Microwave drying

[WH Seelinger]

I know that there have been many threads on Microwave drying of wood, but I tried something earlier in the week that might be helpful.

Started with a rough turned maple bowl, around 13" in dia, left it 1"+ in thickness. it weighted 4lbs 14 oz. Ended up drying it 14 times, it came out 4 lbs. I did let the piece cool to below 80 degrees before the next cycle.

The first cycle was for 5 minutes, I took an infrared thermometer and checked inside and outside of the piece after it finished. 120 degrees F was the highest spot. Microwaving basically boils the moisture out of the wood, so I bumped up the next four runs to 6 minutes. The next three cycles topped out at 180 degrees, the fourth came out at 200 degrees. The next four cycles were at 5 minutes, max temperature went from 180 to 190 on the last cycle. Each cycle, the piece lost an ounce and a half of weight. I did one more 5 minute cycle, and found the bottom of the piece was at 265 degrees.
It had only lost 1/2 ounce.

I did a 3 minute cycle, 180 degrees max and lost .5 oz. 2 more runs at three minutes showed the temperature the same, and .5 oz loss. The final cycle showed the temperature about half way up the piece at 240 degrees, it only lost .2 oz.

I am thinking that there is merit to checking temperature while drying and that 180-190 degrees would be a good point to judge the time of cycle.

WH

 

[John Torchick]

Is the cycle a defrost cycle What setting did you use? Thanks. Found I can get a MWO much cheaper than a dehydrator.

 

[WH Seelinger]

Is the cycle a defrost cycle What setting did you use? Thanks. Found I can get a MWO much cheaper than a dehydrator.

I am calling a cycle, one session in the Microwave. I use the high setting for temperature, the time in the oven is kind of a guess, using the infrared thermometer seems to me, to be a way to see what is happening to the wood to optimize the time.

When I first started drying wood in the Microwave, I left a piece of walnut in too long and set it on fire internally, no flame, but a lot of smoke. Took it outside and it would not quit smoking. I had to dunk it in water.

If you try using this method, read everything you can, go slow, and keep a log of the time and temperature.

 

[Scott Hargrove]

I've done the same with good success ... 5 minutes on high for a medium to large bowl blank, weighing before and after microwaving, and checking with a moisture meter once it gets fairly dry. I've done five minutes at a time until the piece comes out without a lot of steam, and then start backing off the time. I've never had a piece burn at all. I hadn't thought of the infrared thermometer. I think that's a great idea, because it takes out a lot of the guesswork.
I've read about people using low heat for a minute at a time, and that just seemed way to slow and probably completely ineffective. I think your post proves that.
Again, the thermometer seems to be a great idea.

 

[Paul Paukstelis]

I've read about people using low heat for a minute at a time, and that just seemed way to slow and probably completely ineffective. I think your post proves that.
Again, the thermometer seems to be a great idea.

I did quite a bit of experimenting quite a while back and I found the best way to maintain a constant internal and external is to do an initial longer cycle (4 to 6 minutes depending on mass and moisture content) followed by a roughly 10% duty cycle, usually 1 minute on, 10 minutes off (with fan going). Of course, this would be a pain to do manually so I added a Esp-32 microcontroller to run the microwave keypad, letting me set cycle parameters via wifi. Works great, but I almost never use it, so if anyone wants it....

 

[Scott Hargrove]

I did quite a bit of experimenting quite a while back and I found the best way to maintain a constant internal and external is to do an initial longer cycle (4 to 6 minutes depending on mass and moisture content) followed by a roughly 10% duty cycle, usually 1 minute on, 10 minutes off (with fan going). Of course, this would be a pain to do manually so I added a Esp-32 microcontroller to run the microwave keypad, letting me set cycle parameters via wifi. Works great, but I almost never use it, so if anyone wants it....

I guess I'm not techie enough know what an esp-32 is, but being able to control by wifi would be great.

