micro wave drying technique

[allen thunem]

Ok here is my dilemma
xmas is coming and i have over 50 roughed out bowls that is a conservative number. these bowls have been roughed out starting back in oct.
none are dry enough to finish turn. I need a few suggestions on how to speed up the process, i.e. micro wave drying, alcohol drying etc.
can anyone assist?
allen

[Cathy Schaewe]

I've used the microwave on a number of pieces recently for getting Christmas presents to the point where I can final turn them, and it's been very successful. Maple, cherry and walnut. There's an article in the most recent edition of Woodturning about it. I haven't followed those rules, though. They talk about putting it in a bag, and wrapping the edges with cling wrap, and they may turn the power down too, and they do it for a couple of minutes at a time, if I remember correctly. What I've done is put it in the microwave for 35 seconds at full power. Let it cool down completely. Do it again, and again, until it stops losing weight. I do it over the course of a couple of days. Then wait a couple of days. In all cases, the piece actually gained about an ounce after waiting. At that point, it's been completely dry and ready to turn. If there's a crack in the piece to start, though, I wouldn't do it. Never burned any wood this way. YMMV.

[allen thunem]

ymmv?????
im gonna give it a try
most of what im trying to dry is english elm and some cottonwood
and a bit of box elder
tanks cathy

[Cathy Schaewe]

ymmv?????
im gonna give it a try
most of what im trying to dry is english elm and some cottonwood
and a bit of box elder
tanks cathy

Sorry. "Your Mileage May Vary."

[dan carter]

You better make sure your better half is gone when you do the elm; I remember years ago I used the microwave in the teacher's lounge, he heh the principal was really steaming when she came down to the shop. My partner in crime did what all partners do, he took the hit for me; he was the first shop teacher she ran into, boy did I owe him. :-)

[Cecil Walborn]

I have used Microwave drying and worked good. I put a piece of wet box elder in for about one minute on high power then let cool down till cool and repeated it for four times then let it set for four days then finish it. It was good and dry when I finished it. One minute on high did not on this piece burn it any. It maybe a different time on other woods. Would watch and set time to each wood to be sure it does not burn. The also did not warp any so worked great on this piece.

[Bernie Weishapl]

I have used a microwave with some success. I use the defrost cycle and leave the high power for the last cycle or two. I also let them sit for a day or two after and before turning.

[Ron Stadler]

I read an article on drying wood in the Microwave that I found pretty interesting, this man told how he put his bowls in a sealed up zip lock bag, then stuck it in the microwave on High, when the bowl get hot enough steam will form and fill up the bag, when this happens its time to turn it off, now take it out of bag and let out the steam and let cool, oops almost forgot, make sure you record at what time it took to make it steam. Ok now that it cool again, place it back in the microwave but this time without the bag and to the time you recorded, keep doing this for 3 to 4 times on medium bowls and then stick it back in bag and make a new recording of time, the more it looses moisture the faster it is going to get hot and steam released filling up the bag. So now that you have the new time recorded use this without the bag again. All the time you are doing this I would weigh it also after each cooling period. You'll have to play around with this, because different wood and size of your bowl will play a major roll in drying, but I have had a lot of success with it and really like this method rather than just sticking it in the microwave and guessing when its hot enough, but just make sure you stop it as soon as the steam fills up the bag and make regular checks on the time ever so often, well hope this will help, good luck. Also you might think on getting a different microwave for your drying of bowls, some woods stink bad, red oak being one of the worst for me so far, smells like puke.:D

[Josh Bowman]

I can tell you this: If you leave sugar maple in the micowave too long the smoke from the charring wood, will make the whole house smell like a barbeque and the microwave from that day forward will smell the same everytime you use it.

I wouldn't have a clue why I know that:o

[Ralph Lindberg]

I've used both the 30 sec at full power, let cool, repeat and the 10 minute at defrost, let cool, repeat methods (hint weigh between cycles) and had excellent success.

The key is, do it slow and let it cool

I hunted Thrift stores and friends and have three old LARGE microwaves now. When one dies, I will just switch to the next one. I never will use SWMBO's kitchen one

Microwaving is not new, last winter I was going through some old paperwork and found an article from FWW on it, the article was 15 years old and I hadn't even planned on turning back then

[Bob Wolfe]

Here's a tip for someone who might use the "good kitchen" microwave for drying, burns the piece and stinks up the microwave something terrible. Remove the charred piece and either hide it in your shop or destroy it. Make sure there are no more charred bits of wood remaining. Fill a microwave safe container with 1-2 cups of white vinegar. Microwave the vinegar until it boils and then for an additional minute or so. Repeat the boil. While the microwave is still very warm, remove the vinegar and thoroughly wipe down the inside of the microwave with a damp cloth. If it smells like anything except for vinegar repeat the process.

