Updated: Mar 16, 2019
I probably get more questions about my process for drying bowls than I do about any other woodturning subject. There is no shortage of drying methods and I've tried MANY of them throughout my 10+ years of woodturning. Personally, I like turning green (undried) wood not only because it's easier to turn and less dulling on my tools, but also because I love using the natural tendency for wood to move as it dries as a part of my process. I can control how much warping occurs in the final piece by using a variety of drying methods. Sometimes I want a piece to stay relatively stable and sometimes I want it to warp and reveal the amazing textures of figured wood.
I tend to be pretty aggressive when it comes to drying bowls - very few are allowed to dry for more than 4-6 weeks. That is enough time for the majority of the drying to occur. Of course, an aggressive schedule like this will result in a little more cracking than allowing 3-6 months for the drying process to happen, but I've learned what works for me. There are always a ton of caveats and details to be aware of as you experiment with drying; I'm trying to address the basics here. Let me know if there are any details I can expand on in a future post.
Green Turned Bowls
When I turn a piece of green wood, whether freshly cut or partially air dried in blank form, and I'm going for a significant amount of warping, I will turn it to its final form and take one of two routes... I will either sand it and oil it (with Danish oil, Velvit oil, boiled linseed oil, or something similar), or I will allow it to dry a bit, then sand it, then oil it. Oil is a great choice for finishing green turned wood. The oil will soak into the pores of the wood and replace some of the water content, but most importantly, the moisture will still be able to escape. The oil will slow down the drying process and help to prevent cracks in the piece. Using this method, the bowl will continue to dry for several days to several weeks (depending on the thickness of the bowl and the original moisture content of the wood). It's important to strategize the shape of your bowl - if it's on the thicker side, you want the thickness to be generally even and to avoid any sharp transitions in the bowl's shape. Thinner bowls are able to move more freely as they dry and tend to be less prone to cracking. The longer and smoother the curves, the less tendency there will be for cracking. I've been surprised by some of the pieces I've been able to make from green wood simply by using oil to control the final drying stage.
With a twice-turned bowl, you will first rough out the bowl from green wood, then use some method to dry it (partially or thoroughly), then put it back on the lathe and turn it down to its final form. When the rough out goes back on the lathe, the wood will have moved as it dried and the piece will be warped. That "portion" of the warping will be removed while re-turning and any additional movement in the wood will be reduced since there is now less moisture to evaporate before the moisture content reaches equilibrium. This is where controlling the degree of warping comes into play. Depending on how wet the roughed out wood is, I will generally allow mine to dry and lose between 10%-25% of the weight of the rough out before I finish turn it. I just use a digital postal scale to monitor the weight of the rough out periodically and I keep track by writing directly on the piece. It's easier to use grams as your unit of measurement rather than pounds and ounces when monitoring the drying progress. If I have 10 similar cherry pieces drying the same way, then I might only monitor the weight of a few of them and assume the others are drying at about the same rate. If you want to get the moisture content to equilibrium, just keep drying the rough out until its weight stabilizes for a period of time.
Drying Methods for Twice-Turned Bowls
Now for the various drying methods... These include, but are not limited to, paper bags, paraffin wax, wax emulsions, kilns, wood shavings, microwave/oven, and alcohol soaking. I'll address them in the order of least useful to most useful, in my opinion, with the exception of the kiln (since I don't use one).
Alcohol soaking - I do not recommend this method. I tried it a number of times and never found it to be any faster, more reliable, or less expensive than the other methods. And having buckets of highly flammable alcohol and wood soaked in said alcohol could lead to disaster (fire, splashes, breathing it in, or at the very least smelling it constantly).
