Thread: COE question
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Old 2013-11-21, 1:08pm
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Originally Posted by echeveria View Post
Why wouldn't there be glass with different COE's seems to be a better question. Yes, they have different working characteristics, and lampwork is a tiny segment of the glass world - there is industrial, scientific, decorative, household, fusing, casting, commercial, etc. etc., and a lot of it can be used to make beads. The subject is complex, too much so to really cover in a forum post. There is a ton of literature to explain it out there, technical, academic and art based, if you want to really pursue an answer.
Basically, yes, what Kathy said.

With the exception of the 33 COE borosilicate glass used for making laboratory apparatus, it has primarily been the explosion of the art glass market over the last 30 - 40 years, and specifically the market for different colored glasses, that has prompted the present COE "standards". The main reason being that if you make your own glass you can adjust the compatibility however you need to. But if you have to buy your glass from different sources - as the vast majority of glass artists do now - it helps a great deal if those different glasses fit together. If enough potential customers pressure a manufacturer to conform to a certain standard, and the market is large enough that the manufacturer feels it's in their financial interest to do so, they will usually will. That pressure is pretty much why we now have the COE "standards" we do. (Which is a misnomer in itself, because COE isn't the only important issue in compatibility. Strain point - AKA: viscosity - is nearly as important. An example of this is the 90 COE "standard" commonly quoted for Bullseye "Tested Compatible" glasses. In reality, these glasses are not all 90 COE. Some of their COEs are intentionally higher or lower in order for the various colors to be compatible with each other.)

33 COE happens to be the expansion of Corning's code 7740 Pyrex that has been used by laboratory glassblowers for 100 years. Because 7740 was the first widely used borosilicate to market, it became a standard. (Although Schott actually came up with low expansion borosilicate first.) The various boro color manufacturers that have blossomed over the last 30 years have naturally conformed to this existing standard. It would be financially stupid not to.

90 COE glass has been around for a very long time. It is the approximate expansion necessary to make the glass to metal seals required for light bulbs and vacuum tubes. It is also roughly the expansion of window glass and container glass, and a point where there is good compromise between meltability and chemical durability. It has become an art glass standard primarily due to Bullseye developing their Tested Compatible glass at roughly this COE. Like the boro makers, in order to conform to the existing market, subsequent fusing glass manufacturers followed suit with their own glasses in this range.

96 COE as a standard has a more recent history that can be traced to the widespread availability of pre-mixed glass batch for use by studio glassblowers. Somewhere around 1980, as studio glass was becoming very popular, Spruce Pine Batch began selling a glass batch to studios that was developed by glass chemist Dominic Labino. Although they actually make the batch in three standard expansions, it was their 96 COE one (called SP87) that caught on, probably because it fit the widest range of European color bar. (The "87" designation is the calculated COE when using the popular English & Turner expansion factors.) Once again, because it was first to market, and thus held the widest market, it has become a de facto standard, even if a lot of the color manufacturers that were already in business have been late to the party.

104 COE? I'm not sure, except that this happens to be roughly the expansion of Moretti lampworking glass, which has been around for a very long time. As was surmised earlier, this higher expansion was probably settled upon due to it's easy melting in a torch flame. Most lampworking torches in the past were gas/air, rather than the higher temperature gas/oxygen we now use.

In reality, glass expansions run the gamut from zero (titanium silicate glass/ceramic) to over 120 (Sataki).

So that's my glass history contribution for today. Time for me to go do some actual work before my studio becomes history because I can't pay the bills...


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Last edited by glassmaker; 2013-11-22 at 2:55am. Reason: Fixed typo, changing code 7730 to 7740.
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