Why do my glass pieces break in the microwave kiln?

The pieces must be fairly small and clean. Greasy fingerprints and glue can break the glass due to the extremely fast heating in the microwave. When pieces break in the MagicFuse, try adding a pre-heat. Set the microwave oven to 50% power, and fire for 5 minutes. Then fire the microwave at full power. This should solve breakage problems.