1/2 Shelves For More Even Firings?

[clay lover]

I am STILL struggling to get my fairly new skutt to fire more evenly. I have talked to the tec rep several times, nothing is getting better. I still have a cone difference between the middle where the thermocouple is and the top and bottom, even tho Skutt says in their adds that they have prevented that by having hotter elements in the bottom and top to even things out.

 

I do not have a vent, the kiln is outside. on a covered slab.

 

I have a friends who gets even firing tell me she always uses 1/2 shelves for ventilations, seperating them slightly at the middle and staggering them in height when ever possible. she doen't have andy whole shelves so her whole kiln is done this way.

 

I tried this last firing but I only have 4 1/2 shelves, so I arranged them staggered and slightly separated at the center, above and below the thermocouple, with solid shelves at the top and bottom of the kiln. WOAH,, the uneveness was WORSE!! A full cone off, way cooler at the bottom. This was with the top and bottom peeps out, and the lid slightly lifted for the first 1,000*, as Skutt recommends if you don't have a vent.. slow rise, 108* hour, to peak temp of 2000 and 15 min soak. The cones at the thermocouple were way overfired for cone six, the 6 was flat, the 7 down and touching. The cones at the bottom barely made 5.

 

Should I have used the solid shelves around the thermocouple to 'block ' the heat, and the 1/2 shelves at the bottom to 'let the heat in'?

HELP !!!

[Marcia Selsor]

If you have the bottom of the kiln cool, keep the plug in the bottom. Have you tried putting and empty shelf at the top near the lid over the entire load?

I would keep staggering the half shelves as much as possible and avoid having them even on the same level unless you have large platters.

Use nothing shorter than 6" on the bottom level of pots. Maybe have your largest work in the middle so it is less dense. Play with the density of your loads.

See what effects you get.

[clay lover]

Marcia, do you mean that I should put LESS? ware in the middle where it is firing hotter? I have considered borrowing more 1/2 shelves to see if that would help.

Do I need MORE circulation where it is firing cooler?, less where it is hotter?

[Marcia Selsor]

This is an electric kiln correct? Bigger ware in the middle..less shelves.

I am thinking about both ways I have fired. Shorter but not plates on th bottom . Plates can crack from faster cooling on the bottom. You said the hot spot is the center, then pack it thinner i.e....bigger pieces , less shelves, ALso using a shelf at the very top beneath the lid, retains heat. Just MHO.

And stagger the shelves as much as possible. I would use 6" to start, maybe a short 1/2 shelf with a taller one opposite, then reverse the pattern.

It isn't the same as flame patterns but it still is heat distribution. And shelves absorb much of the heat. So distribute them without sealing off radiation. Plug in the peep hole except possibly the top one. Plug it when you are finished. If using a venting plug them all but the top.

M

[clay lover]

Thanks, Marcia,

 

I will try some of these changes next week. this last time I had 4"post on the bottom, a solid shelf 3" posts, another solid shelf, then the staggered shelves, 1 set 2"under the thermocouple, one set 2" above it. then a solid shelf with taller things, on up to the lid with only the thin, tall handles of a shelf full of basket forms ,14" tall but only the bottom 6 " was solid work, just handles the other 8" up to the lid.

 

Every time I do a glaze firing, I think,"This time it will be better, but so far, ........

[Marcia Selsor]

I would try putting the basket handles or tall pieces around the thermocouple. and use 1/2 shelves with the break perpendicular to the thermocouple and not at the thermocouple.

You just need to get to know the kiln and what works. Keep trying and keep notes.

[GEP]

clay lover,

 

I actually disagree with Marcia a little. I think you should put MORE density in the middle of your kiln where it is firing hotter. I agree that your bottom shelf should be at least 6" tall, and same for your top shelf. Put your taller pots there, and pack them loosely, about an inch between the pots. In the middle of the kiln you should stack shorter shelves with shorter pots, and pack them as tight as possible. The extra mass takes a lot more energy to heat up, therefore it will slow down the heat rise in the middle while the top and bottom can hopefully keep up.

 

I'm not the foremost expert in electric kilns, but this is what works for me.

