What am I doing Wrong? - WarmGlass.com

What am I doing Wrong?

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elenor
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What am I doing Wrong?

Postby elenor » Sun Feb 17, 2013 2:52 pm

GLASS1_crop.jpg
First attempt

First try
I need your help. I am trying to duplicate a piece that a client broke. It is 8 1/4 by 12 inches, basically 2 layers of Bullseye with tack fused decoration. 2 attempts have cracked in the kiln, on the way up as the edges are rounded in both cases.
The piece consists of a base layer of Tekta, covered fully with cut pieces that are tack fused to the base. Additional decoration are tack fused on top.
Program for the first one was
200 900 5
250 1325 10
9999 1000 5
60 900 60
50 760 0

After seeing the fist break, I tried again, first tack fusing the first layer of decoration to the base, then after adding frit and some detail , I fired again, slowing to

150 900 5
200 1330 10
then annealed

The Kiln is in an unheated room but in a brick building, the temp last night did not go below 30 and in the morning the room was over 45 degrees. This type of breakage has not been a problem before. I have cut out all new pieces for a 3rd attempt but will wait for your suggestions before firing.
Attachments
GLASS_crop.jpg
Second attempt
Elenor Conn

Brock
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Re: What am I doing Wrong?

Postby Brock » Sun Feb 17, 2013 2:55 pm

Waaaay more annealing required . . .

elenor
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Re: What am I doing Wrong?

Postby elenor » Sun Feb 17, 2013 3:19 pm

OK - I will do that next time but these both broke on the ramp up!!!
Elenor Conn

Brock
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Re: What am I doing Wrong?

Postby Brock » Sun Feb 17, 2013 3:22 pm

elenor wrote:OK - I will do that next time but these both broke on the ramp up!!!


Then you have to slow down . . .

elenor
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Re: What am I doing Wrong?

Postby elenor » Sun Feb 17, 2013 3:25 pm

Less that 100 degrees per hour?
Elenor Conn

Steve Immerman
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Re: What am I doing Wrong?

Postby Steve Immerman » Sun Feb 17, 2013 3:52 pm

I am a bit confused at how many times you are firing these. But, if on an initial firing they are not properly annealed, they may crack from the stress of reheating the next time, even with a reasonable initial ramp.

I'd work on your annealing schedule - use the Bullseye recommended schedule for 1/2" thickness and you should be safe when you go to the next firing.

Steve

Laurie Spray
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Re: What am I doing Wrong?

Postby Laurie Spray » Sun Feb 17, 2013 5:11 pm

It is possible that the original one had stress and that is why it broke while in the possession of the buyer. Sometimes badly annealed pieces can break months later......
Just a thought.....
Laurie Spray

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elenor
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Re: What am I doing Wrong?

Postby elenor » Sun Feb 17, 2013 6:06 pm

Actually it was dropped from a table. But a good thought.
Elenor Conn

elenor
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Re: What am I doing Wrong?

Postby elenor » Sun Feb 17, 2013 6:12 pm

The first one was fired just once and it broke on the way up. The second broke on the second firing, also on the way up but may have been as the result of too little annealing on the first firing. I am trying again.
Elenor Conn

Laurie Spray
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Re: What am I doing Wrong?

Postby Laurie Spray » Sun Feb 17, 2013 6:16 pm

On reheating a tac fused piece I would slow down to no more then 100/hr till at least 800. Perhapsmeven slower......
Laurie Spray

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glass: http://lauriespray.blogspot.com

Tony Smith
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Re: What am I doing Wrong?

Postby Tony Smith » Sun Feb 17, 2013 6:23 pm

A tack fused piece has much greater potential for residual stress after annealing than flat pieces due to the constantly changing thickness. Every edge is a potential crack initiation point. Proper annealing on the tack fuse is critical, because with your second firing, you're trying to heat the varying thicknesses, which creates enough stress by itself. Add to that residual stresses from the tack fuse and you're asking for trouble.

