Fusing with two shelves

[Lori]

I am fairly new to fusing. I have a Skutt 1018 Kilm and have thus far only used one shelf in it, though it has the ability to hold two. The internal diameter of the kiln is about 23" and the shelf I have is slightly smaller than that. Can I put a second shelf in of the same size or must it be a partial shelf? I read somewhere that you need to ensure air can circulate freely in the kiln. Before I buy the second shelf, I'd like some advice.

A second question is - this kiln has pre-programs for different operations and you select the speed of firing based upon the size of the piece being fused. So what if I have 2 pieces that are 8 inches in diameter versus one that is 16 inches in diameter. Would I select the same firing schedule for both scenarios since the two pieces combined is the same size as the one? Or is the firing schedule based upon each individual item in the kiln?

[Jerry]

Two shelves or twelve; it makes little difference as long as you remember what you are doing. Glass needs to be heated evenly. Using a kiln with side fired elements complicates that issue because the heat comes from the side rather than the top, as in most glass kilns. So, you need to slow down your ramp some and everything will be just fine.

As far as the number of shelves, I know one crafter locally who fires into bowl molds three deep daily and rarely has any difficulties. You already said that you understood the air/heat must be able to circulate to each piece of glass. You are correct; now, go try a couple, keep tons of notes, even pictures and let us know how things went.

As far as the schedule goes, plan for the worst case. In your example, it's the larger piece. If you get that one right you will probably do well with the smaller one.

I fire in two kilns regularly for my business. One is a Paragon front loader with elements in the roof; the other is an L&L ceramic kiln where I can use more than one shelf as you've described. Typically I use the L&L for castings and pot melts because they get tall sometimes and it's easier in there. Beside, with a deep casting I WANT the heat coming from the side so the glass inside the mold gets heated evenly.

Go do something neat and tell us about it later.

Jerry

[catdoran]

hi Lori, I also have a ceramic kiln olympic 1818, and I almost always fire two shelves at a time. the lower shelf I position in the middle area of the kiln and the upper shelf about 3/4 to the top. I have never had any problems, and have both fused two shelves at a time and slumped 2 at a time. To be safe, I do follow a pretty slow firing schedule and it takes almost 24 hours to complete a full fuse firing. Slumping goes much faster.
Most of my pieces are about 10 - 11 inches diameter.

[Ann Demko]

Hi. I guess ignorance is bliss. I have a side fire ceramic kiln that I use for both fusing and slumping. I have fused two shelves at a time and base my firing schedule on the larger piece. I don't make any accomodations for firing two items together because I figure if the kiln temp is adequate for one then it should be ok for the other. I do put my larger piece on top because I figure it is slightly hotter ( heat rises). Don't know if that is the case but it has always worked for me. I also make sure my top shelf is smaller in diameter than the lower to help air flow. I guess you just have to experiment and take notes as to what works for your setup. Ann Demko

[bskirwin]

I was a potter before I started fusing glass , and I have a skutt KM 1027 which is larger than your kiln. I often fire 4 shelves deep, but I keep my ramping at around 300rph at the most and I hold often to even out the heating. You will also get to know your kiln and where the hotter areas are and are not..... The highest shelf often does not get as hot, which goes against the laws of physics, but there it is. I also make sure there are at least two layers of elements open to the piece at each level, so that you are getting heating from two elements at least on each level.... Hope this helps. Barbara

[John Kurman]

which goes against the laws of physics, but there it is

Well, no. Your kiln is quite happily following the laws of physics (if laws, as in Laws, there be, but that's a philosopical discussion we need not get into here).

Although heat rises, and you would expect the top shelf to be hotter, you have placed your shelf within a resonant heat cavity (which is what a kiln is, and quite frankly, beats the crap out of the wheel when it comes to inventions). The whole point of a resonant heat cavity is to trap the heat and make it bounce around all over the place within the cavity. Although there is a temperature differential between the top and bottom of your kiln that differential (tens of degrees or so) is negligible compared to the average temperature (thousands of degrees).

Plus, your shelves act as heat sinks, so they retain more heat the more you stack them. They are, in turn, acting as resonant heat cavities - bouncing heat between them. I think, if you observe, that you will see that pieces between shelves tend to be fusing hotter for longer. That is, the temperature between shelves gets a little hotter, and stays that way longer than the top shelf.

[bskirwin]

thanks for that info. You are absolutely right, and now I am better educated because of you. Thanks. Barbara

[charlie]

Plus, your shelves act as heat sinks, so they retain more heat the more you stack them. They are, in turn, acting as resonant heat cavities - bouncing heat between them. I think, if you observe, that you will see that pieces between shelves tend to be fusing hotter for longer. That is, the temperature between shelves gets a little hotter, and stays that way longer than the top shelf.

that's way interesting. why isn't there also a cavity between the top shelf and the bottom of the lid? is it because of the elements in the lid being a heat source, and there is no heat source in the cavities in the middle? what about elements in the sides then?

[John Kurman]

Basically, we have two things going on with heat. Surface area and emissivity.

