TOTAL ECLIPSE OF THE FUN Part 2 | Bullseye Glass Co. | Bullseye Glass Co


Sorry to be dragging my feet on this subject. I have to confess to finding it a little tedious. For me, technical information is like lifeboat maintenance on the Titanic. No one really cares about it (myself included) until something goes wrong.

Then you’d like to know that the equipment doesn’t have holes in it.

Last week I went over basic compatibility testing as it was established at Bullseye and has been practiced in our field, with little or no variation, for the last 25 years. Whether that test is being performed within the factory or the artist’s studio, it is not – and never has been – a measurement of the so-called COE. What is measured is the strain that exists at the interface between the chip and the base glass.

WHAT STRESS?! By Bullseye’s factory standards the four chips on the far right of this bar are considered incompatible. Chips 1570 and 1572 are low relative to the base clear. Chips 1573 and 1580 are high relative to the base. Compare this bar with one in which many of the samples are very obviously out-of-compatible range


The bar above shows test chips fired against a standardized base clear glass and viewed between polarizing lenses.

Whether the stress is high or low and its magnitude can be determined by using a quarter wave plate and by knowing the stress optical coefficient (birefringence constant) of the glass, which allows one to then calculate the amount of the stress.

Rotating the analyzer until the halo of light is extinguished allows the operator to measure the amount of strain. This “angle of extinction” is directly related to the magnitude of the strain in the test glass due to the compatibility mismatch. (The optical retardation is what is actually measured.)

Bullseye accepts and labels as Tested Compatible glasses that show less than 2 degrees LOW and 4 degrees HIGH. It is important to note that these are degrees of retardation, NOT expansion points.


What industry? When Bullseye established its in-house standards no other manufacturer was making a line of compatible glasses designed for kilnforming. The factory set its own standards based on its experience in working with artists and in-house studio practice.

Fat-headed as they were, the Bullseye guys were never so arrogant as to claim to be an industry “standard.”

Within the wider community of glass artists – referred to within the US as the “contemporary studio glass movement” – glassblowers had certainly set no standard. The standard was whether the work broke or not. Torchworkers were primarily using Italian rods but Moretti made no claims of being the industry “standard.”

For standards to provide value they need to be administered by an independent testing body. No such organization exists in the studio glass world at this time. Any standards are those that have been proven by time in the field.

It is the responsibility of manufacturers to make available to the user the standards to which they hold their own products. And to publish the testing method used to support those standards.

NEXT: Why the COE is not a true standard.

13 Responses to TOTAL ECLIPSE OF THE FUN Part 2

  1. Toni J says:


    You might find the subject tedious, but I find it fascinating. I enjoy reading the threads on the WGB, but trying to decipher accurate from inaccurate can be a challenge. I really prefer to read it straight from the horses mouth, so to speak. (I’m certainly not calling you a horse, Lani.) :>) I must admit to impatiently checking your blog every day to see if it was updated yet, because I was very much interested in reading and learning further.

    I’m going to have to go home tonight and check the pictured strip on a different monitor. Maybe it’s my settings on the monitor I’m using right now, but I’m seeing very little halo (and I did adjust the settings) – more so on chips 1570 and 1572. You said the two were low relative to the base strip and the other two were high relative to the base strip, meaning all were incompatible. Will you be explaining this a little more indepth? Sorry for my ignorance, but I’m not sure exactly what that means since I’m not seeing much, if any, halo on the two strips that you said were high relative to the base.

    Thanks, Lani.


  2. Lani says:

    Toni, I’m utterly thrilled that you’re fascinated. I can barely stay awake through stuff like this.

    It’s not the settings on your monitor. The level of stress that we accept for Tested Compatible Bullseye is so tight as to be barely visible on the test bars to many people. The technicians are trained to see halos that I myself often cannot see. (But I’ve been wearing blinders and keeping my face in a feedbag for years.)

    We made the decision years ago to keep the standards this tight in order that the end-user could have more room for error.

    I don’t mean to imply that you need this level of stresslessness (?) in your own work. We’ve seen work with stress that far exceeds our factory standards that is stable years after it was fired.

    As you likely know, the amount of stress acceptable in a work of kilnfired glass depends on many factors: its size, usage and exposure being among the major ones.

