cracking during high fire

[ginnibobinni]

I have a piece I used bubble powder on, using 90 coe bullseye glass. I fired it in 4 segments: 1 at 300 to 1100 hold 30, 2 at 9x to 1700 hold 10 3 at 9x to 1500 hold 30 and 4 at 9x to 960 hold 45 then off. I took it out and heard fine pops, could see very small fissures in the surface, so I refired it to a tack fuse schedule: 1 at 300 to 1150, hold 30. 2 at 400 to 1370 hold 0, 3 at 9x to 950 hold 60 and 4 at 150 to 800, hold 10 then off.

I took it out and again could see less but still some fissures. What can I try now to save this piece? I am really happy with it. it measures about 8" across, sort of round and about 6mm thick.

Thanks!
Ginni

[Brad Walker]

Got a photo?

Sounds like incompatibility, but a photo would help say for sure. If it is incompatibility the cracks will get worse with time and no amount of re-firing will help.

[ginnibobinni]

please try this: https://goo.gl/photos/Q1k7pw2MgT1CTXkE8

I am not sure you can access that. It's on Google Photos...

I used all 90COE Bullseye...

[Brad Walker]

I used all 90COE Bullseye...

Well, no you didn't. You also used bubble powder, which could be made from a variety of non 90 COE components.

From the photo (which I could easily access) it's not clear to my eyes whether the cracking is from incompatibility, from the action of the bubble powder, or something else. Lots of things can happen at 1700F.

Be interested if anyone else has any thoughts to add.

[Kevin Midgley]

The bubble powder was not Bullseye.
Going to 1700 would imho be exceeding where you want to be with Bullseye glass.
The 1700 would have been air temperature read by thermocouple inside the kiln.
the glass would have been a heat sink on the way down and would keep the glass at an elevated temperature for much longer than you think.
Given the anneal temperature was once again air temperature and only for a short period of time not sufficient to dissipate all the elevated residual heat, I'd suggest a lack of annealing.
So you have incompatible material in the bubble powder perhaps used in excess, a high temperature above that needed to fuse Bullseye glass which may have changed the COE's and a lack of annealing.
Did you take into account the thickness and thermodynamic properties of your kiln shelf in determining your short annealing schedule? Obviously not.
You did not specify if the Bullseye glass had been fired before and had thus already experienced heat work.
Simply put, ditch the bubble powder, fire to lower temperatures, use virgin Bullseye glass and anneal longer.
Might want to go and read Graham Stone's firing schedules for glass cover to cover a couple more times, not for the schedules but for the other information on all the pages.
The glass has told you there are limits to what it can take.

[ginnibobinni]

Thanks for the lesson. I am a self-taught hobbyist and did not know of this book. I'll pick up a copy. Guess I'll go on to learn some more lessons. I really like the effect of the bubble powder, so hopefully someday I'll be worthy of working with it.

[Kevin Midgley]

Sometimes it is necessary to be served lessons as a reminder that the glass we work with is special and deserves special treatment and respect. I know I still get served up reminders every once in a while as does everyone who works with the material. Observe and take notes. If your notes are accurate from this disaster firing regarding what you did, you have a starting point from which to make changes in the future. Just like drawing if you erase I line that you don't like, chances are you are going to repeat the same error but if you had left the line you could see where the new line should go.
Stone will/should give you an understanding of glass much greater than found in any other books. For a period of time he took his book off the market as people did not wish to read it and thus understand the firing process.
He did not want to be blamed for the glass problems people were having by just using his schedules instead of understanding what was going on.
If you want to play with bubble powder, you have to understand what is going on.
Take those notes!

[JestersBaubles]

GIven the variation in thickness of the glass caused by the bubbles, IMO, 60 minutes is likely not a long enough anneal and you need better temperature control on the way down. You may have better success by slowing your drop to the annealing zone, increasing the duration of the anneal, and then controlling the temp down to 700 (at which point, it might be OK to turn off). For instance, drop AFAP from the top temp to 1100-ish, and then ramp down slowly (e.g., 50 deg/hr) to 950. Lengthen the anneal to 2 hours. Then, drop 50/hr to 700.

I had a piece (which was a commission, of course), that cracked during the slump at the exact point of a bubble. I had used bubble paints, but it's still a similar outcome. I rarely have ANYTHING crack, because I use very conservative schedules. I've probably jinxed myself for 2017 (though I did have a piece crack just the other day -- too many factors going on and I pushed the ramp rate. Bad me!)

Dana W.