100 deg/hr may be adequate with a properly annealed piece, but if you want to ensure you're not repeating this discussion, you might want to be even more conservative at 50-75 deg/hr. There's no way to know for sure unless you run some tack fuse experiments on a clear base, then look for stress in the glass with a polarimeter. Once you've answered the annealing question, then you can start increasing your ramp rate. Unfortunately, you only know you're ramping too fast when you break your next piece.

I would also take Doc Steve's suggestion and use Bullseye's annealing schedule for 1/2" thick glass.

Tony
The tightrope between being strange and being creative is too narrow to walk without occasionally landing on both sides..." Scott Berkun

Morganica
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Re: What am I doing Wrong?

Postby Morganica » Mon Feb 18, 2013 1:40 am

The big problem, as mentioned, is that you're not annealing nearly long enough on the first firing(s), so you're starting the last firing with highly stressed glass. It doesn't take much to make it come apart.

Annealing, BTW, isn't just how long you soak the glass at 900F or wherever, it's the entire down-ramp or cooling cycle. You always START calculating your firing schedule for the thickest piece of glass in the shortest dimension. If you're annealing two layers of Bullseye with tack-fused decoration that (from the photo) could be 2-3 pieces of glass thick, your annealing schedule should START at four or five layers of glass, or up to 15mm (about 3/5 inch).

THEN you consider that you're doing a tack-fuse. Tack-fuses by definition don't become a single smooth piece of glass--the object of the game is for the glass to stick together so it's solid, but for the individual pieces to retain their shapes and edges. What that means, though, is that when the glass starts cooling and contracting, all those individual pieces try to contract away from each other. If you're cooling slowly enough, they move with the shrinkage. Cool too fast, you literally "freeze" the stress in.

A 4-layer flat-fuse (.5 inches) needs 2 hours at 900F and a starting down ramp at 100dph (see the schedule on Bullseye's site). Tack-fuses can take anywhere from twice to four times as long. As a general rule of thumb you double the thickness of your glass and use the schedule for that (or in this case, 1 inch). If that STILL introduces stress, go to 3X the thickness...and so on. There are some tack-fuses that just aren't going to work, even then.
Cynthia Morgan
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Tony Smith
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Re: What am I doing Wrong?

Postby Tony Smith » Mon Feb 18, 2013 5:26 am

Something else to consider is to increase your hold on the way down to get the entire mass of glass at the same temperature before entering the annealing range. I'm still a bit unsettled about the recent change to 900F for annealing since thermocouple inaccuracy can quickly take you out of range. And where you have so many types of glasses tack fused, you might want to give yourself a bit of margin by starting your anneal at a higher temperature.

Tony
The tightrope between being strange and being creative is too narrow to walk without occasionally landing on both sides..." Scott Berkun

Jerrwel
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Re: What am I doing Wrong?

Postby Jerrwel » Tue Feb 19, 2013 2:39 pm

Replies to your post show how knowledgeable and generous people sharing this board are. There's another great resource that supplies information about the intricacies of various project construction and firing schedules available without going to myriad resources on the internet and that is Graham Stone’s Firing Schedules for Glass available at Warmglass http://www.warmglass.org/servlet/the-1553/Firing-Schedules-for-Glass/Detail. This book is well worth the $$ and time required to study and review and review and review when planning a project and appropriate firing schedules.
Jerry

elenor
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Re: What am I doing Wrong?

Postby elenor » Wed Feb 20, 2013 1:34 pm

Third time is a charm. I slowed everything down and increased annealing time according to Bullseye charts and made a perfectly intact piece. Thank you so very much to all of you who shared tips and advice. You are the most generous folks I have ever met.
Elenor Conn

twin vision glass
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Re: What am I doing Wrong?

Postby twin vision glass » Wed Feb 20, 2013 1:50 pm

=D> =D> =D> =D> =D>


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