The Cracked Plate revisited

Nancy Juhasz · Post by **Nancy Juhasz** » Thu May 29, 2003 12:08 am

The end of last week I asked your advice on why my plate cracked after it was fused in the slumping firs. I took all of the advice and composed it into a better firing schedule. Fused a piece of clear to the back of the plate to hold the two pieces together. Fused it again. Success as in the fusing fire before. I went up and down really slow and in and out of the higher temp as quickly as I could. I slumped the plate again. On the way up no crack, flashed the kiln closed it until cool. When I took the darn thing out it had another crack 90 degree angle to the original crack. Now I went up very slowly and down very slowly. 10 5/8 inch plate base, decoration layer on top of white base, and clear under the white base. I used 3/8 in firing schedule slowed down some. My kiln is a top loading Skutt with top and side coils. The plate was about 8 inches from the top coil. I am thinking that the firing schedules are not the problem but the plate mold. This is the first time I have used this mold. It is a very gentle slope. It is bisque and had 4 holes. I am thinking that the holes are in the wrong places and need to be closer to the center of the mold. Does this make sense? THe 4 holes would form about a 6 inch diameter circle. I am thinking there should be another set closer to the center and 1 in the center. Also I am thinking I should place this mold up on kiln furniture so air can flow better. Any thought are more than welcome

Lisa Allen · Post by **Lisa Allen** » Thu May 29, 2003 12:45 am

I would think that a lack of holes in the mold would be more likely to cause a bubble rather than a crack.........

What are you considering very slow? What does your schedule look like from 960 down? Need more info.

Lisa

Nancy Juhasz · Post by **Nancy Juhasz** » Thu May 29, 2003 1:10 am

Lisa my schedule for the down after 960 was 100/955/40, 100/800/0, 200/400/0, off but my kiln goes down very slowly. I have never cracked a piece of glass in the kiln until this one. Given I haven't done tons of glass but I have fuse a fair amount and slumped a lot of things some of which were fused before they were slumped. Do you think I went down to fast I didn't. This thing is only 2 layers with top decoration that covers less than half the plate.

Barbara Muth · Post by **Barbara Muth** » Thu May 29, 2003 8:07 am

Hi Nanc.

Regarding mold holes... I learned something interesting a while back from Bob Leatherbarrow. Mold holes in the center are NOT well placed. Imagine your piece slumping down. The glass doesn't come rolling down the sides and fill the middle last. It plops down in the middle first (covering and vent holes in the center) and then fills in the sides. You want your vent holes to be in the vicinity of where the last bit of slumping glass will touch the mold.

I always put my molds up on kiln furniture, not only does it help the air flow, as you suggested, but it also reduce the possibility of creating a heat sink. You should definitely elevate your molds on kiln furniture.

Also, if you have top decorations, your annealing schedule should accomodate the thickest part of your glass. How long did you soak the piece at 960?

best wishes,
barbara

Nancy Juhasz · Post by **Nancy Juhasz** » Thu May 29, 2003 8:51 am

Barb I soaked 40 minutes. That seemed to be enough for the fuse. That is also what was recommended by Spectrum. It was all Spectrum glass. Did you ever think that some piece wasn't meant to be? That is how I am beginning to think about this one. I will start all over and try another. It was very Christmas looking so I have time but I want to figure out what on earth went wrong. Maybe I didn't soak long enough since there were 3 layers a lot of places. Here is the design if you can imagine it. It started out 2 layers cracked on the way up on the slump so I added a clear base under the original base layer so now it is 3 layers. Base 10 5/8 inches diameter both clear and white layers. flower petals and center taking up a little over 1/2 of the plate with a crescent 10.5 high and 2-3 inches in the widest place. I think the cracks always start in the crescent and progress with zig zag and half circle across the plate. Do you think given the design that I should have soaked longer. When it was 2 layers I only soaked 20 minutes at 960 again per the Spectrum chart. I'm open for any ideas.

