I just harvested a large white oak. Beautiful wood. I turned out some nice bowls but continue to have issues with it cracking while drying out. Any advice or suggestions would be appreciated. Thanks!
White oak is a bit of a problem because it has those large rays, which are a built-in plane of weakness. Why it's so easy to split for the firewood stack.
Shape is the biggest factor in promoting drying stress. It's going to drop either side of the heart bisector (radial shrink), and it's going to contract across the annual rings (tangential shrink). Won't move much at all along the grain (longitudinal shrink) as
The Wood Handbook, available free at
http://www.fpl.fs.fed.us/products/publications/several_pubs.php?grouping_id=100&header_id=p shows.
See how it moves in ch4
http://i35.photobucket.com/albums/d160/GoodOnesGone/LogEnd.jpg
So try not to leave the bottom too broad, or you'll break open along that bisector at the base area. Don't make the walls too thick or you'll increase the risk at the rim. 10% is about 25% too much for most any wood, so give yourself some shorter dry time and remove more wood while it's wet and easily sheared at the same time. Centering the heart is fine, and visually appealing, but not vital. Avoiding reaction wood will deny you some of the prettiest figure, so accept that it may, very occasionally, find a way to warp that will force you very thin or maybe keep you from getting back to round altogether. Difference will be proportionally greater around the edges of a larger piece, so you might have to settle for smaller. Such is life.
I don't buy the bit that a larger tenon or mortise is required to hold a large piece, because I turn over the bed with the help of the tail, and I don't hack, but slice, which puts less stress on it overall. Allows me to make narrower, more survivable bottoms, where the shorter distance to the air through endgrain allows a rate of loss and therefore shrinkage more in sync with the walls. I even leave a pillar in the center of the hollow side for ease of remounting after drying. It's tough to remember the last drying failure I had. Probably the first time I turned beech, five-six years ago.
I do watch the relative humidity and air circulation, though I don't coat or wrap. Basement floor and a neglected corner work to bring things down easy, shelves to bring them fast. RH never below ~60% for the first month. Week or two at the controlled 50% in the workshop finishes them for final turn.
If you're going to turn once and let them warp, get your thickness under about 3/8" and let them dry in the shop.