Reconciling Temp Differences Between Kilns

[Steve Immerman]

Hi. After 9 years with a Paragon GL22AD, I just added a Skutt Clamshell to my armamentarium. Both great kilns, but I'm finding that the Skutt fires at different temps than my Paragon.

It seems that I achieve the same effects in the Skutt at temps about 40 degrees lower than the Paragon. eg an "almost" full fuse in the Paragon is 1475, while in the Skutt 1440. At 1475 in the Skutt I get a full fuse, similar to 1500 in the Paragon.

This raises a few questions:

Which one is accurate? (I have noticed many times that other board members are working at temperatures lower than I am to achieve the same results as I do in my Paragon.)

Is there some way to synchronize them?

If I have to adjust my firing programs down for the Skutt, do I just have to adjust the process temp? What about the annealing temps? I've been doing an annealing hold in the Paragon at 960 for 9 years with only one annealing break. I have no way of knowing if the temperature difference between the kilns is a linear relationship, and this 40 degree difference continues at lower temps. It might only be a 20 degree difference at slumping temps. Should I anneal hold in the Skutt at 40 degrees lower?

Any words of wisdom for me? It would be frustrating to have to reinvent every firing schedule for the Skutt.

Otherwise, it's a really great kiln.

Steve

[Carla Fox]

Steve:
I have noticed we have a 25° difference between our big evenheat and small skutt. The small skutt does the same fuse at a higher temp.

I didn't reconcile them. I have a series of test tiles I did in both kilns at their corresponding temps so I know which kiln does what at which temp. (Follow that? )

I find they both anneal correctly. The only change I make is in the process temp. I leave the ramp up and anneal temps the same. So far no problems.

This doesn't exactly answer your question, but it's a different approach.

Carla

[slats]

I really could not help you as you have more experience than I do...just to add I have a Jen_Ken 24 and I get full fuse at 1420 with 15 min soak..still anneal BE at 960.....go figure.............

[Brock]

Steve, I have 3 identical kilns with about a 100 degree differential from hottest to coolest. I just compensate for it. Brock

[Tim Swann]

Steve,

Both kilns are accurate to themselves. You could test the accuracy of the thermocouples, but my guess is they are accurate to within there manufactured tolerance. Things that can affect the fuse temperature are volume inside the kiln (air space) and mass of material being heated (shelves, brick lining, glass…). The small kiln I have I mapped to try and answer the same questions you are asking. In my case I had to go as high as 1525°F to get a full fuse. I questioned the sanity of this and started to measure the temperature of all the different surfaces and objects. What I found was the air temperature was 1525°F, but the glass was only at 1450°F. I found I could get these two numbers closer together by going slower and I could separate them by going faster. I found the biggest difference on the ramp up to fusing and less difference through annealing. As with ramping up, the more mass being heated meant the longer it took to cool down. Unfortunately, I think you will have to come up with new firing schedules for the new kiln.

Tim

[Steve Immerman]

Steve, I have 3 identical kilns with about a 100 degree differential from hottest to coolest. I just compensate for it. Brock

Just the process temps? And is the degree difference the same for slumping as fusing?

Steve

[Tony Smith]

Steve there are so many variables involved in thermocouple measurements that the best we can do is document the difference and, as Brock said, compensate for them.

I've set up identical controllers with identical thermocouples placed next to one another in the kiln and have seen a 50° difference between the two. Without going to very expensive process control electronics (read "not user friendly") you'll have to live with the differences.

Tony

[Brock]

Steve, I have 3 identical kilns with about a 100 degree differential from hottest to coolest. I just compensate for it. Brock

Just the process temps? And is the degree difference the same for slumping as fusing?

Steve

Who knows? I fire them by eye. When it's done, it's done. Brock

[Jo Holt]

"It would be frustrating to have to reinvent every firing schedule for the Skutt. "

Hi Steve,

I have 3 Jen Kens: 18" x 18"d, 24" x 11"d, 41" x 13"d ... and each one fires differently & each has its own firing schedule book. My experience with these 3 is that both size and depth make a difference...coolest is the 24" and highest is the 41".

To add to the mixture, I had to re-write the schedules in the 41" when I tossed out the shelves and replaced them with fiberboard.

My annealing time/temp is the same - I cool down very
conservatively.

Jo

[charlie holden]

One major difference between the two kilns is the arrangement of the elements. The Skutt has all the elements in the lid and the Paragon has elements in the walls, (and maybe a few elements in the lid, I don't know.)

