# Microwaving wood



## DonAlexander (Apr 12, 2012)

I'm guessing this fits here as well or better than anywhere else since wood turners often use green wood and few others (that I know of) do.

I've been doing some experimenting drying small pieces of wood. I started with an oak bowl I'd roughed out and wanted to finish...me thinks I got the wood too dry as it cracked while I was turning it. Of course it could be my less than stellar techniques, my somewhat less than perfect tool sharpening skill set, or it could have been a hidden defect. No matter, it was just a bowl from a free log. It was nothing special - a learning exercise. 

I tried drying some pen blanks. This time maple. I had some success here, or so I think. I made pens and they seem fine.

My process has been weigh the wood, microwave it on high until it gets very warm (usually about 2 minutes for small quantities), weigh it, let it cool completely, weigh again and repeat the process until....and here's a catch...I think it's dry. Usually I'm seeing between 15 to 25% weight loss. Usually.

Well two days ago I had a piece of maple with some better than average figure. It was fresh cut and very wet. It was about an inch thick, 9 or 10 inches long and 4.5 inches wide. Being a bigger chunk of wood I nuked it for 3 minutes and got an 11.5% weight drop. After cooling I decided to go 4 minutes since the wood wasn't very warm after 3 minutes. The wood got very warm but you could hold it in your hand. I lost about 9% of the weight. I let it cool over night and when I got my morning coffee I figured I had another 10% of weight to lose - the piece felt a bit heavy and I was guessing as wet as it was to start I'd probably lose more than 25% of the weight. So back in the microwave for 4 minutes and I go drink my coffee. About the time the bell dinged saying the wood was finished cooking I noticed smoke in the family room....hmmm. I mosey into the kitchen and am impressed with the quantity of smoke I've produced, glad Wife isn't home, and I'm wondering why the %$#& smoke alarm isn't screaming.

Well, I can confirm that the wood was cooked from the inside out. There was charring on the outside and a pocket of ash on the inside. I'm happy I didn't go for 5 minutes....

After this, the wood weighed 31% less than at the start. 

So here's the lessons I've learned.

1. Go slower. I'm not shooting to lose a lot of water weight with each cycle of the microwave. Instead I'm looking for a time that gets the wood warm to the touch but not uncomfortably so.

2. I stand watch while the wood is in the microwave. Its just about as interesting as watching paint dry but there's a better goal - wood to turn.

3. I still weigh the wood and I'm looking for a "pausing point" - this would be a place where I suspect the wood is dry and I keep weighing it over a week or two to see if its gaining weight, losing weight, or staying about the same. I plan to have some known dry samples to use for comparison since stable wood (if there is such a thing) will gain and lose water (and weight) depending on humidity (home temps are constant).

I mention this experience for 2 reasons - first, maybe there's a lesson in it for someone who plans to try this and second, maybe there's someone on the board here who can add to what I've learned with their experiences.

Don


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## Treeoflifestairs.com (Jan 9, 2012)

Never thought of this since most of my lumber needs to be much larger than what can fit in a microwave. Interesting concept though. Did you try reducing the power level like you would for defrosting meat?


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## DonAlexander (Apr 12, 2012)

I have not tried reducing power yet, but likely will at some point. The scientist in me is trying to work with one variable, time in this case, and keep everything else as equal as I can....which with wood is literally impossible. 

As I ponder how I can work out time vs power curves, I'm thinking it may be worthwhile getting an remote sensing IR thermometer. I've wanted one of those for a while anyway.<g> Since I'm trying to make water vapor it may be that I can optimize time and power for weight loss.


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## epicfail48 (Mar 27, 2014)

Reducing power would probably help a lot. What I'm guessing happened is the mlisture on the inside of the wood heated too fast for the vapor to escape and caused a rather alarming heat buildup. Lower microwave power should help, but personally I prefer a toaster oven set to 200


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## Hwood (Oct 21, 2011)

Ya I have had smoke coming from the microwave also..


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## Gary Beasley (Jan 21, 2009)

Yep, wood conducts heat poorly so you have to limit the amount of power you apply to it. One item I found interesting was the use of a vacuum kiln with low levels of microwaves applied to enhance the boiling off of the water. I have a stabilizing rig so I can put wood in to help dry it but I have to preheat it in my toaster oven for an hour or so before putting it under vacuum. I've tried it without the heat and the wood actually comes out feeling cold from the low pressure water boiling. Wierd.


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## Mort (Jan 4, 2014)

I've done that a couple times, I usually do 45 seconds and let it sit for 5 minutes. Worked alright for a cherry cup I turned.


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## DST (Jan 10, 2011)

this is not meant to be critical. I just find it amusing how many people have have set fire to wood this way. We are such a hurried bunch. A bowl blank or rough turned bowl normally takes 1 or two months or more to dry depending on temperature. You can decrease it to a day with the microwave but still we have to rush it because its not fast enough.