 

Also, do you have any manual control over the elements in the top and bottom section, separate from the elements in the middle? Or is your kiln is controlled by a computer that makes all of its decisions from that one thermocouple in the middle? If the latter is true, it's hard to imagine that it will ever fire with complete evenness. I have a three zone kiln that fires with a computer reading three thermocouples (top, middle, bottom) and when I watch the kiln fire, all three zones are doing completely different things. If you don't have any separate control over the elements, your best solution might be to select clays and glazes that can tolerate a 1 cone difference.

 

-Mea

[clay lover]

clay lover,

 

I actually disagree with Marcia a little. I think you should put MORE density in the middle of your kiln where it is firing hotter. I agree that your bottom shelf should be at least 6" tall, and same for your top shelf. Put your taller pots there, and pack them loosely, about an inch between the pots. In the middle of the kiln you should stack shorter shelves with shorter pots, and pack them as tight as possible. The extra mass takes a lot more energy to heat up, therefore it will slow down the heat rise in the middle while the top and bottom can hopefully keep up.

 

I'm not the foremost expert in electric kilns, but this is what works for me.

 

Also, do you have any manual control over the elements in the top and bottom section, separate from the elements in the middle? Or is your kiln is controlled by a computer that makes all of its decisions from that one thermocouple in the middle? If the latter is true, it's hard to imagine that it will ever fire with complete evenness. I have a three zone kiln that fires with a computer reading three thermocouples (top, middle, bottom) and when I watch the kiln fire, all three zones are doing completely different things. If you don't have any separate control over the elements, your best solution might be to select clays and glazes that can tolerate a 1 cone difference.

 

-Mea

[clay lover]

clay lover,

 

I actually disagree with Marcia a little. I think you should put MORE density in the middle of your kiln where it is firing hotter. I agree that your bottom shelf should be at least 6" tall, and same for your top shelf. Put your taller pots there, and pack them loosely, about an inch between the pots. In the middle of the kiln you should stack shorter shelves with shorter pots, and pack them as tight as possible. The extra mass takes a lot more energy to heat up, therefore it will slow down the heat rise in the middle while the top and bottom can hopefully keep up.

 

I'm not the foremost expert in electric kilns, but this is what works for me.

 

Also, do you have any manual control over the elements in the top and bottom section, separate from the elements in the middle? Or is your kiln is controlled by a computer that makes all of its decisions from that one thermocouple in the middle? If the latter is true, it's hard to imagine that it will ever fire with complete evenness. I have a three zone kiln that fires with a computer reading three thermocouples (top, middle, bottom) and when I watch the kiln fire, all three zones are doing completely different things. If you don't have any separate control over the elements, your best solution might be to select clays and glazes that can tolerate a 1 cone difference.

 

-Mea

[clay lover]

Mea, If I'de known then what know now>>>........

 

I have the Shutt 1027 with the computor controlls. I don't think I have an override, but this kiln is "ADVERTISED "to have hotter elements on the top and bottom to "INSURE" that it fires evenly, just read the adds.

 

Is my frustration showing?

The kiln is advertised to have a cone fire mode, but it doesn't have the holds that my glazes seem to need. I have been using a 5 ramp program with a slow (108* hr), climb from 1800* to peak temp of 2200. then a hold at peak of 15 min. then controlled cool to 1700*. That program was giving me a correct cone 6 at the thermocouple, but too cool top and bottom. It was when I tried the 1/2 shelf spacing approach the it got way MORE uneven.

 

Another question?????

 

How do I know if the glazes I am using have a tolerence range? The last firing, the clays didn't even mature, the speckles in speckled brownstone didn't show up in the pieces in the bottom half of the kiln. never got to ^6.

[GEP]

If you buy commercial clays, it's easy to choose clays with a tolerance range. Choose something with a cone 4-6 range, or cone 5-7. Don't choose one of those cone 6-10 clays, because they aren't truly mature at cone 6 anyways.

 

As for knowing about your glazes, the only way I know is to ask yourself "does it look right?" Or does it look underfired or overfired? Some glazes are really picky about temp, and some have quite a big range.

 

But the next thing I would try if I were you, is to fire your next glaze load to cone 7. Hopefully the top and bottom will hit cone 6, and the middle will be a little overfired, but find out if your clays and glazes can handle it.