So, if you are firing mostly flat or open faced stuff, the glass is much more exposed to infrared radiation in the Skutt than in the Paragon. This becomes more a factor as the elements glow hotter, (first red then yellow and leaning towards white), and for longer periods of time -- in other words, at higher temperatures. When the Skutt is at fusing temperatures the elements are glowing brightly and the glass is getting blasted directly by IR. In the Paragon you are relying much more on convective heat -- the elements heating the air which then heats the glass because there is less line of sight exposure between the two.

An analogy would be the difference between baking bread and toasting bread. IR toasts the bread much faster while convective baking needs more time or higher temperatures to brown the crust.

Thermocouples read the air temperature and are not nearly as affected by IR as warm glass is.

During annealing the glass, the kiln furniture and the interior of the kiln are already hot. The idea is to control the speed that the heat moves out of the glass back into the air. So it becomes much more a convective process again -- you want to keep the air hot enough that the glass doesn't cool too quickly. You're not trying to pour heat into the glass anymore. The elements only turn on when the air gets a few degrees too cool. The elements might not even get hot enough to glow during the times they are on during annealing, which means there is much less IR bouncing around in the kiln. So the temperatures read by the thermocouples in the two kilns are likely to be more consistent with respect to the actual temperature of the glass.

ch

[Steve Immerman]

Thanks all.

Charlie,

Really good points on the IR/element aspects of the firing.

Steve

[Stuart Clayman]

Glass will slump when it gets to the same temp in each kiln. Your thermocouple is not in the glass. It is measuring Airtemp. if a room is 100 degrees and you walk into it your temp does not raise to 100°. If you walk into a room that is 35° you do not become 35°. If you preheat an oven to 350° how long does it take a piece of meat to raise to that temp? It depends on how large in the oven, how fast the meat heats up, how large is the piece of meat.

The reason that you have to heat something in a little Hot Box to 1500° and something in a larger kiln to 1300° is because the air in the hot box will heat faster.

That is also the reason why one person can slump some thing at 1250 and someone else can do it at 1150° with a longer hold time. It give the glass longer to get to that temp.

So, to answer your question.. which kiln is correct... neither, both, don't care... so the short of it is to listen Brock, and know that one full fuses at one temp and the other fuses at another.... or set program 1 on both kilns to the temp that each needs for a full fuse and then just remember that program 1 is full fuse... or if you don't like that one you can program your controller to add or subtract a certain number of degrees from it's reading. But then again.. your kilns may read 25° off at full fuse but it might be different at a slump temp.

[Bert Weiss]

The temp at which the glass matures is totally irrelevant. Go to the temp that gives you the effects you want in each kiln. Like Brock says, do it by eye.

Heat work is a factor of mass (weight of materials), time, and temperature. Different kilns and kiln setups have differing "qualities of heat". You have to work with what have, to get what you want. Kiln design is a lot about making the effects you want possible.

Annealing is a totally different problem. Most all of us follow some basic directions that essentially were worked out by Ray Alghren and then published by Lundstrom or by Bullseye as guidelines for how to anneal glass. These guidelines are designed to get the entire piece of glass to a temperature where annealing happens, and then slowly lower it's temp through the strain point and then cool as fast as is allowable to end up with a piece of glass with an acceptable amount of strains in it. Ray came up with his schedules by using a chart recorder and several thermocuoples. For thicker work, along with the thermocouples in the air, he placed thermocouples under, in the center, and on the top surface of some glass and then followed the relationship between the air temps and the glass temps. By controlling the air temp, you can make it all happen without all that rigamarole.

Steve, What you need to tune in to, is whether or not your glass is annealing properly in both kilns, using the same annealing schedule. I would suggest doing some testing with identical setups and then maybe the dishwasher test. Before you do the dishwasher test look with a polarizer. In order to do that you must use a clear glass base for the test.

We all hope that you find that the 2 kilns have different top end temps and that they both anneal acceptably using the same schedule. Otherwise everybody needs to worry more.

I would also switch around the thermocouples to see if they are a factor.

I use more sophisticated thermocouples than I see out in the world. Mine are encased inside an inconel steel sheath. The welded thermocople connection is not exposed to the corrosive atmosphere of the kiln. I bought a pair of them for Carol Swann's kiln and they cost $14 each. Not a big deal.

[Tony Serviente]

Steve-Yet another opinion in case you were lacking! I have six kilns, and only two fire the same. Incidentally, the two that give the same results for the same time/temp profile are built the same. One way to get the same results for the same temps, is to vary the time. I could have done that, but it complicates programming and would lead to more errors, and a fuller dumpster. Instead I have about 8 utility profiles, that I use for the work I do. The times are the same on all 8, but depending on the kiln, the target temps are different. What I have found is that with a little experimenting using glass I know making things I have experience with, I can come up with a conversion factor from kiln to kiln. So if I have cooked something in kiln 4, and I want to do the same thing in kiln 2, I just apply the conversion to modify the target temp, and I get the result I want. Good luck.

