korman wrote: ↑
Tue Jan 02, 2018 3:19 pm
How do you prevent the freshly turned shell from cracking while drying?
I am no expert, this is only my 2nd drum, but I have turned a lot of green wood. Wood splits and cracks because of shrinkage as it dries. The most shrinkage is circumferentially or around the growth rings, and is proportional to the length of the growth rings. A vertical grain (or quarter-sewn) board has very minimal shrinkage when compared to a plain or flat-sewn board. There is some longitudinal shrinkage, but it's only a fraction of the lateral shrinkage.
With that in mind, think of all those growth rings around each other like a bunch of rubber bands stretched to their limit. at the center of the tree, there is really no shrinkage. There is not enough length and as you get to the center, there is a solid core that doesn't change. Cut any green log and give it time, it will crack somewhat radially toward the center. Back to the rubber bands, when one goes, a bunch of others let loose.
If you cut a log and remove the center, and keep the thickness similar around its circumference, you minimize the cracking. It's free to change diameter with no interference or tention buildup. That's what I do when I turn a drum. It gets cuts and mounted on the lathe, outside shape completed and center hollowed out, all in one day.
Also, slowing the drying process can reduce splitting. If I can't complete a piece in a day, I put it in a plastic bag until I can get back to it. A lot of turners will turn blanks an put them in paper grocery bags filled with wood shavings to slow the drying process. (sealed in plastic, they'd get moldy)