Barry,
Some other answers have alluded to green wood. Two comments.

Wood isn't always homogeneous, there might have been areas in the wood that
were denser (or lighter) than the rest, and when you removed enough of the
inside of the bowl the difference was enough to cause the vibrations.

The other comment, an 11" bowl does take time to turn, and we all ( I do )
need to take some time out for a 'bio break', lunch, or just sharpening the
gouge. And I have a light close to the lathe. Make sure you turn that light
OFF when you don't have the piece turning for extended periods of time (what
ever that means). The heat from the light can dry one side causing warping,
or just enough drying to cause it to loose balance. For long times (lunch)
wrap the blank with a plastic bag. This will keep some of the moisture in the
wood but it may collect in the 'lower' side of the blank.

mike



Barry N. Turner wrote:
I was hollowing a Sassafras bowl about 11" in diameter this afternoon. The
walls were about 1 1/4" thick and I was using my Crown PM 5/8" bowl gouge
with Ellsworth grind. Near the rim of the bowl, at the beginning of the
cut, I began to experience some vibration.

I put a fresh edge on the gouge and finished with little problem, but still
with some vibration. Then a thought entered my mind........had I switched
to a smaller gouge, say a 1/4" bowl gouge, which would remove less wood per
cut, would it lessen the amount of vibration? Thanks.