Posted by Rob Fleischner on 3/17/2025 to
Great question! Let’s start by admitting we sometimes expect too much of our glass when we build complicated pieces and put them in the kiln.
Let’s start simple. Did you ever look at your transparent sheets of glass (color or clear) and see small seedy bubbles in them? And that’s before you ever fuse it to any other glass. Sorry to say that these bubbles have no chance of going away during the fusing process, we simply don’t make the glass hot enough for these little bubbles to break through the surface. It would probably take another 500 degrees F to soften the glass enough for these little bubbles to break free, but that 500 degrees is almost assuredly going to cause other problems, so don’t get any wild ideas.
Taking it up a notch, do you love textured glass? Well, that texture has lots of nooks and crannies that trap gas when the glass covering it fuses the edges closed. Basically, any peaks in the texture hold the covering glass up while the valleys store the air just waiting to expand and keep the top glass from snuggling down and fusing to the bottom glass. About the best we can do is slow the rate of heating down during fusing so the top glass can (hopefully) conform to the bottom textured layer as it gets hot enough to bond. But don’t be overly hopeful, this rarely ends perfectly. The less the texture, the more it works.
Do you use glue ? Sometimes people put so much glue on that it generates lots of gas while burning off. Generally, if the glue is only at the edges of the pieces, it has a fighting chance of gassing out instead of getting trapped when the edges fuse together. But any glue in the center of the piece is at risk of gassing out after the edges have bonded together leaving a big opportunity for a bubble (and sometimes a huge bubble). To reduce the likelihood of glue induced bubbles, just remember, glue is not your friend. Using almost no glue (and, actually no glue if you are loading your own kiln) is the only defense you have, and using it only at the outside edges of your piece, to combat gas.
Using frit, stingers, sheet glass, etc., to create the perfect pattern, then the spaces between them is prime real estate for gases to hide when you put a layer of clear glass on top of it before fusing. If you want to avoid these gas traps, make sure your pattern pieces fit together very well and leave very little room for gas. In fact, there are two ways to improve the odds of few bubbles. First, don’t cap your piece with clear glass, instead put that clear glass underneath to thicken up your masterpiece. If you absolutely must cap the piece, consider full fusing your pattern before capping so the base piece is as smooth as it can be.
Sometimes artists use a sifter and clear powder between layers to give gases a chance to escape before the edges fuse together. This can be effective, but sometimes what you get are a bunch of small bubbles that are less offensive compared to the larger bubbles you might have gotten.
Another technique is to a small piece of clear glass in each corner so the covering glass is raised across the entire pattern. Doing this can cause the covering glass to slump when the heat is applied and the center of the covering glass will sag into place allowing the top piece to squeeze gas out through the edges before they fully bond. Be careful to put the small pieces all the way into the corners, not a little inset or it can become a trap for the gas.
For now, happy creating and check back for more posts on fused and stained glass art by SW Art Glass.