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## DonAlexander (Apr 12, 2012)

DST said:


> this is not meant to be critical. I just find it amusing how many people have have set fire to wood this way. We are such a hurried bunch. A bowl blank or rough turned bowl normally takes 1 or two months or more to dry depending on temperature. You can decrease it to a day with the microwave but still we have to rush it because its not fast enough.


And as a new comer I'm curious what you base your 1-2 month estimate on for drying a rough turned bowl blank? Everyone from Richard Raffan to YouTube posters seem to think it takes 6 months to a year depending on a bunch of variables.

I suppose its fair to assume I was hurrying *if* cutting (what I understand to be) a 6-12 month process down to a few days or a week constitutes hurrying. The fact is I've recently retired and hurrying was never on my mind. Most of the wood I've dried in the microwave has take 2-3 days and never reacted like the maple blank.

Don


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## Bonanza35 (Jan 20, 2011)

If you haven't been working in green wood for very long it's really hard to put everything you make on hold for six months. After you've been at it awhile, you tend to accumulate blanks and eventually have a selection of dry wood to work with. In the meantime I'd suggest milling extra blanks every time you get some wood so you build up a supply. 
As for nuking, I've charred the inside of a few chunks myself. It takes patience. In my experience it takes longer to reach the point of no more weight loss than I have heard some others say. I heat on high for 1-3 minutes, depending on size, and let cool in a paper bag or covered with a towel to prevent surface checks, weighing between cycles. It's not uncommon to go through 20 or more cycles before it levels out. Remember the thicker the wood, the longer it takes for the center to cool. On the other hand microwaves heat throughout the piece evenly so you can see why the burning takes place near the center of the wood. It just keeps getting hotter with each cycle even though the outside may be cool to the touch. So I say go slow and look at it as a 2 or 3 day process rather than a half day process if it's a sizable blank, although a pen blank may be a lot quicker. Just be careful!


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## Bill Boehme (Feb 9, 2014)

I would like to offer a much better procedure. Your current method seems to be way too much like a "pants on fire" method.
1. Wrap three or four layers of paper towels around the piece
2. Put this in a plastic grocery sack and loosely wrap it around the piece, but do no try to seal it or prevent the loss of vapor.
3. Put it in the microwave and set it for delicate defrost or equivalent.
4. Run the microwave for 2 minutes
5. Open the microwave door, but let everything sit for 5 minutes or until room temperature.
6. Remove the bag from the microwave and unwrap the paper towels from the piece. They will be wet. Spread out the paper towels in a location where they can dry.
7. Turn the plastic bag inside out and use a towel to mop up the water.
8. Wrap new paper towels around the piece and repeat the process.

In the beginning, the paper towels will be very wet and this process also means that some of the water will go back into the wood as the piece cools down. This is intentional. You might want to call this the slow fast method as opposed to the pants on fire method. There is an inescapable consequence of speeding up drying. The faster that the process goes, the greater the warping and splitting and you can chisel that in stone. The method described above provides good acceleration and reasonable yield, but it is labor intensive.


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## Bill Boehme (Feb 9, 2014)

DonAlexander said:


> And as a new comer I'm curious what you base your 1-2 month estimate on for drying a rough turned bowl blank? Everyone from Richard Raffan to YouTube posters seem to think it takes 6 months to a year depending on a bunch of variables.
> 
> I suppose its fair to assume I was hurrying *if* cutting (what I understand to be) a 6-12 month process down to a few days or a week constitutes hurrying. The fact is I've recently retired and hurrying was never on my mind. Most of the wood I've dried in the microwave has take 2-3 days and never reacted like the maple blank.
> 
> Don


I'll give you a summary of my observations. I have been turning for a bit over ten years. I have turned a lot of stuff in that time. Some wood I twice turn and others I do it in a single go. I turn US domestic hardwoods. Every species of wood behaves differently when drying so what somebody on the other side of the world experiences may be different from what I experience. I have recorded and plotted drying data (weight) for enough rough turned green wood items that I now have an intuitive feel and don't need to take measurements for everything.

Of course the size and thickness of a rough-out as well as the species all have an impact. The method of drying also matters. I prefer Anchorseal. An average ballpark figure for a bowl that is 3/4" thick is 3 months ans some species might require up to 4 months. For small bowls with a wall thickness of 1/2 to 5/8 inch should be dry in two months up to three months. You may have heard about a rule of thumb that says one inch per year but that is referring to whole logs or large blocks.

The largest bowl that I have made is 18 inches in diameter and was rough turned to 1 3/4 inch thick. It took nearly six months to dry.


