Unusual plate crack - WarmGlass.com

Unusual plate crack

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Becky C
Posts: 2
Joined: Wed Jul 06, 2016 3:38 pm

Unusual plate crack

Postby Becky C » Wed Jul 06, 2016 4:31 pm

I’ve fused and slumped 5 plates. The first cracked due to an annealing schedule that was too fast. The next two survived. The last two have both cracked in a perplexing (to me) spiral. I haven't been able to find a picture or description of anything like them in the books I have or online.

The cracks occurred out of the kiln, within 24 hours of the slump. One cracked when the plate was picked up after a piece of hot pizza was put on it. The other may have cracked in the dishwasher. (I assumed fused glass plates were DW safe because 1) DW temps are so much lower than the fusing temps, 2) ceramic glazes are glass and are DW safe and 3) the two surviving plates are also surviving the DW. Please correct me if I'm wrong.)


Slumping schedule (in F)


(The T plate is actually my 13yo son’s work.)

Slumping schedule (in F)

The two surviving plates were slumped using the same schedule as the T plate (only one is shown - the other had a white base layer and transparent blue & green with opaque white over that):


I have an Evenheat Studio Pro 17 and used Wissmach COE 96 3mm glass for these projects. All have clear glass bottom layers. All of the designs are in the second layer, except the T and the squares which were in a third (and fourth) layer.

I’m looking for insight into what causes this sort of crack, and what sort of schedule might prevent it. The best I can come up with is something about the light and dark colors, and opaque vs transparent, viscosities and so on. However, the shape of the cracks doesn't seem consistent with that theory.

I’m also curious about the small bubbles in all of these plates (seen as bright spots in the pictures). They were in the original glass sheets and I can’t get them out even with slow rises (50F/hr from 1050 to 1250F) and long soaks at 1250F. Is this a glass quality issue? A firing issue? Some combination? Something else?

Thank you for any help you may offer.

Kevin Midgley
Posts: 712
Joined: Mon Mar 10, 2003 11:36 am
Location: Tofino, British Columbia, Canada

Re: Unusual plate crack

Postby Kevin Midgley » Thu Jul 07, 2016 10:32 am

#1. You need to know more about the characteristics of glass and kilns and your kiln and your molds in your kiln, in particular. You do not say how long you have been firing glass.
There are the Boyce Lundstrom book series, the Warm Glass book available here and if you are going to get serious about glass, the Graham Stone Firing Schedules for glass.
#2 Since you recognize the problem as being annealing, then your schedule is wrong for your kiln and combination of glass and mold and kiln shelf!!!!!
#3 Why are you placing the clear cap on the bottom of these dishes? :-k
The clear should always be on top to act as a barrier between the colored glass and food. :oops:
#4 Food too hot.
#5 Bubbles normal.

Posts: 129
Joined: Thu Feb 10, 2005 6:25 pm
Location: Charlotte, NC

Re: Unusual plate crack

Postby Jerrwel » Thu Jul 07, 2016 9:20 pm

There are... the Warm Glass book available here ....
#2 Since you recognize the problem as being annealing, then your schedule is wrong for your kiln and combination of glass and mold and kiln shelf!!!!!.

Definitely encourage reading Brad's Contemporary Fused Glasshttp://warmglass.org/books and doing the projects as they are presented in the book. Even though a project makes it out of the kiln in one piece, it is not necessarily properly annealed as the post-firing breakage demonstrates.

Becky C
Posts: 2
Joined: Wed Jul 06, 2016 3:38 pm

Re: Unusual plate crack

Postby Becky C » Sun Jul 10, 2016 3:43 pm

Thanks for your replies.

1. You would be correct to surmise that I'm quite new to glass fusing. At this point I'm quite certain that I don't have a clue as to how much I don't know. Thank you both for your book and practice recommendations.

2. I did not recognize the spiraling cracks as being annealing issues - if I had I would not have bothered this forum with my question. The first plate that cracked (not shown) experienced a more classic annealing crack: clean, edge-to-edge that I could confirm as an annealing issue. I had not been able to confirm that the spiraling cracks were due to annealing issues. Thank you for confirming that. I will read up on annealing, experiment and find a schedule that works.


Stephen Richard
Posts: 302
Joined: Sun Jul 06, 2003 4:36 pm
Location: Glasgow, Scotland

Re: Unusual plate crack

Postby Stephen Richard » Sun Aug 14, 2016 4:18 pm

You mention that the T plate was 4 layers. your annealing schedule is far too fast for 12mm thick
Steve Richard
You can view my Blog at: http://verrier-glass.blogspot.com/

Valerie Adams
Posts: 587
Joined: Mon Apr 19, 2004 2:49 pm
Location: Santa Rosa, California

Re: Unusual plate crack

Postby Valerie Adams » Wed Aug 17, 2016 1:06 pm

Hi Becky,

First, I want to commend you on your beautiful circle cutting! I've known glass artists who've got years of experience and still struggle with circles.

So my question to you is, are you positive your clear glass is compatible with your colored glass? I'm not familiar with Wissmach but I know some folks are starting to use it due to cost. Is there a chance you're using clear glass that was made for stained glass instead of fusing (i.e. not compatible)?

I agree with spending some time with Brad's book; many of us consider it the Bible of Fusing. I've been doing this for 16 years and while I own Graham Stone's book, I've never been able to make it through a single reading (sorry Kevin!).

Keep really good notes! I have a binder for each kiln. I record what the project is, what glass is used, the firing schedule, and the results. That way, I've created my own go-to schedules when I do similar pieces in the future.

Good luck!

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