Hi Christy –
I’m in the process of editing this for the site – but here’s some help on bubbles:
BUBBLES BETWEEN LAYERS
When fusing sheets of glass together it is often desirable to reduce the air trapped between layers, thus reducing the air bubbles in the final piece.
There are several excellent and basic techniques for doing so:
Many kinds of glass have one side that is smoother than the other. By placing the glass sheets with a smooth side facing a textured side you minimize space for air while still giving the air paths to escape.
One of the simplest and most effective ways of reducing bubbles is to adjust your schedule so the top glass sheet is able to slump against the bottom sheet before the glass softens enough to stick and trap air. Start the squeeze about 100F above the annealing soak temperature and increase the temperature slowly (50-100F per hour) until you are at the slump temperature. Soak at that temperature for 30 minutes to an hour.
Place small pieces of frit every few inches around the edge of the bottom piece of glass. Rest the top piece of glass on top of these spacers. When fired, the center of the top sheet will sag first and the area of contact between the two sheets will spread from the middle — pushing the air out as it goes.
No, I’m not suggesting you curse at the glass. But by setting up heat dams around the perimeter of the sheets being fused you decrease the chance of the edges getting more heat than the center and “sealing” air between layers. Kiln furniture, strips of fiber board, and cut pieces of old kiln shelves all make excellent dams.
For the best results, consider combining any or all of the above. The use of spacers, for example, works best when combined with a schedule that includes a squeeze.
BETWEEN THE SHELF AND GLASS
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).
Helios Kiln Glass Studio