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## DST (Jan 10, 2011)

DonAlexander said:


> And as a new comer I'm curious what you base your 1-2 month estimate on for drying a rough turned bowl blank? Everyone from Richard Raffan to YouTube posters seem to think it takes 6 months to a year depending on a bunch of variables. I suppose its fair to assume I was hurrying *if* cutting (what I understand to be) a 6-12 month process down to a few days or a week constitutes hurrying. The fact is I've recently retired and hurrying was never on my mind. Most of the wood I've dried in the microwave has take 2-3 days and never reacted like the maple blank. Don


Some of my roughed out blanks certainly do take longer. It takes less during the hot summers and I often have a fan going to keep the air moving. I also built a small kiln that I have been experimenting with. 
Like someone else said eventually you have enough on hand that you don't pay as much attention to the time. Again I wasn't being critical of you. It just isn't the first post about this and several times Ive heard the story of 2 minutes at a time was going good so I decided to try 5. Or something similar. I have a charred bowl on my bench that a turner gave my way before I started turning. It reminds me to take it easy. That being said I am still going to try nuking a few of my own this week and see what happens. Thanks to Bill also for posting his method.


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## duncsuss (Aug 21, 2009)

I think you're making it too complicated.

Weigh the piece *and record the weight.*

Zap it for a minute.

Take it out of the microwave (leave the door open to let the steam out) and set it down to cool.

Weigh it again *and record the weight.

*Compare the two weights -- the difference between them is the amount of water that is no longer in the wood.

Repeat the cycle multiple times and you will see the change in weight get smaller and smaller.

When the change in weight is just 1 gram, I stop and let it sit for a half hour then weigh again -- it usually increases in weight during this time, as it takes up moisture from the air (i.e. it was drier than the ambient humidity.)


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## DonAlexander (Apr 12, 2012)

duncsuss said:


> [/B]Compare the two weights -- the difference between them is the amount of water that is no longer in the wood.
> 
> Repeat the cycle multiple times and you will see the change in weight get smaller and smaller.
> 
> When the change in weight is just 1 gram, I stop and let it sit for a half hour then weigh again -- it usually increases in weight during this time, as it takes up moisture from the air (i.e. it was drier than the ambient humidity.)


Well, let me suggest you may have over simplified the process. Certainly weighing the wood before and after nuking it is necessary but one should also weigh it again after the wood has thoroughly cooled. There's normally a pretty significant weight loss during the cooling period. I'd only do 2 or maybe 3 of your cycles per day and then do an over-night weighing. I have generally notice a significant over-night weight loss early in the process. At some point I'll note a weight increase over-night. That is one signal that I'm done. While the weight losses generally decrease the 1 g loss makes little sense to me as a signal that the wood is dry. A pen blank will show a 1 g weight loss much sooner than a bowl blank but the bowl blank will likely be a lot wetter internally. I usually look for a percentage weight loss over time. About the only rule I have so far is a 30% is the most I've found necessary and sometimes 15% is enough. It depends on the species and how wet it is when you start.

FWIW, I bought an IR remote reading thermometer (on sale at Harbor Freight for $35). I plan to do some scientific wood drying experiments when I get back home. Bill B. has made a few suggestions that I agree with - the goals are to dry the wood without warping it or splitting it. Those are my goals. The idea that, "The faster that the process goes, the greater the warping and splitting and you can chisel that in stone" strikes me as questionable. I don't think speed per se is the enemy. The problems come with uneven drying, and probably internal stresses, which are related to uneven drying and not necessarily drying speed. Slow drying may be conducive to even drying but slow drying may not be the only way to get even drying. So I'm going to be looking for ways to heat the wood evenly using time and power settings and, to the extent I can measuring warpage and splitting.

I'm pretty sure before this process is done I'll have a moisture meter and I'll be cutting blanks to get internal moisture content and temperatures. This isn't rocket science, but it is science and there needs to be methodology. And I know my methods won't satisfy everyone...but that's life. Within some reasonable limits of money and varieties of wood, I hope to get some general methods that will help speed up my wood drying.

I won't post results here because I expect they'll be rather lengthy, full of rather dry and arguable data, and I don't expect that's the purpose of this board. But I will post a link to my methods, my raw data, and my results.

Don


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## duncsuss (Aug 21, 2009)

> Take it out of the microwave (leave the door open to let the steam out) and set it down to cool.
> 
> Weigh it again *and record the weight.
> *


Sorry if it wasn't clear -- I meant "let it cool, then (when it's cool) weigh it again."

I agree completely that they lose significant water content during the cooling period.


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## Bill Boehme (Feb 9, 2014)

Don,

I can understand your skepticism about speed itself being the direct cause of drying defects, however, speed is the vehicle that gets you to the conditions that cause defects. So if one wishes to pick nits, it is the stress gradient that leads to drying defects and there is no better way to set up a steep gradient than by accelerating the process. Speed is responsible because it can't be separated from setting up the gradient. The faster you try to remove moisture by whatever method -- heating or vacuum or whatnot, the steeper the moisture gradient from exterior surface to the interior. If there were a way to dry wood fast without causing defects, then everybody would be doing it.

