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Just took a 14" diameter, 2.5" deep, 7-8 mm thick bowl (BE glass, first time with irid) out of completely cooled slumping mold and it broke in half where piece of transparent irid over tekta butted up against piece of french vanilla. Looking at the split, there is some tekta that slid down between the irid and french vanilla pieces....so there must have been some separation in the initial fusing, which probably caused some stress and the crack. The edge is sharp so I suspect it broke during cooling. I would like to try to save the glass by flattening them and cutting up for parts. Original schedules, which have been successful in my kiln before was
300 F to 900, hold 15
500 F to 1100 no hold
100 F to 1250 hold 60
full to 1450 hold 15
full to 900 hold 60
50 F to 800 no hold
100 F to 700 no hold
300 F to 125 no hold
(its been over 100F here for the last week so I upped the end temp so the kiln cycle would finish at night and cool as much as possible by morning)
Slumping schedule was
200 F to 900 hold 15
100 F to 1175 hold 15
9999 F to 900 hold 60
50 F to 800 no hold
100 F to 700 no hold
300 F to 100 no hold
(ambient today is about 88; it sat finished in kiln for about 24 hours after cycle finished before I moved it and the controller showed 33 hour cycle time which I would have thought was slow enough cooling.)

I've never tried to flatten a slumped piece before and know it needs to be done slowly. Brad's book doesn't suggest a schedule. I'd appreciate recommendations for how to try to do this...ie how slow is slow enough? Do I just place the piece on the kiln shelf and pray or is there some way to ease it down?

I'd use a standard full fuse schedule, but go fairly slowly on the upramp, probably 200dph up to the full-flat fuse, and hold for 15-20 minutes, then take it down. That'll flatten it pretty well.

I'm a little concerned, though, that the crack is so absolutely perfect along that color line. Do you have an irid to irid join, or is there a possibility of some incompatibility along that line?

Beautiful design, by the way.

speed kills
The crack is from the speed of firing owing to the differential of rapid heat absorption between the clear and opaque glass.

Thank you both for answering.
The bottom layer was half french vanilla and the other half was pieced tekta. Top layer was pieces of turquoise irid and french vanilla.
I expected the center strip of irid to react and fully fuse over the gap. I now think I should have held it at mas temp longer to more thoroughly fuse. That joint had enough stress so when slumped it ended up breaking. I still have a lot to learn.....but if I don't have some failures along the way, I won't feel like I am trying to broaden my understanding.

I'll heat the pieces slowly and anneal longer and slower and hopefully be able to cut it up and use it in something else. And maybe try a similar design with only the bottom irid layer pieced.

PS Cynthia - thanks for the compliment....I never know if what I think looks good might appeal to anyone else.

I've never tried to flatten a slumped piece before and know it needs to be done slowly. Brad's book doesn't suggest a schedule. I'd appreciate recommendations for how to try to do this...ie how slow is slow enough? Do I just place the piece on the kiln shelf and pray or is there some way to ease it down?[/quote]
I think you have 2 nice half ..work with that.

I reheated them on a kiln washed shelf . I don't my exact schedule now...will post it when I get back to my studio. I remember it was something like 200F/hour to 1450F then used standard BE anneal schedule. Said the standard prayers to the kiln goddess and now I have 2 flat ovoid pieces, slightly larger than the original diameter due to stretching during the first slump. Now I'm waiting for inspiration to use them in something else....

The heat up rate you used on slumping should be slow enough. If you want to be cautious you could reduce the rate of advance to 100C/hr until about 540C, after which you can speed up.

Here is the schedule used to flatten the pieces.

200F to 1100F no hold
50F to 1250F 60
AFAP to 1450F 15
AFAP to 900F 90
50F to 800F 0
100F to 700F 0
300F to 100F 0