Kiln Shelf Spacing - WarmGlass.com

Kiln Shelf Spacing

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tgotch
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Kiln Shelf Spacing

Postby tgotch » Sun Jan 27, 2013 7:42 pm

My kiln shelf has 1/2" space from the wall to the shelf. I was thinking of cutting a new shelf a bit bigger, which would give me more shelf space for some projects. This would give me only 1/4" space between the shelf and the wall. Anyone see any issue with this?

Also, any good ideas for where to buy a shelf I can cut?

The Hobbyist
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Re: Kiln Shelf Spacing

Postby The Hobbyist » Sun Jan 27, 2013 8:04 pm

No problem if you have skinny fingers.

Jim
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Marty
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Re: Kiln Shelf Spacing

Postby Marty » Sun Jan 27, 2013 8:53 pm

That's a lot of trouble to gain 1/2".
Might be time for a larger kiln.

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Re: Kiln Shelf Spacing

Postby Laurie Spray » Sun Jan 27, 2013 10:10 pm

I am with getting a larger kiln and using this one as is. That mentality has gotten me to have 7 kilns and looking at an 8th!!!! Don't listen to me!
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tgotch
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Re: Kiln Shelf Spacing

Postby tgotch » Mon Jan 28, 2013 12:15 am

Well, my problem is, I just got this kiln. So getting another one right away would not go over well on the home front. :shock:

I don't see it as a huge issue with adding a shelf(unless I am missing something), buy it, cut it and drop it in - save about a grand :). I only need 2 sides to be wider. The other two can stay the same, so I can get the shelf in and out easily.

Anyone have any recommendations for shelves?

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Re: Kiln Shelf Spacing

Postby Laurie Spray » Mon Jan 28, 2013 12:35 am

How big is the kiln....shelf....?
Laurie Spray

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tgotch
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Re: Kiln Shelf Spacing

Postby tgotch » Mon Jan 28, 2013 12:41 am

Shelf will need to be 10 1/2" x 17 1/2" once cut.

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Re: Kiln Shelf Spacing

Postby Bert Weiss » Mon Jan 28, 2013 12:36 pm

With roof elements, during the heatup, the space near the walls is colder than the interior. During the cool down, it is hotter. This is exaggerated if your kiln is made of firebrick, as opposed to ceramic fibers. In order for glass to anneal, the entire piece of glass has to be within 5ºC inside the annealing range. So, it may not be impossible to anneal real close to the walls, if you catch it just right, but it is a LOT more likely that you will not. If you soak too long, the glass will be much closer to the air and shelf temperatures. If they are uneven, the glass will be uneven. If you soak just right, it can work, but you really have no way of really knowing how you did, until it breaks.

The larger the kiln, the more space you need to leave. I would never go closer than 1" in any kiln. In my big bell kilns, I leave 3". If your original shelf is 1/2" from the walls, I would leave 1/2" of that shelf perimeter empty of glass.

Bullseye describes know your kiln tests. You stack a few layers of glass and place these identical pieces at different places inside the kiln. Then you fire them, to somewhere beneath a full flat fuse for the center one. You will be able to compare how the heatwork differs in each spot.
Bert

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Morganica
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Re: Kiln Shelf Spacing

Postby Morganica » Mon Jan 28, 2013 4:00 pm

Seems like a lot of trouble and money for just a half-inch, unless you just need an extra kilnshelf to begin with. Whether it has much effect depends in part on element placement and overall kiln volume. If you've got side elements, the edges of your work will be a bit closer to the elements and get hot faster, with less air circulation to even things out. If the change restricts airflow enough to make a cool spot under the kilnshelf; you might need to adjust schedules a bit and overall fire a bit slower. That means your firings will take a little longer.

That said, 10x17 isn't a huge footprint so the kiln volume isn't going to be such that there's a whole honkin' huge difference (I'd think). If the half inch is REALLY critical, you might think about just getting some fiber board and rigidizing it for only those firings.
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Re: Kiln Shelf Spacing

Postby Kevin Midgley » Mon Jan 28, 2013 4:13 pm

Using fibre board will have a different thermal mass than the kiln shelf that would require compensation during the annealing cycle.

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Re: Kiln Shelf Spacing

Postby Morganica » Mon Jan 28, 2013 5:02 pm

Kevin Midgley wrote:Using fibre board will have a different thermal mass than the kiln shelf that would require compensation during the annealing cycle.

Precisely, and the reason I suggested it.
Cynthia Morgan
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Kevin Midgley
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Re: Kiln Shelf Spacing

Postby Kevin Midgley » Mon Jan 28, 2013 6:37 pm

Just wanted to make it doubly clear to tgotch and other readers.

Bert Weiss
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Re: Kiln Shelf Spacing

Postby Bert Weiss » Mon Jan 28, 2013 6:49 pm

Kevin Midgley wrote:Using fibre board will have a different thermal mass than the kiln shelf that would require compensation during the annealing cycle.

Kevin, I disagree. I never change my annealing programs relative to whether I am firing on the floor, on a fiber shelf, hard shelf, or on sand. Similarly, anneal programs are the same for brick kilns and fiber kilns. Anneal cycles are not precise in that they control the air temperature. The glass and mold follow the air temperature at some different rate. So at any given moment in a program, the glass will be at a different temperature in different kiln setups, but in the long run, the glass will go through an acceptable ramp down to anneal and not break. If we were controlling anneal segment times based on the actual glass temperature, then the time periods necessary would be speeded up with a low mass kiln and shelf.

I have seen published anneal schedules for float glass that are half as long as the ones I use. Maybe they would work? but I don't really need to find out.
Bert



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