Wood Part 2: Green Wood and The Great Microwave Experiment

Green Wood: The Good, the Bad and the Ugly

Let’s talk about green and wet wood. There are a lot of woodturners out there who love to work with green wood, and there are some advantages to doing so. It cuts very easily, produces less dust and allows you to work far more quickly than if the wood was dry. But that’s where the advantages end because once you’ve turned that bowl or other object you are still left to deal with the fact that you’ve got a wet piece of wood that is going to distort, warp, split and crack as it dries unless you do something about it.

This is generally what I end up with when I’ve tried most of the “advice” given by the people who claim you won’t have any problems at all working with wet wood if you follow their instructions and/or use the products they’re trying to sell.

These experts claim this isn’t a problem and that if you do things right everything will work out just fine and dandy. (Personally I think some of these guys have been breathing lacquer fumes too long.) Their solutions to this problem range from painting it with various substances, cutting it in specific ways on the lathe, burying it in buckets full of strange pellets and I don’t know what all else. I’m fortunate enough that I have the time and resources to be able to experiment with a lot of things and I’ve tried most of these techniques and I can tell you from personal experience that none of them actually work very well. At worst they don’t work at all and what you’ll end up with is a cracked and distorted mess. At best your project won’t crack or split, but it will still distort and the resulting object won’t be the same shape it was when it came off the lathe.

Now if you’re making simple bowls some distortion isn’t going to be a problem so you might be able to deal with it. But I like to make boxes or canisters or whatever you want to call them with fitted lids like the one over there on the right. For something like that any significant change in shape is going to result in a lid that doesn’t fit properly. That means I need to work with wood that is dry and stable to begin with.

But the wood I sometimes get is about as far from “dry and stable” as it can get. Yes, I buy kiln dried wood from commercial sources. But I also like to work with a lot of woods that I can’t get that way. Either the commercial sources sell it only as “processed green” and sealed in wax or anchorseal, or it’s wood I just picked up somewhere from someone trimming trees, or that I found at the town compost site. When I get something like that I have to figure out some way of drying it down to the point where it will be relatively stable. Just putting it on a shelf and letting it dry naturally will work. If I don’t mind waiting, oh, months, maybe years. Wood can take a long, long time to dry, especially something like a 5 inch thick slab.

I could build my own kiln or drying system. They aren’t that expensive or complicated to build and there are plans available all over the internet. Some of them look like they might work reasonably well. But I don’t really have room for something like that. Possibly I will build one in the future, but for the time being that is something I would like to avoid.

So now we come to the microwave. There are a lot of people out there on the internet that claim this works pretty slick if you do it right. But there are also almost as many people who claim that microwaving wood is a very bad idea and it can result in a complete disaster, including starting things like the wood, your microwave, and possibly your house, on fire. So I wanted to find out if microwaving wood really works as a way to reliably dry wood.

Microwave ovens are basically radio transmitters, with consumer microwaves operating at a frequency of around 2.45 GHz (gigaHertz). They work because the radio waves bombarding the object they are directed at cause the molecules of the object to rotate or vibrate, which generates heat. That is a very simplistic explanation but pretty close enough for this. But not all materials absorb this energy to the same degree. Water is very good at absorbing this energy, meaning it will generally heat up faster than other materials in the object in a process known as dielectric heating.

So I know that, theoretically at least, microwaving wood to help to dry it should work. In theory the water molecules in the wood will become warmer than the surrounding wood, causing it to force its way out of the wood fibers. Maybe. Emphasis on that last word: “Maybe”.

The problem with most of the videos and other sources you find on the internet is that they rarely provide you with any real quantitative data. With a lot of these guys they’re like “chuck it into the microwave and nuke it for X seconds (or minutes), then wait a while and do it again. If it starts on fire stop.” This doesn’t exactly give me enough information to get decent results. I decided to go about this in a more thoughtful way, measuring moisture content, weight, temperatures and other factors along the way so if I did get interesting results I could repeat the experiment, altering the parameters to try to get better results.

The Equipment

First I needed a microwave. I suspected MrsGF would not appreciate me using the one in the kitchen, so I went out and bought the cheapest decent microwave oven I could find. That cost me $70 including shipping. And to be honest, well, damn, this thing was nice. Nice fit and finish, stainless steel trim, stainless steel interior, lots and lots of options and one of those rotating turntable thingies inside, which I wanted because I figured rotating the wood while it was being nuked would help to prevent hotspots from developing in the wood. (How can they make an oven this nice, ship it all the way from China, sell it for $70 and still make money off it?) I almost felt guilty about using it for this. Almost.

