Why did this break?

[Linda Blackburn]

I took this piece out of the kiln this morning, it was room temperature and it looked great. I set it on top of the three plastic cups you see in the photo and was getting ready to add frit to the piece when it spontaneously broke clean down the center. I wasn't even touching it at the time. The glass is 90 COE. The first firing was fusing 2 clear round blanks. The second firing was tack fusing the flower parts. Here are my schedules:

1st fire:
400 > 1225 > 30
600 > 1490 > 10
9999 > 900 > 60
100 > 700 > OFF

2nd fire:
300 > 1250 > 20
9999 > 900 > 90
75 > 700 > OFF

A couple more things: you may not be able to tell from the photo, but none of the pieces touch each other. There is a small gap between ALL the pieces. Lastly, is this piece worth salvaging? Could I refire to full fuse in hopes it would all go back together?

[Stephen Richard]

This looks like an annealing fracture. You have tack fused pointed pieces on the blank, and this means that more care is needed in annealing. Cynthia Morgan suggests using annealing schedules for 2-4 times the actual thickness when there are angular or pointed pieces. She will be able to give better information than me just repeating her knowledge.
If you treated it as an 18 mm piece (twice uts thickness), you would have needed a 3hr soak at 482C, an initial cooling rate of 25C to 427C, a second cooling rate of 45C to 371C and a final rate of 150C to room temp.

Yes you can refire, but the break will still be apparent in places unless you add to the design in the critical (for visibility) clear places.