Donna Banfield replies: Oak can be a challenge to work with, but it is a species that is abundant in supply here in New England, so if you are a woodturner, you can’t resist. But it is prone to cracking. As the wood cells lose moisture, the wood “moves.” Thinner areas will lose moisture at a faster rate than thicker areas, causing stress, which results in your bowls cracking.
The key to turning a bowl from green wood and limiting your loss from cracking is uniform and consistent wall thickness from rim to foot. In my early woodturning days, I lost plenty of roughed out green bowl blanks because the bowl usually was thicker at the transition point of the curve where the wall met the bowl bottom. If the outside shape of your bowl has a good flowing curve, and does not have steep, straight walls, the interior hollowing will be easier, because the gouge doesn’t have to cut a sharp corner on the inside. Calipers are your best friend—use them.
Other things that I do, especially when turning green wood like oak that is prone to cracking is to use paper bags. Immediately after turning, place them in one or two paper bags. Check them every other day for cracking, and if you see any cracks forming, use thin CA glue to help keep it from opening up. Exchange wet bags for dry ones. After 2-4 weeks, you can remove them from their bags and let them continue to dry. You might also paint the end grains (inside and out) with green end sealer marketed as Anchorseal. Both of these will help slow down the loss of moisture, but your most efficient way to limit loss to cracking is to turn bowls with uniform wall thickness.