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I am trying to make a plate, and I ran into a problem fusing it. The plate is 9" X 9". All glass is System 96.
The firing schedule I used was:
Ramp at 300 deg/hr to 1150 deg
hold for 30 min
ramp at 400 deg/hr to 1350
hold forever
ramp down as fast as possible to 950
hold 60 min
...

The plate consists of a single thickness (1/8") red piece, covered by a single thickness clear piece, topped with a black figure surrounded by clear frit.
The break occurred while ramping up somewhere below 1320 degrees. Only the red glass broke.
I'm wondering what would make the glass separate so violently. Also, the kiln (supposedly) never got hotter than 1340 or so. The plate looks like it is close to a full fuse, which, according to Spectrum, should happen about 1460 deg. Is it possible that my kiln controller is measuring temps inaccurately? How could I test this?
I'd appreciate any pointers the board could give me. Thanks!

Your schedule is too fast, you need to slow it down a lot.

Thanks, Carol. I based the schedule on the firing schedule on the System 96 web site (skipping step 2, as they suggest you may do). How much is a lot, as in slow down schedule "a lot"?

I would not ramp up @300° per hour, you should go no more then 100° on the way up to 1150°.

I toooo feel it was too quick and I never go faster than 100 deg. per hour unless tiny pieces. I even go 50 deg. per hour with thick work. Les

Your piece was uneven in thickness, in that case as everyone mentioned you must slow it down.
Bob

Thanks, everyone, for the suggestions. I'll take it a lot slower next time.

Breakage aside, I'm pretty impressed by the black figure. Did you cut that yourself?

Yes, David, I cut it on a Taurus II ring saw. It was pretty time-consuming. BTW, the piece is an Albanian flag. The guy who requested the plate wanted it as a present to his girl friend's parents, who are Albanian. After sawing out that eagle, I wish his girl friend was Japanese!

Wow. Are the tongues stringer? Or did you actually manage to cut those as part of the larger piece?

Just to be clear, there are lots of cases where a 300 deg/hr rate will be acceptable. With the black, irregular figure on top, this isn't one of those cases.

Tony

David Jenkins wrote:Wow. Are the tongues stringer? Or did you actually manage to cut those as part of the larger piece?

The tongues are cut from separate pieces of plate glass. I couldn't figure out how to cut the whole figure in one piece.