Well. It's embarrassing. But I am unfortunately not a person who functions - at all - without sleep. Therefore, when it came time to cool at hour 22, I left the damper where it was (don't have my notes here but it was probably 1" open) and turned off both burners, left just the pilot on.
That's exactly why I said what I said above.... I've seen that mistake happen with new fuel-kiln firers a gazillion times. Youe are NOT alone.
Like Neil, I used to fire salt a lot... and to get good greys rather than browns... I always crash cooled the kiln deliberately. Works a bit with soda also.