Bubbles in hot glass are usually caused by trapped air between the glass and the kiln shelf or between layers of glass. Here are several ways that air can become trapped:
1) The kiln shelf absorbs moisture from glass separator (the coating applied to the shelf to prevent glass from sticking). Moisture in the shelf turns to steam at higher temperatures causing an air pocket between the shelf and the glass.
2) As the glass softens, it collapses and forms air pockets between layers. The air pockets form around glass pieces placed between larger layers of glass.
3) The weight of glass pieces over the top layer of glass can cause the top layer to press down and trap air underneath.
4) A low spot in the shelf can form an air pocket between the shelf and the glass. Debris such as flakes of glass separator under the glass can also cause air pockets.
Suggestions for preventing air bubbles:
1) Moisture in the shelf and fusing molds can cause bubbles, because the moisture trapped under the glass forms steam. To be sure that a shelf is completely dry, place the empty shelf in the kiln and heat to 300 degrees F. Allow the shelf to cool. Load the shelf with glass while the shelf is still warm. Some people pre-heat large shelves before every firing.
2) Design your fused piece so that air between layers of glass has an escape route. One way to do this is to place glass pieces near the outer edge between the layers. Or keep surface elements such as stringers away from the edges, where their weight can trap air between the layers underneath.
3) Make sure the shelf is smooth and clean before placing glass on top. If the bubble appears over the same area of the shelf every time, it may be due to a low spot or gouge in the shelf.
4) Soak the glass at 1250 degrees F for 30 minutes. You can extend the soak for as long as 1 1/2 hours if necessary. Some artists begin the soak at 100 degrees below the fusing temperature.
5) A sheet of ceramic fiber paper between the glass and kiln shelf helps eliminate bubbles. The paper is porous and allows air to escape from under the glass.
If you’ve had air bubbles, try the above suggestions. Then the next time you fire the kiln, occasionally look at the glass through the peephole. (Wear firing safety glasses.) At the first sign that a bubble is forming, make a note of the temperature, press STOP (if you are using a digital controller), and reprogram the kiln so that at that temperature, you have a 20 minute hold. Then turn the kiln back on.
As the kiln fires on hold, look at the glass occasionally to see what the bubble is doing. Is it going back down and flattening out? If not, leave on hold longer. Once the hold is done, resume firing to the fusing temperature.