Based on a previous comment in this thread, I should mention a common misconception that was intended to simplify in layman's terms how food was cooked by microwave ovens -- the phrase used was, "microwave ovens cook from the inside out". In actuality, the heating is from the exterior towards the interior just like a conventional oven or a crock pot Although there is a characteristic known as skin depth that enables the microwave frequencies to penetrate deeper than the higher infrared frequencies of conventional heating. The other more significant difference that microwave ovens possess is the much greater efficiency due to direct energy transfer to polar molecules such as water and fat. The reason that I mention this is I want to dispel any misconceptions about uniform heating throughout a piece of wood even if it were perfectly homogenous. Even if it were possible to produce uniform heating throughout, it still would result in a moisture gradient from outer surface to center. Obviously, the faster the drying process = the greater the heat = the greater the temperature = greater drying defects.

A moisture meter, digital scale, and IR thermometer are nice toys, but the only tool that you need to determine when the wood is dry is to check the paper towels

I'll keep my hammer and chisel ready for you to borrow when you need it. :laughing:


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## mako1 (Jan 25, 2014)

I've turned a lot of wood.Mostly dry some wet.I don't do turning anymore but when I did I had a VS lathe that was 15" X144".Used to turn a lot of columns and such for very expensive homes.
My curiosity got me when I saw this post.
Why not just use a moisture meter?They have some that are very accurate and reasonably priced.
My point is that it would save you a lot of time dicking around with the wood and more time turning.
It's been a while for me but just curious why this would not be the best way to do it.
When you dry wood quickly you build internal stresses in it to begin with.Especially figured woods.
Why would you not use a meter to check the moisture content?


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## Bonanza35 (Jan 20, 2011)

As chance would have it, my wife needed a project done ASAP and I needed to dry a couple of blanks in a hurry. This one was sycamore with no signs of cracks or checks but still had some moisture. The first heating had steam rapidly spewing from the end grain. After a few cycles it leveled out with no signs of cracks. When I took the first 1/8" of this is what I found. It's cracked up in the dead center of the blank! I can't swear it wasn't like that before but I've turned a bunch from this log and it's been very sound. It really looks like maybe the steam expansion caused interior damage to the blank. Never seen that before.


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## DonAlexander (Apr 12, 2012)

Bill, I don't think we're far apart in our understanding of the problems associated with wood drying. My sense is you agree that you can microwave a piece of wet wood to a usably dry state (or close) over a period of a few days or weeks depending on the size of the wood and how wet it is. I'm simply looking to figure out how to keep the wood drying process as stable as possible. It's entirely possible I'll end up opting for a toaster over or kiln that can hold 140 degrees for ten days with low humidity and good air flow. In any event, I hope to start working on what the limits of a microwave are this week.

Mako1, the scale, the IR temperature problem, and the moisture meter all tell you different things. The scale gives you weight loss and infers how well you might be getting the deeper internal moister out. The moisture meter gives you a reading on the moisture content on the surface of the wood. Together with the scale, you can infer a lot more about the internal dryness than you can with either a scale or the moisture meter alone. The IR thermometer will tell you how evenly you've heated the external sides of the wood (and you can infer something about even or uneven internal heating).

As Bill has noted, the wood is going to dry from the outside to the inside and there will be a gradient. The goal is to minimize that gradient while maximizing water loss. And the $64 question is how can you do this reasonably quickly and have a useable piece of wood when you're done.

I've had a week to ponder this question and I'm beginning to think one of the limits may be the size of the wood that you can quickly dry in a microwave. More on this later.

Don


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## Bill Boehme (Feb 9, 2014)

mako1 said:


> I've turned a lot of wood.Mostly dry some wet.I don't do turning anymore but when I did I had a VS lathe that was 15" X144".Used to turn a lot of columns and such for very expensive homes.
> My curiosity got me when I saw this post.
> Why not just use a moisture meter?They have some that are very accurate and reasonably priced.
> My point is that it would save you a lot of time dicking around with the wood and more time turning.
> ...


Were the architectural columns hollow or solid? Were they hardwood or softwood? If it was solid, was it a stack lamination? I've sen a lot of columns that were hollow, for obvious reasons, and made of softwood that was mitered with a lock joint so that they could be laid up to produce a polygon that was then turned and fluted in some cases.

Moisture meters work fine when you are drying slabbed dimension lumber. But poking a moisture meter into the end or side of a large log tells you absolutely nothing about the dryness more than a couple inches from the surface at best.

I had a very large mesquite log almost three feet in diameter -- extremely large for mesquite. It was on my driveway for five years and I finally decided to cut it up last year. The moisture meter said that it was dry. I was very surprised to see how wet the wood was once I got two about six inches from the surface. The moisture meter was pegged at the maximum reading. Rather surprising given the extremely hot summers here in Texas and that the log was in full sun all day long and off the ground. I used the center portion to make a tall hollow form and it was slinging water. On average, wood dries about one inch per year. That rule of thumb seemed to fit that mesquite log well.

