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I've just had my big side-element kiln retrofit with ceiling elements, so it can be used as a top-broiler, so to speak. I'm using it to slump lots of small pieces of glass, on corrugated stainless steel sheet. Question: how close can I safely place the glass to the top elements? 6"? 4"? I'd like to put them high so I'm not wasting my expensive electricity on heating a large empty space lower down in the kiln, and so the new top elements will bring the glass up to slumping temperature faster. And 2nd question: what's the best way to separate that lower empty space so I'm not heating it: fiber batting? load it solid with firebricks?

Hi Carol,

Most glass kilns are about 11" deep. So when you take the height of the mold, and the thickness of the posts/shelf into account there is as little as 4 or 5". If your top elements are close together and evenly spaced then there should be even heat distribution across the glass sheet. If you are close (like 4") then you might want to slow the ramp rate down (maybe 250F per hour instead of 300).

Don't know about your second question. I would guess that a large shelf elevated on a pile of bricks might be reasonably economical.

Cheers,

Bob

Carol,

How deep is your kiln? I have a tall ceramic kiln, and I slump 4 shelves at a time in it with very good results. That really saves on the electric bill. My tall kiln only has side elements. In my large coffin kiln, which has side and top element I often use 2 layers of shelves. The top layer will full fuse and the second layer will tack fuse at the same time.


Jim

Carol Cohen wrote:I've just had my big side-element kiln retrofit with ceiling elements, so it can be used as a top-broiler, so to speak. I'm using it to slump lots of small pieces of glass, on corrugated stainless steel sheet. Question: how close can I safely place the glass to the top elements? 6"? 4"? I'd like to put them high so I'm not wasting my expensive electricity on heating a large empty space lower down in the kiln, and so the new top elements will bring the glass up to slumping temperature faster. And 2nd question: what's the best way to separate that lower empty space so I'm not heating it: fiber batting? load it solid with firebricks?

Allow 4 to 6". You can be as close as 2", but it's not really recommended.

I'd fill up the empty space on the bottom with fiber blanket, not brick. Brick costs more money to heat up and fiber's a better insulator.

Brad Walker wrote:
Allow 4 to 6". You can be as close as 2", but it's not really recommended.

I'd fill up the empty space on the bottom with fiber blanket, not brick. Brick costs more money to heat up and fiber's a better insulator.

A thread a few weeks ago on craft web gave me the impression that there's a point of insulation thickness (I remember 6") after which it becomes a heat sink and begins to degrade efficiency. But that was folks who run their furnaces and lehrs for days, not hours.

I'm interested in close-to-the-glass top elements for speedy flat glass painting.

[quote="db]A thread a few weeks ago on craft web gave me the impression that there's a point of insulation thickness (I remember 6") after which it becomes a heat sink and begins to degrade efficiency. But that was folks who run their furnaces and lehrs for days, not hours.
[quote]

Sounds counter intuitive, especially given that Unifrax makes 10" thick Durablanket modules. Why would they do that if there was a problem with more than 6" of insulation?

Probably irrelevant in this case anyway. Even if I used brick to fill up the kiln, I'd put a layer of blanket on top of the brick. Even a lone inch would be a lot better than brick alone in terms of reducing the cost of heating the material.

Sounds counter intuitive, especially given that Unifrax makes 10" thick Durablanket modules. Why would they do that if there was a problem with more than 6" of insulation?

Probably irrelevant in this case anyway. Even if I used brick to fill up the kiln, I'd put a layer of blanket on top of the brick. Even a lone inch would be a lot better than brick alone in terms of reducing the cost of heating the material.[/quote]

Just plain air would be a whole lot better than brick. You have to pay to heat up bricks. I would pile up just enough bricks to support a 1" or 2" fiber board. The board will keep the heat from the bricks. I agree with Brad that 4" - 6" up to 12" is a good distance.

Brad Walker wrote:
Sounds counter intuitive, especially given that Unifrax makes 10" thick Durablanket modules. Why would they do that if there was a problem with more than 6" of insulation?

I agree. I asked for an explanation, got back an analogy or two, but never attained an understanding. The theory was stated by an authority, so I just assume its true. (I can't believe I'm saying that).