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Old 2013-11-22, 3:04pm
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ewdb ewdb is offline
burnin' glass just becuz
 
Join Date: Apr 16, 2006
Location: Ottawa, Canada
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Quote:
Originally Posted by glassmaker View Post
I definitely do not mean to disparage anyone, and hope everyone understands that. With that said, my suggestion would be to not believe everything you hear in the glass world. In this case, specifically the "news" you received regarding quicker cooling. There is a good reason you've never heard this before - it isn't true. Opaque cadmium-selenium colors (i.e., the range of yellow/orange/red) have some specific reasons they are more susceptible to cracking than other colors. Losing heat faster than other glasses is not one of them.

Without getting into a long explanation of glass chemistry, the short version is that the COE of Cd-Se glasses can change when they are worked for a long time. The more trips up and down in temperature, or the longer they are held at a high temperature, the more likely their compatibility will change. Generally speaking, they also tend to have a somewhat higher annealing temperature than most other colors. I doubt these are the only reasons you had cracking problems with your beads, but I certainly think it is/was part of the issue.

Also, regarding the use of sodium bicarbonate to change the texture on the surface of the bead. Sodium is a strong flux that increases the COE and lowers the viscosity of glass. So doing this changes the COE of the surface glass and introduces extra stress into your bead, making it more likely to crack. I'm not saying you should never use this technique, just be aware of the issues it can cause if you do. The flip side of this coin is that the bubbly surface created can help absorb the extra stress introduced into the glass by the change in COE. But you still need to be careful.

Brad

P.S. I tried very hard not to comment on this so as not to offend anyone, and I very much hope I didn't. But there is so much hearsay based misinformation floating around the studio glass world that I finally concluded that not commenting on this would have been more of a disservice than anything.
Hey Brad,
Thanks for the fact-based analysis! I was guessing about the cooling based on my experiences - as it seemed to tbe the only thing that made any sense just from watching the glass while i worked (compared to other glasses) plus where it would crack once cooled (usually ends 1st, then anywhere thinner than the rest). i work pretty large and sculpted and for the most part the only beads that break for me (regardless of how careful I am) are opaque yellows & oranges and they don't just crack, they desintegrate - unless i use a clear core.

I anneal at 980F and have a very conservative cycle - any tips for working with opaque yellows and oranges? I'd really like to make a big (5"+) soft glass sculpted piece in oranges/yelows but have always been concerned about the long term stability. The way I work any bead will be constantly being cooled and reheated as i sculpt and decorate each area and regularly flash to refheat the whole piece over a period of an hour or more...
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