An easy way to dry small pieces of exotic wood.

[Monsoon1]

I'm doing a project with some Lignum Vitae, and man that wood moves, warps, cracks, etc, like crazy once it's cut.

So I figured I would do some digging on what it takes to kiln dry the stuff, or possibly make a kiln.

I started looking in the industrial sites, and found some terms for some state of the art equipment that they are using, instead of a kiln. Not recognizing the terms, I kept googling them until I figured out what the components they were talking about were.

It turns out that they're microwaving the stuff.
Yep. That kind of microwave.

So I bought an el-cheapo microwave off of Craigslist, and did some experimenting with about a dozen pieces of lignum that I have in sized ranging from 7/8" to 1.5" thick.

I started with 30 seconds per side, flipping once and immediately putting it back in.
I then scraped the resin off (a lot of resin seeps out!), waited for it to cool, and then sanded.
The wood started showing signs of fine cracks, within several days.

So I repeated the procedure, but this time for 45 seconds, and after it cooled, I did it again on the same wood.
This wood lasted over a week, before it started to show fine cracks developing.

Finally, I did a batch of the wood (1.5" thick) 3x, and 45 seconds per side.
That batch has lasted about a month without the slightest sign of cracks developing.

On woods with much lower moisture/resin levels (pretty much everything else), it would be a really good idea to use a moisture reading tool. Maybe even on all woods.
But on the lignum, the key seemed to be having a minimum of resin seepage during the last session.

Now for the caveat. A couple of pieces warped right away. So you have to start out with thick enough stock to be able to sand down to the size you need.

Other than that, I now have some rock solid Lignum that's sitting with no protective coat on it whatsoever, with not a sign of instability.

[Monsoon1]

Oh yeah, one other thing in case you're thinking of trying this.
After you hit all 6 sides, you need to let it cool completely down. Otherwise if you just keep hitting each side back to back to back, it can get so hot it will burn up from the inside.
(don't ask how I know this...)

And this is really an issue of ymmv, since different wood, different moisture levels, different resin levels, different thicknesses, all will effect how long it can be nuked for.

My first test piece was a 3/8" square x 1/2", that I hit a couple of times for around a half minute. That was nearly two months ago, and it's sat perfectly stable since with no finish of any kind.
Not very scientific, I know. But they're doing this with industrial microwaves, so it's obviously a solid approach.

[Tim McKnight]

I have not heard of using microwaves to dry wood but it seems like the process may be too fast. Most kilns that I am familiar with use much lower temperatures to circulate warm air around wood while pulling the mositure out of the air (at a controlled rate) with some sort of a dehumidifier unit. It usually takes about a month to cycle a batch of wood. The later stages involved adding moisture or steam back into the wood to reduce case hardening stresses that can build up if the wood is dried to rapidly.

[Monsoon1]

Originally Posted by Tim McKnight

I have not heard of using microwaves to dry wood but it seems like the process may be too fast. Most kilns that I am familiar with use much lower temperatures to circulate warm air around wood while pulling the mositure out of the air (at a controlled rate) with some sort of a dehumidifier unit. It usually takes about a month to cycle a batch of wood. The later stages involved adding moisture or steam back into the wood to reduce case hardening stresses that can build up if the wood is dried to rapidly.

I wondered about that too. But not only do they use microwave ovens, they're also fairly high power.
The one I read about was something like a couple thousand watts. But they're huge. Big enough to do much larger lumber than would fit in a normal microwave oven.

Maybe it has something to do with how a microwave heats up the wood from the inside out.

[Kitchen Guitars]

I got yelled at for drying in an attic because it was too fast. I think if you take years, not minutes you'll have better results

[Tim McKnight]

Now that I have had a chance to think about the process it might actually make good sense. Most of the stresses induced into wood during the drying process comes from the wood drying to rapidly on the outer surfaces thus adding to the case hardening affect. If the wood were dried from the inside out this may actually be more efficient at reducing these external stresses and quite possibly eliminate the post processing treatment?

[RonS]

I'm familiar with this process.
A lot of woodturners think this is the Holy Grail.
It does work, but in a very limited way.

Dense and/or oily woods like Ebony, Bubinga and Cocobolo seem to problems. (Lignum Vitae falls into this category) I heard a story from a woodturner who said that nuking ebony was like placing a potato in a microwave and forgetting to pierce the skin. (BANG).

I tried on a maple bowl that I turned on a lathe. It looked good for a year or so then started to develop problems like warping and developing hairline cracks. For a wooden bowl it looked good but for instrument grade wood it is a major problem.

[tkuane]

I would like to point out:

Microwave cooks from the outside in, just like most other methods. Because microwave heats things up so quickly, more stress is introduced.

The only thing special about microwave is that it doesn't heat up anything without water molecules. Wood has water molecules all round. Microwaves do not suddenly decide to heat up when they reach the 'inside' of something. They heat up the first water molecule that they come into contact with, which is on the surface most of the time, especially with wet wood.

[Monsoon1]

tkuane, it might not heat literally from the center first while it progresses outward, but it heats the water and the highest concentration of moisture is going to be in the middle of the wood.

I know for sure it heats lignum more in the center, as I burnt a piece up, and the smoke was pouring out from inside the piece. It literally opened up a small hole on the side, and the smoke was just pouring out of there. If i'd had this in the kitchen, it would have caused big problems, believe me.
Very weird to see.

[Monsoon1]

Originally Posted by RonS

I'm familiar with this process.
A lot of woodturners think this is the Holy Grail.
It does work, but in a very limited way.

Dense and/or oily woods like Ebony, Bubinga and Cocobolo seem to problems. (Lignum Vitae falls into this category) I heard a story from a woodturner who said that nuking ebony was like placing a potato in a microwave and forgetting to pierce the skin. (BANG).

I tried on a maple bowl that I turned on a lathe. It looked good for a year or so then started to develop problems like warping and developing hairline cracks. For a wooden bowl it looked good but for instrument grade wood it is a major problem.

This is probably related to the piece that lit up on me. It was a thinner piece, and I simply let it get too hot.

Needless to say, the resin and moisture can vary from wood to wood. So to me, this is just another learning curve of figuring out how to work a particular wood.

And I'm pretty sure a moisture reader would be really handy in figure out when it's been hit enough.

As far as the wood developing some problems along the way, my experience while limited, is that you might have needed to hit it again. Even if it's just 20 seconds a side.
I've been monitoring over a dozen pieces I did with a magnifying glass, and I think I got it right on these. Of course, only time will tell.