Question: Ive noticed that some of my newly-turned bowls have warped, and one also has cracked slightly. This has never happened before. I have not changed my woodturning or finishing techniques recently, so I cannot figure out what Im doing wrong. Is there any way to prevent this?
Answer: If your bowls are warping, chances are the wood is green or not completely dry. The wood that you used may have appeared dry, but the fibers inside may still have been wet. If your wood is not actually green but only slightly moist, it may be possible to prevent warping by simply sealing in the moisture after turning your bowl. Finishes such as epoxy, varnish or lacquer are best for this. While oils provide a nice looking finish, they are ineffective in reducing warping, as they do not seal in moisture adequately and the wood still dries too quickly.
Wood usually warps and/or cracks due to uneven drying, but there are methods you can use to help green wood dry more evenly. One way is to place the freshly turned bowl wrapped tightly inside two or three brown paper bags for several weeks to slow down drying.Another method to prevent warping and cracking is to turn the bowl in two stages. First rough turn the bowl, leaving the wall thick, and then store the bowl in paper bags for several months. Alternately, you could apply a product such as Anchorseal to the bowl and store it. Sealers such as these slow evaporative moisture loss and reduce checking and warping as the wood dries. Once the wood is dry, you can turn the bowl to its final size.
You could also try your hand at turning natural edge bowls, so you wouldn't have to worry if they warp. To learn more about working with green wood, Turning Green Wood by Michael O'Donnell is an excellent introduction and resource. You may also enjoy Turning Bowls by Richard Raffan, which also includes natural edge bowls.