Plate Cracked While Slumping

[Nancy Juhasz]

My plate cracked during the slumping phase.

I know this has been asked a number of times before but I thought I flowed a good firing schedule. I am giving you the glass used and the firing schedules.
Glass was Spectrum 96 Opal
12 5/8 inch diameter base circle
crescent measuring 2.75 wide at center and 10 5/8 inches tall
oval shaped flower petals and a round center.
The crescent was laid on the base circle to the edge and the flower was laid on the rest of the base glass.

Fusing Firing Schedule: this schedule was according to what Spectrum suggested
500 dph to 1000 no hold
AFAP to 1440 hold 10 minutes
AFAP to 1000 hold 10 minutes ( I crack the kiln lid about 4-5 inches to
quick cool)
300 dph to 955 hold 20 minutes
200 dph to 800 no hold
120 dph to 400 no hold (the 120 was a mistake I misread the chart
it should have been 400 dph)
cracked kiln lid to room temp.

All looked great at this piont. I was very pleased.

Put in plate mold that was kiln washed. The blank was just a tiny bit smaller than the mold. You could see mold all the way around. When I went to see how the slumping was coming there was a big crack about 1/2 inch wide to 1/4 in wide from high on the crescent through one of the flower petals on through the base to the edge about 1/3 of the plate was cracked off. This peek was was at 1225 right at the beginning of the hold for the slump. Imagine my suprise. Here is the slumping schedule which again was the one suggested by Spectrum. So much for trusting Spectrum firing schedules.
400 dph to 1000 no hold
1200 dph to 1225 hold untiled desired slump in my case at 1 or 2 minutes
AFAP to 1000 hold 10 minute
300 dph to 955 hold 20 minute
200 dph to 800 no hold
400 dph to 400
would have cracked lid of kiln to cool.

Where did I go wrong? Is there anything else with the schedule in the part I never got to? I don't know exactly when the crack occured since I am not a peeker until slumping on slumped things or until quick cool on a fuse. I am thinking perhaps since this had been fired once already perhaps I should have gone up a little slower maybe 300 dph to 600 hold 5 minutes then 500 dph to 1000 hold 5 minutes. Do you think it had anything to do with the fact that where there wasn't design the plate was only 1/8 in thick? Should I next time make the base 2 layers of glass? I am really baffeled. Thank you ahead of time for any suggestions. I thought I was getting a pretty good handle on this fusing and slumping thing. Nanc

[Jerry]

Some time back Spectrum suggested a firing sked that was sooooo far out of line that I may have been the only one dumb enough to follow it. Since then they've cleaned stuff up and usually give pretty good advice. Your sked seems a little fast but not enough to have caused the crack you described. But it does sound like a thermal fracture to me.

Typically these things occur in the first 500 degrees. Many slumpers go slowly to 500, hold for maybe 30 minutes and then go to 1000 with another hold. Yor piece at twelve inches is just large enough to get in trouble depending on the number of layers.

All I can say is to try something similar and slow down at the lower temps. Otherwise, call a Hawiian Kahuna to exorcise the Menehunie you've obviously got in your kiln. He visits me pretty regularly too.

Jerry

[Cliff Swanson]

Nanc,

The trick is to have the entire mass of glass heat up as uniformly as possible. You didn't mention what kind of kiln you are using, but if your kiln has side elements, only with no elements in the top/lid, the perimeter of your base and cresent may have heated at a different rate than the center, and the resulting stress could cause a crack. Even if your kiln does have top/lid elements, I think a piece that's 12+ inches in diameter and somewhat asymetrical with regard to the placement of other design elements probably should be heated more slowly than your schedule. I've fired numberous pieces of Spectrum glass, anywhere from 8" sqaures to 15" diameter circles, at 400 dph to 1000 using a Jen-Ken 24"x11" kiln with top and side elements, and none have cracked... at least not yet. Maybe just back off the 500 dph ramp to 1000 a bit.

