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What happened: Pattern Bar plate broke in slumping

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What happened: Pattern Bar plate broke in slumping

Postby jtensminger » Thu Oct 20, 2016 6:02 am

My 12" x 12" plate inexplicably broke in half. The composition of the plate was 4 pattern bars in the middle (6"x7" area overall). The bars were surrounded by 3/8" on-edge strips. Overall thickness of the piece was 3/8". The initial assembly and firing were good, everything properly fused, looking strong. However, the top surface (just the on-edge strips) were a little rough, and I used a 220 diamond hand pad to gently smooth it. I was afraid that fire polishing would not eliminate abrasions from the pad, so I opted to sift a layer of clear, fine frit over the surface and refire. I used BE tack-fuse schedule, 1425 top temperature. I guess it was too low because the frit did not completely melt, but the resulting "pebbly" texture was kind of interesting, and the underside looked very good. For slumping, I used a BE 12" square mold, #8637 and BE pattern bar slumping schedule: 300/1180/:10, 9999/900/1:30, 100/700/:00, 9999/70/:00. The piece split in two pieces, and the fissure was not even aligned with the pattern bar joints. I can see that the middle of the piece started to rise instead of slump which caused the break. My firing program was correctly entered, and I vouch for my kiln. The mold was dry. The pattern bars were comprised of BE transparent blues, greens and neo-lavendar shades. The strips were midnight blue. What went wrong?
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Re: What happened: Pattern Bar plate broke in slumping

Postby marykaynitchie » Thu Oct 20, 2016 9:45 am

How disappointing to have a piece break after so much cutting, firing and coldwork! One thing we need to understand is if the piece broke on the way up to process temperature, or after.

If you were slumping, and the pieces don't fit together, then your piece broke on the way up. Is this the case? This would be a thermal shock.

How was your mold set up in the kiln? On stilts, or on the kiln shelf?

Do you have a subscription to Bullseye's videos? The video "Why Did It Break?" might help you determine the cause. https://videos.bullseyeglass.com/videos ... -it-break/

This video actually shows a piece breaking during a slump firing, and explains annealing shock and thermal shock.

We look forward to learning more about your piece, and helping you with this!

Mary Kay
Mary Kay Nitchie
Bullseye Glass Co.

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bit.ly/BullVideos
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Re: What happened: Pattern Bar plate broke in slumping

Postby jtensminger » Sun Oct 23, 2016 11:24 am

Thank you Mary Kay. I had forgotten about the "Why Did It Break" video. Per your comments, it seems the piece broke on the way up. Interesting discovery: After I posted my issue, I discovered that our house incurred a power outtage in the middle of the night. I do not know how long it lasted or what segment cycle was interrupted. I have been kiln-forming for 6 years and this is the first time a piece of mine has broken. Maybe that's a good record but sure wish it had happened to a less elaborate piece.
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Re: What happened: Pattern Bar plate broke in slumping

Postby marykaynitchie » Mon Oct 24, 2016 10:33 am

That is such a bummer! If you are doing pattern bars, you must have a saw, right? Have you thought about cutting up the broken pieces and making something else out of them?

Mary Kay
Mary Kay Nitchie
Bullseye Glass Co.

Subscribe to Bullseye kiln-glass videos at
bit.ly/BullVideos
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Re: What happened: Pattern Bar plate broke in slumping

Postby Kevin Midgley » Wed Oct 26, 2016 8:10 am

If you have a battery backup on your kiln controller it can sometimes save a kiln firing by enabling the controller to "think" the power never went out on a short power blip. It won't save you on a long power outage however. Check with your controller supplier about the pro and con issues involved. However consider the value of saving a firing of glass in which there are hours of work involved. Digitry controllers can do the above firing save. Additionally a high quality battery backup unit may protect your expensive controller from damage from 'dirty' electric power.
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