Bubbles in BE from shelf side

[Traci]

I recently started using BE on kilnwashed shelves and have just started experiencing bubbles from the kiln shelf side. I had done a dozen test firings with all sizes, on the shelf and on paper, and only had one bubble before, but now I've gotten 2 in a row (projects over 6" in diameter). Can you please review my schedule and let me know if I'm doing something wrong? I'm heartbroken - test pieces are all fine, now actual projects are getting ruined...........

Evenheat coffin kiln, kilnwashed prefired shelves
BE glass - 1 project had 13" base with small squares on top (2 quarter sized bubbles), and 1 project 10" single layer base with 1" edge strips and frit design in center (1 cup sized bubble)

Firing schedule:
500 dph to 1000, 300 dph to 1300 with 10 min soak (slowed to 200 dph on second try), afap to 1475 with 10 min soak, afap to 960 with 30 min soak.

Any help would be most appreciated! These projects are for other people, and I feel so bad not being able to control this...........

Thanks, Traci

[Lisa Allen]

Check your shelf with a straight edge and see if it is level. I don't think it is your schedule, I'm betting your shelf has some minor depressions that are trapping air in the larger pieces. The solutions are going back to using fiber paper or getting a new shelf.

Also, you might try flipping the shelf over and see if that side is better.

Lisa

[Cynthia]

Evenheat coffin kiln, kilnwashed prefired shelves
BE glass - 1 project had 13" base with small squares on top (2 quarter sized bubbles), and 1 project 10" single layer base with 1" edge strips and frit design in center (1 cup sized bubble)

Firing schedule:
500 dph to 1000, 300 dph to 1300 with 10 min soak (slowed to 200 dph on second try), afap to 1475 with 10 min soak, afap to 960 with 30 min soak.

Any help would be most appreciated! These projects are for other people, and I feel so bad not being able to control this...........

Thanks, Traci

Check your shelf as Lisa recommends, depressions in the shelf are often the cause for bubbles, but so can ramping up too rapidly through the 1350 to full fuse range. Paper will eliminate either cause as the problem.

Are you saying that you are firing with single thickness (3mm) in places? If so, I can see that bubbles would or could be a problem. At around 1400 is where bubble trubble often starts and single thickness along with AFAP to full fuse can both be culprits.

Two things to try, ramp up at a slower rate to your processing temp. AFAP, particularly if you have side elements can cause the edges to seat and trap air. Baffle the side elements with fiberboard or strips of mullite dams if you are inclined, but I would just slow down to 500 dph to full fuse. If you have single thickness this glass will thin or spread to try to achieve and even full thickness of 6mm. This can be manipulated to effect, or it can give you grief. Reconsider your design and construction and see if you can get the effect you want with a more solid layering of glass (If indeed you are full fusing with single thickness in places).

[Traci]

Thanks Lisa - I hope that's it! This is my first serious run with BE, and I had lost all confidence in my scheduling abilities. It is a new kiln and new shelves at a studio I'm helping get up and running with fusing, but I thought I had done enough tests. Guess not....

[Cliff Swanson]

Traci,

Firing on paper generally solves the shelf bubble problem, but if you prefer firing directly on a prepped shelf, the only way to elmiminate air trapping (provided that your shelf is indeed flat, as Lisa and Cynthia mentioned) is to slow way down between 1000-1250 to give the glass a chance to squeeze down to the shelf and elminate trapped air while it's softening over that range, and before ramping faster to your process temp.

Here's a schedule I use that has eliminated shelf bubbles for me:

400 dph/1100 for BE or 1000 for S96/0 hold; 60 dph/1250/0 hold; 600 dph/process temp/10 min hold (usually); AFAP/1050/10 min hold; anneal

Most of my work requires 3-4 firings, and I use this schedule for all except the slump.

Hope this is helpful,

Cliff

[Lynne Chappell]

I have had problems when I used a lot of design elements on top. Like an allover pattern of squares on top of a single layer of glass. I figured there was enough volume to even out the thickness as some squares were two layers high and indeed there wasn't any deformation of the shape. However the bottom layer seemed to rise up off the shelf and the extra weight of the design areas stayed down. I've also had big bubbles form in thinner areas of pieces that had a double layer for bottom. Other pieces that were more uniform in thickness would be fine. In the end, I got a new kiln shelf and haven't had this happen again.

[Traci]

Thanks for all your helpful responses - after much studying of the end result, I believe it is a combination of issues that has prompted the bubbles. I do not have a flat shelf, as predicted, and I firmly believe the single layer of white with a second layer around the edge was the perfect setup for the elementary school "parachute effect" - the edges slumped down first and sealed in any air underneath, which had no where to go but up.......after slowing my fusing ramp segment and using paper, my next projects went fine.

Thanks again, I will not give up!
Traci

[Tony Serviente]

While using paper used to solve bubble troubles, the new thin fire is much denser than the old, and I've had to work through some problems I have not seen in years. Generally, the bubbles arise when the glass size exceeds 12 x 12. The new paper has some great attributes, but letting air out is not one of them.

[wiredthings]

I came to this site through a friends suggestion after talking with her about bubbles appearing under my larger pieces (10" X 10" & 12" X 12"). I have read a lot the postings, but still haven't found anything that seems to really address what seems to be happening to my pieces.

I have been fusing for over 2 years, and until this week was using the old fiber paper under my work. (I make vases, bowls, sun catchers, pendants, wind chimes, plates, etc.) I never had a problem with bubbles with the old, more "breathable" thin fiber paper, regardless of the project size. Yes, I did have to deal with the shrinkage issue, but that was not as bad as the huge, domes bubbles I've gotten in my last 2 projects.

I recently purchased a roll of the new BE fiber paper and, in my last 2 firings (doing everything the same as before) both pieces have been ruined by huge bubbles coming up from the shelf side. The first piece, a future vase, was a single layer of BE 90 COE with a "picture" on it. The center had been left alone as a single layer of glass to allow for a smooth vase bottom after slumping. This is where the bubble formed.

After the first piece was ruined, I consulted others and we decided that the new fiber paper was too dense to allow the air to escape. After much discussion (everything was the same except for the paper), I decided that I should "pin-prick" the fiber paper under the project. I pricked the paper all over. I also cut a slit from the edge of the paper to the center of the project to give the air an additional avenue of escape. Since this new paper doesn't shrink, I was sure I had hit on the perfect solution.

Alas ! My next project had 2 large bubbles ... one very thin which I deliberately broke, and one which was not thin enough to break. These bubbles were to one side of the split and not dead center of the project like the last one was.

It seems that this new paper is causing a major problem with my big pieces. The smaller ones, 3" X 5" and 5" X 7", are just fine on the new paper and I LOVE to finish it gives the backs of my pieces.

Anyone have any ideas how to get past this bubble issue? It has been suggested that I slow my firing process to allow the air more time to escape. Is that all it will take? Is there anything else I can also do. The glass is to expensive to ruin, and each of my "picture" vases takes about 6-12 hours to put together before firing! That's a lot of waste! (Well, the glass is not a total waste because I can break it down for other projects, but that's not the point, is it.)