Cliff

[Strega]

I had the exact same thing happen to an otherwise beautiful bowl about the same size. You can sometimes figure out where the crack happened by whether the edges are round or sharp---Brad's chapter on "trouble shooting" talks about it. But I am certain mine broke because the glass was too thin, and that is probably true for you as well. Broke my heart! You really need two layers, especially over 12 inches of surface. One solution for next time to make it thicker would be a layer of clear underneath it all. Clear opal is nice too...
If you want to try and reuse the piece, you could fuse it to clear at a high temp and reslump--the cracks would remain, imbedded. You could name it 'broken blossom"....

[Dani]

And your cool-down might have been a bit fast.... I haven't had a plate crack sinced I stopped venting entirely.... just use overspray to prevent devit, let things cool down naturally overnight. No more probs... wish the rest of life was that simple and easy.

[Jerry Barnett]

Do you think it had anything to do with the fact that where there wasn't design the plate was only 1/8 in thick?

Absolutely.

Should I next time make the base 2 layers of glass?

That will help, and use their schedule for 3/8" thick.

The alternative is to ramp at no faster than 120 to 1000 for the slump. A 90 ramp rate would be safer.

Also consider slowing the annealing range ramp for the initial fusing from 200 to 150.

Jerry

[Nancy Juhasz]

Thanks for all the advice. In the future I am going to do what I think is right and not listen to Spectrum's firing schedule. I certainly will slow way down on the way up for the slump. I will come down a little slower and not pop the top until a little cooler. I might even add that second layer of base glass. I had an instructor that ramped up fast at around 800 dph and poped the lid at about 500 on the way down. I have had luck with that on smaller things but certainly was afraid to try the fast a ramp on larger things.

Cliff you asked what kind of kiln. It is a Skutt 17 diameter and 13 deep. Top and side elements. The piece was about 8 inches from the top element.

Nanc

[Jerry]

Nanc
Some time back Spectrum suggested a firing schedule for a 12" 3 or 4 layer platter where the initial heat up from Room Temp to 1000 was 900 degrees per hour. Well, I followed their recommendations with a three layer piece and I heard the explosion in the next room!

The bottom of the three layers exploded in a "Pie" shape. They actually moved over 3" away from the center. It wasn't all that ugly, but certainly not what I wanted.

All the advice you've gotten is on the mark. Glass likes to be heated uniformly regardless of top or side elements. Side elements? Well, slow down; that's not too complicated.

I think you've reached a good conclusion about your up ramp schedule so take a shot at it and let us know how it goes.

Good Luck,
Jerry

[charlie]

Nanc
Some time back Spectrum suggested a firing schedule for a 12" 3 or 4 layer platter where the initial heat up from Room Temp to 1000 was 900 degrees per hour. Well, I followed their recommendations with a three layer piece and I heard the explosion in the next room!

The bottom of the three layers exploded in a "Pie" shape. They actually moved over 3" away from the center. It wasn't all that ugly, but certainly not what I wanted.

All the advice you've gotten is on the mark. Glass likes to be heated uniformly regardless of top or side elements. Side elements? Well, slow down; that's not too complicated.

I think you've reached a good conclusion about your up ramp schedule so take a shot at it and let us know how it goes.

Good Luck,
Jerry

this also happed to me just yesterday. what i haven't seen mentioned is that the glass traps cooler air underneath it inside the mold. i ramped at 500 dph and it shattered before it hit 400. slowing down to 250 dph solved it.

[lyndasglass]

Nanc,
Let me add one more suggestion to the mix.
I make 12" single layer pieces with multiple layer design elements stacked on top.
In some places my glass may be 4-5 layers thick and in others only 1 layer. In Graham Stones book on kiln schedules, he talks about a "catch-up" firing. Which is a slow heat up(200 degrees/hour) to 400 degrees and then soak for 10-20 minutes and then a little fast (say 250degrees/hour) up to 700 degrees. This give the glass time to evenly heat (remember your thermo coupler is only reading the air temperature in the kiln and not the actual temperature of your glass). Since I have switched to this type of schedule I haven't had any problems with my glass breaking when I reheat it for slumping.