 

Don't be frustrated! I don't think this range of temperatures is all that unusual. Just remember all the ancient potters firing with wood and dealing with variables much bigger than this. They learned how to adapt and so will you!

 

-Mea

[Marcia Selsor]

clay lover,

 

I actually disagree with Marcia a little. I think you should put MORE density in the middle of your kiln where it is firing hotter. I agree that your bottom shelf should be at least 6" tall, and same for your top shelf. Put your taller pots there, and pack them loosely, about an inch between the pots. In the middle of the kiln you should stack shorter shelves with shorter pots, and pack them as tight as possible. The extra mass takes a lot more energy to heat up, therefore it will slow down the heat rise in the middle while the top and bottom can hopefully keep up.

 

I'm not the foremost expert in electric kilns, but this is what works for me.

 

Also, do you have any manual control over the elements in the top and bottom section, separate from the elements in the middle? Or is your kiln is controlled by a computer that makes all of its decisions from that one thermocouple in the middle? If the latter is true, it's hard to imagine that it will ever fire with complete evenness. I have a three zone kiln that fires with a computer reading three thermocouples (top, middle, bottom) and when I watch the kiln fire, all three zones are doing completely different things. If you don't have any separate control over the elements, your best solution might be to select clays and glazes that can tolerate a 1 cone difference.

 

-Mea

Maybe you are right...Figuring out stacking in kilns takes trial and error. I agree that more densely packed takes more energy. I was thinking of the radient heat and more densely packing would hold it... Theory, not practise. I am most familiar with gas kilns with 35+ years experience there but have been firing ^6 electric in large ovals lately. The ovals seem to fire pretty even. The tall skinny electrics IMHO are harder to keep even. I had understood the problem was hot in the middle..but thought it was densely packed.

Marcia

[Chris Campbell]

I think this is kind of like blaming the camera for bad pictures ...

It takes a long time and a lot of practice to get better than your camera!

 

I cannot say enough good things about controlled cooling ... firing down.

This does wonders for preventing cracks and improving glaze results.

It also gives you a nice even firing from top to bottom.

 

I fire my Skutts up and down on all but my bisque firings and the difference

from top to bottom is half a cone ... The center is fired just a bit higher to

make sure the bottom is right on.

 

If you have computer controls check the manual for the controlled cooling

program and try it. You might like the results.

 

All my shelves are half shelves simply because I cannot handle those full sized

ones ...

[clay lover]

Thanks Chris, for the suggestions.

I do have the manual for the kliln, I had not used the cone fire function because it does not have a hold at peak temp, I had been told by Skutt that I needed to hold for 10-15 min at 2200 for more even firing.

I have also talked to a man at a large S East show with beautiful cone6 elec glazes that said, "Lady,just hit ^6 and START and go back to the house".

[Marcia Selsor]

That is great advice Chris. I have been experimenting with my Axner kilns with Barlett controllers. And I am discussing this with other ^6 electric firers concerning the iron reds and yellows.

Soaking does help and firing down and holding to enrich glazes has made some startling improvements on certain glazes. I have also been working with underglazes and clears where the down firing doesn't show much impact on that particular glaze. And as is true with so much in clay...practice leads to getting it right.

[hansen]

Just a couple of thoughts, I just finished a cone 8 firing which was not only uneven top to bottom but under-fired, and the kiln took WAY too long. Time for new elements and a tune-up. So I share your frustration.

 

"Soak and hold" is one of the key rules to getting more even temps. Areas of greater density of wares will take longer to heat, but will hold heat longer. Holding heat longer is the big bug with electric kilns, they are really fast-fire kilns. "Ramping down" is another thing that will help quite a lot. I would research these topics. The big advantage to programmable controller units is that you can program in the firing curve you want. You'll probably have to do that.

 

What the man said is true if you are using neph sy or volcanic ash with lots of gerstley borate substitute. We covered that topic already, however. A straight calcium glaze with just a little potash will be underfired one minute and runny the next, and is particularly difficult in an electric kiln.

h a n s e n

 

Thanks Chris, for the suggestions.

I do have the manual for the kliln, I had not used the cone fire function because it does not have a hold at peak temp, I had been told by Skutt that I needed to hold for 10-15 min at 2200 for more even firing.

