I hope someone has seen this before and can tell me where to look for the problem.
Two bubble-up firings. The first is fine and was slumped into a plate. Next was 4 smaller pieces on the same schedule. All have multiple cracks. The cracked one in the lower right is even the same glass. What happened?
Everything was the same except the size of the glass (7” vs 4 ½”) a different kiln shelf and I noticed the total time in the kiln was 8:02 vs 8:20 for the ones that cracked. Cracks are sharp so it happened on the way down, and some places are really stuck to the kiln shelf. Thermocouple bad? Controller bad?
Schedule:
400/hr to 1100 hold 10 min
AFAP to 1700 hold 45 min
AFAP to 925 hold 60 min
100/hr to 750 off
I'm with Brad. Especially with a new kiln shelf, if the kiln wash is too thin (or nonexistent), the glass sticks and tears itself apart as the kiln cools. Are there bits of kiln shelf stuck to the bottom of the glass?
Tony
The tightrope between being strange and being creative is too narrow to walk without occasionally landing on both sides..." Scott Berkun
More kiln wash seems like an easy fix. My notes say I used 8 coats on both firings, but I have been known to be senile. I will try again and see what happens.
The experts were right. Not enough kiln wash. My notes say 8 coats of kiln wash, but my guess is I was distracted with something and picked up the wrong shelf. Thanks for the help. This BB may be one of the best "tools" I have.
As Brad mentioned it takes 1700 to do a Bubble Up. Brad originated this at Warmglass and teaches classes for it. A great class and I highly recommend it. Lots of fun.
Mark