Deb Libby · Post by **Deb Libby** » Thu May 29, 2003 9:52 am

Nanc - I've been struggling with a similar problem .... my plate design is quite different than yours but again has alot of areas with multiple layers. Some areas have as many as 5-6 layers, while other areas are just the blanks; the raised areas are textural and not full fused. I've tried slowing my up and down rates right down to 100 degs/hr. with 30 min. soaks at several points .... still no luck .... have finally decided there must be too much difference between the various areas of design.

The flat fuse is okay (and there is no stress evident under the polarized film) but the slumping process hasn't been successful yet. The one time I thought I had succeeded (this last schedule mentioned above), the piece cracked somewhere lower than 750 which was where I had last peeped in the window and all was fine.

Seems there is too much stress/lack of even heat transfer within the piece during slump/cooling .... which leads me to worry about breakage somewhere down the road (like after a sale).

Jane Lindell · Post by **Jane Lindell** » Thu May 29, 2003 9:54 am

Just a few ideas:

What temperature are you slumping at? Do you really need to flash cool it? If your slump temperature isn't in the devit range there is no reason to accelerate the cool down to annealing - you are risking thermal shock.

I would also elevate the mold off the shelf for even heating and cooling.

Is there any substantial difference between this mold and others you have had success with? Is the glass grabbing onto the mold somewhere?

Have you successfully fired the same glass used in the crescent decoration before?

For a thicker piece I would take your anneal ramp down to 700 rather than 800.

Keep trying

Barbara Muth · Post by **Barbara Muth** » Thu May 29, 2003 9:56 am

Nanc, check out this info in the tutorial. A picture of the crack would immensely help us help you.

http://www.warmglass.com/Troubleshooting.htm#cracked

Also, check out this page on schedules from Spectrum. You should have followed a schedule similar to the one for a 3/8 inch piece.

Lisa Allen · Post by **Lisa Allen** » Thu May 29, 2003 10:01 am

Nanc wrote:Barb I soaked 40 minutes. That seemed to be enough for the fuse. That is also what was recommended by Spectrum. It was all Spectrum glass. Did you ever think that some piece wasn't meant to be? That is how I am beginning to think about this one. I will start all over and try another. It was very Christmas looking so I have time but I want to figure out what on earth went wrong. Maybe I didn't soak long enough since there were 3 layers a lot of places. Here is the design if you can imagine it. It started out 2 layers cracked on the way up on the slump so I added a clear base under the original base layer so now it is 3 layers. Base 10 5/8 inches diameter both clear and white layers. flower petals and center taking up a little over 1/2 of the plate with a crescent 10.5 high and 2-3 inches in the widest place. I think the cracks always start in the crescent and progress with zig zag and half circle across the plate. Do you think given the design that I should have soaked longer. When it was 2 layers I only soaked 20 minutes at 960 again per the Spectrum chart. I'm open for any ideas.

My only guesses now would be an incompatibility with the crecent piece of glass or too much difference in the thickness throughout the piece to allow for even cooling. With my pattern bar pieces, I have pretty big differences too, with the border being 1/4" and the center being a bit over 3/8ths. For those I ramp up at no more than 200 dph and I ramp down below 960 at 60dph. Takes forevr to fire one, but I have had one blow up on the way up and on the way down when I went any faster than what I mentioned.

Also, i always raise my molds up off the shelf with kiln posts, so the holes can do their work and so that the air can circulate more evenly under the mold.