    All I hope to do here (and in the coming entries) is to clarify a little of what is meant when we all talk about “compatible” glass. And most importantly to lay some groundwork for problem-solving in this field. – Lani

  3. Morganica says:

    I’m with Toni; I can’t say I live for this stuff, but I certainly enjoy it and appreciate any overengineering that lets me take risks with glass. Heaven knows I take ‘em.

    Your post, though, kinda leaves open a question: What are the chances that we’d ever seen a standards body for art glass, i.e., someone to certify “tested compatible” for the industry? I note that Bullseye isn’t the only glass company now using that term…but that leaves open who’s testing how with what and which.

    And is it ever likely to happen? Seems like there’s a lot of variance between companies and not much incentive to develop an “industry standard.”


  4. Lani says:


    I’m pretty sure that you know the answers to our own questions.

    Given the size of our field, I doubt we’ll ever see a true standards body. Without one, I think it’s pretty important that manufacturers define their product claims very clearly.

    If we call our glasses “Tested Compatible” or “Tested to 90.0″, or “96 COE”, I believe we need to provide users the testing protocols behind those claims.

    Knowing the product is essential to knowing how to use it. – Lani

  5. davidknox says:

    Hi Lani

    A comment related to my comment on the first part of this blog- If a glass formula, by chemistry and process is found to be repeatedly ‘compatible’ to the standard, what then would make it suddenly fail the compatiblity test the next time you made it? Are the compatibility tests in fact a QA procedure for your process control or raw materials? Or, are you suggesting that inherently, any particular otherwise known and proven compatible glass, as an amorphous crystalline structure, occassionally and randomly forms into a sheet that is not compatible due simply to the way the molecules have lined up during relaxation to a firm state? Basically it woke up with a bad-hair day and is full of stress? By the way, as a former laser guru, your bifringence tests appear at first blush to be Very stringent given the medium and the methodology. good for you- and us and I thank you for your concern. This is a big topic- thanks for bringing it up.

  6. I’d bet on the “process” bit for plunking in a bit of randomness, ie. “incompatibility”, on occasion. I’ve seen the guys make the glass. Dip into the furnace, plop a blob of molten glass on the table, knead it with kneading thingies (what ARE those called, Lani?), run it through the laundry wringers, and into the annealing furnace.

    It’s that kneading bit that I can see introducing the weird stuff now and then. Maybe the kneading is off by 5 seconds, maybe the air temperature was a little bit off, maybe the lead kneader wasn’t holding his mouth Just So…

    If the glass manufacturing was done by machine I’d bet the incidence of off-standard would be virtually non-existent. With wet-ware (that’s you and me) involved there is always going to be some variance.


  7. bertglass says:

    Heatwork can alter the chemistry which effects compatibility. Bad hair is for us…

  8. Lani says:

    For the benefit of my Geekmeisters David and Gary,

    I apologize in advance for the length of this. Like I said, this stuff is painful for me. But I like you guys and appreciate your interest, so here goes:

    Glassmakers are fond of saying that “you can’t take a glass formula across the street”

    Gary, what you call “wet-ware” is certainly the variable that is the most difficult to control – although neither how the glass is “kneaded” nor the facial contortions of the caster affect compatibility (as far as we know!). Small scale glass manufacture is hugely labor intensive. We’ve got thousands of double-checks built into our systems to allow for the inevitable human error.

    Here’s just some of the other stuff that can screw with the consistency of glassmaking:

    The FURNACE life: glass “eats” furnaces. Ours last for about 500 (day) melts each. A furnace at the beginning of its campaign is different in size and composition from the same furnace at the end of its life. We need to make adjustments to the formula for the same glass based on the age of the furnace.

    The heat value of the FUEL changes from day to day, sometimes from hour to hour. Whether the fuel we get is 1050 or 1020 BTU/cubic foot is outside our control

    Believe it or not, the BAROMETRIC PRESSURE can affect how a glass melts.

    Those are just a few of the more obscure variables. But Gary’s right that it’s people that make the biggest difference in a small-scale operation like ours.

    And when it’s human error, the offending human is often not one of our own (who are as near perfect as they come IMO!).

    We once got a load of soda ash delivered by a driver, who contrary to the protocol we specify (and pay for), had not cleaned out his tanker of the residue of an earlier delivery when he blew over 40,000 thousand pounds of soda ash contaminated with bentonite into our silos. It was over 2 days and almost $100,000 worth of lost product before we traced the incompatibility (and breakage) back to that driver “cutting corners” on his job.

