How to prevent medium to small bubbles in glass fusing

Bubbles can be caused by many different things.

Uneven stacking of glass can result in air trapped between layers. To prevent this from occurring, check the placement of all the glass pieces to insure they are sitting properly on the base. If possible fill up the small openings between the different glass pieces with a clear glass.

Second, check the glass prior to fusing. Some glass may already contain bubbles inside, which may or may not affect the outcome.