It’s all about finding a way to allow the air to escape.
1. Design the piece so air has a chance to escape — smaller pieces to cap rather than one large.
2. Use small pieces of glass on the outside edges between the top and bottom layer so that the middle of the top layer drops to the bottom layer of glass first, pushing air out toward the edges as it continues to drop. These are commonly called “chads”.
3. Incorporate a bubble squeeze into your firing schedule. Plenty of info on this site, other forums, and “google” about how to accomplish a bubble squeeze when fusing.
4. Your inclusions should be as thin as possible to avoid bubbles and also the glass breaking because of COE differences.
Jester’s Baubles Fused Glass Designs