I have also talked to a man at a large S East show with beautiful cone6 elec glazes that said, "Lady,just hit ^6 and START and go back to the house".

 

 

 

[Kelly Savino]

A few suggestions:

 

First, check your elements. They don't go all at once, and they will weaken and slow down long before they give it up completely.

 

Second, check your thermocouples. It's easy to bash one with a kiln shelf and they can get worn and corroded.

 

Next, stagger half shelves. A full shelf makes a heat dam that separates top from bottom. The more space between right and left shelves, the better.

 

Consider that you may be firing too fast. Our guild kilns, instead of firing to ^6 and shuttign down, get set to ^5 with a 20 minute hold. It evens out the kiln heat.

 

Finally -- is your kiln vented?

 

Best of luck!

Yours

Kelly

[Marcia Selsor]

To add to everyone's advice..Mea, Chris, Kelly, Eric, and others. I went back and was reading some of your original posts saying you got 2.5" brick walls rather than 3''. I personally think the thicker walls actually do help with retaining the heat inside the chamber and may actually help make the kiln fire evenly. My new kilns have 3.5" plus fiber insulation. I know people who have removed the steel jacket and placed fiber insulation around the kiln and covered it back up. You could also place a piece of fiber on top of the kiln to help insulate. Also, the shelf at the top, over the load inside the kiln will help to slow the cooling as well. Also spraying ITC on the walls , floor and lid will help.

I think your kiln is leaking heat, #1. That is the most critical. Insulate it.

Marcia

[clay lover]

The Skutt manual says to leave out the top and bottom peeps and crack the lid till it reaches 1,000#. then lower lid. Doesn't that leak alot of heat?

 

What is ITC? amd where would I spray it? inside?

 

 

i am soaking and firing down, from 2200 down 500#/hr to 1900, then 125#/hr to 1700, then off. Are you guys recommending a more controlled fire down than that?

I did a continuity check on the elements. and I treat the thermocouple like gold, it hasn't been bumped. No one uses the kiln but me.

When the kiln is firing, if I remove the peeps and look in at every level, the elements are all the same color, does that indicate anything? But I can put my hand near the peep hole and the air coming out the bottom peep is noti cably cooler that the other[peeps.

 

Kelly , if I could get ahold of more 1/2 shelves, should I line up the gaps or rotate each level, crisscrossing the gaps from one shelf up to the next.

[Marcia Selsor]

The ITC is sprayed onto the lid. The walls with the elements should not be sprayed until you change the elements. There is a different ITC for metal (element)

If you have all three plugs out, (this is true on gas especially) the bottom will be pulling air in and the top pushing out. At least the pressure is usually much less on the bottom than the top.

I have only been doing Ox. firings for about 8 years and the firing down for much less than that, three I think.(Actually used oxidation in the 70s for a few years for crystalline glazes.) I hold at ^6 for 20-30 minutes, I just drop mine til 1850 and hold it for 1/2 an hour. This has really improved the iron reds. People all do this a little differently. Some insist on 1900. One friend soaks for 2 hours at 1800 or so. See what works for your glazes.

My kiln came sprayed with ITC. Very well insulated with fiber and thicker brick walls. It fires pretty even. The tall skinny one is a 1/2 cone off . The oval is even.

[Kelly Savino]

I line up the gaps, so the heat can weave back and forth through the open spaces.

[clay lover]

Well, I made changes as suggested, but nothing improved

.

I used the 1/2 shelves in the bottom and top, seperated as much as I could without having them too close to the sides. Bottom set 6" off the floor, with solid shelves and more ware above and below the thermocouple by about 2'. Solid shelf at top. Bottom and top plugs in, 3 plugs out at center where T.C. is.

 

I had the elements checked and the thermocouple changes by my instructor, he ran some diagnostic programs, and checked alot of things and didn't find anything wronge. He is as puzzled as I am, says the same kiln at school doesn't have these issues, and is loaded every which a way by students who don't even know what a thermocouple is, kilns fire within 1/2 cone even, top to bottom, using the same glazes as I use at home.

