kiln shelf bubbles

[Susan B]

I am new to fusing. The last three pieces I have done, about 8x10 inches, have all had a rather large, rather annoying bubble in them. This is a bubble from under the bottom layer of glass.

After doing the second piece, it crossed my mind that the bubble might be due to the kiln shelf itself. So the third piece was placed on the shelf in the same position as the second. And yes the bubble was in the same place.

What is the probablility there is a flaw(?) in the kiln shelf that is causing this?

I have read that primers might cause these types of bubbles. I am using the HotLine brand of shelf primer. and I did NOT re-prime between the second and third piece.

[charlie]

it's high probability that it is caused by the shelf. put a straightedge across it looking for dips. you can either replace it, or use fiber paper, as a solution.

[revjerry]

It's a sad commentary, these kiln shelves. It's the result of sending our jobs to countries where labor is cheap and quality doesn't exist.

I went through the very same issue about a year ago when I bought two new shelves from Evenheat. Bubbles the size of grapefruits. I put a level on the shelf and was absolutely amazed at the amount of daylight I could see under it. I called Evenheat and complained. They checked EVERY shelf they had in stock and told me the every one was warped. Since they started buying their shelves from central america, this has been the problem. At least they refunded my money.

I called every shelf supplier I know (my wife is a potter so I know quite a few) and every one of them said they had to stop buying their shelves from Americal sources because the forign shelves were so much cheaper. When I explained the problem, they all admitted that none of their shelves had the quality they used to.

That's the history, now what to do about it? I've done several things. First, a thick piece of fiber paper stops the bubbles. By thick I mean any other than thin fire. I do a lot of flat fusing to add borders on pot melts and rakes so I've got to be able to do this without bubbles but I don't like the texture from fiber paper or lava cloth.

I beat the problem by making my pieces thicker; three layer border plus the center disc. Also, I slowed down my schedule and didn't go as high. I've found with Spectrum '96 I can get a great fuse at 1450 with a one hour soak and no bubbles.

You might find a carpenter with a belt sander wide enough to grind your shelf down but don't count on them being too anxious to help.

You aren't the first or the only person to have this problem. Maybe if enough of us complained to ONE supplier, we could get some decent quality again. Probably not; it's become the American way.

Jerry

[Brad Walker]

This isn't a foreign supplier thing, it's a matter of the people who are buying the shelves from the suppliers not requiring that the shelves be flat. When the importer cares enough (or knows enough) to make that a priority, they can get perfectly flat shelves (no matter where they're made). The shelves that Bullseye sells (which are both flat and foreign made) are proof of that.

Remember, most kiln shelves are made for potters, who don't care if the the shelf is flat or not. It's our responsibility to educate the people who supply us shelves. If they know that flat is a desireable characteristic, they'll make sure their suppliers (wherever they're located) get the job done.

[charlie]

You might find a carpenter with a belt sander wide enough to grind your shelf down but don't count on them being too anxious to help.

Jerry

belt sanders up to around 36" are pretty common at wood working places, like ones that make custom kitchen cabinets or sell hardwoods.

you can also level shelves by grinding them together. you need 3 to ensure that they come out flat. just put two of them together with a little bit of 200+ blasting grit in between, and rub them against themselves for a while. switch off between all three, and it wouldn't take too much to get them flat because they're pretty soft. you'd wind up with 3 flat shelves. if you only use 2, you'll get one concave and one convex.

[Brock]

This isn't a foreign supplier thing, it's a matter of the people who are buying the shelves from the suppliers not requiring that the shelves be flat. When the importer cares enough (or knows enough) to make that a priority, they can get perfectly flat shelves (no matter where they're made). The shelves that Bullseye sells (which are both flat and foreign made) are proof of that.

Remember, most kiln shelves are made for potters, who don't care if the the shelf is flat or not. It's our responsibility to educate the people who supply us shelves. If they know that flat is a desireable characteristic, they'll make sure their suppliers (wherever they're located) get the job done.

It's been done. Buy the very good, flat shelves from BE. No problems. Brock

[Bob]

Yes the quest for flat kiln shelves is our version of the holy grail. However once you find a good shelf they are true for years. So it pays off to be fussy. The fitrst time I went to the local ceramics supply store with a straight edge they scoffed. They still snicker but at least they acknowledge the shelves aren't flat.

Bullseye sells shelves that are consistently good quality. They are pricey but worth it.


Cheers,

Bob

[dee]

I am new to fusing. The last three pieces I have done, about 8x10 inches, have all had a rather large, rather annoying bubble in them. This is a bubble from under the bottom layer of glass.

