Technical COE question

[lbowman1]

This isn't so much a need to know as just to satisfy my curiosity.

Thermal shock is caused by stresses in the glass and that stress can be caused by using glass of different COEs. Correct? BUT glasses with a fairly close COE can be used together. Is there still stress in the glass after annealing when two close but different COEs are used? Or does it somehow even out? How about powders and things that say "for use with all COEs?" Do they make a difference in how glass reacts to stress after being annealed?

It amazes me how the glass companies come up with so many different glass rods with a uniform COE, especially with all the colors and things that get added to them. I'm surprised it doesn't make more of a difference in the strength of the glass.

Lori

[CO_Phantom]

Thermal shock is a different beast than compatibility.

Thermal shock is just as its name implies--due to the bead getting too cold too fast (or too hot too fast in the case of a drinking glass). Thermal shock has nothing to do with COE.

Compatibility is from using different COEs. You will find MAJOR stress if you were to use, say a boro rod and a moretti rod. If you looked at that under the...is it really called a "stress-o-meter"? Anywho, you would see a large halo, and probably many many visible cracks. As you get closer in the COEs, that halo gets smaller, to the point where the same glass from the same rod should be nonexistent.

To answer your questions: 2 different COEs will indeed introduce some amount of stress. Say you make a bead of 104COE and rolled it in some 96COE frit, like we all have done. You will be introducing stress to that bead by adding that different COE. That is why there is the "10% rule" that generally says it's ok to add a different COE in small amounts. And generally those beads are ok. But if you were to look at them with the stress-o-meter you would see a small halo around the technically incompatible glass.

Powders that say "for use with all COEs" are probably not glass, and so have no bearing on the stress from COE and incompatibility. Other inclusions can also cause your beads to crack, such as...copper tubing (I believe mine was thermal in the beads I had crack for this method), CZs (still have yet to figure out why those are cracking). You have to be careful with the amount of your inclusions as well.

-Amy

[RSimmons]

COE refers to the linear coefficient of expansion of a material - how much it will expand under specific conditions. Thermal shock breaks come from the outer and inner parts of a glass piece expanding or contracting at different rates and building up stresses very quickly. Mixing different COE glasses makes this much worse, though technically it's not 'thermal shock'. There can be residual stresses left after annealing if the glasses in question are different enough. The COE for most of the soft glasses is an approximation rather than an absolute measure and you do get incompatibilities in some. Evidence what happens when you try to use Mustard Yellow in a murrini. Different combinations of flux components help get different colors to play well together - these often affecting the viscosity of the glass as much as the COE. Lead is great for smoothing out the bumps in the compatibility curves, but there are lots of other issues with using it. Most companies are working on fluxes that don't contain lead but will get the glass to behave.

Powers powders and frits that work with 'all COEs' may contain lead to help with the flexibility of the glass and it's usually assumed that there will be a very small amount used on a percentage basis. Some powders don't contain glass at all and are pure chemical colorants without silicon dioxide. I'd advise testing them carefully before going into a production run or making things to sell.

I've found that CZs 2mm or less are usually OK - anything over that cracks every time in my experience. I think it's that mass thing again.

Robert

[lbowman1]

Thanks for the info. That does make sense. It contradicts some of what I've been told and read in books but I've been getting the feeling "thermal shock" is kind of a catch all phrase for whenever things go wrong. As in "Why did your bead break?"

"I dropped it into a wood chipper. Must have been thermal shock."

All things expand and contract when heated and cooled and they all do it at different rates. It still amazes me that rods that have multiple colors swirling through them hold together as well as they do. It must be a fascinating job designing the glass that we use.

Lori

[Mr. Meker]

Wood chipper lol.

A polariscope (Stress-o-Meter) lets you see stress in clear glass. They can be purchased or home made. There is a youtube on making one.

[lbowman1]

Cool. I'll have to look into that.

Lori

[houptdavid]

A polariscope (Stress-o-Meter) is nothing more than a flashtight with a difusing lens and 2 polarized filters.

Here read this...
http://www.bullseyeglass.com/pdf/technotes_tipsheets/TechNotes_03.pdf

Edited to add...

http://www.warmglass.com/Glass_compatibility.htm

[ArtcoInc]

FWIW, a polariscope works by allowing you to 'see' the way light is 'bent' when going through glass with stress in it. Thus, it can only work with *transparent* glass. Light doesn't go through opaque glass :)

Malcolm

[pam]

Lori, in an ideal world it would be really simple to tell thermal cracks from incompatibility cracks, but it's not always so easy. Normall thermal cracks go in a straight line, worst case scenario the bead breaks perfectly in half. Cracks from incompatibility are like a spider web of cracks, cracking from the incompatible glass out into the compatible glass. However, it's not a perfect world, and sometimes stress from incompatibility can actually cause a bead to crack in half. Sometimes thermal cracks don't go all the way though a bead, and just leave a crack line.

Sometimes a slight incompatibility can be made to work by very slow annealing, as it allows the molecules of the glass more time to align themselves. And sometimes not. Sometimes thermal cracks as well as incompatibility cracks can show up immediately, and sometimes it is years down the road.

DON'T YOU JUST LOVE GLASS!

[lbowman1]

Oh I love glass even though it is daunting at times. After all a really drastic thermal shock at the torch can result in an explosion. It's getting easier though and the easier it gets the more I love it.

It's helpful to understand how different glasses react to each other and the inclusions in them. I'm sure that will cut down on my frustrations too.

Knowledge is the most valuable tool of all.

Lori

[pam]

Oh I love glass even though it is daunting at times. After all a really drastic thermal shock at the torch can result in an explosion. It's getting easier though and the easier it gets the more I love it.

It's helpful to understand how different glasses react to each other and the inclusions in them. I'm sure that will cut down on my frustrations too.

Knowledge is the most valuable tool of all.

Lori

Ah, the perfect glassworker!