If you rough turn a piece of wood and then Anchorseal it to let it dry, one of the first things that you will learn is that a moisture meter will give widely different readings at different parts of the rough turning. If the turning is large, the moisture meter may not be accurate. For example, I rough turned an 18 inch box elder bowl to about 1.75" thickness. Moisture meter readings were useless. Weighing the rough turning periodically gave me an absolute answer when the wood had reached EMC. I didn't give a rat about the numerical value of the moisture content, I just needed to know if it had reached EMC. When the weight had stabilized, I knew it was dry. Here is a plot of the weight as the bowl dried.









It started at 13 pounds and the final weight was about 8.5 pounds. It was dry in about 80 days, but I didn't get around to the final turning until a few days later. I see that the image was resized to the point that it can't be read.


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## mako1 (Jan 25, 2014)

Bill Boehme said:


> Were the architectural columns hollow or solid? Were they hardwood or softwood? If it was solid, was it a stack lamination? I've sen a lot of columns that were hollow, for obvious reasons, and made of softwood that was mitered with a lock joint so that they could be laid up to produce a polygon that was then turned and fluted in some cases.
> 
> Moisture meters work fine when you are drying slabbed dimension lumber. But poking a moisture meter into the end or side of a large log tells you absolutely nothing about the dryness more than a couple inches from the surface at best.
> 
> ...


 I have done a lot of them both ways.Some glued up slod coloumns that 24' long poplar.Most were glued up polygons 10-12'.
Thank you for the detailed explanation.


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## Steve Neul (Sep 2, 2011)

I don't microwave wood. I eat it raw, mostly what floats through the air. Occasionally I eat a larger piece comming out of the table saw but mostly stick to dust. :laughing:


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## Al B Thayer (Dec 10, 2011)

If I may add. My microwave on defrost just turns on and off for that cycle. Rather than staying on. I believe the power level is high when it's on. So it blasts high then sets with the fan going. Then blasts and so on.

Al


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## DonAlexander (Apr 12, 2012)

Al B Thayer said:


> If I may add. My microwave on defrost just turns on and off for that cycle. Rather than staying on. I believe the power level is high when it's on. So it blasts high then sets with the fan going. Then blasts and so on.
> 
> Al


You're missing a real treat. Rather than cold dry saw dust you could be enjoying the bold smokey tastes and aromas of mesquite or red oak, or the more subtle flavor profile of maple or cherry. Well, in my case the maple aroma wasn't subtle...

Don


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## Bill Boehme (Feb 9, 2014)

Al B Thayer said:


> If I may add. My microwave on defrost just turns on and off for that cycle. Rather than staying on. I believe the power level is high when it's on. So it blasts high then sets with the fan going. Then blasts and so on.
> 
> Al


.... and?

That is also what a conventional oven does and what a crockpot does.


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## DonAlexander (Apr 12, 2012)

So I've done some reading on drying wood, albeit not with a microwave. It's interesting how wood dries and what stages of drying results in cracks. Some of it may apply to drying wood with a microwave.

Based on what I've read I've modified my technique a bit and I'm observing what's going on. I have no conclusions yet but I can assure you the microwave (at least my over-priced GE) doesn't heat wood evenly. I've been trying "low and slow" (not a great way to fly but a better way to dry).My test subject is an 8" by 3.5" cherry bowl, rough turned at about 20% moisture content (MC). 

Five minutes on power level (PL) 4 got the wood to about 150 degrees, depending on where you measured it. There's at least a +/- 25 range there. I also lost about 2 1/4 percent moisture after microwaving. After it cooled I'd lost 5%+. I'm down from about 20% MC to a bit under 15% MC. Encouraging results. 

One of my goals here was to find a time/power setting that didn't create steam. I didn't want temps over 180 and I didn't want to hear any hissing. I succeeded.

I'd heard that MW excited water molecules to create heat. I'm not sure I believe that, but I won't rule it out. I repeated the 5 minutes at PL 4 and the bowl, starting with less water, came out warmer. So I can interpret that to mean most anything but I think I'm going to start looking at reducing the PL next time around to keep the temps down. I didn't get any hissing and my average temps were well under 180 but in spots I hit 190. I'm guessing that's not going to be good for stability. So far no cracks, checks, or significant warping.

I like the way cherry turns - wet or dry it cuts cleanly, smells great, and looks better. I just hope I don't over cook this bowl.

Don


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## Bill Boehme (Feb 9, 2014)

DonAlexander said:


> ....I'd heard that MW excited water molecules to create heat. I'm not sure I believe that, but I won't rule it out.......


