ECLIPSE OF THE FUN Part 3 | Bullseye Glass Co. | Bullseye Glass Co


I’ve got a brilliant vet. She’s coming to our home to help Ed “pass over”. I wish there were a glass doctor who’d do the same thing for the sick old myth of the COE.

Fat chance. This misunderstanding about compatibility and the Coefficient of Expansion has been around for longer than many people have been fusing. They were raised on it. Sadly, it may be around forever, mucking with basic technical information in our field and making it harder for users to solve problems when they occur.

We’ve never labeled Bullseye glass as “90 COE”. At the beginning, we labeled the sheets – and still refer to them today as – TESTED COMPATIBLE. Due to the confusion that arose when other companies entered the market and adopted the term we’d coined, we now label our sheets BULLSEYE COMPATIBLE. My two earlier blogs in this series define exactly what this term means.

Maybe the COE thing won’t die because most people don’t really understand what it is. Many fusers – especially beginners – think that the COE number indicates that a glass expands and contracts at a particular rate and that for another glass to “fit” in the fusing process it must expand and contract at the same rate. Sounds reasonable, right?

Not exactly. Let’s back up and start with what the COE really is.

The coefficient of expansion is a number that indicates the percentage of change in length per degree Centigrade change in temperature. This change is measured over a specific temperature range. Without specifying what that range is, the number is meaningless.

The temperature range most commonly measured is in the area of 20 – 300C (68 – 572F). Within this range the change is constant or linear, hence the more appropriate term for the COE as it is used in our community is “LEC” or Linear Expansion Coefficient.

At higher temperatures – those for instance where fusing occurs – the COE is not linear and will be dramatically different (for Bullseye it is closer to 500). In those higher temperature ranges, the viscosity of the glass (i.e., its resistance to flow) is at least if not more important than the LEC in determining compatibility.

The COE (LEC) of a glass is measured using an instrument called a dilatometer. Although we own one, it is NOT how we test for compatibility between glasses. Nor, to my knowledge, is it used by any other colored glass factory to test for compatibility in their daily production.

Dilatometer, open.

Dilatometer, closed.

Neither of these qualities ALONE – neither the COE nor the viscosity – can be used to predict compatibility when assessing a broad palette of opalescent and transparent glasses. It is a COMBINATION of the two that will determine how glasses behave when fused together and then cooled.

That relationship between viscosity and expansion is what manifests as stress on the fuse tests that we do at the factory and that users can do in their own studios.

Years ago Dan wrote a rather thorough article on this topic. If my babble here isn’t enough, you can learn more by reading TECHNOTES 3 here.

So, if matching COEs doesn’t insure compatibility, how did this misunderstanding get started? Tune in next time for some finger pointing.

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