three large bubbles

[maggie may]

Hi,
I'm new to fusing and am having success fusing and slumping rectangular pieces, about 4 x 7 using two layers of BE glass with different size pieces on top.
The last two round pieces measuring 12" in diameter with the same
glass structure have gotten three large bubbles in the center area, just about in the same places. I am using a Paragon Fusion 8,
Firing schedule is 200 to 1100 holding 30, 100 to 1300 holding 0, 600 to
1495 holding 5 minutes, flash vent, then 150 to 750.
When I look in the peep hole there are no bubbles until after 1300.
Reading Bullseye's knowing your kiln I think it might be heating on
the sides too fast and trapping the bubbles. Do you think it would
help if I lifted the kiln shelf up higher. By the way, I am fusing
small pendants along the edges of the shelf and they are coming out
beautiful.
Any help would be greatly appreciated. Thanks, Marilyn

[Kitty]

hi marilyn. check your kiln shelf with a straight edge, like a yardstick on its edge, and see if it's flat. i had a warped kiln shelf when i started fusing, and it created large bubbles. there's a ton of stuff written about this in the archives. use the "search" button above, and you can learn all kinds of stuff.

[maggie may]

Thanks Kitty. I have checked my shelf and it is flat.
I'm thinking that maybe I will get a smaller shelf as mine is
very close to the edge of the kiln and some posts that will
raise it a little higher. am I having trouble with the larger
circle because they are larger? Maybe I should also go to
1450 instead of 1495.
Marilyn

[Jerry Barnett]

Marilyn, I assume the bubbles are occurring between the glass and the shelf. There are a number of possible causes, but those bubbles can be eliminated by firing on fiber paper. However, I would continue to fire the pendants directly on the kiln-washed shelf as their back side will be smoother.

I assume you have the shelf up on posts now. I don't know the element positions for the Fusion 8 kiln. If there is a single side element near the floor of the kiln, elevating the shelf further may help, but I don't think that is the problem. I don't think a smaller kiln shelf will help.

I would suggest a couple of changes in your schedule. In the second step I would ramp to 1250 instead of 1300 and hold there for at least 30 minutes. This is intended to eliminate as much air between the layers of glass as possible. By "flash vent" I would mean opening the kiln to rapidly drop the temperature to, say, 1100. If that is what you are doing, I would suggest just opening the kiln long enough to observe that you have gotten the results you want, and then let it drop at its own rate to 960 and hold for 30 minutes and then power down to 750.

The bubbles will not form until 1300, or higher than that, because the glass is too stiff at a lower temperature to move much.

The higher the temperature, the easier it is for bubbles to rise through the glass. Depending upon your kiln, you may be able to get the fuse you want at 1450, but that may be too low. In general, 1495 is not an excessive temperature for full flat fusing.

Hope this helps.

Jerry

[maggie may]

Jerry,
Thanks for the help. My kiln has top and side elements. I will
try tomorrow fusing on the paper and use your suggestions for
firing.
I'll let you know how it turns out.
Thanks again,
Marilyn

[Kitty]

marilyn, i dont know why these things seem to happen when we're first starting out, but i had an experience much like yours. firing on fiber paper or thin fire is one way to overcome the bubbles. in my case, i bought a new shelf, and with a strong push from a couple of people on this board, i finally graduated to bullseye kiln wash, and have not had any trouble since. i might add, for my jewelry i use 50% alumina hydrate and 50% kaolin mixed together for kiln wash. best wishes to you to solve the problems. believe me, i know how disappointing it is to see these bubbles and have a ruined project.