 

When I ran the firing, I used a slow,108*/hr rise to the last 200*, peak at 2200, 15 min soak, 400*/hr drop to 1900. Then 125*/hr to 1700, off.slow cool to 4 Same schedule as before. Only changes were the shelf arrangement and putting the bottom plugs back in.

 

Results:

cones at the thermocouple hard 7, ^6 flattened, 7 down and knuckled over. This is a cone hotter at the center than I had b4 with the same firing schedule. top and bottom a solid ^6. So basicly, the kiln fired a full cone hotter, but just as unevenly. A full difference from center up and down.

 

What do ya'all recommend now?

 

I know I'm supposed to keep trying, but I don't know what to try next.

 

What is ITC, we've not heard that expression used.

[Guest JBaymore]

 

 

Just finally read this thread.

 

Heat transfer in an electric kiln is predominantly radiative, with conduction thrown in there too. (Even on one with a vent system.) For stacking, you have to think like electromagnetic radiation to get a handle on the prime heat transfer. Radiative heat transfer works a lot like the way we understand that light behaves. Line-of-sight is important. So for example, you can create "shadows". Also important is the inverse square law (the further away from the source... the WAY lower the power availabe there). So the interior center of a shelf will heat slower than the edges near the elements. As is the idea of reflective surfaces. Heat energy can reflect (or emmitt .....too complicated to get into) off certain surfaces. So a big piece with a cone pack wedged between it and the electric element near a wall likely is showing you the impact of heat work in only THAT micro-clime in the kiln... not the whole kiln. The energy got concentrated on the ware and cone pack there...... and is not typical of the rest.

 

Understanding how thermal mass acts is also important to stacking and firing. It takes the 100% application of well less than one BTU to raise one pound of most clay materials one degree F. The more pounds of clay you have, the more BTUs you need there to raise the temperature by one degree. If the BTUs available are the same, but the mass of clay is less, the greater the temperature climb of that mass will be in the same unit of time.

 

Surface area exposed to a radiative heat energy source also figures in here. An object with 100 square inches of surface exposed to a given radiative source will absorb twice as much heat energy in a unit of time as an object with 50 square inches of surface, given the same mass.

 

On the cooling side of life, the larger mass of clay will be storing more BTUs for any given temperature. Since to cool it has to give this off to the surrounding environment, and since the emissivity of a low mass clay object and a larger mass clay object is the same for any given temperature, the object with the same surface area that has more mass will cool at a slower rate. This energy can end up affecting nearby pieces as they, having already cooled more rapidly due to lower mass, actually absorb heat energy from the higher mass (and higher temperature) object. Posts and shelves and the kiln structure itself figure in here too.

 

So from the above, TIME in the firing cycle can be an important factor. Rate of climb, and soaking can have a dramatic effect. But they interplay with the other stuff.

 

Think about all that stuff as you are loading. Distribute the load based on how heat enegy will flow in that kiln.

 

Probably the most important thing I can think of that you can do right now is to install a local pickup kiln vent system on the kiln. Not one of the hoods located over the kiln.... but a downdraft pickup that is directly connected to the kiln. That will add some convective heat transfer into the equation which is going to help even things out.

 

 

Try stacking the 1/2 shelves in this kind of manner......

_____________

][ ][

][ ][_____________

][ ][ ][

_][________][_ ][

][ ][ ][

][ ][ ][

_][________][__________ ][_

][ ][ ][

 

And so on. No two 1/2 shelves on the same layer level. Start the load off with a full shelf for the bottom layer.

 

Set the bottom shelf on 1 1/2" to 2" tall posts off the kiln refractory floor. You don't need more. Probably could use 1". Don't stack ware closer than about 2" from the top of the lid surface....and that is the top part of taller pieces... not a layer of plates.

 

If at ALL possible, do not line up the edge of any shelf with the groove that the elements are running in. Or do this with as few shelves as possible, for sure.

 

Do not crowd the elements with overhanging wares. Do not place large vertical flat-sided pieces near the edges of the shelves so that they basically block the energy radiating from the elements. Keep the posts away from the elements by at least a few inches. Do not "crowd" the area of the thermocouple with wares in a manner that is not similar to the rest of the load. Also... do not leave the area more "open".

