New Kiln - glass cracking

[Hannah Love]

I've recently purchased a Paragon GF kiln after two years firing with Quick Fire kilns. I'm trying to figure out at what stage I messed up two fires. I've had one that worked on smaller pieces in the new kiln.

Bulleseye COE 90 glass on all glass except Austin Thin Films 90 Tye Dye accents
Base layer cobalt
Mid Layer frit and tye dye and some stringers
Top Layer Clear

Fire Schedule:
Step 1: Fire at 300 per hour until 900 and hold for 45 minutes
Step 2: Next go to 1250 and at 1250 go up 100 degrees holding for thirty minutes
Step 3: Target temperature 1475 with 10 minute hold
Step 4: hold at 960 for 60 1 hour
Step 5 allow kiln to cool 50 degrees per hour until reaches 800
Step 6 Allow kiln to cool 100 degrees per after it reaches 700

Hopefully, I've attached a picture of what occurred following this schedule. I'm seeking a suggestion on where to modify the schedule. The cracks I've seen before when pieces cooled too quickly in the quick fire. The pocket of shattered glass (not just cracked is different). Any comments or ideas on what to try differently would be appreciated.

[Morganica]

New kiln shelf too? Have you had any trouble with kilnwash on that shelf? Did the cracks in both pieces happen at about the same point on the kilnshelf? Is there any kilnwash stuck to the back?

This looks like some kind of stress fracture, where the glass is shattering at the stress point and the cracks are spidering out from there. If it is, then either there's something in the glass at the center of the fracture that's incompatible...or something's sticking to the kilnshelf and preventing it from evenly contracting as it cools.

If the cracks are occurring in the same place, especially if the two pieces used different glasses at that point, I'd blame the shelf. If not, I'd look carefully at my glass.

You can try a slower anneal (I typically soak any project bigger than a pendant for at least two hours), but unless there's something else going on I really don't think this is an annealing problem.

[Valerie Adams]

Because you encased frit, stringers, and a non-bullseye glass in between your layers, I'm guessing you blew a big bubble that burst.

[Marty]

Hannah- to add to your troubles, you need to rethink your schedule on the way up. Why the hold at 900? Step 2 makes no sense to me.
You need a reason for every step. I'd slow down on the way up, hold for a bubble squeeze and a good soak at 1100 or so (especially if you're layering all kinds of things), and then go to process temp in a reasonably expeditious manner.

[David Jenkins]

Is all that crumbly-looking cobalt unfused frit?

[Hannah Love]

The small glass bits were a solid piece of glass with some frit in the middle layer. It's now going to be all frit.

The shelf was new and I do think a stress fracture makes sense.

[Studiodunn]

I see dichroic components to the design...Could it be possible that neighboring dichroic pieces would cause this effect when the dichroic coatings touched? Just curious if that may have been a factor in this...

[Cheryl]

I'm with Cynthia. Looking at your piece the first thing I thought was, it stuck to the shelf. On your schedule, when using so many colors (as I frequently do) I think it's worthwhile to anneal at both the upper (920) and lower (720) BE ranges.