Microwave drying

[Roger Chandler]

I read an interesting post on the WC forum from a gentleman who evidently worked in forestry and was involved with controlled burns and such. He states that adding a damp sponge in a Microwave oven with a bowl or wood would help prevent fires from occurring.......stating that they did that every year on determining the condition of the "fuel" in the forest and never had a fire.

That brings me to a video I saw a couple weeks ago on youtube where a turner was microwave drying his roughed out bowls on the defrost setting for a minute or so, setting it in front of a fan to blow on it for a few minutes, repeating this process for maybe a couple of hours, and got his roughouts dry enough to final turn without cracking.

Does anyone on SMC use microwave drying for your roughouts! If this is viable, I would be glad to get a Microwave for my shop. Actually, when I mentioned this to my wife the other day, she said....get a new one for the kitchen and you take the one we have and put it in your shop! :D

[Eric Gourieux]

Roger, last year I purchased a huge microwave (the under-the-counter, or built-in type) for $5. It works great. I have tried the defrost, cool, defrost cycle and it seems to work pretty well. I haven't done it enough to give you specific times, etc, but you have re-sparked my interest. I have read that you don't want to heat the bowl until it is hot to the touch. This will probably take some experimentation with your m-wave and may differ between wood species.

[Roger Chandler]

Roger, last year I purchased a huge microwave (the under-the-counter, or built-in type) for $5. It works great. I have tried the defrost, cool, defrost cycle and it seems to work pretty well. I haven't done it enough to give you specific times, etc, but you have re-sparked my interest. I have read that you don't want to heat the bowl until it is hot to the touch. This will probably take some experimentation with your m-wave and may differ between wood species.

Thanks for that info, Eric.......when you do your next one........if you would not mind, take a note or two about the drying times you use and what cycle. Also note the wattage of your oven. Mine is 1100 watts.......some are less, some a bit more, but it would be interesting to narrow the thing down to some approximations to begin with.

Much appreciated!!!

[Ralph Lindberg]

I actually do that a lot. I put the word out a few years ago and had several friends donate their old large capacity microwaves. I actually wore the first one out.

I run mine at full power, but very short periods, between 30 seconds and a minute. I then let the wood completely cool before repeating. I took a form I was playing with from 175 grams on 120 over the course of the time I was chopping up some Walnut (see my post on being productive in the new year). Then dried the walnut blanks for pens and bottlestoppers while I cut the Madrone

[Jon McElwain]

I used to work in a geology lab doing soil testing and such. There were several tests that required microwave drying of materials. Everything from wet concrete to road gravel, sand, and topsoil (lots of organic content and wood). I am comfortable with the idea of drying wood in a microwave oven, however, I would warn that exceedingly high temperatures can be achieved while doing so.

Drying wood should be done in short periods of time and with sufficient cooling time in between. Like Ralph said above, full power at 30 seconds to a minute at a time works well to dry wood. I suspect that drying wood on defrost would allow you to extend the time for a much longer period so that there was not so much handling. Instead of 30 seconds of full power followed by 3-5 minutes of cooling, you could potentially put a piece in the microwave on a low setting and let it run for 10 minutes or something like that. Experimenting and monitoring temperature would be critical before a full 10 minute burn!

When we dried materials in our lab, it was important that we not dry them too fast because the steam generated inside a rock could cause the rock to burst into pieces. Likewise, if dried too fast, wood might pop as the steam is trying to escape quickly. When you burn wood in a fireplace, pitch pockets can pop. Not a situation that you would want to have occur in your wife's kitchen microwave!

I have heated wood to the point of scalding as well. I once had a brown spot develop on a piece. It was starting to burn because I had heated it too long. I backed off and completed the drying in shorter periods with sufficient cooling in between. I also used a plastic microwave cover over one piece and ended up melting a little hole in the plastic. Let's just say I've become more conservative with the microwave drying.

If your goal is to dry wood for finish turning, I would recommend getting a kitchen scale of some sort to track the moisture loss. Dry your piece for a minute or so at a time and cool in between. Record the weight between each cycle. When the wood is no longer loosing weight, the piece is dry and you should stop the microwave process. At this point, if you keep going, you are probably at the most risk for scalding your wood.

Here is a link (http://www.sawmillcreek.org/showthread.php?177783-Elm-Pot&highlight=) to a small pot that I finish turned wet, then dried rapidly in the microwave. I was going for a warped pot, and I was hoping for a small crack. I succeeded on both points!

Hope that helps!

Jon

[John Keeton]

Roger, this thread (http://www.sawmillcreek.org/showthread.php?136006-25-Ky-Coffee-Tree-more-lessons-learned!/page2) is the only piece I have used the microwave on, though I wouldn't hesitate to do it again on a rough turning of wet wood. It is just that I don't use much wet wood. I had been turning only a few months when I did that piece. Needless to say, I have learned a lot and may not incur some of the problems I had with that one.

