TOTAL ECLIPSE OF THE FUN? | Bullseye Glass Co. | Bullseye Glass Co


Third, and last, on my list of “Unbeautiful Things” is the manner in which kilnforming is being marketed today. I have some pretty strong opinions on this, so if you’re not ready for another rant, you may not want to keep reading.

This is what STRESS looks like. It happens when glasses that are not “compatible” are fused together. Understanding it is the foundation of glassforming.

IMO reducing a rich and fascinating field to a litany of assurances that success will come from buying a glass with a “wider margin of error” and a kiln that a dog can program does a disservice to the many exceptional artists who have worked to advance the reputation of the medium and to the beginners who deserve to be educated, not harvested.

If technical information is manipulated in order to sell products, some of that misinformation eventually becomes imbedded and does permanent damage to the body of knowledge on which we all depend.


You’re looking at the results of a standard factory compatibility test. The upper image is a fuse test seen by the naked eye. The lower image is the same test bar viewed through polarizing lenses. This test does not measure the COE of these glasses.

But it gets worse.

To now disagree with some of the misinformation circulating in our marketplace draws down charges of being an elitist. Since when did having opinions about measurable and demonstrable phenomena equate to being a snob? Since when did calling for evidence and accuracy in a technical field isolate one from the mainstream?

I get the impression that some people would like me to quit yabbering about this topic. That the endless advertising puffery is somehow acceptable but my own request that users look at “charts, graphs and firing schedules” is petty and evidence of some deviant, anti-social character trait. Or a fear of competition.

Well, sorry, folks, that’s not what this is about. And the rant isn’t over yet.

In this and future entries, I’m going to try to reconstruct the foundations of our field and to discuss discoveries made in the intervening years. At the risk of boring the old-timers and irritating those looking for the E-Z button, I’m starting with the most fundamental issue in our field: compatibility.


Nothing – unless defined. That’s why, when over 25 years ago, Bullseye developed the world’s first line of glasses specifically formulated for kilnwork, we called it “Tested Compatible” and defined the test.

We did not call the glasses “Tested to 90” or “Compatible to 90.0 COE”, because the test used to establish compatibility does not measure the COE of the glass. It measures the stress or “fit” existing between the glass being tested and a base standard.

This test, in our experience, is the most relevant one for the kilnformer’s needs because it replicates –on a smaller scale – the actual conditions of the fusing process itself.


At Bullseye, the testing process looks like this:


Every day close to 1500 sheets of glass, individually cast in as many as 35 different colors, styles, and textures, roll off the end of Bullseye’s continuous belt annealer.

Here at the end of the lehr a 1/2” square chip is cut from each tenth sheet and placed onto a 2-1/2” wide strip of a base clear standard glass (called T-glass).

Because a glass changes predictably through its melt cycle in the furnace it is not necessary to test every single sheet. If sheet #140 passes the fuse test and sheet #150 passes the test, sheet #145 – and all its neighbors – will also test as compatible.


The base clear glass onto which the chips will be fired is a standard that has been maintained since Bullseye began testing for compatibility over 25 years ago. Any chip that tests as “compatible” to the base clear is considered to fit within Bullseye’s line of compatible glasses.

At the end of a shift, the test bars are fired to a standard program that is appropriate for the size of the work being fired. The firing cycle is:

1200F per hour to 1000F – No Hold
AFAP to 1500F – Hold for 15 Min
AFAP to 960F – Hold 40 Min
120F per hour to 800F – No Hold
240F per hour to 700 – No Hold
Cool at the kiln’s natural rate, unopened, to room temp

Certain glasses – reds, yellows and oranges in particular – have a tendency to shift their internal chemistry when fired for extended times, more than once, or to temperatures higher than a typical full fuse (1450 – 1500F). They may, under these conditions, become incompatible. With these glasses, the test is therefore performed (on the same chip) three times.


One of the 27 Paragon GL-24s in use at the Bullseye factory.

The kiln we use for testing is a Paragon GL24. We have used these kilns, or their predecessor, the GL22, for almost 20 years in our testing protocol. This is not an advertisement for Paragon (although we like the kiln a lot). I mention it in order to emphasize that the most critical element in any testing program is consistency. Same base glass, same schedules, same equipment.

The slightest change in a variable to any testing protocol can alter the results in unexpected ways. We have tested close to one million sheets of glass to the same standards over the years since compatibility testing was first introduced at Bullseye.



After the glass has fully fused at the standard cycle it is removed from the kiln and allowed to sit at room temperature until the bars are “read.”

Each fired test bar is then placed on a light box above a polarizing filter and viewed through a second filter, rotating the top filter until the bar appears at its maximum darkness. At this stage, stress between the chip and the base glass will show as a halo, first in the corners of the chip, or eventually encircling the entire chip if the stress is extreme. Chips that exceed this extreme level of stress will actually break out of the bar – at which point the stress is relieved by cracking and no halo will be visible.

Old-timers to kilnforming will recognize this test as the one Bullseye introduced to the glass community in Glass Fusing Book One (1983) after having implemented it at the factory. In the years since, it became the industry standard and has been reprinted, and adapted by individual users and other factories for more than two decades.