As an electrical engineer who has worked on radar systems, I can assure you that what you read is true. The frequency of microwave ovens is 2450 MHz which just happens to be the right frequency to get water molecules all excited. We all get excited by different things ... what can I say? :laughing:

The water molecule is polar if you remember that from chemistry class. The size of the water molecule is just right to respond to the energy of the microwave frequency. You might also think about weather radar systems. They "see" clouds because clouds are ... lets see, is it cotton candy, whipped cream, cigar smoke, or water? No looking on other students papers. Time is up -- pass your papers in.


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## Ron Rutter (Jan 18, 2011)

"I have no conclusions yet but I can assure you the microwave (at least my over-priced GE) doesn't heat wood evenly.'

Gee Don, maybe that is because the moisture content isn't consistent.


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## DonAlexander (Apr 12, 2012)

Ron Rutter said:


> "I have no conclusions yet but I can assure you the microwave (at least my over-priced GE) doesn't heat wood evenly.'
> 
> Gee Don, maybe that is because the moisture content isn't consistent.


Ron, That certainly explains why the sap wood is hotter and wetter than the heart wood. But there are other discrepancies that I'm not sure it explains.

Bill, I appreciate your explanation of how microwaves work and as a pilot, I know clouds are made out of cotton candy which appears out of thin air when the sugar level in the atmosphere reaches the dew point. Seriously however, microwaves surely excite other molecules, too but likely to different degrees...right? It would seem that in one of my earlier experiments, the maple cells got pretty darned excited after the water was gone. Are there minerals that might be subject to MW excitation?

Don


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## Bill Boehme (Feb 9, 2014)

DonAlexander said:


> Ron, That certainly explains why the sap wood is hotter and wetter than the heart wood. But there are other discrepancies that I'm not sure it explains.
> 
> Bill, I appreciate your explanation of how microwaves work and as a pilot, I know clouds are made out of cotton candy which appears out of thin air when the sugar level in the atmosphere reaches the dew point. Seriously however, microwaves surely excite other molecules, too but likely to different degrees...right? It would seem that in one of my earlier experiments, the maple cells got pretty darned excited after the water was gone. Are there minerals that might be subject to MW excitation?
> 
> Don


... and most people think that the propeller on the front of a plane is to make it go through the air, but as a former pilot, we both know that it is actually a fan to keep the pilot cool.

And, you are right about water not being the only thing that gets excited about (or is it "by") microwaves. Other polar molecules, notably fat in meat, love microwaves. As you noticed sugars might also absorb energy from microwaves, but it might be the water in the sugar. Try microwaving some dry granulated sugar to test that idea. Keep in mind that "dry" sugar contain "bound" water molecules linked to the sugar molecules. Certain ceramic materials also cac be heated ... there was one that was sold for use in microwave ovens so that it could act as a heating element that would make a microwave oven work somewhat like a conventional oven. Most things will heat very little if at all -- saltine crackers, bone dry wood, and paper to name a few.

When drying wood, don't count on heating from anything other than the free and bound water in the wood. When drying wood in the microwave, you will find that after the water is gone, the wood can be heated, but then your self-contained fire extinguisher is empty and if not being very careful (as in drying on max power for too long), you could wind up with maple flambé.

One other thing not previously mentioned is that very fast drying will always result in creating internal stresses in the wood -- sometimes even slow drying does as well. When you turn wood with internal stresses, the equilibrium is upset and the wood will move or crack in the process of restoring equilibrium.


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## Bonanza35 (Jan 20, 2011)

I'll be honest, Bill, I'm still hung on your mention of Crock Pots. Seems an awful lot like a little mini kiln if you think about it. It might just work. 

Fascinating about the cotton candy too. I never got instrument rated but always wondered what was in there.


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## Ron Rutter (Jan 18, 2011)

One other thing not previously mentioned is that very fast drying will always result in creating internal stresses in the wood -- sometimes even slow drying does as well. When you turn wood with internal stresses, the equilibrium is upset and the wood will move or crack in the process of restoring equilibrium. 
__________________
Bill Boehme

DON. I hope you appreciate the significance of this information.


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## Treeoflifestairs.com (Jan 9, 2012)

Ron Rutter said:


> One other thing not previously mentioned is that very fast drying will always result in creating internal stresses in the wood -- sometimes even slow drying does as well. When you turn wood with internal stresses, the equilibrium is upset and the wood will move or crack in the process of restoring equilibrium. __________________ Bill Boehme DON. I hope you appreciate the significance of this information.


This is actually what is trying to be avoided by microwaving the wood. Yes, when drying wood in a kiln the outside of the wood is dried before the interior and thus the outside may crack from shrinking while the interior is still plump with moisture. Microwaving, by contrast, heats throughout and therefore (hypothetically) will dry more evenly. I say hypothetically because I'm sure everyone has microwaved their food before and had parts of the good warm and parts still cold. I think the objective of this kind of method needs to be to dry it slow enough to dry it evenly (using a lower power level) but still much quicker than using a kiln or even just allowing it to air dry.


