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Kiln Overfiring


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Hello! 

I recently got a second hand kiln, and I've done around 4 glaze test firings now, but they keep overfiring. I wonder if anyone has any tips? My target is cone 6 1220C. For each test, I have put in a 6 and 7 cone (please see pictures https://imgur.com/a/fxAzkLB ). They are definitely improving, as more and more definition can be seen in the cones. I think test 4 shows that the kiln is getting to about cone 8 now, maybe? But the clay is still blistering a little, and it's suppose to be ok between 1140-1280C. 

Test 1: 

1st: 190C/hr to 1100C (5hr47)

2nd: 60C/hour to 1220C (2hr)

Hold: 20mins

This was the worst, the test pot I put inside bloated and got quite toasty in colour on the base.

Test 2: 

1st: 160C/hr to 1000C (6hr15 )

2nd: 60/C to 1220C (3hr40)

No hold

Test 3: 

1st: 160C/hr to 1110C (7hr03)

2nd: 60C/hr to 1195 (1hr24)

No hold

Test 4: 

1st: 180C/hr to 1110C (6hr06)

2nd: 60C/hr to 1190C (1hr30)

No hold. 

I'm thinking it could also be affected by only having one pot in the kiln, instead of it being full? It's a 50L Keramikos kiln. But I'm scared to put a whole kilns worth in incase they all get overfired! Aha. Could it also maybe be a problem with the thermocouple? 

Thank you for any advice,

Shannon

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 I would suggest following the cone chart and firing the last 100c at the rate shown in the column or 60 c for the center column. If it fires too hot you can always drop to the cone 5-1/2 peak temp, still maintain the last 100c firing rate and dial that in. this is easy enough to do until you test a solid cone six.

Oddly though things begin to over fire when the kiln fires slower than that rate. As kiln elements wear out, this does happen as their final segment where glazes mature actually is slower than the prescribed speed. Your calculations for the time are spot on, but is that what it is actually happening in real time?

Finally thermocouple offsets are made for the occasion so it is possible that the thermocouple is incorrect. How accurate is it at room trmperature?

 

 

EE1136E0-ECFA-440C-A5E9-29738834EB1E.jpeg

Edited by Bill Kielb
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2 hours ago, Bill Kielb said:

 I would suggest following the cone chart and firing the last 100c at the rate shown in the column or 60 c for the center column. If it fires too hot you can always drop to the cone 5-1/2 peak temp, still maintain the last 100c firing rate and dial that in. this is easy enough to do until you test a solid cone six.

Oddly though things begin to over fire when the kiln fires slower than that rate. As kiln elements wear out, this does happen as their final segment where glazes mature actually is slower than the prescribed speed. Your calculations for the time are spot on, but is that what it is actually happening in real time?

Finally thermocouple offsets are made for the occasion so it is possible that the thermocouple is incorrect. How accurate is it at room trmperature?

 

 

EE1136E0-ECFA-440C-A5E9-29738834EB1E.jpeg

Hello,

Thank you so much for your reply! 

That's great, thank you, I'll try dropping another cone and see what happens.

Hmm, I wonder if the elements are worn, as I set up a camera and checked the temp every hour against what it should have been. I found that the kiln follows the ramp spot on, until it gets to about 1170C and then it slows down, and added about an extra 15 mins to the total time. Would lowering the final temp help with this too? 

I didn't have an accurate room temp at the time, but when comparing the kiln against the outside temp, it seemed about right! I'll have to try and measure that more accurately. 

Thanks again! 

Shannon

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17 minutes ago, ShanRums said:

I found that the kiln follows the ramp spot on, until it gets to about 1170C and then it slows down, and added about an extra 15 mins to the total time. Would lowering the final temp help with this too? 

Really interesting find and firing to cone 5 with a 15 minute hold gets you to cone 6 so firing with the equivalent of a slow rate, very likely gets you cone 6 or more. 

If that’s the issue, then the elements will continue to degrade and it will be very hard to get it to fire properly. Since your kiln was lightly loaded, If all this is true, fully loaded would make it worse.

Generally when elements wear by 10% they need replacement so measuring to see if their resistance has risen by 10% or more from their new value is a good way to confirm.

we are not sure all the elements are working though, so be sure they ALL are as well to rule that out.

Edited by Bill Kielb
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10 hours ago, Bill Kielb said:

Really interesting find and firing to cone 5 with a 15 minute hold gets you to cone 6 so firing with the equivalent of a slow rate, very likely gets you cone 6 or more. 

If that’s the issue, then the elements will continue to degrade and it will be very hard to get it to fire properly. Since your kiln was lightly loaded, If all this is true, fully loaded would make it worse.