 

[Dennis J Gooding]

For what it is worth, note that the IR thermometer will report the surface temperature of the piece, not the internal temperature.

 

[WH Seelinger]

For what it is worth, note that the IR thermometer will report the surface temperature of the piece, not the internal temperature.

I understand we are talking surface temp. If we took a pan of boiling water and checked it with an IR thermometer, it should read 212 degrees, when the water boils off, the temp will rise.

 

[Kent Reisdorph]

so I added a Esp-32 microcontroller to run the microwave keypad, letting me set cycle parameters via wifi. Works great, but I almost never use it, so if anyone wants it....

Of course you did.

 

[Darryl Fective]

Is the cycle a defrost cycle What setting did you use? Thanks. Found I can get a MWO much cheaper than a dehydrator.

I got one for free off Facebook Marketplace.

This is very interesting. I also have the impatience bug, but haven't tried any extended times, just 30 second blasts. I haven't checked temps other than by feel. I don't always let it cool completely before the next round, either. If only I took notes... I've also been experimenting with a soaking roughed blanks in denatured alcohol first.

 

[John Torchick]

I got one for free off Facebook Marketplace.

This is very interesting. I also have the impatience bug, but haven't tried any extended times, just 30 second blasts. I haven't checked temps other than by feel. I don't always let it cool completely before the next round, either. If only I took notes... I've also been experimenting with a soaking roughed blanks in denatured alcohol first.

Many times a MWO at the curb only needs a fuse. Remove the back and replace for a couple of bucks.

 

[John K Jordan]

One thing I wold like to be educated on about the process of drying by microwave or even with gentle heat in a light bulb refrigerator kiln.

I've done microwave drying on small pieces and one thing I don't like about it - when to decide it's dry how do you know if is actually at at EMC? (equilibrium moisture content) for the specific environment? Since many pieces are kept indoors when finished, turning the piece when at EMC will prevent it from gaining or loosing moisture and warping when moved to a conditioned indoor envronment. Of course, if you turn green and let it warp when drying, none this matters unless you want the final piece to sit flat on the table wittout rocking.

If it comes out of the microwave (or makeshift kiln) with greater moisture than EMC, it will lose mositure when moved into a conditioned house. If it comes out COMPLETELY dry with a lower moisture than EMC (something I definitely don't want), it can gain moisture when moved to a conditioned space after turning. It either case it might warp some in the recipient's house. (This is less likely with end grain turnings like vessels and lollow forms that it is wilth face grain bowls and platters.)

This potential problem could, of course, be avoided if the piece is removed from the microwave then left in a home-conditioned environment until it reached EMC then finished turned.

OR if the microwaved piece has is tested for EMC by a useful method.

Moisture meters? Pin-type are can be inaccurate since they only measure the resistance right at the surface, or as deep as you can drive the pins. Pinless type will measure deeper but require a flat surface on the wood. They also need to be preset to the species of wood being tested. (I have both types and have done experiments.)

I solve the EMC question by air drying all blanks in my climate conditioned shop where temperature and humidity is very close to the typical living environment. (I track the drying by weight - when the weight quits dropping or starts to fluxuate a small amount with the seasons I know the blank is dry. That way, when I turn a piece and give it to someone who will keep it inside I know it's not going to warp.

The same thing would work with rough turned bowls - and I always do that - keep the rough-turned bowl in the shop either for what I've learned is enough time OR track the weight until it stabilizes. Then when I finish turn there is never a problem when it's moved to final resting place in a house. I don't sell pieces but have visited many friends who have my pieces. All were rock-solid on the table.

The huge downsize of drying this way is it can take a long time, years for large blanks.

And this is not a problem for pieces dryed properly in a professional kiln following a proven schedule of humidity and temperature. I wonder mostly about the casual shop and shed "kilns" with light bulb heat, and pieces dried with microwave heat.

I understand a piece that turned at a high or low EMC may be more stable if finished well, but moisture can travel through many types of finish. Small pieces, of course, can have proportionatly smaller movement.

