The process of elimination

I'll looked through Warm Glass and discovered that there are three potential reasons for the turtle cracking.

• I might have cooled the glass too quickly. My one-size fits all firing schedule was based on using Bullseye glass, which should be annealed at 900 degrees. Spectrum should, instead, be annealed around 960.
• I might have warmed the glass up too quickly. That's less likely but it's easy to slow it down, just in case.
• The glass might not be compatible. That's completely possible, given that Susan helped me guess on the manufacturer for some of the glass. 

So I've changed the firing schedule to the following, and crossed my fingers.

Starting Temp	Ending Temp	Rate Per Hour	Minutes to Hold
Room temp	1100	250	0
1100	1240	150	30
1240	1480	max	10
1480	960	150	60