KCofCA We have experienced
We have experienced the same thing in our glass class but not consistently with the same slump mold or glass. The first guess was the hole sizes, maybe make them larger by a small amount. Our best guess is maybe the firing schedule is too close (Jen Ken Kiln) for some of the glass (all 90 COE but different colors and embellishments) on some of the slump molds. Maybe a slower, longer firing schedule?!? If you find an answer we would appreciate it. So far it comes and goes at random. It’s hard to calculate a random variable. Keep your journal.