Lisa

charlie · Post by **charlie** » Thu May 29, 2003 10:41 am

Nanc wrote:Barb I soaked 40 minutes. That seemed to be enough for the fuse. That is also what was recommended by Spectrum. It was all Spectrum glass. Did you ever think that some piece wasn't meant to be? That is how I am beginning to think about this one. I will start all over and try another. It was very Christmas looking so I have time but I want to figure out what on earth went wrong. Maybe I didn't soak long enough since there were 3 layers a lot of places. Here is the design if you can imagine it. It started out 2 layers cracked on the way up on the slump so I added a clear base under the original base layer so now it is 3 layers. Base 10 5/8 inches diameter both clear and white layers. flower petals and center taking up a little over 1/2 of the plate with a crescent 10.5 high and 2-3 inches in the widest place. I think the cracks always start in the crescent and progress with zig zag and half circle across the plate. Do you think given the design that I should have soaked longer. When it was 2 layers I only soaked 20 minutes at 960 again per the Spectrum chart. I'm open for any ideas.

how thick is this mold? a fusing soak at 960 is different than a slumping hold; the slump hold must be longer if the mold is thick, because the mold is holding a lot more heat in it. it could take significantly longer to lose that heat, than your shelf does during a fuse. also, if it's sitting on the shelf, then there is no air cooling under the mold, and thus it'll take even longer to cool.

charlie · Post by **charlie** » Thu May 29, 2003 10:44 am

Deb Libby wrote:The flat fuse is okay (and there is no stress evident under the polarized film) but the slumping process hasn't been successful yet. The one time I thought I had succeeded (this last schedule mentioned above), the piece cracked somewhere lower than 750 which was where I had last peeped in the window and all was fine.

NO PEEKING between 1000 and 200 unless you're using a window! the air currents from doing so can be causing your cracks.

Nancy Juhasz · Post by **Nancy Juhasz** » Thu May 29, 2003 11:51 am

Charlie,

The mold is about 1/4 in thick. looks like a plate that might be found in ceramics bisque only with 4 holes in it. I think that I really need to get it up off the kiln shelf. Next try will be on kiln furniture.

This has been the biggest learning curve I have had for quite a while. I am really beginning to be able to think things through more clearly since all of this and the help everyone has given me. A few more of these disasters and I will be a first class warm glasser. LOL I will work at posting a picture of the thing this week-end. I need to set up someplace to hold the pictures. Did I hear right that I can't use Yahoo for some reason?

Barbara Muth · Post by **Barbara Muth** » Thu May 29, 2003 11:52 am

charlie wrote: how thick is this mold? a fusing soak at 960 is different than a slumping hold; the slump hold must be longer if the mold is thick, because the mold is holding a lot more heat in it. it could take significantly longer to lose that heat, than your shelf does during a fuse. also, if it's sitting on the shelf, then there is no air cooling under the mold, and thus it'll take even longer to cool.

in which case venting to cool is probably not a good idea at all. You are creating stress from the flash cool on the top surface of the glass with the heat on the bottom surface of the glass.

I say elevate the mold on kiln posts and stop venting.

Barbara

charlie · Post by **charlie** » Thu May 29, 2003 12:00 pm

Nanc wrote:Charlie,

The mold is about 1/4 in thick. looks like a plate that might be found in ceramics bisque only with 4 holes in it. I think that I really need to get it up off the kiln shelf. Next try will be on kiln furniture.

This has been the biggest learning curve I have had for quite a while. I am really beginning to be able to think things through more clearly since all of this and the help everyone has given me. A few more of these disasters and I will be a first class warm glasser. LOL I will work at posting a picture of the thing this week-end. I need to set up someplace to hold the pictures. Did I hear right that I can't use Yahoo for some reason?

last weekend i just used a bowl mold i made. 3/8" thick clay slab. very shallow so it only contacted the (fiber) shelf in a very small spot. no holes drilled in it. 20" rough diameter 2 layer glass blank, all b.e. my annealing schedule was

9999 960 45

120 850 0

200 700 0 off

if it was 3/8" in places (anneal for thickness, not diameter) i would have used:

9999 960 90

60 850 0

120 700 0 off

i don't look, nor open the peephole plugs at all, once it leaves 1480 fusing temp.

Nancy Juhasz · Post by **Nancy Juhasz** » Thu May 29, 2003 12:35 pm

Thanks Charlie