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Mending a crack

For discussion of processes related to using Bullseye glass, including kilnforming and kilncasting, torchwork, blowing and stained glass.

Mending a crack

Postby Kerry7140 » Tue Dec 29, 2009 5:55 am

My piece cracked in the kiln yesterday and I could really use some advice on what, if anything, I can do to repair the piece or disguise the crack.

I used an Olympic coffin kiln with internal dimensions 64" x 24" and elements in the top only. I fired the piece twice, both times on 1/2" fiber blanket. T

First firing, I full fused opaque white (0013.30) on bottom to light sky blue (1414.30) on top. The dimensions of this piece are 16" x 28". During this same firing, I full fused my tree with thin black (0100.50) on bottom + CBS & Dichro Magic (various colors on thin black) + clear Tekta topper. The results were perfect.

Second firing, I tack fused the tree + black vitigraphs + leaves to the full fused base. The firing schedule I use for tack fusing components to a full fused base:
1. 280 to 1100 hold 30 min
2. 9999 to 1375 hold 10 min
3. 9999 to 960 hold 2 hours
4. 50 to 720 hold 20 minutes

When I opened the kiln, the piece had a vertical crack to the right of the tree trunk that went from top to bottom. It also had a horizontal crack from the left edge to the point where it intersected the vertical crack. It had obviously cracked on the way up, because it had healed itself. But the crack is still obvious.

So my question is what can I do now to make the crack less obvious?

I have photos of before and after both firings that I would be glad to send.
Kerry7140
 

Re: Mending a crack

Postby marykaynitchie » Tue Dec 29, 2009 11:10 am

Here are photos of Kerry's piece mentioned in the post above.

Edited 12/30/09: Kerry sent one more image which shows the cracks outlined with a silver Sharpie pen. This is the topmost image in the group.

Mary Kay
Attachments
IMG_0164.jpeg
IMG_0164.jpeg (43.73 KiB) Viewed 19335 times
1 Tree before firing #1.jpeg
Tree before firing
1 Tree before firing #1.jpeg (28.64 KiB) Viewed 19408 times
7 Kiln after firing #1.jpeg
Kiln after firing
7 Kiln after firing #1.jpeg (26.63 KiB) Viewed 19408 times
10 Tree before firing #2.jpeg
Tree before firing
10 Tree before firing #2.jpeg (32.56 KiB) Viewed 19408 times
11 Tree after firing #2.jpeg
Tree after firing
11 Tree after firing #2.jpeg (38.08 KiB) Viewed 19408 times
Mary Kay Nitchie
Bullseye Glass Co.

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Re: Mending a crack

Postby marykaynitchie » Tue Dec 29, 2009 2:29 pm

Hi Kerry,

Ted Sawyer, of Bullseye's Research and Education department, provided some advice. He wasn't able to see the cracks very well in the image, but it looks like the piece thermal-shocked on the way up in the fusing cycle and then healed on the way down, as you already observed. If you were to do this project again, he suggests slowing down the initial heating rate for that final tack fuse. Instead of 280 degrees F per hour, we recommend no faster than 100 degrees F per hour, up to 1000 degrees F.

Why must most tack-fused pieces be heated more slowly than fully fused pieces? Because the arrangement of glasses is characterized by thick and thin areas, and these are difficult to keep at a uniform temperature during the heating up phase. The thicker areas stay cooler than the thinner areas, resulting in thermal shock. (Slower initial heating rates are also necessary when doing a full firing of a piece that is assembled with areas of uneven thickness.)

We don’t have any really good ideas for mending this piece, since the only way to do so would be to fully fuse it together – which will destroy the relief quality of the tree. If you were to infill the crack with frit and tack fuse it, it would have a different texture and to some extent color than the rest of the background. If you were willing to modify the design, then you could cover the cracked areas with stringer or other design elements.

If those options don't seem like acceptable solutions, only way that we can think to make the piece seamlessly look as you originally intended is to remake it entirely.

For more information on how Bullseye glass behaves, see our "TechNotes 4: Heat and Glass" at http://www.bullseyeglass.com/education/#techbook

I hope this helps explain what happened. Let us know what you plan to do next!

Mary Kay
Mary Kay Nitchie
Bullseye Glass Co.

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Re: Mending a crack

Postby Kerry7140 » Wed Dec 30, 2009 8:57 pm

Mary Kay,
Thanks again for your suggestions. I will follow Ted's instructions to ramp my initial segment at 100 degrees per hour to 1000. And, I decided to apply medium frit in the area of the crack to disguise it, then tack fuse to 1375 (which was the same temp as my original tack fuse).
Attachments
IMG_0169 - 2.jpg
Frit applied to mask crack - pictured before re-firing.
IMG_0169 - 2.jpg (151.83 KiB) Viewed 19283 times
Kerry7140
 

Re: Mending a crack

Postby Kerry7140 » Sat Jan 02, 2010 12:59 pm

Mary Kay,
I followed Ted's suggestino and used frit to cover the crack. The frit turned out to be a lovely enhancement and the crack isn't noticeable. My client loves the piece. Following is the firing schedule I used:

1. 100 dph to 1000 degrees hold 0
2. 350 dph to 1375 degrees hold 10 min
3. 9999 dph to 1000 degrees hold 0
4. 50 dph to 900 degrees hold 2 hours
5. 50 dph to 600 degrees hold 0

The firing took about 20 hours, and was well worth the time. Picture is attached.
Attachments
14 Caryn's tree finished small.jpg
Completed piece with frit covering crack. Crack is well mended and hardly visible from back, as well.
14 Caryn's tree finished small.jpg (152.92 KiB) Viewed 19122 times
Kerry7140
 

Re: Mending a crack

Postby marykaynitchie » Sat Jan 02, 2010 3:30 pm

Kerry,

Nicely done! Not only did you repair the cracks, but I think your solution with the frit makes the design and feel of the piece even stronger. Thanks for letting us know how your project turned out.

Mary Kay
Mary Kay Nitchie
Bullseye Glass Co.

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Re: Mending a crack

Postby Paul Tarlow » Tue Jan 05, 2010 2:23 pm

I'll add to the above that all the dichro probably wasn't helping when the piece broke.

In much same way that variable thickness can contribute to thermal shock, large areas of irid and dichro also seem to exacerbate uneven heating. I suspect this is because those surfaces reflect much of the heat that the non-metalic glass surfaces absorb.

In your case you seem to have the worst of all factors -- thicker sections that are also largely dichroic.
Paul Tarlow
 

Re: Mending a crack

Postby Kerry7140 » Tue Jan 05, 2010 3:46 pm

Thanks Paul. Yep, the reflective quality of the dichroic was probably the final straw in my piece's break...in addition to thick, uneven glass and ramping too quickly.

I've been working in fused glass for several years, but have only had my own kiln for about 18 months. If experience is the greatest teacher, firing this piece was an incredible learning experience. The folks at Bullseye were so supportive. And fortunately the outcome was great and my client is pleased.

By the way, I'm a Dallasite and am familiar with your studio in Austin. Next time I'm in town, I'll stop by and introduce myself. It sounds like you have fostered a great glass community.
Kerry7140
 

Re: Mending a crack

Postby Vicki Floyd Willhite » Wed Jan 20, 2010 10:14 am

Thank you so much for sharing this. I learned a great deal from this one problem and both the advice given and the solution found. I am very pleased with this forum and am looking forward to all future posts! :D
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