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I am trying to learn how to drape over a SS bowl with a single 1/8 in glass 9 in in diameter. I am using a paragon 7 kiln. program is as follows
200 1150 10
200 1220 0
400 1240 0
400 1270 20
full 950 90
200 700 10
full 100 0
The circle sheet broke in half about 1000, the second time I tried it, it broke around 1100. There was a small curve in the second try. I am kinda thinking the glass is too close to the top heaters. I have 1 inch posts and a 1/2 inch shelf.
Sure would like some help. Thanks in advance.

My standard temps & hold times for drapes are:

Ramp to 1100, hold 30
Ramp to 1225, hold 10 (or watch and hold until "done")

It may just be that you need more time at 1100 for the temp to even out. 1270 seems a little high... at least in my kiln.

Dana. W

I will also point out that if you ramp at 400 to 1240 and don't have a hold, and then ramp at 400 to 1270, that ramp to 1240 is superfluous!

Dana W.

If your glass has broken on the way up, the rate of advance was too fast. Slow it down

With a single sheet of glass, other than the multiple ramps which aren't necessary, this should be a simple slump. Are you getting the bowl kiln washed enough? How do you know it broke at 1000 and 1100? Are you looking? Or did you hear it? If you are looking and you have such a small kiln, it could be that you are thermal shocking it from the room air coming in.

Mike

JestersBaubles wrote:My standard temps & hold times for drapes are:

Ramp to 1100, hold 30
Ramp to 1225, hold 10 (or watch and hold until "done")

It may just be that you need more time at 1100 for the temp to even out. 1270 seems a little high... at least in my kiln.

Dana. W

Missing some significant data here: Rate of ramp is probably more important than the end temperature. Hanging out longer at 1100 isn't as good as keeping the glass at thermal equilibrium all the way up, at least not if she's trying to stop cracking--it's about like running THIS light but stopping twice at the next one.

Unless there's a reason to change the schedule at 1100 (say, you need the glass to relax very slowly to the slumping temp, so you drop temp, or you think the glass has enough heatwork and you want to speed up for some reason), do the ramp-up in one slower segment.

But that's more than likely not the problem here--she's giving plenty of clues. It's a Paragon 7 kiln--6.5 inches deep. She's got a 9-inch piece of glass she's trying to drape, so the mold is probably 3-4 inches high. That means she's got clearance, at most, of about 3 inches, probably less. She's got 1-inch posts with a half-inch shelf on top, so subtract 1.5 inches from that. Her glass is probably within an inch or so of the elements, and the center is sitting on stainless steel while the edges are hanging out in space.

First thing I'd advise is to get rid of the kiln posts and shelf. It's a slump--you don't need the shelf. Just make sure you've got kilnwash on the floor (NOT THE SIDES) of the kiln.

That should give you a little more distance from the elements. Then go slowly--drop to 200dph all the way to your slumping temperature, and when the kiln passes 1175F, start watching. You'll have added significantly more heatwork to the glass, so it should start slumping sooner.

If it breaks again, try two things: First, see if slowing down to 100 or 150dph helps. And/or you might try propping up the edges of the glass with kiln posts. (kilnwash where they contact the glass). The edges may be coming down before the center is ready and encouraging a crack down the middle. If you can prop the edges up in maybe 3-4 places, so just the barest edge of glass is on the post, the glass will have more time to get soft and should slide off the post naturally when it's ready.

Cynthia,
Sure wish there was a "like" button. I always learn so much reading your replies.

I forgot to state that I had put a 1/32 fiber sheet on top of the SS because I thought the SS was heating up faster than the glass and causing a hotspot where the glass touched the SS. Thanks to all for the help. I will take out the kiln posts and shelves and ramp at 150 up to 1250, hold 30(when slump is good I will skip to anneal) and start watching at 1100. Thanks so much Bill Harrell

With all of the good advice I received, I have made 2 bowls by drapeing. The glass is no longer breaking. I do have 2 small problems, the SS pot I am using causes black splotches on the glass and I can't remove them. For the second bowl I used a sheet of kiln shelf paper and reduced the SS to one small splotch. The third bowl is firing right now. I used a double thickness of shelf paper so I am hoping to eliminate the SS splotches. The second problem is that the edge of the bowl does not conform to the SS bowl, it gets within 1/4 inch of the bowl and stays out leaving a curl in the edge. This occurs down past the curve of the bowl, so I have reduced the diameter of the glass by 1/2 inch to keep the glass on the curved part of the pot. Any suggestions will be helpful. (I did sand and kiln wash the bowl all three times, I spray on kiln wash at about 500 degrees and get a good coat.) Thanks again Bill Harrell

[quote="wharrel"]I forgot to state that I had put a 1/32 fiber sheet on top of the SS because I thought the SS was heating up faster than the glass and causing a hotspot where the glass touched the SS. Thanks to all for the help. I will take out the kiln posts and shelves and ramp at 150 up to 1250, hold 30(when slump is good I will skip to anneal) and start watching at 1100. Thanks so much Bill Harrell[/quote]

the steel acts like a heat sink more probably, causing a cool spot in the middle. it's shadowed from the top elements, so is heating slower than the glass rate.

Thanks to all for advice. I now have 2 nice bowls, green iridescent and most of the iridescent is on the inside of the bowls and is very pretty. The SS mold seems to have stopped turning black so I am trying a third bowl with no shelf paper. Thanks again-Bill Harrell

And my apologies, BILL, for assuming you were a "she." Sexist thing to do.

Morganica, no problem, there is no way to determine gender from a name. Thanks for all of the help. Bill Harrell