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I want to create a large 11-inch bowl. To learn, I'm using scraps from last year's snowflake project. I arranged 2-inch diamonds, half-inch sqares and diamonds, and 2-inch rectangles as tightly as I could. I filled the gaps with smaller shards and frit I was making on the fly. I arranged this in a rough 11-inch circle and put an 11-inch disc of Bullseye Tekta COE 90 on top. All glass is Bullseye COE 90, by the way.

Here's my firing schedule (from THIS SITE):
Ramp 1 - Rate/400, Temp/1000, Time/.20 (20 minutes)
Ramp 2 - Rate/400, Temp/1150, Time/.15 (15 minutes)
Ramp 3 - Rate/50, Temp/1250, Time/.20 (20 minutes)
Ramp 4 - Rate/850, Temp/1450, Time/.15 (15 minutes)
Ramp 5 - Rate/AFAP, Temp/950, Time/.60 (1 hour)
Ramp 6 - Rate/400, Temp/100, Time/0

I think the large bubbles are from escaping air from all the little pieces of glass. I think the Tekta melted very nicely and formed a perfect seal. I'm using a ceramic shelf in a Paragon 14-inch clamshell kiln that has wash painted on it rather thickly.

The bubbles had no bottoms and were just thin glass at the top. I've broken out the tops of the bubbles, placed some more scrap at the bottom, and used cut pieces of Tekta to cap. Plenty of room for air to escape this time. It's in the kiln right now cooling after last night's refire. I'll see what happens tonight.

FINALLY TO THE QUESTIONS!

1) How do I prevent this from happening in the future?

My thought is that since it was a full fuse of clear uncoated glass, that I could have cut the Tekta a few times to give the air a place to escape. Is this a sound approach?

2) My Christmas project is an 11-inch Poinsettia Round Texture Mold [LINK HERE]. What can I do to keep the large bubbles from forming when I place my cap glass on top of all the frit?

Bill,
Put the solid piece of tekta on the bottom and there will be no air entrapment (bubbles.) On your schedule, there is no benefit to hold at 1000. You also skipped the anneal. The time at 950 is not the anneal. It is to equalize the temperature. The anneal occurs when you drop from 950 to 750 at 80 deg./hour. Slower for much thicker pieces.

Warren

Warren Weiss wrote:Bill,
Put the solid piece of tekta on the bottom and there will be no air entrapment (bubbles.) On your schedule, there is no benefit to hold at 1000. You also skipped the anneal. The time at 950 is not the anneal. It is to equalize the temperature. The anneal occurs when you drop from 950 to 750 at 80 deg./hour. Slower for much thicker pieces.

Warren

Thanks for the feedback. I'm glad I posted the schedule.

Here is the modified schedule according to your recommendations. I am not renumbering the ramps only so I can illustrate where I have removed steps.
Ramp 2 - Rate/400, Temp/1150, Time/.15 (15 minutes)
Ramp 3 - Rate/50, Temp/1250, Time/.20 (20 minutes)
Ramp 4 - Rate/850, Temp/1450, Time/.15 (15 minutes)
Ramp 5a - Rate/AFAP, Temp/950, Time/.60 (0 minutes)
Ramp 5b - Rate/80, Temp/750, Time/.60 (0 minutes)
Ramp 6 - Rate/400, Temp/100, Time/0

Feel free to clarify.

If your design pieces are only one layer thick you may not need the "bubble squeeze" from 1150 to 1250. Your 15 min. hold at 1150 is not necessary, but it wont hurt anything, just wasted time.

Warren

Soak time at ramp 5a should be 60minutes, not none.

You might also want to take a look at:

http://www.bullseyeglass.com/methods-id ... glass.html

which talks about Bullseye's recommended annealing cycles.