Next is a wood moisture meter. These things used to be ridiculously expensive but prices have come down to where you can get a pin type moisture meter for around $25 – $35. I’m not going to go into the differences between pin and pinless models or how they work and all that. You can find that info yourself if you’re really curious. The one thing I do want to emphasize is that you shouldn’t trust these things too much. They tell you only what’s going on at or near the surface of the wood. The outside of the wood could indicate it has, oh, 5% moisture level, but just a short distance farther into that hunk of wood the moisture level could be significantly higher.

The best way to measure how much moisture a piece of wood has given up is with a scale. That’s how I keep track of wood I have drying naturally on the shelf. I periodically weigh it, the write down the weight and date. When it stops losing weight that means it has probably dried down to a point where I can consider actually using it. A decent scale isn’t that expensive. This one cost me $27 from Amazon. And it is surprisingly accurate. I used to repair and certify scales for commercial use and I still have my test weights, and this thing was accurate down to something like 0.001 of a gram. It can handle weights up to about 22 pounds according to the manual.

The next piece of equipment is a contactless thermometer because I want to measure how hot that piece of wood gets during this process, and, hopefully, keeping it from getting too hot and starting on fire. Mine is a Fluke but you can get brands that are a lot cheaper than that which will still work reasonably well, especially for something like this.

I want this for three reasons. First I’m going to be handling the wood that I put in that oven so I can weigh it and I want to measure the temperature before I grab the thing so I don’t end up in the ER with burned fingers. Second, I want to do everything I can to make sure that nothing starts on fire. I want to stop this immediately if the temperatures of the wood get too high. And third I want to measure the temperature of the wood at various stages of this process to see how much it heats up after specific periods of time being zapped in the oven.

The final piece of essential equipment is that fire extinguisher up there. Seriously. I’m about to chuck a piece of wood into a microwave oven. I don’t think it will start on fire, not if I am reasonably cautious. But it is far better to be prepared and have it ready to go rather than try to run and grab it after things start smoking.

Sidenote: You should have one, probably several, in any case. If you are a woodworker, you’re working with power tools with motors that can overheat, even produce sparks. You’re sharpening tools which can also produce sparks. So you have a variety of potential ignition sources scattered through your entire wood shop, which is also filled with wood, wood dust and wood shavings. So yes, you need a fire extinguisher. They aren’t that expensive. Buying a few extinguishers and having them located in convenient locations in your shop, garage and house is a hell of a lot cheaper than losing your shop, your garage or your house to a fire that you might have been able to prevent. End of rant.

The Test Subject

The final element in all of this is the wood. I want a chunk of wood that is typically sized. Most commercially available bowl blanks are about 6 inches square and 2 – 3 inches thick, so I needed one about that size. I also wanted a piece of wood I didn’t like very much. So I ended up with a block of cherry because it was the right size, was wet, and I don’t like cherry very much to begin with so if things went horribly, horribly wrong, which was a distinct possibility, I wouldn’t care too much. In hindsight cherry was probably not a good choice for this experiment. But I’ll get to that later. If I remember. (I probably won’t.)

The piece of wood I’m working with here is in the photo up above sitting on the scale. It’s approximately 6″ square and 3″ thick. It’s been sitting on the shelf for several months now and it’s pretty wet. You’ll note that it is coated in what looks like wax. If you buy wet/green wood from commercial vendors this is how it will come, coated in wax or Anchorseal to prevent it from drying too quickly and cracking. You’ll note that at the start of this it weighs 1,411 grams. That starting weight is important because how much weight it loses during this process will tell me how much moisture it has given off during this process.

Gads that was a mess.

I didn’t think that chucking a hunk of wood coated with wax into a microwave was a good idea so first I had to get that wax off. I did that with a hand plane. It was really messy and I had to stop to clean my plane several times but it wasn’t hard to do and only took a few minutes.

Once I did that I proceeded to microwave it, checking the temperature and weighing it after each step to see what happened.

