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Glass cracking

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Glass cracking

Postby ryoder » Fri Oct 28, 2016 9:51 am


I have been working on building the glass program in my art classes. We have had great success in our fusing class at the high school from both the help I have found on the forum and from our local glass center. Lately though I have run into an issue and I am hoping to get some more help.

The issue I have had glass fused projects cracking. I will put the details below and insert a photo as well.

Project Information:

1) A description of the problem.

Glass cracking after it has been fired a second time.

2) If possible, a digital image of the piece is and kiln is often helpful.
I will insert one below.

I get a server error when I try to attach the photo

3) Style codes (i.e. 1101-30), production dates, and sheet numbers for all of the glasses in the project in question.

Tekta Clear glass 001100 0380 possible dates 7/9/14 or 6/18/14 (I have a large bin of clear scraps that we use so not all of them have the date stickers attached.)

ruby pink tint 001831-0030 3mm
powder blue 000108-0030 3mm
Stringers Clear dichroic

On the wreath
Tekta Clear
spring green 0014426-0030 3mm
Averntine green stringers
Tomato Red Opal Frit Course 00024-0003-F-P001

4) Design of the piece (dimensions, arrangement)
Square: 7 1/2 inches with clear base and glass tack fused on top to create the design

Wreath: Approximately 4 1/2 inches diameter clear base with Cut green pieces on top layered to create the wreath.

Kiln and firing info:

Both Pieces were tack fused once without issue.
Both students added some final touches (stringers onto the square, red round fit onto the wreath) then tack fired again. The cracks and/or separating did not happen until it was tack fused the second time.

Here is the tack fuse schedule.

ramp 1 300 to 1250 hold 30 minutes
ramp 2 400 to 1350 hold 10 minutes
ramp 3 960 to 960 hold 1 hour
ramp 4 100 to 700 hold no

1) Description of the kiln (type, interior dimensions, controller, etc.). Describe location of elements.

paragon kiln square interior shelf 14 inches by 14 inches. Computer controller with elements only on the walls not the ceiling.

2) Firing cycle for this project (including the annealing portion of the cycle)
see above
Kiln Furniture and supplies

1) What kind of kiln shelf are you using? mulliete

2) What are you using for a shelf or mold separator? (Thinfire, fiber paper, shelf primer, etc.)

Bullseye kiln wash

3) Where is it situated in the kiln during firings?

Center of the shelf in center of the kiln

4) Describe any additional kiln furniture used (dams, molds) and the separators used.
no other kiln furniture used

Optional (but encouraged):

Do you have a hunch about what could be done differently to avoid the problem? The only think I can think of is that it looks like a crack from glass that would not be compatible. I don't see how that could be the case because we use only Bullseye fusable glass. The clear is all that the two pieces have in common in terms of materials.

Otherwise it would have to be something with the kiln/firing. This is the same schedule I have been using so I am stumped. I have fired pieces more than once numerous times and have never had this happen.

We could be causing this so I can avoid it in the future.
Could either of these pieces be salvaged/ fixed? If so I'll take any suggestions
Posts: 5
Joined: Tue Oct 29, 2013 3:43 am

Re: Glass cracking

Postby jestersbaubles » Fri Oct 28, 2016 10:58 am

Since there was no image it is hard to tell, but keep in mind that a tack-fused piece requires slower ramping and longer annealing than a fully-fused piece of the same dimensions. Also, your annealing temp of 960 is not what is recommended by Bullseye for annealing their glass -- I believe they recommend 900 degrees.

If the edges of the cracked glass are rounded, then the piece likely broke before reaching the top tack-fused temp, so too fast of a ramp rate may be your problem.

If the edges of the cracked glass are sharp, then it likely cracked after the tack-fused temps because of improper annealing.

Without seeing the piece my guess is it is an annealing crack because of annealing at 960. Lower the temp to the proper annealing temp and increase the hold at that temp by 30 minutes.

Dana W.
Dana Worley Fused Glass Designs
dba Jester's Baubles
Posts: 188
Joined: Thu Jan 21, 2010 6:29 pm
Location: Logan, UT

Re: Glass cracking

Postby marykaynitchie » Mon Oct 31, 2016 9:02 am


Dana is right, tack-fused pieces need slower heating and longer annealing cycles than fully fired pieces of uniform thickness. You can learn about that here from this video from our subscription series:

Why Did It Break?
http://www.bullseyeglass.com/education/ ... k-190.html

and from this free article "Heat and Glass" http://www.bullseyeglass.com/methods-id ... glass.html

If you are working on building a glass program, the video series will be extremely valuable to you and your students, to help students understand the material of glass, and how it works under conditions of heat. I guarantee it will pay for itself in preventing broken or otherwise failed projects.

Mary Kay
Mary Kay Nitchie
Bullseye Glass Co.

Subscribe to Bullseye kiln-glass videos at
Posts: 1091
Joined: Thu Oct 13, 2011 5:39 am

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