Air Bubbles in Fused Glass
Some people like air bubbles in their finished glass pieces. They claim that they give the work character and are even attractive in their own right. It's even possible to work with bubbles by manipulating them between layers of glass.
But most people think of bubbles as a nuisance. They'd like their finished pieces to be bubble-free and as smooth and untroubled as a piece of plastic. This isn't always possible, but there are some techniques that will help minimize bubbles in the glasswork.
The first step is to identify the kind of bubbles you have. There are many different kinds, from large ones that pop like a balloon and leave a hole behind to very small bubbles trapped between the layers of glass. Each of the types of bubbles has different causes and solutions.
-- Bubbles in unfused, unheated glass.
Sometimes a "new," unfired sheet of glass will have small bubbles captured inside. These bubbles, called "seeds," are manufacturing defects. They cannot be eliminated. All you can do is avoid buying this kind of glass next time.
-- Large bubbles, sometimes breaking through to form a hole.
These can be caused by uneven kiln shelves. Gases given off by the kiln wash can also be a contributing factor. If unchecked, the bubbles will increase in size until they pop, leaving a hole behind. Prevent this problem by first checking to make certain that your kiln shelf is level. Use a straightedge to check for warping or distortion in the shelf. You may need flip the shelf and try firing on the other side. Sometimes a new shelf is the only solution.
A second way to attack this problem is to slow down your fusing schedule between 1100 and 1300F. Some people add a lengthy soak at around 1250F to help give the bubble time to escape. Another solution is to fire on fiper, rather than kiln wash. Many of the solutions in the next section will also help with large bubbles.
-- Small bubbles between layers of glass.
These are the most common types of bubbles, caused by air that has been trapped between the layers of the glass. It's very difficult to totally eliminate these bubbles, but using the following techniques can minimize them.
-- Fire slowly between 1100 and 1300 degrees F.
This allows the air to escape. You might also consider soaking for a few minutes at the point in this range where glass softens (try around 1240F).
-- Cut small pieces, rather than large ones.
The smaller the piece of glass, the less likely it is to trap air.
-- Avoid unintentionally creating pockets that can trap the air.
Fuse with smoother sides of glass touching, rather than textured surfaces. Try to leave ways for trapped air to escape.
-- Fire on fiber paper, rather than kiln wash.
Fiber paper allows more air to escape from the bottom of the glass.
-- Support the edges of the glass with small beads.
This is especially useful for large glass sheets. It involves supporting the edges of the glass with tiny, nugget-sized, pieces of glass. Use glass that is the same color as the base glass. Place the pieces in three different places along the outside edge. This will give the air additional room to escape. Also, make sure you soak the glass as described in the first bullet above. Here's a good description of this technique from the Warm Glass archives: http://www.kilnforming.com/cgi-bin/wgarchive.pl?read=12381