    Of course there are manufacturing controls available for lots of the variables. In small scale glass manufacture, however, the cost of many of those controls can be prohibitive.

    Bottom line, humans are still the critical variable in mixing, melting and forming a broad color palette of (relatively) small quantities in glass. There is simply not the market today that would allow for the production efficiencies of larger scale manufacture.

    Show me a factory making large quantities of fusing-compatible glasses and I’ll show you a factory that is dependent on a smaller factory to provide a complete color range – a factory trying to do what we do. And – I’m confident of this one – not doing it nearly as well.

    Bottom line, all of the variables mentioned above (and more) mean that compatibility in colored glass cannot simply be predicted. It has to be TESTED after the fact – regularly, consistently and strenuously.

    As always – humbled but arrogant, Lani

  9. *ouch* bentonite contamination. I can imagine what throwing random amounts of (Na,Ca)0.33(Al,Mg)2Si4O10(OH)2·(H2O)n into the mix would do!

    In one way, though, it’s that bit of human-induced randomness that makes life interesting. I can imagine a situation where a piece of glass comes out of the annealer and fails the compatibility test. BUT… that glass is so cool and funky looking that someone wonders… “Gee, I wonder if I could CONTROL that funkiness”. So, what was a bug eventually becomes a (compatible) feature. In a big and faceless production facility that “Gee, I wonder…” would never happen.

  10. Lani says:

    Re Bert’s comment:

    “Heatwork can alter the chemistry which effects compatibility. Bad hair is for us…”

    Most definitely. The glass at the top of the furnace, at the beginning of a melt, is different (typically higher in expansion) than the glass at the bottom of the furnace, at the end of the same melt.

    My hair has its own set of problems…thank you.

  11. davidknox says:

    Hi Lani

    Interesting topic and blog- true synaptic fodder for a glass dude in obvious need of and little desire for, a larger social existence. To be very blunt, which I often am to a fault, given the dynamic of viscosity in glass compatibility, it is a miracle that you can claim/create a glass compatible with an entire line of glass. Theoretically, having a glass compatible to a given standard- 1101 or whatever- does not make it necessarily always compatible to all other glasses that are compatible to that standard. No news to you and the big-brains at Bullseye. Given that- and the countless variables that we add into the mix in what we make with it, you may be a bit too hard on yourselves through these tests. Why?- because what you are testing for is one large variable- I realize it is the premise of your company and all your pioneer work- but it only one and does not and cannot address the viscosity dynamic in terms of the variables. And yet it still works. Thanks

    I gotta go. this problem- or non-problem- is mind-boggling when you put it in the context of lots o’ variables.


  12. Lani says:

    David, good points, of course.

    “Theoretically, having a glass compatible to a given standard- 1101 or whatever- does not make it necessarily always compatible to all other glasses that are compatible to that standard.”

    This is precisely why we are so tight (some say excessively so) on our range. It allows the samples at either end to be close enough.

    “…what you are testing for is one large variable… but it [is] only one and does not and cannot address the viscosity dynamic in terms of the variables.”

    I would argue that our compatibility testing DOES in fact address the viscosity variable, and is the very reason why this chip test – not a dilatometric reading of the COE is the basis for determining compatibility. Is the COE what you consider the “premise” of our company? I’m not sure I’m understanding you.

    But keep reading. Maybe I haven’t ranted enough yet.

    And thanks for pointing out that my social life sucks. You’re right. It’s this blather or the kitty wake. – Lani

  13. davidknox says:

    Hi Lanio-

    A few misunderstandings there-

    1. The social life I was commenting on was my own- without regrets (I think). My dog won’t even give me the time of day anymore.

    2. I wasn’t suggesting that your COE was the basis of your company: I was stating that the compatibility test is how you created the products of your company in terms of fusing- which then pretty much became the standard by which we all work. And it works!

    3. I see your point about the tightness of the test being the reason that you feel the full range of differing viscosity in the glasses will work. So generally, there must be some glasses that pass almost everytime- those that flow well with the standard and ones that are on the edge all the time- those that don’t. Maybe I’m missing something but the bottom line is that few, if any of us, have ever had compatibility issues with your glasses (that I know of)- so something must be working.

    Very interesting topic-

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