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annealing and stress

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annealing and stress

Postby william » Thu Feb 25, 2010 12:12 pm

Hi
I would like anybody's thoughts on this. How doe's the size of the glass change the annealing and cooling of larger pieces of glass. I setup my kiln to process a piece of glass 16" x 99" x 1"' I fired a test piece 16" x 36" x 1" to check my annealing and cooling program and how my mold material looked in the glass. the test piece came out fine no stress showing in the glass body. My question, I know that the mass of glass in the larger piece will hold heat longer and take more time to give up the heat in the annealing and cooling process how do others account for the difference in writing a program for the larger piece. I am sure many of you have fired small projects that come out fine and anneal properly but when you fire the same type of project but very large
different forces are at work in cooling the glass.
Thanks for your ideas
Bill
william
 

Re: annealing and stress

Postby bertglass » Thu Feb 25, 2010 3:30 pm

Bill

I totally ignore area and anneal based on thickness. If you have thick and thin spots, this drives longer anneal times. I have a large kiln and I will anneal a piece 40" x 84" x 10mm the exact same as I would 12" x 12" x 10mm. The controller will do whatever it has to to maintain the anneal schedule as programmed. The larger mass of glass will ask for more power to get the same job done. You don't have to change the program at all.

This paradigm works until you get down to around 1" square at which point you can go faster.
Bert Weiss
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Re: annealing and stress

Postby Twin Vision Glass » Thu Feb 25, 2010 4:17 pm

But also perhaps be aware that the size of your kiln and the configuration , element placement and cool corners must also be taken into account.(also how you place your shelves) Have you done a multi-zoned firing schedule with 4 "x 4" x 1 " blocks placed throughout the kiln and in every corner to make sure you are getting even heating. Just a thought. (I am sure you have already done this, but if not; Do take the time to do so. It will tell you much about the environment inside your kiln)
Leslie
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Re: annealing and stress

Postby william » Thu Feb 25, 2010 8:35 pm

Hi
I am not having problems, I am just asking if anyone has thoughts on determining glass mass variations for larger pieces, example you can melt a 12" x12" x 3" block shape open face and anneal and cool for a 3" thickness and melt a 48" x 48" x 3" block shape same open face and have that mass cool very differently, how might or would you adjust for these differences or would you as Bert said run the same program for both.

Bill
william
 

Re: annealing and stress

Postby bertglass » Fri Feb 26, 2010 7:31 am

william wrote:Hi
I am not having problems, I am just asking if anyone has thoughts on determining glass mass variations for larger pieces, example you can melt a 12" x12" x 3" block shape open face and anneal and cool for a 3" thickness and melt a 48" x 48" x 3" block shape same open face and have that mass cool very differently, how might or would you adjust for these differences or would you as Bert said run the same program for both.

Bill


3" thick is an interesting challenge. You must be able to circulate heat beneath the mold. The glass will be losing more heat through the bottom than you can add through the top. You also have to take the mold/dams in to account. However, if both setups are the same, I'd use the same anneal program. As I said, the controller will take care of matching the cooldown profile. My understanding is that kiln configuration matters during the heatup, relative to thermal shocking the glass, but during the anneal, schedules are the same for side elements as they are for top, etc.

The best test to see if your anneal is successful is coldwork. If you can saw, grind, and polish the edges, you are probably good.

If somebody knows I am wrong about these issues, I'd certainly like to hear about it. I learned the thing about pendants, when a pendant maker piped in.
Bert Weiss
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