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I am slumping a 9 mm piece of glass 51 inches by 24 inches over a stainless steel mold. it cracked at the annealing stage. it formed around the mold well but cracked in several places.
The temperatures I used were 150 degrees to 300 and hold for 15 min.
250 degrees to 800 no hold, 150 degrees to 1100 no hold, 200 degrees to 1200 hold 5 minutes, full to 900 degrees- hold for 5 hrs. 100 degrees to 700 degrees to 70 degrees
I suspect the annealing temperatures were not correct. What do you think?

Carol,
I don't think your annealing temps are bad. It all depends on the shape of the mold. If your mold has 2 sharp angles as opposed to gentle slopes (such as strait up, strait across, strait down) the glass would be prevented from shrinking as it cools. It must be able to shrink or it will break near the angles. Are you sure it broke post shaping? Was the mold coated with kiln wash? Have a picture?

Warren

What a bummer! I am always in favour of moving thru the anneal zone slower, eg: instead of your AFAP (full) to 900F.... I drop AFAP (full) to 1000F, then 80F/hr to 900, then hold for anneal (5hrs seems excessive for 10mm) then 60F/hr to 700F, then 100F/hr to 500, then 125F/hr to 250F, then off and do not open kiln till 80F. That would be my suggested program. The SS is suppose to shrink more than the glass so as Warren stated the shape of the mold has to be considered. I hope you can flatted; refuse; then reslump.

The mold has no sharp angles. Image jpeg is attached.. I used MR 97 mold release which I have used before with this mold. I have slumped several other pieces of 6 mm that turned out fine. Thank you for your help.
Carol Ann

Don,
Thank you I will try your schedule. I will be able to flatten and refire.

I have been using stainless steel molds for quite a few years now and after quite a bit of experimentation in the beginning, I now only slump into the molds. I did all sorts of bends for china cabinets to windows for buildings and found bending into a curved stainless mold much preferable than bending over a stainless mold. When I tried bending over a simple curved mold, I found that gravity worked against me as far as getting the final edges to conform to the mold. There was a decreasing amount of weight pushing those last edges down. When you fire into a mold, you have the entire weight of the glass pushing down until the end when it has reached the bottom. You also don't have to worry about the area where the glass is resting on the mold heating up too fast and cracking.

Years ago I had trouble slumping over arc shaped molds. The glass was resting on the mold in a way that there was only a fine line where the glass rested. I would get fractures on the centerline probably because the glass was a little cooler on that line. The outside (wings) started to bend before the centerline was hot enough.
I read that the cure was to suspend the glass on four corners with kiln posts, keeping the glass slightly above the slump mold so that the glass was hot and moving when it first touched the mold. Because the kiln posts were far out, only an 1/8" or less of the glass rested on the posts. Once the glass moved down it fell off the posts or the posts fell over. You can remove the posts using tongs once the slump is well underway. With the full arc you are making you may have to press the glass wings down using wet wood or a stainless spatula and Kevlar gloves. Never had a fracture after using that procedure.

Warren

Thank you Warren and Terry I will take your advice into consideration. I have used this mold several times in the past without any problems but always with 6mm instead of 9mm. Could that make a difference?

If you slump into the mold instead of over the mold, the extra thickness results in extra weight and only hastens the slump. At least in my experience that is the way it works. I have found 3/8" float to bend faster than 1/8" for this reason.

Glass does not crack when hotter than the strain point. So, it either cracked on the way up or the way down. Most likely, it was on the way up, from thermal shock. The glass would then slump and the cracks would widen. If it cracked on the way down, it would just be cracked, and the shape would conform with the mold, other than the amount it changed as the stresses released.

Glass balanced atop a steel mold is pretty prone to thermal shock. The mold gets hot in the place it contacts the glass.

Terry is right on. Gravity is your friend. When draping over, the weight is supported in the middle. When slumping in, all the weight is pushing down. And yes thicker glasses are easier to get to move. Companies that make curved glass for cabinets always slump in, and often they use a dummy piece of glass coated with talc, and the finish piece sits on top of it. The bottom piece will clam up on the edges, as it slips down the mold.

I did quite a bit of draping over a cylinder mold made of Zircar RS 100. I didn't have trouble with thermal shock. My problem was setting the glass perfectly on the mold. But I did have to fire pretty hot to get the glass to conform to the mold.

Thank you very much for all of your input. It was all very valuable. I appreciate your expertise. According to my assessment the piece did slump fine and the cracks were clean and fresh. I heard the noise in the kiln after the slump cycle. Would you have any advice on changing the temperatures on the cool down? Is the annealing cycle too long?
Anything else you could offer would be appreciated. I am refiring the piece flat to begin again.

Carol- Why are you making yourself crazy with that elaborate (and nonsensical) schedule? 100 to 150dph up to 1200F, hold as long as necessary (look inside) and anneal.
Your annealing looks ok but I'd go slower from 900 to 700. If your kiln cools to room temp fast, you'll want to control that range as well.
Try insulating the mold from the glass with fiber paper (not thinfire)- I use about ⅛" thickness. That was a lesson learned from Doug Randall when my 12mm thick pieces were cracking on my ⅛"thick steel molds.

I'd love to know where that schedule came from.

Here is how I might anneal a 9mm piece of Bullseye or Spectrum.
AFAP > 920 hold 2 hours
920 > 820 take 2 hrs (50 dph)
820 > 620 take 1.5 hours (133 dph)
620 > 220 take 1.5 hr (266 dph)

For float, I add 80ºF

Bullseye suggests 2hrs anneal hold for 10mm.

I certainly appreciate everyone's input and help. It has certainly been most valuable to me. I plan to design more of these large pieces so this has been helpful.
Thank you for your time and energy. Carol Ann