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Disaster! Is this a kiln problem?

Posted: Wed Jun 01, 2016 4:12 pm
by Mark Wright
I hope someone has seen this before and can tell me where to look for the problem.

Two bubble-up firings. The first is fine and was slumped into a plate. Next was 4 smaller pieces on the same schedule. All have multiple cracks. The cracked one in the lower right is even the same glass. What happened?
Everything was the same except the size of the glass (7” vs 4 ½”) a different kiln shelf and I noticed the total time in the kiln was 8:02 vs 8:20 for the ones that cracked. Cracks are sharp so it happened on the way down, and some places are really stuck to the kiln shelf. Thermocouple bad? Controller bad?
Schedule:
400/hr to 1100 hold 10 min
AFAP to 1700 hold 45 min
AFAP to 925 hold 60 min
100/hr to 750 off

No problem here
1st plate.jpg


Disaster
4 in kiln cracked.jpg


Spread out
4 spread out.jpg

Re: Disaster! Is this a kiln problem?

Posted: Wed Jun 01, 2016 4:24 pm
by Brad Walker
Probably not enough kiln wash. Most likely, sticking in some places.

Re: Disaster! Is this a kiln problem?

Posted: Thu Jun 02, 2016 6:20 am
by Tony Smith
I'm with Brad. Especially with a new kiln shelf, if the kiln wash is too thin (or nonexistent), the glass sticks and tears itself apart as the kiln cools. Are there bits of kiln shelf stuck to the bottom of the glass?

Tony

Re: Disaster! Is this a kiln problem?

Posted: Thu Jun 02, 2016 8:35 am
by Mark Wright
More kiln wash seems like an easy fix. My notes say I used 8 coats on both firings, but I have been known to be senile. I will try again and see what happens.

Re: Disaster! Is this a kiln problem?

Posted: Fri Jun 03, 2016 5:43 pm
by Valerie Adams
Why are you firing to 1700°?

Re: Disaster! Is this a kiln problem?

Posted: Sat Jun 04, 2016 7:23 am
by Tony Smith
Good point.

Why are you firing so hot? You're in the domain of combing temperatures... well beyond where kiln wash breaks down.

Tony

Re: Disaster! Is this a kiln problem?

Posted: Sat Jun 04, 2016 7:56 am
by Brad Walker
This is a high temperature firing. Like combing, you need to fire hot to get the particular effect you're after.

Re: Disaster! Is this a kiln problem?

Posted: Sat Jun 04, 2016 11:59 am
by Mark Wright
The experts were right. Not enough kiln wash. My notes say 8 coats of kiln wash, but my guess is I was distracted with something and picked up the wrong shelf. Thanks for the help. This BB may be one of the best "tools" I have.
As Brad mentioned it takes 1700 to do a Bubble Up. Brad originated this at Warmglass and teaches classes for it. A great class and I highly recommend it. Lots of fun.
Mark

The latest with 8+ coats of kiln wash. No cracks!
Bubble Up retry.jpg