Drying rough turned bowls for finish turning

Since I started turning pens a few years ago, my woodworking has leaned more and more toward woodturning. I still do other projects but I've found Woodturning to be very addictive, and turning bowls, for me anyway, has been even more so.

I started turning bowls last summer after I got my Turncrafter Pro midi lathe for my birthday. The lathe I had before that was a small mini lathe that barely had enough power to turn pens. Actually, that isn't really right, it is just about all you'd need if all you are doing is turning pens. But I wanted to do more, so when I got the midi lathe it was like going from a Yugo to a Lexus. It has more power and is so much more stable than the small one. Unfortunately, I'm still limited to less than 10 inches in diameter for bowls, but that's a good maximum to have while I'm learning...I suppose.

Anyway, now I've been turning bowls for about a year and I'm getting a little better at it. One thing, though, about turning bowls is that the turning is much easier if the wood is green. The moisture in the wood both lubricates the chisel as you are cutting, and it keeps the dust down a little compared to dry wood. The problem with turning green wood, is that it still has to dry before you can finish it. I've done a lot of reading online on turning bowls and what to do about drying your rough turned bowls. Most everything I've read recommends that you rough turn the bowl to an even thickness all the way around, and that should be equal to about 1/10th the diameter of the bowl. In other words,. if you are turning a 5 inch bowl, you should turn it to approximately 1/2(0.5) inch thickness consistently from edge to bottom. This doesn't include the tenon on the bottom of the bowl that is there for the chuck to hold on to while turning. After that, comes the drying process.

As the wood dries, the moisture moves within the wood. The moisture in the surface fibers evaporates to the air, and the moisture in the fibers in the inside moves toward the outside This process continues until the wood has reached what is called the Equilibrium Moisture Content, or EMC. That is the point where the air around the wood won't draw any more moisture out of the wood. One problems is that the end grain of the wood will lose moisture much faster than the face grain causing uneven loss of moisture and warping or cracking of the wood. Another problem is that in the open air the moisture will leave the surface fibers faster than the movement of the moisture inside the wood can replace it causing the surface to want to shrink more than the inside also causing degradation. Some woods will degrade more than others, and figured woods seem to be some of the worst. One simple method is to place the wood inside a paper bag or wrap in brown craft paper and let it sit for a couple months to dry. This creates a "micro climate" within the bag and keeps the surface fibers from drying out too quickly and minimizes degradation. Other methods involve boiling , immersion in alcohol, and even immersion in dish soap. One method says to place your bowl in a microwave oven on high for 45 seconds to a minute and then weigh it and record that weight. Repeat this process after it cools and again weigh it and record the weight. Continue this process until you see no weight loss for at least three cycles. This means the wood has reached equilibrium with the surrounding atmosphere, and it's ready for final turning and then finishing. I tried the microwave method but didn't have much luck with it. It may be that my microwave is underpowered, I don't know.

What I have been trying is a combination of the microwave method and another method I've tried before using the conventional oven. What I do is rough turn the bowl and then place it in a paper bag, roll the top of the bag down and tape it closed. Then I turn the oven on to 250 degrees and place the "bowl-in-bag" into the middle of the oven. Then I wait until the oven beeps, indicating that it has preheated, then turn it off. Sometimes I'll let it sit for a few minutes on before turning the oven off but I don't leave it on for very long...at least, not on purpose. After the oven has cooled down I'll remove the "bowl-in-bag" and when it is cool to the touch I'll weigh it and write that weight down. I repeat the process and continue until there is no further weight loss through three heating cycles. I've had some actually gain a few tenths of an ounce after sitting for a while which, again, tells me it's ready to finish turning.

So far, with limited trials, this has worked quite well for me. There is, however, one other factor that I've had in these trials. I had previously decided to try some Pentacryl wood stabilizer to prevent cracking which had plagued several of my previous attempts at bowl turning. So far I've tried this method with an 8 inch box elder bowl and a smaller one of bradford pear. Although both had some previous cracks that I had sealed with CA, or super glue. No further cracks appeared after I started this process. As I said, I had purchased some Pentacryl and I covered both of these thoroughly. After letting the solution dry out for a few days I then decided on trying the oven drying method.

Right now I'm in the process of drying a small maple bowl (about 6 inches in diameter) which hasn't had the Pentacryl treatment and I will try more as the results dictate. If the ones without Pentacryl start showing more tendency to crack, I'll most likely go back to painting on the stabilizer again.

I don't profess to know a lot about this subject, I'm just attempting to relate my experiences here, hopefully without being too wordy and boring the hell out of anyone reading it. But then, it could be a cure for insomnia. My results so far look promising, and I will relate further results later. I wanted to at least get another entry into my blog, and hopefully make some sense to the reader.

Feel free to comment and/or correct me on anything you read here. As I said, I'm no expert, just a guy learning and relating my experience.

See you next time...
Larry