stress...in the glass that is!

[mistahead]

ok so im wondering something here about the stress caused from working/over working glass and how it affects the final structure of a peice? so lets say i take a tube and put a bunch of marias in it then heat it all up and start giving it a slight bend when it starts cooling a bit, now there is a lil surface devit that i can polish out but im guessing theres prolly some more devit inside the tube yes? so after the peice is finished and annealed properly is there still stress in the glass? would it show up on a polariscope? (ive never seen or used a polariscope) . i guess it would help if i understood exactly what devit was i just see at as a blemish caused from stress that i can usually chase out with a lil flame.

[somberbear]

ummm if i remeber right its taking the sodium etc... out to the point where it crystlizes.... so it should just be on the out side where the flame hits it... im sure if you heated it up alot you could get it to gassify on the inside as well...

and if it is annealed properly it should have as little stress in it as it can...

a polariscope is handy but sadly only works with clear as far as i know....

[Brian Newman]

Also, the outside cools before the inside, so any cold strech marks tend to be on the outside of the tube. Those solid antlers I made show stress right where I added the points when I look at it under a polariscope. The flame changes the glass enough to give a permanant, crisp line of stress where two pieces join.

[glassman]

I have a few thoughts that I would like to add. Devit, crystal growth, phase separation; each are a consequence of different causes, yielding different end results but all track the same physical process. That is a crystal grows on a nuclei. Devit usually happens as a result of volatization of some fluxes leaving behind some extra sodium and a silica rich crystal grows. A “cold stretch” is a mechanically induced phase separation; a process that occurs when the glass is to cool to “mix” thermodynamically, but not cool enough to prevent the glass from separating into pools of similar materials. When not stretched it may look like a finger print that can’t be wiped off. The droplets are forming around nuclei. A final note, the phase separation process extends deep into the glass, often to the inside of the tube as you speculated. In is not a simple surface process.

And yes, you were right to ask about the structural integrity. The cold stretch, or devit or whatever name you want to call is a completely different glass from the mother glass and is no longer compatible to the larger mass. It is a flaw. It is best to remove this bit, but not always possible. In the long run this degradation will compromise the piece, albeit that could be 20 or more years (or 20 hours depending on location and amount.)

I hope this might clear up some of the similar but different information and the different but similar information that floats around.

Henry