ECLIPSE OF THE FUN Part 4 | Bullseye Glass Co. | Bullseye Glass Co


After dragging you tediously through how we test for compatibility, what the COE is, how it is tested and what it does NOT tell us, the obvious question is: So, who ever suggested that matching COEs could identify compatibility in the first place?

We did.

GETTING OUT FROM UNDER SOME SERIOUSLY OLD INFORMATION. Written almost 25 years ago by my partner Dan and his then-partner Boyce, Glass Fusing Book One was the first – and is still the most definitive – book ever written on the subject of kilnforming. Today, even Annie is too smart not to dig out from under that old story.

Yes, this entire mess started at Bullseye. We made a mistake. We (actually, it was Dan Schwoerer and Boyce Lundstrom. I wasn’t here at the time, Your Honor) believed that matching the LEC would insure that glasses “fit” when fired together.

They even went so far as to write “The range of fusing compatibility is plus or minus 1. Therefore, Bullseye #101F [now #1101F] with a coefficient of 90 would be fusing compatible with other glasses whose coefficients range from 89 to 91. Outside this range, undue stresses will develop in the glass.”

They soon learned that this was only true if the base formulas of the glasses being matched were the same. It was NOT true when mixing different glasses like opals and transparents. They learned, in fact, that sometimes the COE had to be different for the glasses to match.

How did they learn that lesson? The hard way, of course.

They’d been having a horrific run of a blue and purple streaky glass, a combination of an opal and a transparent. It was breaking up as it came out of the annealing lehr. The breaks were obvious incompatibility – along the interfaces of the two colors.

2105 – The glass that taught the Bullseye guys that matching COEs does NOT insure compatibility.

Samples of the blue opal and the purple transparent were sent off to a testing lab to check their expansions. The report showed that the LEC of the opal was slightly lower than that of the transparent. Problem solved: the formulas were adjusted in order to match their expansions.

Result? The breakage got WORSE. The COEs matched perfectly, but the two glasses were now even MORE incompatible.

Now what? When in a corner it’s sometimes a good idea to ram it into reverse. They reformulated the opal to make it even LOWER – to increase the differences in the COEs of the two glasses.

It worked. By making sure that the COEs did NOT match, they could insure that the two glasses fit.

But, before anyone gets the idea that the authors of Glass Fusing Book One were idiots, please read the original text. They knew that the COE wasn’t quite so simple. They cautioned that:

“…some glasses that have the same coefficient number, as determined by a laboratory, do not always fit each other when fused together.”

They went on to point out that how a glass behaved in the upper temperature ranges was also important.

But the damage was done. The EZ-users, the salesmen, the Fusing-for-Dummies set just grabbed the COE and made it into the astrological sign of our industry: “What do you use? 90 or 96?” It got worse:

“If you can fuse it to Bullseye and it doesn’t break, it must be 90!” Who cares if the manufacturer says it’s 94? Let’s just label it 90. It’s easier to sell that way.”

“Hey, this company says their glass is even closer to 90 than Bullseye. They say theirs is 90.0”

“Don’t you know that the standard in glassblowing is 96?” “Think of all the incredible combinations you can do by mixing all those color bars with a 96 glass!” “Wow, what FUN.”

A random sampling of chips from color bars by Reichenbach, Zimmerman and Gaffer fired against a base System 96 clear glass. Is this the same “compatibility” as the standard that Bullseye established for kilnforming so many years ago?


In the rush to make it all look so E-Z, users are being led astray from the basics of our field: the understanding of what compatibility means, especially as it relates to kilnforming. They are also being deprived of the opportunity to learn about a truly fascinating material. Glass, and the properties that make it so interesting to work with, can’t be reduced to marketing bytes without the user losing some of the rich potential that it holds for exploration, discovery and true creativity.

In the commercial carnival that has erupted in our field, essential, valuable, and necessary technical information is being sacrificed for sales dollars.

Does anyone care? You tell me.

Ed didn’t. He got pretty tired of living in a house full of obsessive glass fanatics and decided to bag it.

PS. Lest anyone think Ed ended it in a grocery bag, he didn’t. He was still alive – although a bit zoned in this shot – just enjoying one of his favorite activities on the day before his Big Check Out adventure.

5 Responses to ECLIPSE OF THE FUN Part 4

  1. The physics of materials, and that’s really what we’re talking about here, are fascinating. I suppose what might be seen is that if you took ALL the glass that Bullseye makes and plot the COE on a curve you’d get some sort of normal (abnormal?) curve centered on 90. What ARE the variables? COE is obviously one, viscosity at various temperatures is another, surface effects like surface tension would be something else, heck I’d even suppose the chemistry of the glass could occasionally do weird stuff. Neat Stuff… thanks Lani!

  2. Lani says:


    The MAJOR variables in determining compatibility of glasses are the COE and VISCOSITY, and the tug-of-war between the two. (Surface tension is of much less importance and tends to follow the viscosity). And of those two major variables, VISCOSITY is more important. You can have glasses that are 10 expansion points apart that will still fit each other. Our experience indicates that you can be off by as much as 10% in expansion difference, but not more than 5% in viscosity.

    To turn this technical reality upside down for purposes of product branding and the genericizing of material serves little purpose other than a commercial one. (Sorry, Gary, I know I don’t need to rant at YOU, but I’m delighted that it seems to amuse you). ;-) Lani

  3. Heh, heh… it’s all a clever ruse! I make you rant at me, deflecting your ire from the Gentle Souls that would shatter like a piece of poorly annealed glass (oh my, did I really say that?).


  4. Lani says:

    You did. And the grenade that you have fallen on salutes you. – L

  5. Steve says:

    It may be that you are going on to give much more information about viscosity, but I would make the point that until viscosity is understood better and more widely by the users, there will be a tendency to fall back on the CoE. You have given the essence of why we have to pay most attention to viscosity, but we do not have the vocabulary to describe it. That vocabulary may be words or numbers (or a combination of these and other symbols). And it will take some educating to get people familiar with the terminology.

    I don’t think it is impossible to change the direction of thinking. It needs influential promoters. You and Bullseye are good at presenting information. A number of influential makers are talking of viscosity rather than expansion. But how will we describe the particular viscosities that we are wanting to indicate?


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