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strangemagic

New-To-Me Refurbished Kiln Overfires. Where To Go From Here?

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Hello,

I recently acquired a used Skutt 1227 and want to ensure its in good working condition before I begin firing my orders, which are piling up. Unfortunately the kiln seems to be overfiring and my trouble-shooting hasn't helped.

 

I'm new to firing but have read up on what to do. We started by replacing the elements and the thermocouple, and did the following for an 04 test fire:

 

-packed the kiln with furniture

-placed a set of cones on each shelf (03 04 05)

- set the kiln to run at 04, medium speed, with no hold

 

Kiln ran for a duration of 7.32 hours and reported reaching 1948 degrees. The cones however all melted down (image attached). I'm inexperienced here, but does this seem unusual? I saw some discussion about calibration, so is that a normal part of the process that I'm missing?

 

(One other thing I will note incase it could play a factor is that we rewired the kiln to be single phase and are running it on a 50 amp breaker. It is quite close to maxing out the breaker which I understand is not good, but I don't know if that's something that could play in to overfiring.)

 

Thanks in advance for your input!

post-79831-0-12206000-1475377342_thumb.jpg

post-79831-0-12206000-1475377342_thumb.jpg

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When you are firing a new kiln with only furniture, there isn't enough thermal mass for the controller to accurately measure/control what is going on. It is good that you fired the new elements in an empty kiln to set them and oxidize them for longevity, but the heat work is not normal. Load a full bisque load plus witness cone packs in several locations and fire it again. This will give you a better indication of the calibration. A bisque load is preferred not only because it will be faster, but bisque is less sensitive to a cone over or under; glaze is sometimes more sensitive and you don't want to mess up a whole load on calibration that turns out to be off. Once you have a baseline from the witness cones, you can go into the controller settings and adjust either cone offset or thermocouple offset. Read the manual for instructions on that.

 

As for the 50 amp circuit, the kiln will only pull 48, so the breaker may or may not blow. But the issue is that the wire size for 50 amps is rated only for intermittent current draw with a longer period of cooling between draws. In a kiln, the power draw is almost constant towards the end of a glaze firing, (less so for the lower temperature bisque firing) and so the wiring may overheat, and maybe the breaker will pop too. To avoid such problems, the electric code requires such appliances to be on a circuit that is rated at 125% of the nominal amperage of the appliance, or 60 amps for this kiln. That includes not only the breaker, but the wiring between the breaker and the kiln must be at least 6 ga., or heavier if the run is longer than 50 ft.

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How old is the thermocouple? If it has not been replaced recently, I would start by replacing it. Then go ahead and load up a bisque and see how it goes. Like Dick said, it may be fine with a full load. If not, then you may need to set up a thermocouple offset if it's firing hot at all temps, or a cone offset if it's only off at cone 04.

 

The breaker size has nothing to do with the firing temp. But it should be on a 60 if you want to be up to code.

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looks like a well cared for kiln, you are lucky to have found such a good one.  noticed that your shelf looks brand new, no kiln wash on it.  it will help if you use a protective barrier to keep your shelves from damage from possible glaze runs.  there is a great recipe for kiln wash on the forum and it involves calcined EPK so if you are going to bisque things, maybe put some in a bowl and calcine it now.

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Thank you for your responses!

 

Yes, the thermocouple is new, standard Type K. No kilnsitter, just the thermocouple to read the temp. Also, I stopped being lazy and kiln washed the shelves ;)

 

Dick: I did wonder how much the temp would be affected by not having the kiln full. Yesterday I loaded the kiln with mugs, though perhaps not full enough because the results seem about the same. I loaded three shelves with about 10 mugs each. The capacity is 9.9 cubic feet so I could have fit twice as much on the shelves. This time the fire ran about 8 hours, just half an hour longer than the empty fire and reported the same bisque temp. The cones are just as melted down as before, and the wares are visibly fired beyond the usual bisque look. I'm wondering if this still seems normal given the amount I filled the kiln. I can start looking into offseting the thermocouple but I'm feeling pretty nervous about potentially overfiring at glaze time!!  :blink:

Any suggestions on next steps?

 

 

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Thank you for your responses!

 

Yes, the thermocouple is new, standard Type K. No kilnsitter, just the thermocouple to read the temp. Also, I stopped being lazy and kiln washed the shelves ;)

 

Dick: I did wonder how much the temp would be affected by not having the kiln full. Yesterday I loaded the kiln with mugs, though perhaps not full enough because the results seem about the same. I loaded three shelves with about 10 mugs each. The capacity is 9.9 cubic feet so I could have fit twice as much on the shelves. This time the fire ran about 8 hours, just half an hour longer than the empty fire and reported the same bisque temp. The cones are just as melted down as before, and the wares are visibly fired beyond the usual bisque look. I'm wondering if this still seems normal given the amount I filled the kiln. I can start looking into offseting the thermocouple but I'm feeling pretty nervous about potentially overfiring at glaze time!!  :blink:

Any suggestions on next steps?

Assuming that you followed the Skutt info on phase conversion and it started and ended as a 240v kiln, than no change in elements values are needed.  You should look at what offset might have been set for the thermocouple to begin with. You could fire to cone 06 and see what the cone 05,04,03 look like. Cones go from 022 to 01, than 1 to 14, 022 the coolest and 14 the hottest.

The kiln looks in great shape.

David

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david, if this person is new to firing, make sure the cone number mystery is explained.  large numbers with a 0 are cooler than small numbers with 0. 

 

strangemagic, do you have a cone chart to tell you temperatures?

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We're still troubleshooting and haven't had much luck. I'll be calling Skutt asap to see what they can tell me.

 

We felt we had found the culprit when my partner looked at the thermocouple and tightened some wires that seemed loose. He fixed a mistake of the red and black wires going into the wrong place, so they are now as the directions say they should be (Sorry I'm translating what he told me on this). After that I filled the kiln with sacrificial seconds and set it to a slow speed cone 6 fire. Once again all the cones (5, 6, 7) melted to puddles and some amount of ware stuck to the shelves. 

 

 

post-79831-0-10127400-1476058797_thumb.jpg

post-79831-0-10127400-1476058797_thumb.jpg

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Can you measure how fast it is rising per hour?

Can you place a series of cones in front of the peephole and log the temp. against cone melt? You could fire successfully by witnessing the cones even though the thermocouple is dicky.  Analogue or digital meter?

Usually for a glaze firing you start slow then increase the input at a known rate. Not usual to just put a kiln on medium and let it go....

Not looking at the cones till end of firing is very risky in an unknown kiln.

I know this is not helpful for your current problem but switching a kiln on and walking away is really not best practice. Not being helpful or critical just advising here if you are new to firing as you say.

Read up on various firing schedules, start logging your firings thoroughly then you will notice when changes to your kiln occur.

Good luck with  the thermocouple/meter fix.

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I also would toss in there is no need to fire to high cones and melting ware onto shelves until ya get it firing right. I mean once its dead on low 08ish with the cones then you can re-fire to proper bisque temps and if those cones fall correctly THEN run a cone 6 glaze load. I do get you did it above because you thought you had the problem fixed though. At least thats what I would recommend anyway.

 

Good luck, sorry for your troubles.

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