If you're lucky you'll get a thank you for cleaning the microwave. I was left with a sparkling clean microwave and the only smell was vinegar which dissipated over the next couple of days. I hope this helps somebody out of a jam.

[Lee Koepke]

A word of caution regarding microwaving wood that I have read ..... since the wood heats from the inside, its possible that its charred where you cant see it, thus carrying the (probably slight) potential for continued ignition process.

The article recommended you never leave the wood unattended until its very cool .. and then I'd still be cautious where you put it for the next day, just to make sure !!!

[Reed Gray]

50 plus bowls before Christmas??? Probably wishful thinking. One alternative to microwaving them would be to finish turn them now. They will warp, but might be dry enough to easily sand out. Other than that, a 1/4 inch bowl will be dry in 7 to 10 days. Try one to see.

robo hippy

[David Gilbert]

I have been successfully microwaving some of my bowls. I turn my bowls to final thickness and shape and sand if they aren't too wet. I let them warp as they want as I microwave them. I normally use the bowl in a plastic bag method for 30 - 60 seconds per cycle at full power. I take the hot bag with the bowl out, dry out the bag and let the bowl cool a little and dry slightly before weighing it. When I first start I can lose 5 - 10 grams per cycle. The bowl is still warm when I put it back into the bag and re-zap if for the next round. As you approach total dryness the weight losses drop off and the bowl will start to distort more. I try to stop before I get it too dry. If I go too far, after the bowls are cool they will actually pick up moisture from the air and the weight will increase. Over drying the bowl can also really stress the wood.

My hypothesis is that once the bowl is warm or hot and it is losing water that the heat helps to relieve the stress in the wood. For that reason I don't let the bowl cool down to room temperature. If I need to leave it overnight then I leave it in the bag and let the water equilibrate. In a recent case with a poplar bowl that weighted about 400 grams, it took about 15 cycles to get it down to its final 210 gram weight.

All of the comments above about charring the inside of the wood prompted me to do an experiment. I took a cutoff from one of my apple bowl blanks and zapped it for 30 second intervals at full power. This wood was about 6 x 2 x 2 and weighed 320 grams so it was much thicker than any bowl I might make. I drilled a hole for a metal thermometer that I inserted after taking the block out of the microwave. Here are the results (sorry that the software pushes all the columns together):

Time Weight (grams) Temperature (in Fahrenheit)
0 320.2 68
30 319.7 140
60 316.2 205
90 310.7 210
120 304.0 217
150 298.2 226
180 294.4 244
210 292.0 257
240 289.0 270
270 287.0 288
300 285.4 302
3600 287.2 96

I was pretty surprised that after four cycles that the temperature was above the boiling point and that it kept raising with each 30 second cycle. I stopped after 10 cycles and let the block cool. After an hour the block had gained back almost 2 grams of water so I had gone past the equilibrium point. Since my basement is about 50% relative humidity at 65 degrees, my Lee Valley moisture content chart says at equilibrium, my wood should be about 9.3% water. Back calculating, this block started out at almost 22% water.

When I cut the block in half I didn't observe any charring on the inside. There was a hint of something burning after the tenth cycle but nothing very strong. My guess is that if you kept heating it up that in a couple of more cycles there could be some charring. 300 degrees is pretty warm but charcoal production must start somewhere above this temperature.

My recommendation is to use full power for short times and measure the weight loss as you go. You will be able to tell by how the bowl feels as you approach the endpoint. The largest bowl that I have done like this was about 15 inches by 5 inches tall and it only took 45 seconds to inflate the bag. I wouldn't go longer than a minute.

There is one other thing that I feel that I must disclose. Some of the woods that have a disagreeable smell when turned will have a very strong (and worse) smell when microwaved. I more or less got used to the smell of the swamp poplar but my wife didn't. This tree made some very pretty bowls but the logs aren't allowed in the house because of the smell.

Cheers and happy holidays!
David

[Ron Stadler]

A word of caution regarding microwaving wood that I have read ..... since the wood heats from the inside, its possible that its charred where you cant see it, thus carrying the (probably slight) potential for continued ignition process.

The article recommended you never leave the wood unattended until its very cool .. and then I'd still be cautious where you put it for the next day, just to make sure !!!
Absolutely true, had this happen to me once when it began to smoke around the spigot area after I removed it from the microwave, luckily it happened right away and no harm done, but it was burning from within and if it were'nt for the smoke I would have never knew. Thats why I say to make regular checks on your microwave time as described above in the method I use, you must throw whatever your drying back in the bag ever so often to see at what time it begins to steam again, enough said, just be careful and like you said never leave it unattended.

[Paul Williams]

Sugar maple isn't the only wood that will burn in a microwave and smell up the house. I have heard that can happen if you get over enthusiastic with mircowaving cherry. If you see smoke don't open the microwave door until you take the microwave outside. It really smells up the kitchen if you open the door inside.