dry wood cracking issue

Last springI cut a sycamore drift log, sealed one end and left the unsealed end sitting directly on my concrete shop floor. I left the chunks long enough that I can cut off the checking. Over the last year the wood has spalted beautifully and I'm trying to work my way through the pile to reclaim that section of my small shop. I screwed the log to a 6" faceplate centered on the heart to get more grain uniformity around the piece. My technique wasn't stellar when I turned the first piece so I did a lot of sanding. Over the next 2 days it developed a large crack from the bottom to about 1/2 way up the side. I figured it had to do with the heat from so much sanding. I just filled it in with coffee grounds and CA and it blends in beautifully with the spalting. This weekend I turned another HF, and thanks to Reed Gray I was able to get a much better finish cut and only spent a few minutes sanding from 100 to 320 grit so I expected it to not crack. Sure enough, the next morning it split from the bottom just like the other one. I also got 3 cracks in the top while I was turning - I think these happened because I turned it to about 3/16" and the dry wood just couldn't handle the vibration.



So I have a few questions I was hoping y'all could help me figure out. As to the crack coming up from the bottom, is there a way to stop that from happening? I always leave the bottom thicker than the rest of the piece, and with the wood being really dry I figure it is easier to relieve pressure by cracking than moving. If that is the case I can just go thinner in the bottom, but is there anything else I can try? And for the vibration cracks at the top, how can I stop that? I thought about taping it or wrapping it in cellophane, but I'm not sure how much that will help. Any ideas?



Here are some pics of the piece I just turned to give you an idea what I'm working with:

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dry wood cracking issue