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Problem with big bubbles under the glasses (full fuse)

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Problem with big bubbles under the glasses (full fuse)

Postby margareta » Fri Sep 15, 2017 2:37 am

I have a friend who has been teaching glass fusing classes for many years. Lately (the last year or so) there is a problem that he can´t solve: The fused glass comes out with very big, ugly bubbles. Not between the glassheets but under them. (See picture)

We have tested his glass, shelf primer and firing schedule in one of my kilns and the result there is perfect. Now I wonder what we can do more to locate the problem.

He i using Bulls Eye glass (of course), Bulls eye Shelf primer (the pink powder), ceramic shelves that came with the kiln once. The primer and shelf is really dry before use. The size of the fused objects concerned is about 30-50 cm.

He didn't have the same problem using firing paper, I think. But he can't use this any more in his teaching, because of the health risks. So shelf primer it is.

His firing schedule is (C°):
345°/h to 650°, h 45 minutes
220°/h to 810°, h 4 minutes
Full to 540°/h, h 2 hours
30°/h to 420° END

He has been using this schedule during many years. The only difference between his rutin and mine, is that his kiln is bigger. (And I work in a rather damp environment - his class room is very good in that regard – big and fresh.)

I think the primer is OK, the firing schedule is OK and the glass is OK. Remains the shelf and the kiln as a source of the problem, we guess. Can the shelves be too old? Or could something have changed with the kiln? Or what have we missed?

I really hope someone can help him solve this problem. Or at least give us an idea of what the problem can be.

Many thank's in advance!
/Margareta in Sweden
Full fused glas with big bubbles.jpeg
Full fused glas with big bubbles.jpeg (84.83 KiB) Viewed 16327 times
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Re: Problem with big bubbles under the glasses (full fuse)

Postby marykaynitchie » Fri Sep 15, 2017 1:15 pm

If the bubble is happening in the same spot every time, then it might be a kiln shelf that is not flat. This can be checked by taking a straight edge (like a ruler or straight board) and running it across the shelf, looking for gaps between the shelf and the edge. An uneven kiln shelf can trap a bubble beneath the work.

If the kiln shelf is flat, then his kiln might be firing hotter than yours, and sealing the edges of the work down against the shelf before the air escapes. Try a slower, lower cycle.

Here are some resources for you to study.

Video: Beating Bubbles (part of our subscription series) https://videos.bullseyeglass.com/videos ... g-bubbles/

Article: TechNotes 5: Volume and Bubble Control
http://www.bullseyeglass.com/methods-id ... ntrol.html

Article: TipSheet 7: Platemaking Tips
http://www.bullseyeglass.com/methods-id ... -tips.html

Please look these over with your friend and let us know if one of these suggestions is helpful!


Mary Kay
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Bullseye Glass Co.

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Re: Problem with big bubbles under the glasses (full fuse)

Postby jestersbaubles » Sat Sep 16, 2017 9:21 pm

Has he changed his style of design/lay-up recently (or the size of the pieces)? One of the things you will notice is that all of the large bubbles, and many small bubbles, lie along the lines of the flower stems and the thin blue border. It is hard to tell, but my guess is that these design elements are sandwiched between two layers of glass. Even if these are painted enamels, this creates a prime area for bubbles to form. If they are glass elements sandwiched between two other layers, you most certainly have conditions for trapped air.

Also, if I have converted the firing schedule correctly (my brain thinks in Fahrenheit, especially when it comes to ramp rates!), you are ramping very quickly to a temperature that is just below a bubble squeeze. By my calculation, you are ramping at 621 deg F to 1202, holding 45, and then ramping 400 deg F to 1490. There is no proper bubble squeeze in this schedule. I work in System 96, but a typical schedule for me is:

200 to 300 deg F/hr to 1000, hold 30 minutes
50 deg F/hr to 1250, hold 60
200 to 400/deg F to process temp (for me, typically about 1460)

Stopping at 1000 degrees and holding allows the temperature of the glass equalize just past the strain point. The very slow ramp up to 1250 and the long hold allows the glass to begin sagging in the middle first and moving out toward the edges, pushing the air with it as it goes, before the edges seal. If he has side elements in his kiln and your smaller one has only top elements, or, if his kiln can heat more quickly to temperature than yours, then that might be why you see a difference in the two kilns (not to mention temperature differences).

You don't mention anything about the lay-up of the glass, so this is just my best guess. Additional information might change my answer :).

Also, a word of caution. You are annealing at way too high of a temperature at 1004 deg F; Bullseye glass recommends 900 deg F (482 C).

Best, Dana W.
Dana Worley Fused Glass Designs
dba Jester's Baubles
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Re: Problem with big bubbles under the glasses (full fuse)

Postby edwardcantarella » Mon Mar 19, 2018 7:36 am

"345°/h to 650°, h 45 minutes"

Basically this part of his schedule is too low for a bubble squeeze and accomplishes almost nothing as glass is not soft enough at that point. Change 650 c. to somewhere between 680-700 c. and do the same hold.
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Re: Problem with big bubbles under the glasses (full fuse)

Postby mattt » Tue Apr 03, 2018 2:37 pm

I'd be heating at 222C per hour to the bubble squeeze at around 680C, then holding there.
The time that you hold for depends on the size of what you're making and how likely it is to trap bubbles. (If, as Dana suspects, you have stringers etc. between sheets of glass, there's a strong risk of trapping air.)
Given that he's having problems with bubbles, I'd try extending the bubble squeeze to a couple of hours. (If this works, he can then progressively shorten it in 15 minute increments in subsequent fuses to see how short a bubble squeeze still gives results that he's happy with. I'd also definitely agree with Dana that your friend needs to be doing his annealing at 482C for Bullseye glass.
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