Blister and air bubbles in fusing result

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    I am new to fusing and have done about 3 successful fusing firings.  This weekend, I did have some problems.  One firing, the center of the work blistered.  I researched and came up with the shelf was probably a bit uneven.  I cleaned and rewashed the shelf.  This seemed to help quite a bit.  The next firing I did left a large air bubble.  Is it possible to fill in the blistered piece and put a clear glass behind it and refire?  Is it possible to refire the air bubble piece to remove the air?  or have I just learned a couple of lessons?  I have another fusing to do tonight. so I will be making sure everything goes right.

    Thank you for your input.


    Hi Christy –

    I’m in the process of editing this for the site – but here’s some help on bubbles:


    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.


    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).

    Helios Kiln Glass Studio


    jim boles

    Very early in my fusing experience I researched bubbles and gave up because there are quite a few things that cause or assist in making them. Some of which are: shelf not dried out from wash; uneven distribution of glass causing relative ‘weak’ spots where there is less glass; and firing schedules to name a few.

    The shelf side of glass will always be textured or matt no matter how smooth you try to make the shelf. So why not just go with it. Fire on top of a thin fire matterial or other shelf type papers or fiber products. You will never get bubbles since air will escape via the shelf lining, and learn to love the various texture possibilities you can come up with by using all sorts of fiber based products and doing things with or on the fiber/paper.

    As far as saving the piece that has a raised bubble. You can put it back in the kiln and raise the piece to bending temp. Use a stainless steel soup ladle or some such tool to push it down by rolling the ladle over the bubble, then follow a regular schedule to bring the temp down to room temp.

    Since your a newbie I need to tell you to turn the kiln off when reaching into it and use a kevlar glove or some such. After you pushed it down, then turn the kiln back on and continue with the down ramp part of the schedule.

    Jim Boles


    during a full fuse I ended up with air bubbles about a dime size raised on my glass.  Can I refuse to get rid of bubbles or chip a piece where the air bubble is and refuse or just consider it botched and scrape the glass?  Any suggestions?  Thank you.

    I just read Jim’s posting about refiring and using a stainless steel spoon at bending temp.  Would bending temp be the highest temp?  Thank you for any suggestions and help.


    jim boles

    No reason you can’t refire it. It might settle back down, but it will likily have tell-tale marks or thinness. The list of causes are too nummerous IMHO to mess with. The simple solution is not to use kiln wash/primer, and instead use thin fire or other type of fiber paper. I bought a 100 foot role of the stuff 10 years ago and I stil have 50 foot left. You can fire multiples times on 1/8 fiber, then save the scrape for other projects. I often use the scrapes to form heavily decorative textures on the back of the glass. looks absolutely great if you use irid glass on the the fiber.

    As you might read elsewhere about the ‘back side’ of projects… celebrate the textural effects by creating your own approach. Using primer on the shelf is boring… besides being troublesome at times.

    Jim Boles


    Any suggestions on firing times for a full fuse two layers of glass, using a 12 key 146 Paragon Kiln.  I think the firing times I”m using is just not cutting it, having bubbles.   Any help is greatly appreciated.


    I have fired pieces for a full fuse and would like to refire to get rid of small bubbles.  I am trying to figure out if it’s already been fused, how do I do a bubble squeeze when the edges are already sealed?  What is the schedule for a slower refiring with bubble squeeze?    


    If I were using the firing times below, during which phase is “bending temp” to open kiln and use stainless steel spoon to try to flatten air bubble…. below is the firing times that I will use. 



    FULL Fuse



    1     300   1150    30

    2    400   475    10



     3    9999*   950   60

    4    150   800    10

    Or do you have a suggested firing schedule?

     3    9999*   950   60

    4    150   800    10

    Or do you have a suggested firing schedule?



    Stephen Richard

    I think you are better to do the squashing of the bubble in the range of 1200 – 1250F (650-675C).

    Stephen Richard

    blogs at: and

    Stephen Richard

    I am trying to figure out if it’s already been fused, how do I do a bubble squeeze when the edges are already sealed? “

    As you have guessed, if the edges are sealed, no amount of bubble squeezing will do the job you want.  It has to be done on the first fusing.  The bubbles in the fused piece are there for life, unless you want to drill them out.

    The bubble squeeze is in the range of 650-677C.  This determined both by the glass and personal preference.  You can do a soak at the temperature you choose, or you can take an hour to do the 25C up to the desired temp, with or without a soak.

    Stephen Richard

    blogs at: and

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