In August I provided some suggestions for reducing bubbles between sheets of glass when fusing. Today's GlassFact covers another problematic situation – eliminating the bubbles that can occur between your kiln shelf and the glass. These are the bubbles that can rise up through your work, ripping a large hole through the entire piece.
Here are some common causes and what you can do to address them:
Damaged Shelf
Bubbles form when air gets trapped. If you are fusing on top of a shelf that has nicks or pits then the trapped air can cause bubbles at full fuse. Keep in mind that air expands significantly more than glass. Even a tiny bit of air can become a large bubble at 1500°F.
The best way to diagnosis this problem is to pay attention to where the bubbles form in relationship to the shelf. If you frequently get bubbles six inches from the top and ten inches from the left edge of you shelf then you probably have a shelf defect at that spot.
Similarly, shelves that are warped can trap air as well. A warp that causes bubbles can be too subtle to detect with visual inspection. The best test for this problem is to flip your shelf over. Bubbles tend to be caused when the glass rests over a "valley".
There are several solutions for damaged shelves
- avoid fusing over the shelf "pits"
- fill shelf scratchs and nicks with kiln-wash
- fire on fiber (of shelf) paper - this will provide an escape path for the air
- flip warped shelves
If the shelf cannot be salvaged, consider cutting it up with a tile saw and using the pieces for dams and kiln furniture.
Firing too fast
As was the case with bubbles trapped between layers of glass, fast firing can also trap air between the glass and the shelf. The same solutions described in Part 1 (link below) will help avoid this problem.
Uneven Heating (dam!)
Another problem that can lead to air between the glass and the kiln shelf is uneven heating. For example, if you are firing glass close to the side elements the edges of your work may fuse together before the rest of the glass – trapping air like a balloon. Solutions for this problem are the same as described in Part 1 for glass-to-glass bubbles.
Kiln wash (or other) off-gassing
Some materials will give of gasses at high temperatures. The gas, like air, expands and if trapped can rip a hole through your work.
If you believe you are having this problem consider trying a different brand of kiln wash or pre-fire the kiln shelf. Kiln wash formulated for firing glass is less likely to cause this problem than kiln wash formulated for ceramics (where off-gassing is less likely to cause problems).