[Brian and Jenny Blanthorn]

Hi. After 9 years with a Paragon GL22AD, I just added a Skutt Clamshell to my armamentarium. Both great kilns, but I'm finding that the Skutt fires at different temps than my Paragon.

It seems that I achieve the same effects in the Skutt at temps about 40 degrees lower than the Paragon. eg an "almost" full fuse in the Paragon is 1475, while in the Skutt 1440. At 1475 in the Skutt I get a full fuse, similar to 1500 in the Paragon.

This raises a few questions:

Which one is accurate? (I have noticed many times that other board members are working at temperatures lower than I am to achieve the same results as I do in my Paragon.)

Is there some way to synchronize them?

If I have to adjust my firing programs down for the Skutt, do I just have to adjust the process temp? What about the annealing temps? I've been doing an annealing hold in the Paragon at 960 for 9 years with only one annealing break. I have no way of knowing if the temperature difference between the kilns is a linear relationship, and this 40 degree difference continues at lower temps. It might only be a 20 degree difference at slumping temps. Should I anneal hold in the Skutt at 40 degrees lower?

Any words of wisdom for me? It would be frustrating to have to reinvent every firing schedule for the Skutt.

Otherwise, it's a really great kiln.

Steve

I go 4 the heat / work

Heat absorbanivity of the kiln / shelves n set up

Then the direct radiation or not

Remember its the temp of the glass U want but U get the temp of the thermoucouple

There is a test U can do where U put strips of glass in a kiln then take it up a a certain rate then a little bit of maths gives the aneal temps

IF U cant find it mail ask on this thread in a few days n I will find it 4 U

I suggest U do this in each kiln

Brian

[Steve Immerman]

This discussion has made me think of my kiln in an entirely different way. Tell me if you agree:

I had thought of the kiln as a box who's function was to heat the air around my glass, and that hot air brought my glass up to a certain temperature.

But actually, it's a machine that transfers energy into the glass, and then controls the loss of energy from the glass. Although we measure the air temperature in the kiln (because it's easier to do than measuring the kinetic energy of the glass molecules) heating air is not the real goal of the kiln. It just so happens, that we can use the air temperature in the kiln as a guide to what is happening to the glass.

Hmmmm?

Steve

[Tony Serviente]

I would agree with most of that Steve, except I am not convinced that the air is contributing much to the temp. measurement. Try shading a thermocouple from the elements and the result will be dramatically affected. I think what is being measured is primarily the IR impinging on the element, and being reflected off the glass and the interior of the kiln. I would love being able to run the experiment of firing in a vaccuum kiln just to see how different the temps would be to get the same result. Been thinking of trying to build one for years-Maybe I'll find the time before Halley's comet returns.

[Bert Weiss]

I discovered an interesting result. When firing 2 layers of clear float on a sand bed in a kiln with roof elements, The bottom layer of glass heats up and slumps before it sticks to the top layer. This leaves a little bubble where there is a depression in the mold. What is happening is the sand is heating before the glass surface due to it's absorbtion of IR energy. The bottom sheet of glass is actually hotter than the top sheet, so it moves first. I imagine that if I were firing opalescent glasses this would not happen.

At any rate it is a good assessment that there is more going on, than simply the air temp.

[Tony Serviente]

Along with Berts really interesting result is what I have seen when I fire on stainless versus mullite. Stainless fires hotter. I have to drop the temp at least ten degrees to get the same result as I do for my 3/4" mullite shelves. The metal is reflecting the IR more effectively, and contributes to the heating of the glass. Bert-You going to try this with opal?

[charlie holden]

Along with Berts really interesting result is what I have seen when I fire on stainless versus mullite. Stainless fires hotter. I have to drop the temp at least ten degrees to get the same result as I do for my 3/4" mullite shelves. The metal is reflecting the IR more effectively, and contributes to the heating of the glass. Bert-You going to try this with opal?

Or the mullite has a higher thermal mass and is soaking up more heat than the steel before it reflects any back.

If you want to really see the effect of IR fire a base layer with lots of little pieces of glass on top to a full fuse in a top fired kiln. But build a wide bridge over one end so you create an IR shadow.

I think, but it's really just conjecture, that the IR effect may have something to do with how conductive the glass is. Glass is an electrical insulator at room temperature but becomes conductive as it softens. So the IR may have more effect on the glass above the softening point than below.

ch