[Dan Hintz]

A word of warning... since wood isn't the best conductor of heat, do not rely on the outside being holdable as indicating the interior is just as cool. You could set a piece on fire internally (well, it would char) if you just waited for the outside to cool down.

[Roger Chandler]

Wow.....this is becoming a very informative thread! I hope to give this a try......perhaps my wife will like the new unit! :rolleyes::D

Question.......if drying a roughout....lets say a bowl form, should the bowl be turned upside down in the oven, or with the opening at the top?

[Roger Chandler]

Another question......since microwaves vary and wood might heat unevenly, might it not be best to use a microwave oven that has a turntable?

[Fred Belknap]

Roger I have used microwave drying quite a bit. My method came from someone on the web so it isn't original to me. I put whatever I am drying in a plastic garbage bag and and fold the top under the turning. I microwave on high for two minutes then take it out of the bag and cover with something to keep it from cooling to fast, in the shop I use an old Carhart coat. I let it cool for 5 minuets then turn the bag inside out so there will be a dry spot for the turning. I also weigh and record the weight. You need to be pretty persistent in the process to get good results. It takes several cycles to get it dry, the bowl stays hot and heavy gloves are recommended. There is a lot of steam generated in the bag so you need to use caution with that. I have dried bowls up to 14" and smaller things like lids that tend to warp a lot. It is a lot of trouble but it does produce good results . I have dried oak, cherry, maple and walnut and maybe others. One thing that can happen is the turning may get two dry, it is kind of a guess when it is dry enough. It usually takes 8 to 10 cycles, I watch the difference weigh between cycles starts to drop off and that is when I usually stop. I let it set in the shop enviorment for at least three days to see how it reacts, sometimes it will gain weight (to dry) and sometimes it will loose weight (not dry enough) . It is quite a bit of work but sometimes I need something next week instead of a couple months down the road.
You may need to adjust the time on for different microwaves, mine is an old one and I don't have any idea of the wattage. The person who gave me this method used a seal plastic bag, when it swelled but didn't burst was what he used for the cook time. I always use full power. I have tried other processes but this one gives the best results.
edit: My microwave doesn't have a turntable, sometimes I take out the glass dish to get extra room.

[Scott Lux]

+1 to everything Jon Said. 30 seconds to a minute, cool down, weigh, repeat.

Fred's point about the plastic bag and covering while drying is good too. Although if I had a Carhart, I don't think I would let a chunk of wood wear it. Be sure to reverse the bag between cycles.

[Dan Hintz]

The person who gave me this method used a seal plastic bag, when it swelled but didn't burst was what he used for the cook time. I always use full power.

Although it traps the moisture and will likely take more cycles, it's actually a great method... when the bag starts to puff, you know you've made the water hot enough to boil and turn to steam.

[Steve Schlumpf]

Roger, when I turned the top to this piece, The Tear (http://www.sawmillcreek.org/showthread.php?195822-The-Tear&highlight=), I used fresh cut wood. Had to, the idea of adding a cone shaped lid came after the vase had been completed. I rough turned the top and then microwaved it on full power (1100 watts) for 35 seconds. It was warm to the touch when I removed it, but nothing extreme. I let the wood cool for about 5 to 10 minutes while I went around and did other stuff. Used the microwave 10 to 15 times before it felt light enough in my hand that I considered it dry. Nothing scientific about the process but it does pay to be patient!

[Roger Chandler]

Roger, when I turned the top to this piece, The Tear (http://www.sawmillcreek.org/showthread.php?195822-The-Tear&highlight=), I used fresh cut wood. Had to, the idea of adding a cone shaped lid came after the vase had been completed. I rough turned the top and then microwaved it on full power (1100 watts) for 35 seconds. It was warm to the touch when I removed it, but nothing extreme. I let the wood cool for about 5 to 10 minutes while I went around and did other stuff. Used the microwave 10 to 15 times before it felt light enough in my hand that I considered it dry. Nothing scientific about the process but it does pay to be patient!

Good info, Steve......thanks! I have a scale that weighs to the 10th of an once up to 55lbs. That along with some experimentation ought to prove useful!

[kevin nee]

I use the microwave all the time. 1 minute a day full power. I dry pieces of burl for pen blanks. Pieces of wood cut in circles from 2+ inch boards. Roughed out bowls etc. Life has been much better around here since I got the shop microwave

[Brian Finney]

Roger, I have used microwavedrying a number of times – suggest that you don’t get it too hot otherwise youhave heat checks (cracks) in the wood. Onmy microwave defrost is 10% power and if the piece is thinnish I would usedefrost for 1 or 2 minutes and leave for 10 minutes and cycle. Thicker pieces youmay be able to use 30%. It can take hours to get the wood dry, but it does seemto stop movement in the green wood. Somesuggest weighing the wood after each cycle and I am told when you see a marked drop inweight the m/c is optimal - never doneit, I just feel the wood if it feels wet it’s still too wet.
As an aside, if the second use microwave fails without smokethen it could be the internal fuse blown. Ebay has them. If it fails with smokeget a new one.