Problems with a Stainless Drape Mold cracking at corner

Things formed in the kiln.

Problems with a Stainless Drape Mold cracking at corner

Postby samanthahookway » Sat Apr 14, 2012 12:09 pm

Hi!
I am seeking advice on a stainless slump over form. It worked for about 4 runs and now for the past 3 runs the glass is cracking on the corner (the bottom corner of the glass vase or the top corner of the stainless steel form). I have tried slowing down my program on the way up and the way down. I have tried to double my Boron Nitrate Spray. Any ideas what is going?
I am using a stainless steel mold that has like a deflated square shape at the top (so the corners are very sharp). The glass is about 10 x 10 in and only 2 layers thick.

My last program was
Rate 250 temp 600 Hold 10min
Rate 150 temp 960 Hold 10 min
Rate 300 temp 1290 Hold 10min
Rate 9999 temp960 hold 1.5hr
Rate 80 temp 800 hold 1 hr
Rate 150 temp 300
off

Any advise would be much appreciated!
Thanks So Much,
Samantha
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Re: Problems with a Stainless Drape Mold cracking at corner

Postby Bill Brachhold » Sat Apr 14, 2012 1:07 pm

Hi Samantha !!

Welcome to Art Glass Answers.

One of the big things we have to deal with, as glass artists, is the COE of the glass -AND- anything else introduced into the 'equation'. In this case, your mold becomes a component of the COE issue.

Stainless steel has a much higher COE than glass, usually in the 180 range. This means it expands about twice as much as say, Bullseye or Spectrum, -and more importantly- shrinks twice as much when the mold and glass cool.

In general, if the glass is OUTSIDE a stainless steel mold, everything will be OK. Think a piece of glass slumped OVER the bottom of a stainless cocktail shaker. By over the bottom, I mean the cocktail shaker upside down on the shelf. I think its pretty easy to visualize that as the glass and mold cool, that the mold will shrink *more* than the glass, causing the glass to release from the mold.

Any glass slumped INSIDE a stainless steel mold, will be a big problem, because as the whole mess cools down in the kiln, the stainless steel mold shrinks MORE than the glass, getting what I call a 'death grip' on the glass. If your mold is really, really shallow, then as it cools, the higher shrinking mold *may* push the glass up and release it. There is not a concrete, scientific answer as to what the shallow angle can be, because there are many factors that affect the ability of the glass to release from the mold, and the mold push it loose.

All that being said, I would ask you is this stainless steel mold *intended* to be a glass mold, or is it a *found object* that you'd like to use as a mold ?? If it is intended to be a mold by the manufacturer or seller of this mold, and you are using it as said manufacturer/seller suggests, then you might want to contact them. Additionally, I've had *much* better success using kiln wash on molds, instead of boron nitride. It's a lot cheaper too; I use the Bullseye pink shelf wash.

Why not post two pictures of the mold; one of just the mold, and the other with the glass on the mold. Also, is the glass cracking on the way up, temp-wise, or on the way down ??

Bill

PS - this looks OK, except it is overly conservative.

Rate 250 temp 600 Hold 10min
Rate 150 temp 960 Hold 10 min
Rate 300 temp 1290 Hold 10min
Rate 9999 temp960 hold 1.5hr
Rate 80 temp 800 hold 1 hr
Rate 150 temp 300

I would change the above to:

Rate 9999 to temp 800 hold 15 minutes
Rate 600 to 1290 hold as long as you need to get desired slump
Rate 9999 to temp 960 hold 45 minutes (I vent the kiln to try to do this in 10-15 minutes)
Rate 80 to temp 825, hold 5 minutes (I usually use 120DPH, ie 2 degrees per minute, but slower won't hurt)
Turn kiln off and let cool to room temp
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Re: Problems with a Stainless Drape Mold cracking at corner

Postby samanthahookway » Sat Apr 14, 2012 6:07 pm

Hi! Thanks!
Yes the mold is made to be a commercial slump mold and yes to is a drape over not an into mold. Using System 96 glass.

I believe it is cracking on the back side of the program. The first time I had the problem though it definitely was on the way up (as the piece broke off and then slumped to the shape of the mold/and shelf). Since then the cracks have only been on the corner of the glass at the sharpest bend and I believe it to be on the way down. It looks and seems to be that way.

Image
Image
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Re: Problems with a Stainless Drape Mold cracking at corner

Postby Bill Brachhold » Sat Apr 14, 2012 7:04 pm

I think you are saying it is cracking at the sharp bend at the top of the mold, which becomes the bottom of the slumped object.

And, I think that that is being caused by the flat bottom on the mold. I cannot see the whole mold, only the side of it ?? I would try kiln wash, instead of the spray, and see if that makes a difference. It is *possible* that a tiny bit of the glass is sticking to the mold, and then when the mold shrinks more than the glass during cooling, it "pulls" on the glass and cracks it. You might just want to paint a little bit of kiln wash around the bottom edge, and do a test run with some scrap glass. make sure your test glass though, is the same thickness as what your normal glass would be. If nothing else, fuse two pieces of the same glass, to get to your usual thickness.

I've done many slumps over a cocktail shaker, but the ones I have, have a more rounded corner/edge on them.
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Re: Problems with a Stainless Drape Mold cracking at corner

Postby samanthahookway » Sat Apr 14, 2012 10:19 pm

Yes! I will try the kiln wash. It is a very sharp turn and a very sharp corner of the mold. Sorry the picture got cut off- somehow it got squared up in the process of posting.

Thanks first thing I will try is the kiln wash!
Samantha
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Re: Problems with a Stainless Drape Mold cracking at corner

Postby Bill Brachhold » Sun Apr 15, 2012 10:48 am

Hi Samantha,

Please let us know the results of your experiment !! I know that when I used boron nitride, I had problems with it releasing as easily as kiln wash. I know that you get a much better "finish" with BN, but if a lot of stuff breaks, whats the point ??

Also, there is a powder you can sprinkle on top of shelf wash, to keep glass from sticking. Maybe that stuff sprinkled on top of your mold, might help. I need to research it to get you a brand name.

Bill
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