So does anyone have a problem with the long-term stability of relatively large finished pieces after drying if the wood is finish-turned with too much or too little moisture? Or do pieces go away after a sale and there is no opportunity long-term feedback? Does anyone who sells bowls turned ater being dried with the microwave method check back a couple of years later with the buyer?

Or maybe I'm being overly picky and no one else cares about this.

JKJ

 

[Scott Hargrove]

One thing I wold like to be educated on about the process of drying by microwave or even with gentle heat in a light bulb refrigerator kiln.

I've done microwave drying on small pieces and one thing I don't like about it - when to decide it's dry how do you know if is actually at at EMC? (equilibrium moisture content) for the specific environment? Since many pieces are kept indoors when finished, turning the piece when at EMC will prevent it from gaining or loosing moisture and warping when moved to a conditioned indoor envronment. Of course, if you turn green and let it warp when drying, none this matters unless you want the final piece to sit flat on the table wittout rocking.

If it comes out of the microwave (or makeshift kiln) with greater moisture than EMC, it will lose mositure when moved into a conditioned house. If it comes out COMPLETELY dry with a lower moisture than EMC (something I definitely don't want), it can gain moisture when moved to a conditioned space after turning. It either case it might warp some in the recipient's house. (This is less likely with end grain turnings like vessels and lollow forms that it is wilth face grain bowls and platters.)

This potential problem could, of course, be avoided if the piece is removed from the microwave then left in a home-conditioned environment until it reached EMC then finished turned.

OR if the microwaved piece has is tested for EMC by a useful method.

Moisture meters? Pin-type are can be inaccurate since they only measure the resistance right at the surface, or as deep as you can drive the pins. Pinless type will measure deeper but require a flat surface on the wood. They also need to be preset to the species of wood being tested. (I have both types and have done experiments.)

I solve the EMC question by air drying all blanks in my climate conditioned shop where temperature and humidity is very close to the typical living environment. (I track the drying by weight - when the weight quits dropping or starts to fluxuate a small amount with the seasons I know the blank is dry. That way, when I turn a piece and give it to someone who will keep it inside I know it's not going to warp.

The same thing would work with rough turned bowls - and I always do that - keep the rough-turned bowl in the shop either for what I've learned is enough time OR track the weight until it stabilizes. Then when I finish turn there is never a problem when it's moved to final resting place in a house. I don't sell pieces but have visited many friends who have my pieces. All were rock-solid on the table.

The huge downsize of drying this way is it can take a long time, years for large blanks.

And this is not a problem for pieces dryed properly in a professional kiln following a proven schedule of humidity and temperature. I wonder mostly about the casual shop and shed "kilns" with light bulb heat, and pieces dried with microwave heat.

I understand a piece that turned at a high or low EMC may be more stable if finished well, but moisture can travel through many types of finish. Small pieces, of course, can have proportionatly smaller movement.

So does anyone have a problem with the long-term stability of relatively large finished pieces after drying if the wood is finish-turned with too much or too little moisture? Or do pieces go away after a sale and there is no opportunity long-term feedback? Does anyone who sells bowls turned ater being dried with the microwave method check back a couple of years later with the buyer?

Or maybe I'm being overly picky and no one else cares about this.

JKJ

I don't want anything I give away or sell to warp after it leaves my hands.
I wonder about local climate (south Georgia vs. Arizona, for example) but I would think that modern HVAC probably equalizes that.
In my limited experience, it depends a lot on the wood species, and even the particular tree. For example, I've turned live oak vases from whole rounds from large branches and had no problem with warping at all. I've turned others from another tree and everything warps and splits eventually. Both with MW drying. Maybe I just haven't dialed it in enough for consistency.
Several large black olive (jucaro) pieces gave me no problems, and the same for a few pieces turned from Florida mahogany. The pecan around me warps more than splits, but it warps radically, almost comically. I've had pretty good success with sufficient MW drying, though.