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I am trying to use some of my left over glass by fusing them into circles to be used in a future project. I fired a 12" x 3/8" round using the following firing schedule:
275-1150 hold 15 minutes
300-1250 hold 10 minutes
300-1450 hold 3 minutes
full to 960 hold 1.30 minutes
50 to 800 hold 2.30 minutes
100 - 0

I took the piece out at 150 and it was fine I layed it on the table on a sheet of corrugated paper and 15 minutes later it cracked down the center.???
What am I doing wrong? Can anyone give me an idea how to prevent this from happening?
Thanks
Janice O

take out cooler.
wrap in towel

A conservative approach would be to work with the Bullseye annealing thick slabs chart. It begins at .5 inch. I would suggest subtracting 25% from the times in this recommendation. Since anneal times are not linear, this would yield a schedule a bit longer than necessary, but not too much longer.

Do not stop your program at 800, make it go quite a bit lower. The kiln will stay under control, until the program is going faster than the kiln can lose heat. This will harm nothing. Even if this should happen to produce an error message, nothing happened to mess up the firing schedule.

Notice that the Bullseye chart does not begin the anneal soak until 900, then the timing kicks in. Remember that during the cool down, the glass can be 50º hotter than the air temperature that the controller sees.

The anneal schedule for Bullseye is also applicable to System 96.

Wait until the kiln is below 100.

I would never ever take anything out at 150 the shelf is still screaming hot!!! I let my kiln cool right down I just walk away....... and at about 50 I open it and I still wrap it in a towel if the pice is important to me:)

C or F?