Loving Lampwork Beads

Posts Tagged ‘soft glass’

What is Glass COE?

So what is glass COE? You see it listed in auctions and Etsy, you hear references to COE, but what does it mean? Let's talk about it, and why it's important.

What is Glass COE?

COE, when talking about glass, stands for coefficient of thermal expansion. In other words, how much and how fast glass expands and contracts, based on its makup. Since different kinds of glasses do use different ingredients, each rod of glass has its own rate.

In general, you'll most often see the following COE numbers:

104 -- soft glasses like Effetre, Vetrofond, Lauscha, Double Helix, CIM, Precision...I could go on and on. It's by far the most popular type of glass for lampwork beadmaking.
96 -- Uboros, System 96 and CIM are three names you'll see most often, but there are others. And while some people include the Zimmerman and Richenbach glasses here, they really do have a COE closer to 94.
90 -- Bullseye is the glass of note here.
33-34 -- Borosilicate. Also known as pyrex and boro. Northstar, Glass Alchemy, Momka's are three of the top names in colored boro, while Simax and Pyrex are two of the clears.
Other -- This category includes the real softies like Satake and Kinara, plus Czech (Ornela) glass. They have COEs anywhere from 106 though 120.

Glass Rods, Mostly COE 104

The most important rule if you don't want broken beads: do not mix glasses of two different COEs in the same bead. Although shortly you'll see some exceptions to this rule.

COE Versus Viscosity

Now if COE was all you had to worry about, life would be grand in the bead-making world. But sometimes you'll find that two glasses with the same COE don't play nice together. And the culprit is usually viscosity.

I have a hard time explaining viscosity with how it relates to glass, so I'll use an example with foods. Take a tablespoon of milk and a tablespoon of honey and pour them into a bowl. The milk pours out quickly, while the honey takes its time oozing out of the spoon. The honey is more viscous than the milk.

(Okay, it's not a perfect example, but now you can get a mental image of viscosity.)

A good glass example of viscosity is comparing Effetre pastel white against Lauscha clear. Effetre pastel white melts fast and can get drippy very easily. Lauscha clear is much stiffer and is more like honey when it melts.

While I don't know for certain, I suspect that viscosity effects COE; in other words, a more viscous glass has a lower COE. And within a glass type (example, glass with a COE of 104), the tolerance is generally 2 points. So what that means is that a glass can be labeled 104, but actually have a range of between 102 and 106.

So, if you combine a 104 glass that's actually 102 and a 104 glass that's actually 106...you get incompatibility and a cracked bead.

Is it Ever Safe to Mix COEs?

I want to say first off is that there is a lot of controversy over this. Some say that you never, ever want to mix glasses of two different COEs, even in minute amounts.

Others say that it's acceptable to mix a tiny bit of a leaded glass (usually a frit made from a 94-96 COE glass) with, say, a base bead of 104 glass. The emphasis on the word tiny.

The only thing I can say about this is from my own experience as a bead-buyer and a bead-seller. I have beads from around 2002 that have some frit on them that appears to be the 94-96 glass that are unbroken. I have made beads that include a tiny bit of 94-96 COE glass that for 5 years haven't cracked or split.

Does that mean they won't crack or split 20 years from now? Don't know, but you can check back for a report then!

In my case, I work mostly with COE 104. Therefore, I label each and every rod that is not COE 104 with its color and COE. Glass can all look the same after awhile; best to play safe.

Well that's it for today with the topic of "what is glass COE". Here are some examples of lampworking glasses in various COEs and colors.