You'd think by now I ought to know this:

[David Jenkins]

Questions regarding the term "COE":

Is this a measurement of Linear expansion? Volumetric Expansion? Something else?

What are the units of COE as it's used in the glass fusing context?

Isn't a COE number relevant to some particular temperature interval? And if so, is there agreement among manufacturers on the definition of that interval?

Thanks.

[Bert Weiss]

Dave I forget the exact answer because it doesn't matter.

It would be linear expansion because it is expressed in centimeters squared, not cubed. It is measured within a certain temperature range, which is relatively low. The reasons it doesn't matter are many. First the number expressed tends to be some vague average of pieces tested. Second. COE is only one of the critical factors that determine compatibility. COE and viscosity have to be balanced to achieve compatibility between glasses. PPG Staarphire is listed as COE 90, but is never compatible with Bullseye as viscosities aren't balanced.

The key thing to remember is that everything has a coefficient of expansion, molds, glass, everything. Compatible means you can heat and cool them without one breaking or trapping the other.

[Rick Wilton]

It is a measured change from 20 - 300 degrees Celsius

Although as Bert said it WAY more complicated than just COE

[David Jenkins]

@Rick:

I'm presuming that what you're referring to as "way more complicated" is the issue of compatibility - correct?

[Brad Walker]

Questions regarding the term "COE":

Is this a measurement of Linear expansion? Volumetric Expansion? Something else?

What are the units of COE as it's used in the glass fusing context?

Isn't a COE number relevant to some particular temperature interval? And if so, is there agreement among manufacturers on the definition of that interval?

Bullseye has a pretty good technote on compatibility: http://www.bullseyeglass.com/images/sto ... tes_03.pdf

And this is worthwhile reading, too: http://www.davebross.com/GlassTech/glas ... ility.html

And if you want a history lesson: http://talk.craftweb.com/showthread.php?t=3397

[Tony Smith]

Questions regarding the term "COE":

Is this a measurement of Linear expansion? Volumetric Expansion? Something else?

What are the units of COE as it's used in the glass fusing context?

Isn't a COE number relevant to some particular temperature interval? And if so, is there agreement among manufacturers on the definition of that interval?

Thanks.

You're asking some straightforward questions that have been somewhat sensitive in the past. I'll see if I can help.

COE is the same as CTE (coefficient of thermal expansion) in the engineering world and is a dimensionless unit typically expressed as 10-6 in/in/F or 10-6 m/m/C. It is a measure of linear expansion over a specific temperature range. For glass, we use 10-6 m/m/C. COE 90 refers to 90 x 10-6 m/m/C... More simply, a meter long piece of 90 COE glass will expand 90 micrometers (almost 0.1 mm) for every degree C of temperature rise over some temperature range. Bullseye defines that range as 20 to 300C for their products. In an ideal world, we would have a single value for COE that would extend all the way up to fusing temperatures. Unfortunately, glass has other properties that are equally important in establishing the compatibility of glasses. The Bullseye Tech Note 3 that Brad provided the link to explains Bullseye's position on compatibility in more detail.

Tony

[David Jenkins]

@Tony:

Thanks. This thread seemed to drift quickly into discussions of compatibility - my original questions were posed solely to find out more of the innards of 'COE' - especially as the term is bandied about in this and other contexts.

(Showing exponents is a challenge, isn't it?)