There is another crack on the base but a smaller stand should take care of that as long as it does not go too deep. Likely I would have been better off just cutting the log down the large crack into a couple of smaller pieces or for that matter burning the whole thing, but stubborn goes a long way in my mirror.
So having decided to leave the upper area wide and cut in on the bottom I now have a very tapered vase in the works.
With that in mind I cut as far into the vase as I could,
until I reached as far as a reasonable base would allow.
I then affixed a flourescent bulb on a lamp fixture and rigged it to fit inside the vase. A flourescent gives less heat than an incandescent bulb and is less likely to cause cracking problems. This bulb is about the equivalent of a 60 watt incandescent.
Then I began to turn the outside of the vase until the light shone through at an even rate. It is best to work down about an inch at a time so as to leave lots of wood under the cut for support and to dampen vibration.