Cracks on bottom of bowl blanks

Several of the green turned bowl blanks I've done lately have cracked across the bottom. I typically leave a uniform inch to 1 & 1/2 inch thickness on the sides and bottom depending on the size of the bowl and then I add a tenon on the bottom. That adds almost 1/2 inch to the thickness of the bottom. Could that be why I'm getting cracks? Do I need to cut the bottom of the bowl thinner if I'm going to add a tenon?

You could try going a tad thinner, but what I really would like to know is if you are using any anchorseal on them after you rough them out, or any other method to slow down the drying process through the end grain.

Dave, I do put anchorseal on the blanks, both inside & out. That helped a lot, but still getting cracks across the bottom of some bowls. Most of these are oak, so that my be part of the problem. Several people tell me they have a lot of problems with oak cracking.