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## Bill Boehme (Feb 9, 2014)

Treeoflifestairs.com said:


> This is actually what is trying to be avoided by microwaving the wood. Yes, when drying wood in a kiln the outside of the wood is dried before the interior and thus the outside may crack from shrinking while the interior is still plump with moisture.


And the stresses created by fast drying in a microwave oven or any other method of fast drying by heating will be greater than slow air drying. You are confusing the intent of drying dimension lumber where the grain is running the length of the board with a turning blank where you have to deal with shrinkage and warping in all dimensions. Internal stresses created during kiln drying of lumber that is being constrained from warping will be dimensionally stable because the internal stresses are in equilibrium. The wood won't be cut the way that turnings are that throw the internal stresses out of balance and create warping.




Treeoflifestairs.com said:


> Microwaving, by contrast, heats throughout and therefore (hypothetically) will dry more evenly.


That is a completely mistaken assumption. A convective oven uses infrared wavelength energy to heat food (or other things) ... just a different wavelength range of electromagnetic radiation. The difference between the two is skin depth, but regardless of that, they both heat from the outside in. The "heating from the inside out myth" is primarily the result of marketing in the early days of microwave ovens attempting to come up with an oversimplified description for consumers that "explained" why the oven itself didn't get hot.

Generally, when woodturners microwave bowls, they are turned to completion and very thin. They are then microwaved with the expectation and purposeful intention of getting far greater warping than if the bowl were simply wrapped in paper and allowed to slowly air dry.


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## DonAlexander (Apr 12, 2012)

Well, I have been running more experiments drying wood in the microwave and it really isn't as clear cut as you might imagine.

I had a decent size cherry log that I made four bowls out of. They were 7-8 inches in diameter and 4-5 inches tall (deep?). Two I left in my rather humid basement to dry and two I dried in the microwave. The microwaving got the roughed out bowls to 6.5 to 8% moisture content as measured with a "General" brand moisture meter from Lowes. I measured AFTER turning the bowls to final size. Originally they were at 15-20% moisture content with the heart wood being 15-17% and the sap wood being 18-22%. All roughed bowls had small cracks that originally emanated from the pith. Microwaving "low and slow" didn't seem to make the cracks any worse. They turned nicely, similar to kiln dried cherry. The kiln dried cherry I have seemed a bit harder - that may be just the difference in the tree they came from or it may be an affect of the process. They two roughed bowls that were air drying developed major cracks.

I also dried 2 pieces of oak that were about 4-5 inches long and 2.25 - 2.75 inches in diameter. The starting moisture content was 17-18%. Again a low and slow process and no cracking occurred. I'm in the process of finishing one as a small pepper mill and the other I've drilled and will turn shortly.

Lastly I dried a rather large (for me) oak bowl that was rough turned to a tureen shape (bulge in the middle). It too was 17-28% moisture content to start. Low and slow microwaving got in down to about 8% but I have not finished turning it yet. It had one small crack to start. If it grew, it was insignificant and may be turned away when I complete the turning. This piece of wood started out weighing 1,400 grams and finished at just under 1,000 grams as I recall.

My low and slow process isn't nailed down in stone but generally speaking I'm using a power level of 4 (options are 1-10 with 10 being full power) and microwaving it long enough to get the temperature up to 140-150 degrees F on AVERAGE. Usually this takes 3-4 minutes. I allow the wood to cool at least to <79 degrees (or more) and repeat the process until the wood measures 10-12%. On bowls it seems if the inside is 140-150, the outside will be 25 degrees cooler and the bottom of the bowl will be 25 degrees hotter. I can't explain this difference but I've seen it after every heating event on three different bowls. The bottoms of the bowls had a tenon and started with higher moisture content. That would explain the higher temperature to me. Why the inside and outside temperatures were so different when the wall thickness has be 1/2 to 3/4 of an inch escapes me so far.

One other odd thing I've noticed is the outside of the bowls will have a moisture content measurement of 10-12% when I'm done microwaving. After doing the final turning the moisture content has been 7-8%. This suggests to me that either my moisture meter is very temperature sensitive or the wood dries from the inside out. My results are VERY tentative at this point. I have just a few data points on a few varieties of wood...but they results are consistent.

One other comment on the 'low and slow' process - I usually only do 2 or 3 warnings at power level 4 before the temperature starts to increase with the same time. I'd have thought with less moisture, at a given power level and time the temps would have dropped. The opposite has been the case every time. So I drop the power level to 3 and keep going. The bowls took 3-4 minutes on power level 4 to reach my target temperatures and eventually 3 minutes on power level 3 to get there. I am trying really hard to keep from hearing any hissing like I did on my earlier experiments. I figure steam is going to result in problems.

