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Crack the Case –Your Guide to What Went Wrong Inside Your Kiln


Mystery and magic attract us to glass fusing. It’s the mystery of making something new and the magic of melting materials that hook us. I’ve asked numerous students, and most admit that opening the kiln after firing is their favorite part of fusing glass. We look forward to it with the same childhood glee we have when opening a crisply wrapped gift.

When the project comes out as expected, or sometimes even better, angels sing, unicorns exist and a rainbow glows brightly over your studio. On the rare occasion that the project doesn’t come out well, our disappointment is heartbreaking. We ask why, what was different, where did I go wrong? We rack our brains looking for answers.

All the while, the answer is right there in front of you.

You just have to recognize what your broken glass is telling you.

Let’s take a simple project and break it. Our sample project is made of two full layers of glass, plus an accent layer. The base layer is clear, and the second layer is a pretty purple and the third, partial or accent layer is a pattern glass with hints of purple.


To better understand why glass breaks, let’s do a little review.

There are two basic stages to the firing process: heating and cooling. During heating we take the kiln from room temperature to 1200° – 1500°, depending on the type of fused effect we want. Glass is sensitive to temperature change between room temperature and 1000°. To avoid thermal shock, which is breakage due to rapid temperature change, we heat the glass slowly from room temperature until we reach 1000°.

Over 1000° we can heat the glass quickly to the desired temperature. At 1265°, the glass will slump and take on the shape of a mold. At 1365° glass sticks together, otherwise referred to as tack-fuse. At 1465°, the glass will completely melt into one smooth piece. This is considered a full fuse. (These are System 96® temperatures.)

Once we have the desired effect, we cool the glass. The process of heating the glass causes stress within the project. We relieve the stress by holding the glass at an annealing temperature for a period of time. This allows the glass in the project to equalize at that temperature. (The System 96® annealing temperature is 950-960°.)

After fusing, during the cooling phase, the glass is sensitive to temperature change from 1000° down to room temperature. And the project now has greater value because it’s almost done. To avoid breakage due to thermal shock, after the annealing hold, we slowly cool the glass down to room temperature.

Read between the lines. 

Heating Breaks

This is an example of what a break would look like if the glass is heated too quickly. The clear base layer and the purple layer broke in two on the way up. They’re visibly separated on the kiln shelf. Yet the accent layer bridges the crack and the project is indeed fused and held together by the accent layer. Layers one and two break because they are larger and therefore have more mass. The greater mass is more sensitive to rapid heat. The accent layer is smaller, less mass and therefore remains intact. A break like this happens inside the kiln. This is a prime example of breakage due to heating the glass too quickly.


It’s possible to break the project apart and refuse it slower, but the finished product will likely be different from the original design.

The fix: slow down. Heat the glass at a slower rate. I heat projects from 4” coaster size to 12” bowls at a rate of 300° per hour with dependable success.

Cooling Breaks 


An annealing fracture has a very specific look. It has a tight curve that rolls into a broad gradual curve. (It almost looks like a smile, but not one that any of us would look forward to seeing.) The broad curve is followed by another tight curve. In my experience, the break usually runs vertically through the project. The glass pieces have been fused together and the break is clean through all three layers. The two broken pieces are generally separated by a sizable gap. The gap is evidence of the internal stress in the fused glass. An annealing break can occur inside the kiln or weeks later. This type of break is the result of rushing the annealing time, peeking into a hot kiln or from opening the kiln too early and chilling the glass.


You can re-fire the broken glass and re-anneal it, but the finished project may have a visible scar where the glass was broken. You can add frit or extra accent pieces to hide the seam.

The annealing temperature for any specific family of fusible material is constant. The length of time a specific project needs to be held at that temperature is determined by the size of the project and the number of glass layers used to make the project.  A large project, like a 12” bowl, will require a longer anneal time then small project like a 1” pendant.

The fix: consult the glass manufacture’s firing guides and be sure to include the appropriate annealing temp and time in your firing schedule. Here’s the hard part. Don’t peek! Or open the kiln below 1000° until it is at room temperature. I anneal projects that are 12” is size or smaller, made with two layers of glass plus an accent, like the example here at 960° for 40 minutes.

Incompatibility Breaks 

Compatibility breaks are really upsetting because they’re so easily avoided. We do our best to sort and organize our glass by COE, but sometimes a mix-up results in a mystery break. Knowing what to look for can save you from having any future accidents.

C.O.E. (Coefficient of Expansion) this refers to the rate at which glass expands and contracts when heated and cooled. Fusing compatible glass has been formulated, manufactured and tested for compatibility. All the glass in a single project must be of the same C.O.E. so they expand and contract at the same rate during firing. If they are not, stress cracks or breakage will result.


A glass incompatibility break usually has more than one crack and it often happens a short time after the fused glass comes out of the kiln. The materials literally repel each other and so the broken pieces are angular and sharp.

The fix: Pay close attention to the manufactures labels, store materials by C.O.E., clearly mark scrap and work with only one family of fusing compatible products at a time. Don’t trust that materials given to you are compatible unless they’re marked as such.

Refusing the glass is not an option. But if you want to feel better, take a hammer to the project. Then use the pieces to make a mosaic or to line the bottom of a fish tank.

