I've turned a few things green on a lathe and I expected them to to do odd things, and they warped and/or cracked.
But what useful techniques are out there that this could be minimized or even controlled? I don't know if it is possible, other than letting the wood dry to a more stable moisture content before turning it.
I have a relative who uses something akin to a steam box, without the steam, to control the moisture moving out of his green turned pieces. It is like a wooden cabinet lined with plastic, with small adjustable vents like on a cheapo charcoal grill. He allows them to stay relatively moist for a period of weeks, loosly wrapped in plastic sheeting. There is often a wet rag or open water cup in the cabinet.
By slowly allowing the moisture to escape the pieces change shape slowly, thus avoiding cracking. He does a final fit, finish and seal after the piece is down to something lower moisture content (I will try to find out specifically what % moisture content).
The best strategy I've seen is to rough turn, let dry, then finish turn.
The rule of thumb is to turn to a thickness 1/10th of the finished diameter. So a 10" bowl would be rough turned to 1" wall thickness.
Then pack the rough turned bowl in shavings in a paper bag, and set it aside to dry in a controlled fashion. Every couple of weeks, check for moisture level and/or change in weight.
The key though is consistent thickness. I've had bowls crack when the bottom was thicker than the sides- I assume the sides are drying faster, and therefore shrinking / putting stress on the piece until.. crack.
Another thing to consider if you're cutting your own bowl blanks-- consider taking the pith out. In some woods, pith dries at a different rate than the other heartwood and definitely different than the sapwood. If you're buying bowl blanks, the pith is likely already cut out.
I've only done this a couple times, but I have good luck turning a rough form (as @TxTurner notes) and then setting the bowl aside for a few months.
My technique is, I keep all the shavings from the bowl I just turned and stuff those, along with the rough bowl, into a (plastic) shopping bag that I tie off. I'm careful to pack the bowl in the center of the bag, so there's plenty of moist shavings evenly surrounding the bowl. I also don't tie the bag so it's airtight - I leave a little gap at the tie so air can circulate.
I check on the bowl every two months or so; this lets me know if I can proceed, and also helps rotate the shavings around inside the bag...
By the way, "know if I can proceed" is still a thing I'm not totally sure about. I've taken bowls out after four months and still found them quite true when chucked up (and none of those bowls have subsequently cracked), but I'm not sure how this would work for a super-thin second turning, or very green wood, etc.
I know one turner who just puts them in the loft of a shed, and another who has a "green room" where he just puts all his turnings for drying. I'm not sure if he just calls it that, or if he has an actual humidity-controlled green room like what some people use for growing plants.
I've also heard several turners talk about wrapping up each piece in a paper bag.
I have turned a few green bowls cross grain after turning I sand them down. Then, I use a old micro wave oven to heat them up some, 1- 1 1/2 mins. at a time. then take them out, and set them on a shelf for a while to cool. reheating them several times over a mater of days. have had good luck with that. Thinking of getting Wood Juice to put on some to see how that works. but so far cutting board oil has been used with good results.