In terms of time, I'd need to check my notes but typically I'm looking at 6-8 cycles over 2 days for the bowls. I think the larger (1 kg oak bowl) took 3 days to reach my target moisture content.

So if drying a bowl in two days is "slow" then my current process qualifies. It seems fast to me - the bowls I roughed out 2 months ago are still at 18%. We'll see where this goes.

Don


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## Bill Boehme (Feb 9, 2014)

DonAlexander said:


> .... The microwaving got the roughed out bowls to 6.5 to 8% moisture content as measured with a "General" brand moisture meter from Lowes. *I measured AFTER turning the bowls to final size.* Originally they were at 15-20% moisture content with the heart wood being 15-17% and the sap wood being 18-22%.....


I ought to point out that a turning will lose a consider amount of moisture during the final turning if you are turning it relatively thin. This is because the high speed spinning will increase evaporation as well as cause moisture loss by centrifugal force. On top of that, sanding on the lathe generates a lot of heat that will be the coup de grâce on drying.

The starting MC that you stated is often considered "lumber yard dry". If you are going to do microwave drying, it's greatest benefit will be to use it on freshly cut and then freshly turned *green* wood -- meaning that it is from a _living _tree. By the time that most domestic hardwoods are in the 15 - 20% MC range, they are fairly safe for turning without cracking although there might still be a little warping. Of course that isn't dry enough for things like boxes and other things that must be absolutely stable.



DonAlexander said:


> I also dried 2 pieces of oak that were about 4-5 inches long and 2.25 - 2.75 inches in diameter. ....


I am guessing that your oak is red oak. While white oak has more beautiful grain, it just loves to twist and crack at the slightest provocation whether air drying, kiln drying, DNA drying, or microwave drying. However, it might have already been stabilized if it also had a MC in the vicinity of 20%. I suspect that the pice of oak that you dried might show a greater moisture content in the middle when it is drilled. For all of its difficulties, white oak is one of my favorite woods to work with.



DonAlexander said:


> Lastly I dried a rather large (for me) oak bowl that was rough turned to a tureen shape (bulge in the middle). It too was 17-28% moisture content to start. ...This piece of wood started out weighing 1,400 grams and finished at just under 1,000 grams as I recall.


If the oak were _freshly cut green and wet_, the weight change from there to EMC would be more like 2:1.



DonAlexander said:


> ... On bowls it seems if the inside is 140-150, the outside will be 25 degrees cooler and the bottom of the bowl will be 25 degrees hotter. I can't explain this difference but I've seen it after every heating event on three different bowls....


Just a WAG, but the greater surface area of the exterior might account for the temperature difference. Since the bottom is all side grain, heated moisture isn't going to be able to pass through it nearly as well as the side.



DonAlexander said:


> One other odd thing I've noticed is the outside of the bowls will have a moisture content measurement of 10-12% when I'm done microwaving. After doing the final turning the moisture content has been 7-8%.....


See my first comment in this post. It isn't because the wood is heated more in the middle than the outside. Taking temperature can be a red herring because evaporation will cause the exterior to be cooler.



DonAlexander said:


> One other comment on the 'low and slow' process - I usually only do 2 or 3 warnings at power level 4 before the temperature starts to increase with the same time. I'd have thought with less moisture, at a given power level and time the temps would have dropped. The opposite has been the case every time.....


Hmmmmmm! Maybe you're going from boiling to baking. :laughing:

Just keep the fire extinguisher handy. :icon_smile:


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## Gary Beasley (Jan 21, 2009)

Bill Boehme said:


> That is a completely mistaken assumption. A convective oven uses infrared wavelength energy to heat food (or other things) ... just a different wavelength range of electromagnetic radiation. The difference between the two is skin depth, but regardless of that, they both heat from the outside in. The "heating from the inside out myth" is primarily the result of marketing in the early days of microwave ovens attempting to come up with an oversimplified description for consumers that "explained" why the oven itself didn't get hot.
> 
> Generally, when woodturners microwave bowls, they are turned to completion and very thin. They are then microwaved with the expectation and purposeful intention of getting far greater warping than if the bowl were simply wrapped in paper and allowed to slowly air dry.


Don't be confusing convection ovens for microwave ovens, they are different machines. As pointed out earlier microwaves heat by excitation of molecules in the target, not by infrared radiance which is a much shorter wavelength which doesn't penetrate but is absorbed at the surface.


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## Bill Boehme (Feb 9, 2014)

Gary Beasley said:


> Don't be confusing convection ovens for microwave ovens, they are different machines. As pointed out earlier microwaves heat by excitation of molecules in the target, not by infrared radiance which is a much shorter wavelength which doesn't penetrate but is absorbed at the surface.


:surrender:

Thanks for clearing that up -- I must be really confused.

Let's see who was it that mentioned the excitation of molecules, anyway?


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