I haven’t used a clamshell that size, but it is certainly big enough to handle a few pieces at once. I wonder if there is a problem with the elements. It sounds like it is heating unevenly. The owners’ manual should have some info on element arrangement,and you should be able to talk to the company about the problem. They may be able to help you find a kiln repair shop or a glass studio that builds or works on its own kilns to help you pinpoint and correct the problem. Sorry I can’t be of more help.
Fused Glass, Wood, Beads, PMC, Ceramics
Usable and Wearable Art
Art for the Sake of Art