Microwave/Oven - This can be a fun way to play around and experiment with your wooden creations, but until you get the hang of it, expect to lose some pieces to cracking. Also, if for some reason you decide to do the alcohol soak, DO NOT combine it with this method of drying. If you do, you're asking for trouble (alcohol vapor is extremely flammable). Using the microwave or an oven, you will likely be able to hear the wood cracking as it loses steam from deep inside the wood. You will probably have more luck with this method if you use it with thinner bowls (since they can move more freely without cracking). I would recommend microwaving at 10-15 second intervals, then let the piece cool down for several minutes before returning it to the microwave. An extra small piece might be better with 8-10 second intervals. I've even speed-dried pen blanks in the microwave for commissions where my client is providing wood that's not at equilibrium moisture content yet.
Kilns - I know that a lot of woodturners use homemade kilns of various configurations to slowly dry their roughed out bowls and vessels. I am very much a "keep it simple" kind of guy and have never bothered to build a kiln for drying... I just haven't found it to be necessary for my process. I'm sure that kilns work just fine and plenty of turners like them, but they take up a fair amount of space. If you're just starting out woodturning, I recommend trying the methods described below before investing the time, money, and shop space in a kiln. If you do want to build a kiln, there are plenty of instructions on the internet describing how to make one.
Paraffin Wax - Paraffin wax (basic candle wax) is available in slab form at many art & craft stores (check online for the perpetual 40% off coupons that certain retailers always run). Paraffin wax is a great option for sealing the blanks that you cut for longer term storage (and slow air drying), but not so great for drying roughed out bowls due to the difficulty in applying it and the amount it can take to coat a bowl. I air dry a fair number of blanks in addition to keeping a stock of green wood, since sometimes a project isn't suited for green wood. My turning blanks that I want to air dry for several months or years get paraffin wax on the end grain and Anchorseal 2 on the rest of the blank. This works relatively well in my experience.
Wax Emulsions - When I say "wax emulsions" I'm referring to products like Anchorseal 2 which is generally available at specialty woodworking stores. This is one product I always keep on hand. You simply brush it on (or put on nitrile gloves and just rub it all over the roughed out bowl) and let it dry. (Side note: dogs really seem to love licking this stuff while it's wet, so keep it out of their reach.) Your sealed rough outs will definitely dry more slowly, but still fast enough to crack if left for longer periods of time. You have to keep an eye on them every several days and keep a bottle of CA glue handy to stop any cracks that you notice. If they're drying too fast, combine this method with one of the following methods.
Wood Shavings - Drying your rough outs in a large bin or paper lawn/leaf bags full of wood shavings is a great inexpensive option. I don't use this method very often anymore since it takes up a fair amount of shop space. I like my shavings to have a little moisture to them when I first place the rough outs inside so the shavings don't draw the moisture out of your bowls too quickly. Basically, just bury the pieces in the shavings and wait (check on them periodically to make sure there are no cracks forming). Personally, I think the heavy paper lawn/leaf bags are better since they will let some moisture pass through them, versus a plastic bin that won't.
Paper Bags/Craft Paper - This is my go-to method for drying, often in combination with Anchorseal depending on how quickly I want a rough out to dry. Breaking down those heavy lawn/leaf bags is a great way to get large pieces of paper to wrap large bowls (they're usually double layered, so you can get a lot of paper from a single bag. I also use standard grocery style paper bags. Either way, I will always wrap my rough outs in at least two layers of the paper. Another possibility is to just put a bunch of bowls into the large bag and fold it a few times to keep it closed. You will want to check on the rough outs every few days in the beginning to make sure they aren't drying too quickly and possibly cracking (CA glue is always handy for rescuing a cracking bowl).
To summarize... You have probably picked up on the fact that the goal is slow down the drying process by whatever means possible and to different degrees depending on your preferences. (Fun thing to try... Hold a roughed out bowl up to your ear early in the drying process and you will hear a crackling sound. Sometimes I've even heard the pop of a bowl cracking on my drying shelf.) In the beginning, monitor the pieces at least every few days as you dry them until you get a feel for it. Cracks can occur even after several weeks of no cracking. Drying bowls is just something that takes trial and error. What works for me and my process may not work for you and your process, so experiment... and let me know if you have other methods you find useful.
Thanks for reading! (BTW, I am not sponsored or compensated in any way by the makers of Anchorseal; I just like the product.)