Let’s get the E-word out of the way.

Emissivity is physicist-speak for how well and how quickly a material will transfer heat. This depends upon the type and internal arrangement of the material, but we can ignore this. (As an aside, some materials used in space and high tech industries, are near perfect reflectors of heat). Metal, for example, has higher emissivity than refractory material like a kiln shelf (ignoring silicon carbide). The electron shells of metals have something to do with it, but mainly it’s the densely packed atoms of metal that allow high emissivity.

Dense refractory material, like a kiln shelf, has higher emissivity than fire brick. Fire brick has higher emissivity than air. Why? Air ain’t as dense, and so there are fewer atoms to absorb and emit heat. What’s fire brick? Lots of air, with some refractory material surrounding it. What’s kiln shelf? Not as much air.

Okay, now surface area. This determines how many atoms are available to absorb and eventually re-emit IR photons. For a side-fire kiln, the sides of the kiln shelf absorb less heat than the faces. True, the sides are closer to the elements than the center of a kiln shelf, but they still have less surface area to absorb heat. (I’m ignoring heat diffusion within materials, which gets into all sorts of hairy math involving Reynolds’ numbers and Boltzman’s constant and stochastic models and probability and statistics and so on).

So once you get all those IR photons bouncing around in the kiln, chances are higher that they will hit a shelf face rather than a shelf edge. And shelves being relatively thin, chances are that the IR photon – even if it was absorbed at an edge – will find it’s way back out of the shelf on a face, rather than an edge. So over the long haul, the heat tends to bounce between the shelves.

So what about the top and bottom? Well, surface area again. A “square inchâ€

[Lori]

I had good success in full fusing pieces on both shelves. I put a large piece on the top shelf and two small circles on the bottom. But it when it came to slumping the same pieces, the top large piece slumped perfectly. The two small circles barely started slumping on the bottom shelf. Anyone have any ideas on why this would happen? I would have thought if anything that the larger piece would not slump if any of them were going to have problems.

[Paul Tarlow]

I had good success in full fusing pieces on both shelves. I put a large piece on the top shelf and two small circles on the bottom. But it when it came to slumping the same pieces, the top large piece slumped perfectly. The two small circles barely started slumping on the bottom shelf. Anyone have any ideas on why this would happen? I would have thought if anything that the larger piece would not slump if any of them were going to have problems.

The larger piece on top has two things going for it:

- more direct heat from lid elements

- more weight to help move the glass

- Paul

[Lori]

My kiln does not have lid elements - only side. What do you suggest the next time - should I heat the kiln to a higher temp or should I fire smaller pieces separately from larger pieces and adjust the temp accordingly?

[Bob]

Lori,

I'd slump the two sizes separately. One of the learned statements from Graham Stone, one of the early contributors to the Board who was a tremendous wealth of knowledge, pointed out that slumping rate is a function of span (not spab)... the width of the unsupported glass on the mold. The wider the span the lower the slumping temperature.

Either the large piece slumps first and the smaller are underfired, or you have to overfire the large piece to get the smaller pieces to fully slump. The consequence of overfiring is the possibility of leaving marks from the mold on the bottom of your bowl.

Hope this helps.

Cheers,

Bob

[Lori]

Thanks for the advice. I"ll slump separately next time.

[Nancy Juhasz]

Lori, I'm a newbie to this warm glass thing also. I was so bummed when I found out that I couldn't fill the kiln with various things at one time and have everything comeout just right. What I have found to be true with my kiln is that if I want a full fuse on one piece and a contour or relief fuse on another piece and I am useing glass from the same mfg. I can put the relief fuse on the bottom shelf and the full fuse on the top shelf and all comes out well. My kiln has side and top coils. Nanc

[Lori]

What is a countour or relief fuse? I'm not familiary with those terms.

[Kathie Karancz]

Hey guys: I too am thinking about the Skutt 1018 because I would also like to make ceramic tiles (the 1018 is a kiln that is deep enough to fit tile setters and gets hot enough to do ceramics but is also a glass kiln). As my current kiln only has elements on the top, am I going to have to relearn everything because I want a side and top firing kiln??? What problems am I going to run into??? Now I'm scared....
Kathie from Calgary

[Nancy Juhasz]

Lori, relief or contour fuse is like a tack fuse. It just depends on how much you want the edges to round down into the bottom layer. Honestly when they are in the bottom shelf I take what ever happens since I can't see what's going on. This is the terminology I picked up on theSpectrum firing sched. Nanc

[charlie]

Basically, we have two things going on with heat. Surface area and emissivity.

<big snip>

I, uh, I pruned a lot of what might be construed as potentially confusing and unnecessary technical jargon out of this explanation. Hope I did not sacrifice clarity for brevity. If you don?t buy into this explanation or still not clear, I?ll try again tomorrow.

- In Chicago, Once-Upon-A-Time physics geek,

John

actually that was very clear and answered the question beautifully. please feel free to delve into physics-speak. there's a lot of techies loitering here.