I have to admit that this was a very tedious process. At first I only cooked it for 30 seconds at a time, but it quickly became apparent that wasn’t doing much. Eventually I started increasing the times up to 3-4 minutes. That length of time brought the temperature of the wood up to approximately 180 degrees. At that temperature the wood was uncomfortably warm to the touch, and it was obviously giving off moisture. Parts of the wood became visibly wet, indicating moisture was migrating from the inside of the wood to the outside surface. I kept going, keeping extensive notes about times, temperatures, weights and the visual appearance of the wood during the process. I won’t bore you with all of that data.

There was no doubt that the process was forcing the wood to give up significant amounts of moisture. The exterior of the wood became visibly wet at some points during the process.

Now in hindsight I probably should have done this with more patience but dear lord this got boring fast. I didn’t want to take the wood up to a temperature higher than 180 because I could only measure the surface temperature of the wood and it was possible the interior was much hotter. So I let it cool down to about 80 – 90 degrees before repeating the process. This was possibly also a mistake in hindsight and I’ll tell you why at the end of this. After fiddling with this for a couple of hours I went and did something else until, well, until the next day when I realized I really needed to get back to work on this.

It gave off significant amounts of moisture. It was losing weight with almost every cycle in the oven. And finally it reached the point where it was no longer going down in weight when I put it on the scale. In total it lost almost 500 grams in weight, and the moisture meter registered 2% when I tested it.

Slight end grain cracking but nothing serious.

So, the results. Did it actually work? Well, yes. Sort of. Microwaving it did indeed dry it down. It was by no means a fast process, but I was making tests and notes and doing other things in between and certainly could have wrapped this up a lot sooner if I hadn’t gotten bored and wandered off.

But there were issues. First there was cracking in the endgrain. That had started before I was even halfway through the process. I rather expected that, however. And the cracks seemed shallow and superficial. Once I started cutting the wood on the lathe I found that was indeed true, the endgrain cracks did not extend more than 1/2 inch into the wood. Endgrain is always a problem when it comes to wood. Even if you let the wood dry naturally there is going to be some cracking of the endgrain unless you resort to sealing the ends with Anchorseal or paint.

There was also considerable discoloration on the surface of the wood. But this was cherry. One of the reasons I don’t like cherry is that it often does get blotchy looking, so I had rather expected this to happen. And some of that discoloration was due to the fact some of the wax it had been sealed in had remained in the pores of the wood near the surface.

Also the block of wood was now distorted in shape as you can see in that photo up there. Still that isn’t going to be a real problem either because this is going to get turned into a bowl anyway.

The only way to see what’s going on inside of the wood is to put it on the lathe and start turning it. So here’s what I ended up with in the photo below

The wood was indeed dry and stable. But as you can see there was a problem. There three significant cracks in the wood running diagonally across the grain.

Does this mean the experiment was a failure? No. Not at all. In fact I’d consider this experiment successful. Or at least successful enough that I’m going to try it again in the future.

The wood did indeed get dried down to the point where it was stable. The sample bowl did have those three cracks, yes, but the resulting bowl has been sitting on the shelf now for three weeks now and shows no signs of warping, twisting or otherwise distorting. No new cracks have appeared, only those three. So the process did indeed take a wet piece of wood and dry it down in a relatively short period of time.

As for those three cracks, to be honest I’m not all that concerned about those. The wood could have had internal flaws to begin with. Or, more likely, I overheated the wood. When going back over my notes I see that twice the external temperature of the wood reached 190+ degrees during this. That means it is entirely likely that internally the temperature could have been well over the boiling point of water which means that pockets of steam were being produced inside of the wood which could have caused enough stress to crack the wood. I was also allowing the wood to cool down during the process. That heating and cooling cycle almost certainly was introducing stresses into the wood that could have caused problems.

I’m actually pleased and encouraged by the results of this. There are still a lot of variables to take into consideration. What would happen if I used the “defrost” cycle on the oven for an extended period of time rather than just nuking it at full power? Would that result in a more gentle process that wouldn’t put as much stress on the wood?

What if I tried to maintain a stable temperature, say 150 degrees, rather than letting it cool down between cycles? That also would result in less stress on the wood and might have prevented those three cracks from developing.

Overall I’m very encouraged by the results and I’m probably going to try this again. But I need to make better notes, have more patience and do it in a more methodical fashion to make it easier to get easily reproducible results. If I do it again I’ll keep you posted.

Meanwhile, if you want to try this yourself, do I need to tell you to be very, very careful? From some of the things I’ve seen on the internet it appears to be very easy for this kind of thing to go bad very quickly.