 

Scatter cone packs all over that kiln and fire again. Record the variations in end heat work around the kiln. That likely will be time and money well invested (this is true for anyone reading this with any kiln..... you'll likely be amazed....or depressed ........ if you have never done this). When debugging a clients kiln, I sometimes might use 50 cone packs in a 50-60 cubic foot kiln. And remember that this test only shows you the END RESULT. It does not tell you ANYTHING about the even-ness of temperature at any temperature lower then the end. And at the end it tells you cone (heat work) anyway, not temperature.

 

Try a glaze firing of all bisqued wares on a program of a linear 100 degree F an hour climb to the desired end point, with a programmed cooling of 50 F per hour for the first 3 hours of the cooling cycle then let it drop naturally. Use the "scattered cone packs" test for information on results.

 

 

Backtracking a bit...........

 

Continuity is not the issue on the elements.... it is the resistance value of the element that is important. That determines the exact heat output of a bank. And to test EITHER continuity or resistance you MUST remove the connections to the kiln switches and totally isolate the elements.

 

(I have to mention that electrical safety is a prime concern here... make sure the kiln main is OFF... and cannot be accidentially switched back on by anyone. Then test all three leads of the 220VAC for power before assuming it really is all off.)

 

Once you've isolated the various elements... compare them to the manufacturer's specs for each bank of elements (they are not always the same)..... and if they are more than 20% off this value... replace them.

 

In dealing with the kinda' "obvious" things that are often so easily missed.... for a computerized controller to work corretly all of the kiln's main element switches need to be set to full "on" (or at 100% ) to work correctly. Check that this is so. Also if the kiln has vairable switches on the banks of elements... it is possible that one of them is "shot". I don't want to get into how to test/preplace these here..... if you know what you need to know about electrical matters to do this safely... you already know how to do this.

 

Also... what kind of shelves are you using? How much clearance is there between the edge of the shelf and the side wall of the kiln? What kind of posts are you using? How many do you use per 1/2 shelf?

 

What is the voltage rating listed on the metal plate on the kiln unit? What is the supply voltage you have on your main panel (you might have to ask your electritican)? Has the voltage at the kiln unit been tested under load? Is the kiln hard wired into the electrical service ot is it plugged into a receptacle?

 

Doing this stuff via postings on a board is like being a doctor trying to diagnose illnesses from afar on the telephone. It makes it FAR more difficult.

 

best,

 

.....................john

 

EDIT: Crap... my nice carefully spaced keyboard drawing didn't show up right! SOrry.

[GaragePotter]

Your kiln is not working right. It is supposed to fire reasonably evenly, and a cone difference between the middle shelf and the top and bottom is too much of a difference. It is not a matter of stacking the shelves differently. You have tried that, and it hasn't helped. It is not a matter of needing to make extensive modifications to your kiln, which would void your warranty. It is a new kiln, so wear isn't a factor. I would call Skutt again and tell the tech that you've tried leaving out various peeps and you're still having trouble with the evenness of the firings.

 

I have a 3-year old 1027 that I fire regularly, and over time I've started having some trouble with the evenness of the firings top to bottom. Some parts might be starting to wear on my kiln. But when it was new, it fired evenly enough that I never had a problem with the glazes maturing at the top or bottom when the middle shelf's cone read a good cone 6 firing, and even now I can put the bottom shelf up a few inches and stop loading a little before the top and get pretty good glaze maturity throughout the kiln. It sounds like your kiln is firing so unevenly that you can't really use anything but the middle of the kiln, and that's not how the kiln is supposed to work. Skutt should fix it under warranty.

[clay lover]

Well, time for an update.

My instructor got on the phone with the Skutt tec and they had several long conversations on my behalf. Photos of a years worth of highly uneven cones were exchanged. Greatly appreciated.

 

Upshot is, Skutt agreed that since I have been having these issues since day one , even tho my warranty has expired, they sent me a new touch pad and controller.

 

Results, 1/4 cone difference between shelves, all thru the kiln. What I was expecting from the kiln when I bought it.

 

So it was a kiln malfunction, and now I can get on with my pot adventure.

 

If my instructor had not intervened on my behalf, I expect I would have been dead in the water, nothing I would have done would have evened out the kiln. I appreciate all your advice, and efforts to help me with it at the time.