resolution.  As thin pieces dry quickly, you must also weigh them promptly after sawing.  For these reasons, the 1" along the grain piece is the suggested procedure...less critical when cutting and weighing.