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I have a 15 " diam round project that worked the first time but cracked twice on a second version. It has several approx 8"x2" pieces of black irid in a center cluster with trans blues on outside.

Here is mycurrent annealing schedule from a high temp of 1500
full speed drop to 960 soak 40 min 60 deg/hr to 900 soak 40 min 60 deg/hr to 600 cool as kiln cools

On my 2nd try I went to 45 degrees /hr for a cooling rate but had more cracks
I saw that bullseye had faster cool rates for 1/2 thick than I have for 1/4
and a final cool rate of 1200deg/hr all the way to 75 Degrees.

any suggestions for alternate schedule

Joseph Tracy wrote:I have a 15 " diam round project that worked the first time but cracked twice on a second version. It has several approx 8"x2" pieces of black irid in a center cluster with trans blues on outside.

Here is mycurrent annealing schedule from a high temp of 1500
full speed drop to 960 soak 40 min 60 deg/hr to 900 soak 40 min 60 deg/hr to 600 cool as kiln cools

On my 2nd try I went to 45 degrees /hr for a cooling rate but had more cracks
I saw that bullseye had faster cool rates for 1/2 thick than I have for 1/4
and a final cool rate of 1200deg/hr all the way to 75 Degrees.

any suggestions for alternate schedule

When did it crack?
On the way up, on the way down? or after it was taken out of the kiln.

Your anneal schedule looks ok to me.
maybe incompatible glass snuck in?

Jim

No sign of healing so pretty sure it cracked on the way down.

I'm fairly certain it wasn't incompatible glass. the cracks originate from the edges of the black irid. I think its a temp differential from black to translucent blue. Some of the blue is uroburos 90 but haven't had a problem with that before.