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Mosaic UFO Bowl Crash Landed (why'd it break?)

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Mosaic UFO Bowl Crash Landed (why'd it break?)

Postby tjdempster » Sun Apr 08, 2018 5:42 am

Hi all,
My husband and I are fairly new to glass (a little over a year) and this weekend, we had our first major fail.

Because we are learning, we have been bouncing around to all sorts of techniques. I made a 12" mosaic style bowl. I cut very small 3mm shapes to make my mosaic. I worked inside a steel round form. Pieces were tight in some areas, but space was visible in most places. I capped it with a 3 mm clear. First firing was a high tack fuse. Slumped in a 12" round mold, somewhat shallow. Followed Bullseye recommendations for deep form 3-step process, but after the first firing, it looked great, so I stopped there.

The first bowl was a hit, sold immediately, but I saw issues I wanted to remedy. I made a second piece, 10" this time. I wanted more definition between pieces so this time I spaced the glass a little further apart. I also worked on top of the clear blank this time. I did another tack fuse but held it for less time, and I was very pleased with the results.

Even though it was a smaller piece (10" instead of 12") I slumped in the same mold, the same schedule. When I removed it, a piece broke apart in my hand. The rest of the bowl had cracks forming all over it. Cracks did not necessarily fall between mosaic pieces. Cracks were sharp. Because I still make rookie mistakes, I realized I had left a peep hole plug out. I figured this was likely my problem-- that the kiln cooled too quickly.

This is the slump schedule we used:
250 to 1200 hold 05
Afap to 900 hold 1.00
100 to 700 hold 00
Afap to 70

We knew it would not be a sellable piece, but I was quite attached, so we put it back in the mold and fired this schedule:
250 to 1225 hold 30
600 to 1420 hold 05
Afap to 900 hold 1.30
75 to 700 hold 30
100 to 500 hold 15
Afap to 70

When I removed the piece this morning, there was a brand new crack traveling the length of the entire piece.

So, what are we doing wrong? I have watched bullseye video on breaking, and figured it was due to uneven glass thickness, but we felt we compensated for that in the 2nd firing?

We are using a skutt 818 with elements around the side only. I am having trouble uploading an image but will keep trying. (The image is available through the link below. It is the only image I have and is pre-firing. It does illustrate the distance between the glass pieces, if this is important)

https://drive.google.com/file/d/1EHzoS8 ... p=drivesdk
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Re: Mosaic UFO Bowl Crash Landed (why'd it break?)

Postby marykaynitchie » Tue Apr 10, 2018 1:49 pm

Sorry you are having trouble with your piece. What are you lining the round steel mold with as a release to keep the glass from sticking to the metal? Normally people use a lining of fiber paper (not ThinFire) to allow some ease for the steel ring to expand and contract as it heats and cools.

Mary Kay
Mary Kay Nitchie
Bullseye Glass Co.

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Re: Mosaic UFO Bowl Crash Landed (why'd it break?)

Postby tjdempster » Wed Apr 11, 2018 7:00 am

Thanks for your response! I'm sorry, I was probably not clear. I actually didn't use a steel former on the piece that cracked. I only brought it up in regard to the first piece to illustrate that the glass was snug and tighter fitting than the second piece which cracked. I guess I was wondering if inconsistencies in the thickness of the glass since there was more space between the mosaic pieces could have made the piece vulnerable to cracking.

I guess a much more specific question I have is, should I alter my firing schedules for mosaic type pieces? My tack fuse looks great, but wondering if the nature of mosaic means that I should slow down firings, increase annual times, etc?

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Re: Mosaic UFO Bowl Crash Landed (why'd it break?)

Postby edwardcantarella » Sun May 13, 2018 12:14 pm

My thought was 1420 is a little low to try and fuse something cracked for a healing (without a cap most cracks try to pull open, it's like 2 pieces of glass each rounding their edges over independent from each other). Like an "m"

Side element only is hard going for glass work - heat is only hitting a small edge so there is a lot of temperature inconsistancy across the larger surface. Explaining top elements in most glass kilns. The ones with both side and top usually only turn on the sides above 1000f or so, when you are beyond the strain point.
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