The Dreaded Hole         

The dreaded hole in the project is caused by heating the glass to fast while firing on a very smooth surface like a primed ceramic self or Thin Fire fiber paper. As the glass is heated the perimeter of the project becomes soft first. It seals to the shelf like a suction cup. The air trapped under the glass expands as it’s heated and pushes up on the glass forming a bubble. The glass bubble becomes thin, it pops and heals over at the full fuse temperature.

You can fill the hole and refuse the glass; however, a blemish will be visible in the new piece. If your design is organic the new detail may add interest. But if your pattern is pictorial, and unless the hole is perfectly located to represent the sun, not so much.


The fix: add a segment to your firing schedule. In the heating phase of my firing schedule I have a hold at 1300 ° for 60 minutes. Holding at this temperature softens the glass in the entire project uniformly. This extra step is well worth the wait as it eliminates inconvenient eruptions and those nasty holes.


Fortunately, the fusing spirits grant me many successes and very few mishaps. But every so often, no matter how hard I work, through no fault of my own, a single project or piece seems destined to failure. My general rule it to try to rework or salvage a failed project once and on special rare occasions twice. If at that point I’m not happy with my results I abandon the piece and start over from scratch. Hard earned experience has taught me, additional efforts are a waste of time and resources. My best advice when that happens is, let it go.

Here’s a quick example. I was building a huge 12 foot by 16 foot wall sculpture with 14 fused glass circles ranging in size from 12 inch to 48 inch across. All of the glass was special ordered from Uroboros. It came in a single crate. Five of the circles were made with the same color glass. All of the pieces were fired in the same kiln using the same firing program.

For some unknown reason, one of the five circles broke into two pieces after fusing. It cracked as neatly as an egg. No problem, I re-fired the two broken pieces onto a new, single piece of clear. I opened my big clamshell kiln, when it was at room temperature, the re-fired glass looked great. Happy with the result, I went back to my worktable a few feet away.

It never happened before and hasn’t happened since. I was alone in the studio, just me and my glass.

Out of nowhere, the lid on the clamshell came crashing down. I rushed over to the kiln and opened the lid. No big surprise, the fused glass circle was broken and in two neat pieces again. Dam isn’t the only word I used. My rapid fire expletives would’ve made a sailor blush.

I got even by dumping the uncontrollable pieces in the trash. Nope, I didn’t even trust them in a smaller project. Bye, bye! I made a totally new circle and it’s still intact ten years later.

With luck, this is the first time you’ve been introduced to these various ways that glass can misbehave. Hopefully, you’re now armed with the knowledge to avoid such disappointment.

You can confidently explore more in-depth techniques and more advanced projects.

But for added safekeeping, before closing the lid on my kiln I blow kisses to my darlings. You might try it.

Happy Fusing!


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12 thoughts on “Crack the Case –Your Guide to What Went Wrong Inside Your Kiln

  1. Love the article very infirmative

    1. Hi Diana,
      Thank you for taking the time to comment. I appreciate hearing from enthusiastic fusers like yourself.
      Happy fusing!

  2. Wonderful information. Such clear problem solving.

    1. Hi Becky,
      Thank you! I’m glad my ut-ohs can help you avoid them.
      Keep on fussing!

  3. Hi I was looking at your crack the case blog and thought perhaps you have an insight into my problem.
    I am a into glass fusing in a small way and a couple of times i have experienced large bubbles appearing in projects. I am talking about 4″ dia bubbles. The last one was a very simple plane red balloon shape for a clock, and all I was doing was fusing stringers to identify the minutes and there it was, not quite in the middle.
    This piece was a single layer of CO90, normally i would have had a base layer of clear with the cut pieces of various colours laid on top. Do you have any ideas as how this could have occurred.

    1. Hi Mick,
      Bubbles are caused by a variety of reasons. In my experience, it’s good practice to assemble your project with two layers of glass. You could also slow down your firing to allow the entire project to reach a uniform temperature. I fire at 300 degrees per hour to 1300 degrees and hold there for 40 minutes. This hold helps minimize the bubbles. Then I heat 500 degrees to my target temp. I use COE96. Full fuse for COE96 is 1465. I suggest you refer to the COE90 glass manufacturer’s firing schedule for guidance. Good luck! Lisa

  4. Hi Lisa,
    I am brand new to fusing. For some crazy reason I decided to take up fusing during the pandemic thinking it would be relaxing! Lol at times I have found it frustrating, others amazing. I have basically had all these issues. I am happy to have had stumbled across your article. It is very helpful.
    Thank you!

    1. Hi Linda,

      It’s nice to hear from you! I’m glad you found my blog post helpful.

      You might like my eBook titled, Get Fired Up! It’s a comprehensive beginner guide to glass fusing. You can buy it on my website
      Keep on fusing it only gets better.

      Happy fusing!

  5. Hi Lisa, thanks for all of your information!
    I have been fusing glass for many years and I just started having this problem of black veining and smudges on my tack fusing. I’d love to send you a photo.
    I hope you can help.

  6. […] Crack the Case –Your Guide to What Went Wrong Inside Your Kiln […]

  7. Great article for those of us who have fused a piece or slumped one to find a crack . Mine have largely occurred when slumping a piece of varied thickness and by probably heating it too rapidly as if it was a standard 6mm piece . Thanks for your input

    1. Hi Bob, Thank you for your comment! Lisa

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