Shelves that have gouges or pits can give rise to bubbles from trapped air. Since air expands much more than glass, it will force its way out through the most plastic material. At fusing temperatures, this is the glass.
To determine if this is the problem, note where the bubbles form in relation to the shelf. If it is always in the same area, there is reason to believe it is related to the shelf. By noting the location you now have an area to inspect for damage.
If you can see no damage, it may be that the shelf is warped, or has a low spot. These can trap air, just as the pits and gouges can. But these are difficult to determine by direct visual inspection. You can place a straight edge on the shelf and look for any gap as you move the edge along the shelf.
Possible solutions are:
- Avoid fusing over the shelf "pits".
- Fill shelf scratches and nicks with kiln-wash.
- Mend the shelf with cement fondue or other refractory materials.
- Fire on fibre paper - this will provide an escape path for the air.
- Flip warped shelves, as the opposite side is likely to be equivalently bowed, but in the opposite direction. The degree of bowing is imperceptible, so will not affect the appearance of the fused result.
-Grind the shelves flat. This can be done commercially with a milling machine, or you can do it manually. Place two shelves with their concave faces together with some sandblast grit between. Rub the shelves together and this will reduce the convex areas on each to flat.