After doing the second piece, it crossed my mind that the bubble might be due to the kiln shelf itself. So the third piece was placed on the shelf in the same position as the second. And yes the bubble was in the same place.

What is the probablility there is a flaw(?) in the kiln shelf that is causing this?

I have read that primers might cause these types of bubbles. I am using the HotLine brand of shelf primer. and I did NOT re-prime between the second and third piece.

in my small kiln i was having trouble when i first started firing coaster size pieces with getting huge honkin' bubbles and those shelves are VERY flat - problem was in the firing schedule - you don't say how fast you are ramping up but for an initial fusing with multiple layers i don't ramp any faster than 350 dph to 1000 and 125 or 200 dph after that - going fast isn't always a good thing....

and i have no problem with bubbles on my decidely un-flat shelves with hotline hifire primer, 1st, 2nd or 3rd use of a shelf between preppings...

D

[Cynthia]

in my small kiln i was having trouble when i first started firing coaster size pieces with getting huge honkin' bubbles and those shelves are VERY flat - problem was in the firing schedule - you don't say how fast you are ramping up but for an initial fusing with multiple layers i don't ramp any faster than 350 dph to 1000 and 125 or 200 dph after that - going fast isn't always a good thing....

and i have no problem with bubbles on my decidely un-flat shelves with hotline hifire primer, 1st, 2nd or 3rd use of a shelf between preppings...

D

Ditto. This is a simpler and more cost effective solution to try first. I altered my schedule (for my Jen-ken with side elements) rather than seeking the holy grail. If you have side elements either slow your ramp rate down, soak at 1200 to let your glass squeeze that trapped air out, or baffle your work from the direct heat of the elements...use fiber paper. You can also try flipping your shelf and firing on the other side to see if that is a better surface.

[Bob]

Dee and Cynthia,

Your suggestions are definitely a viable way of working around the problem. If I had my druthers I would rather buy a new shelf and save approximately 2 hours of fusing during the ramp through the brittle zone room temp to 1000F (I fire as much as possible at 600F/hr in this temp range). It would save electricity and allow me to recycle the kilns for more frequent firings. I avoided the fibre paper route because it is costly over time, it is a health hazard, and it doesn't give the smooth back I like.

A flat shelf also gives me a small amount of peace of mind when firing projects. A couple of ruined pieces is a fair amount of material and time.

Your suggestions are definitely viable. I'm just putting a different spin on "economy".

Cheers,

Bob

[revjerry]

Brad,
Your comments are well taken but you might wish to talk to some other kiln suppliers before you make blanket statements about them.

Evenheat and Paragon both said they had been aware of the problems glass crafters were having and had put their supplier on notice, all to no avail. They were buying cheap and getting cheap. Notwithstanding Bullseye and their purchasing power, the other guys don't seem to weild that kind of influence.

From time to time, I visit a pottery supplier in Portland, Maine, with a straight edge in hand. They know what it means and may make subltle comments under their breath, they've come to respect what I'm doing and don't give me any grief; especially since I'll buy a shelf if I find a good one - I have five kilns.

Jerry

[Siyeh]

I bought a new Skutt clamshell in Januaray and the first three times I fused in it I had honking bubbles--four 8" square plates each time and 1-1/2" - 2" bubbles in each one. Major disappointment. So I changed my firing schedule. I now fire everything 400-600 /hr 0 - 1250. Then I hold at 1250 for 45 and ramp to 1450 at 400 /hr. Not only am I not getting the big bubbles, but I am getting very few little bubbles in pieces up to 18" diameter. I occasionally get some devit, and I can usually get rid of it with a thick coat of FuseMaster Super Spray on a re-fire.

Brenda Griffith
Siyeh Studio

"I love deadlines.
I especially like the whooshing sound they make as they go flying by!"

[Alice]

I'm a total novice here and I have been having the same problem, bubbles here, there and everywhere.
I haven't had a chance to check if my kiln shelf is level but I have been firing slower (the information, and inspiration, I have attained from this warm glass site is second to none, thank you!).
I have noticed though (and I haven't a clue if there is any relevance here at all to my own problem) that at times when I have opened the kiln door slightly to check if a piece is 'done', it has been fine. Five minutes later or even sooner, there is a bubble!!.
I have also had bubbles occur when I haven't opened the kiln door, but does this somehow encourage them ( a draught somehow getting under the glass, or a kiln with a wicked sense of humour!).