Hi again, these are just my thoughts as I am not in your studio but ! I still feel if you slow down to 100 Deg. Per Hour (or if a full kiln, even less if thick pieces) you will not find it breaks on the way up.(unless the flat fire was not annealed correctly
) (and the outer sides really dump heat)I have had really good success with this slow down approach. They are such wonderful pieces (and also strip cut with dark and light glass side by side) and when I make pieces that have taken so much care like these, I really want them to work. Slowing down does add a few hours BUT at the end , you still have all the glass and the beautiful pieces intact.(especially when side fire is engaged .) You had mentioned it did not happen before. Perhaps you where fortunate. (still try alittle more central in the kiln perhaps too.) Just fire 4 plates instead of 5 at once (or 3 instead of 4)
I purchased a hand held Pyrometer that was suggested in Tech notes 7 . I am sooo pleased I did as I now know my kiln is firing almost perfect to the GB-1 . This gives me great comfort to know my kiln is not the culprit as far as temp. goes. Now I just have to make sure that all other variables are considered when something else goes wrong and not worry it was the temp.(the kiln is just another tool and we have to understand all it's ups and downs ) I strongly suggest this tool to everyone. Puts one more piece of the puzzle together. (also when you are firing on the lower end of the kiln , where is your thermocouple . ) perhaps it is reading the center of the kiln and you are on the bottom. Just a thought as well.
P.S. I also notice you have a kiln shelf right beside the long broken vessels, I wonder if you are changing heat flow with that large shelf and perhaps cool air comes out from underneath the kiln shelf and hits the side of the vessel. Hmmm! raise it above this by an inch . It still gives you lots of room to the elements. This might help the flow of air better.