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For the last couple of months I've been concentrating on larger pieces (that is as large as my 18" kiln allows). Mostly pot drops. I've been working with WG now for over 3 years and really felt pretty confident as I embarked on some of these projects. My experiences have been less than satisfying/successful and I really need some advice.

I'm posting the last 2 pot drops I've completed.

http://photos.yahoo.com/glassgnut62


The orange one "mind over magma" is approximately 15" and about 1/2" thick in the middle. This piece broke 3 times. Once after each firing. Out of TOTAL frustration I called Spectrum as I use 96. Never had any problems before. I was told I wasn't annealing long enough. They suggested 6 hours. The last time I fired it I ramped 75 dph up to 1100, then 1200 dph to 1450 held for 3 hours, full power down to 960 held for 15 hours then 50 dph down to 75. As you can see from the photo, it obviously broke on the way up and partially healed itself.

The other one I just took out this morning after almost 2 days of firing. This one is 13" and about 1/4" thick. I ramped up full power to 1675 (pot drop with small pieces) and held for 2 1/2 hours. Full power to 960 for 8 hours then 75 dph to 750 held for 2 hours then 50 dph to 75. When I left the studio yesterday the temp was at 149 and no crack. Then I come back to this.

Out of the last 5 projects I've done that have been at least 12" square or larger 4 have cracked/broken at least once and some more. I called Marty at CD and he said it probably was my ceramic shelf causing shock. So I bought Duraboard. I used Duraboard as my shelf for the last firing on both pieces shown. No difference. They still broke. The only thing I can think is my controller isn't maintaining the temps it's displaying. OR Sysytem 96 sucks for larger projects. Please help! I'm getting impatient and worried about everything I do!

Christyn

Christyn,
Boy I know how frustrating it can be when something you have done in the pass is no longer working for you. I would try and fire something simple to see if your controller is working right. Also, when your temperature was at 149 did you peek or do you have a window. One thing you have to remember is your glass is always hotter than the temperature reading in the kiln coming down. Good Luck .......Claudia

Are these the same color combos you've done smaller pieces with? Just because they're tested fuse compatible doesn't necessarily mean large pieces like this will marry well in a melt. Some colors are more persnickity than others. Your firing schedule seems so conservative, it's hard to imagine annealing is an issue. I'd be interested to learn what finer minds than mine have to say.

Those look like annealing cracks to me, even though your schedule is conservative. The orange one may have cracked on the way up because it wasn't annealed properly in the previous firing.

Are you baffling the glass? It may be a situation of the pot shading the pour from your lid elements so the glass is getting much more IR from the wall elements on its edges, and therefore staying hotter around the edge while it cools in the center. Annealing is complete by the time you get down to 750, so you can speed up from there. If you went 75 dph from 960 to 750, you're not gaining anything by going 50 dph the rest of the way down.

Do you have a polarimeter? If so you should do a pour, using the same schedule and amount of glass, with transparent glass. That way you can look at it for stress around the edge.

There's no kilnwash stuck to the back is there? Sometimes that means that the glass has swallowed enough kilnwash to introduce incompatibilities.

ch