The following tip comes from Wally Dickerman of Green Valley, Arizona.

"Several years ago a friend gave me a piece of dry, spalted but almost rotten birch burl. I mounted it on the lathe and rough turned it into a hollow form, about 8 inches in diameter. In spite of being very careful, several areas had badly torn, nearly rotten wood. I decided to try to save it. I mixed a 50/50 solution of white glue and water (white glue is mostly water anyway). Using a plastic container intended for paint, I soaked it for a little over 24 hours. I removed it, and allowed it to dry for several days. It had soaked up a lot of water, and had warped quite a bit. After remounting it on the lathe I barely had enough wood to get it round. The opening had been a bit over an inch, but since it warped oval, it was now over two inches after rounding it. The soft wood had firmed up nicely. There had been several cracks, and all of them had disappeared. The wood had swelled, and of course I was soaking it in glue, so they sealed permanently. A real plus. I saved the glue solution in a plastic jug and I've done this several times since, with varying degrees of success."

Here's a picture of the resulting birch hollow vessel.