The book has an extensive Trouble Shooting section. This page provides some extra advice.

Cracking Enamel

There are a few reasons noted in the book on why your enamel may crack, but another reason may be thermal expansion. Higher expanion means contracting faster during cooling, which could cause cracking. Thus, slowing down the cooling process may stop the cracking.

If nothing else helps, these procedures, both of which slows dows the cooling process may help; try one then the other. If one works, employ the working process again after each subsequent firing.

• Upon taking the piece out of this kiln, put aluminum foil over the piece
• Leave the piece in the kiln, turn off the kiln, and leave over night