deep bowl cracking and splitting when slumped

[pcarbaugh]

I have previously slumped several bowls using a metal sink mold. The last two bowls have cracked and split apart at the bottom of the bowl. Nothing has been changed in the firing schedule. I have moved from Montana to New Mexico (3000' to 6000' elevation) I made three bowls since moving and they all turned out fine. The only change is the environmental temps are higher this time of year. All flat pieces have fired well, even those that were fired twice. Pieces slumped into shallow molds have been fine as well.

I have a two year old Skutt kiln GM1414 (oval)

Any suggestions. I am on a time crunch to make several more bowls for an upcoming show.

Thanks

[Mary Kay Nitchie]

We may be able to provide advice if you can provide your slumping cycle and describe how the mold is arranged in the kiln, and other details. Is the mold set on posts, or on a kiln shelf?

Mary Kay

[charlie]

this is almost always a too rapid heat up. slow down the ramp up a lot (no more than 100f/hr). the deeper the bowl, the more this is a problem.

[pcarbaugh]

The kiln shelves are in three parts. i take out the middle section and place the metal mold on a small metal ring (that came with the mold) directly on the kiln floor. So it is floor, ring then mold. The firing schedule is in 8 segments:
Rate 300'F to 250'F hold 25min; 300'F to 500'F hold 25 min; 300'F to 750'F hold 25 min; 600'F to1150'F hold 10 min; 600'F to1225'F hold 10 min; AFAC to 1000'F; 30'F to 970 hold 2:00; 60'F to 750'F hold 10 min.

Again nothing was changed from the 8 previously successful slumps.

Thanks for the help.

[Twin Vision Glass]

In my humble opinion ! tooo fast. I have had much better success with 100 deg. per hour lately.(with almost 100 percent success!) Also on the kiln floor with side fire can be abit of situation because as the glass slumps , the bottom of the kiln can be cooler. Just a thought though.
Leslie

[Twin Vision Glass]

P.S. Not sure why you are holding at the different temps. Just 100 deg. per hour up to 1220 F. Hold 20 minutes , and then start downward schedule. (oh yes, and I anneal at 900 F. now. )
Les

[pcarbaugh]

OK.....but why have the other bowl slumps turned out perfectly?? it makes sense that the bottom is cooling faster with these two but why the change? The kiln is only 13 inches deep so if I raise the mold wouldn't the top get too hot?

[Twin Vision Glass]

Oh sorry, I did not know the kiln was only 13 inchs deep. You could try a baffle tooo so that the sides are not heating up tooo fast. Are you using different colours. What is the thickness. Hope we can help you out. (and why did the first ones work, maybe check under polorized film just to make sure. What did the break look like. Did it break on the way up or down. Did you Peek!!!! (I am sure you did not!!! but more info is always super for the investigators!!!! (or helpers!!! tee Hee!!!)
les

[Twin Vision Glass]

P.S. try doing an anneal test tooo and see if anything got jingled while you where moving. You just never know. It is always good to do a test for different levels in the kiln tooo. I truly would slow down though as It can always be good to be cautious as it takes so much work to create your piece. Alittle while longer to slump is a good thing to do.

[pcarbaugh]

The crack is at the very bottom. A split about 6 inches long and opening up to 3/4-1 inch at the widest point. The piece must be slumping almost to the bottom and then maybe it cools down too fast at the bottom then splits? Or splits just before reaching the bottom? Is there away to attach a photo to the forum? I took pictures just as it was found when I opened the kiln. Like you suggested, an anneal test is in order because nothing else was changed since the successful firings. The confusing thing is that everyhing else I am firing is coming out just fine. Some one suggested that I change the relays? I am new at all this and don't even know how often they should be changed or how to do it.