You thought that was it? No. At the factory, testing doesn’t end with a polariscope reading. In order to quantify the stress, to determine whether it is compressive or tensile, and ultimately to determine whether it fits within Bullseye’s standards for compatibility, the technician makes a quarter wave plate reading.

STAY TUNED: without getting too geeky about this information, I’ll try to wade through the quarter wave plate measurements, Bullseye’s standard, and most importantly, what “Tested Compatible” truly means to the studio user. And – for those of you who enjoy watching me go ballistic – I will, once again, try to explain why equating the coefficient of expansion with compatibility is a distortion of this fundamental issue of kilnforming.


  1. What, no white lab jackets??? What kind of operation is this anyhow… And a “teaser” paragraph at the end of the blog promising to Reveal All in the next episode.

    Thanks for (starting) to go into depth on this. COE / compatibility and its siblings are some of the most difficult things to explain to folks. Is the base glass you use for your testing compatibility against anything different from the Usual Glass, or is a Special Mix that’s got some properties that are unique? Here’s an off the wall question… have you ever taken a hunk of “compatible” glass from back when you started putting on the label and compared it to today’s standard?

  2. bertglass says:


    This is really important fundamental info that most people are not exactly aware of. To the best of my knowledge Bullseye is the only company that goes through this extensive level of testing. The System 96 or 84 or5 manufacturers don’t do these tests to every 10th documented and numbered sheet. Bullseye’s premium cost is directly related to this procedure.

    When anybody wants to educate a newcomer to what testing means, this page is an excellent example of how and why.

    Rant away! It helps.

    I look forward to learning more about the quarter wave.

  3. Morganica says:

    There’s a difference between ranting and providing useful information. This is one of the better explanations of stress-testing glass I’ve seen, and the pictures are especially useful. Most interesting reading, but I wouldn’t call it a rant.

    Now for quarter waves…and are you going to talk about (ulp) viscosity?


  4. Lani says:

    Cynthia, I like to start a rant with useful information. There’s so little of it around anymore that I think it adds a bit of an exotic flavor.

    We’ll definitely get to viscosity. It’s our favorite little orphan puppy. – L

  5. Lani says:

    “.. have you ever taken a hunk of “compatible” glass from back when you started putting on the label and compared it to today’s standard?”

    Gary, why would we want to do THAT?!

    Seriously, a number of years ago, one of our favorite Watchdogs, Rudi Gritsch of the Glasfachschule in Kramsach, Austria, who has been fusing with our glass for a couple of decades, informed us that he had tested sheets from the early era against our current production. His tests showed that the current and the ancient glass all fit.

    Of course, “drift” in the standard is an obvious concern. And we’ve heard rumors in our industry for years about how “Bullseye is gradually shifting its expansion and not telling anyone.” We just filed that one with the story of your stolen kidney.

    Answers to the rest of your questions will be covered in coming entries….once I’ve finished with the teasers…. – L

  6. glassbear says:

    I am enjoying your blog! A rant would be something like “read the free manual!, expletive” It is very hard to convince students about compatibility until they run into a problem — such as their piece breaks apart while sitting alone on a shelf. Could you tell us more? Of the three coe 90 glasses available (B, U, and W), — they do not have the same annealing point — is this a factor? It seems to me that it must be so. I think that your post is the basis for an excellent technote. Thanks.

  7. Lani says:

    “Of the three coe 90 glasses available (B, U, and W), — they do not have the same annealing point — is this a factor? It seems to me that it must be so.”

    - and you are correct. The annealing point reflects the viscosity – not the COE – of the glass. This must ALSO be taken into consideration when determining the “fit” between different glasses. I will get to this in future entries. Meanwhile, thanks for staying tuned, and for pointing out what is an important but hugely neglected aspect of glassforming. – Lani

  8. sunnystrapp says:

    Gee L. what can I add to all of this? Have you tried Hershey kisses fritted down and fused at 230ºC? Its compatable to old formula S. (Not to B,U, and W of course).


    Profound-ity to follow.

  9. Lani says:

    G….S… Here I thought I was dealing with the absurdity of reducing compatibility to a COE. Now suddenly I find whole glass factories being reduced to alphabet soup.

    C, this is where it leads, U all.

  10. davidknox says:

    Hi Lani-

    Maybe an obvious question but you would think that if you maintained all the variables in your manufacturing process on a given glass number that had already proven itself compatible through these kinds of tests, it would remain compatible until something changed within that process or within the raw glass material. I guess I am wondering what would make it suddenly “fail” the compatiblity test if all else was constant in your process?

    Cheers- David

  11. Lori G. says:

    Keep yammerin’! It’s important!

  12. Casey says:

    As a scientist by training, I 100% support you on this post. In fact, I made an account on here just so I could comment on it and tell you so. I am trying to find exactly what makes glasses compatible or not, and the information out there is misleading and confusing, also complicated by people who don’t understand science. Thank you for writing this!

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