Generally when elements wear by 10% they need replacement so measuring to see if their resistance has risen by 10% or more from their new value is a good way to confirm.

we are not sure all the elements are working though, so be sure they ALL are as well to rule that out.

Ahhh okay, interesting. So the extra 15 minutes makes a big difference! 

The ebay seller said that they had recently changed the elements, so I was hoping it wouldn't be them. But I'll buy a multimeter today and give them a check over. 

Thanks for your help! 

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You are whistling up using your firing schedules!!! Doubt elements are worn if you are acheiving 160 and 180 deg C/ hr.

Seriously overfiring. What controller are you using?

What state is your thermocouple and pyrometer in?

Place your cones in line with peephole and taking a log of you firing and readings throughout the firing, turn off when cone 5 is over and cone 6 is touching its toes, cone 7 will be on the bend. Use an eye protecting goigles.

By logging the hourly rises in temp coupled with the C/ hr amount through to cone 6 touching toes will give you the guide you need for future firing.

Your log book will be your best friend.

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4 hours ago, ShanRums said:

The ebay seller said that they had recently changed the elements, so I was hoping it wouldn't be them

If it’s not making 60c per hour at the end then definitely check them. Actually no harm in knowing how to check them. Again though, make sure they are ALL glowing.. if one set is out because of a relay or connection it can also be a reason it slows down excessively at the top temperatures even though it was an empty kiln.  If what you saw on camera and have been confirming with cones is correct, it appears under powered right now. How fast it goes in the beginning can be less material indication. If it cannot achieve its end firing speeds when it needs to have all power available, that is very material. The task is just to confirm why. Loose wires and bad relays, improper wiring are a thing. Make sure all elements are glowing as well, one of the simplest of things to check first.

 

Edited by Bill Kielb
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Sure, elements may be wearing but the kiln is being overfired. The chart Bill provided shows the different end result firing at different rates. Logging and turning off using observation will give a guide to adjusting the controller to get the heatwork required.

If controller, thermocouple dicky, new elements expensive and poss not required.

OP could be at 1220 or above by 5 hrs firing by orton rate of firing chart, just saying.

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@Babs
Hmm, not sure where this is going but if the kiln does not make 60c per hour in the final segment nearly empty it likely will degrade rapidly from there until no amount of extra time will make it work. If this is as it appears, calibrating may work for a short while but very likely won’t work on a full kiln. If true, It cant make a suitable rate empty, when full its rate likely will be unusable even with the best of recalibrating. Calibrating and compensating won’t make it fire faster at the top temperatures. All speculation though, And hopefully a minor problem like a set of elements out, bad relay, bad connection. I think finding out is the prudent course of action though.

Edited by Bill Kielb
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8 minutes ago, Bill Kielb said:

@Babs
Hmm, not sure where this is going but if the kiln does not make 60c per hour in the final segment nearly empty it likely will degrade rapidly from there until no amount of extra time will make it work. If this is as it appears, calibrating may work for a short while but very likely won’t work on a full kiln. If true, It cant make a suitable rate empty, when full its rate likely will be unusable even with the best of recalibrating. Calibrating and compensating won’t make it fire faster at the top temperatures. All speculation though, And hopefully a minor problem like a set of elements out, bad relay, bad connection. I think finding out is the prudent course of action though.

The Op was initially writing of overfiring her kiln. Target cone 6

Guess that is my focus.

The images of the cones show mega overfiring issues.

The time taken, test 4, shows 1hr 30, 10mins more ...but it is an empty kiln, no mass would affect the time imo.

Wonder what the elements are made for as they are being asked to hit around  at least cone 9 by look of those cones.

Maybe banks of cones on each shelf, shutting off when peephole cones indicate, get to find out what is happening in there.

Clay is blistering also...

Test one temp. rise in last ramp is spot on

Test2: ditto

Test 3: 1 minute faster....

Test 4: 10 mins out

Needs to focus on reading the cones imo. May safe countless firings way above what is needed.

What is this kiln rated at re cone to fire to or what can be drawn,  re electricity .

Just thinking, not scientist.

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With cones on the shelf so that you can see them from the peephole, run it at 150C/hr up to 1050C, then 60C/hr to 1250C, and watch the cones, When cone 6 drops, note the temp and shut it off. Then do another firing with the same program with the last 100C at 60C/hr to whatever that temp was and see if it works. It's really far off for thermocouple calibration, sot best to just figure out what temp you need.

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@Babs @neilestrick

It was mysterious as her times and ramps seemed good especially at the end of firing, but when I asked if these were actual or just calculated she did watch this and explained:

Hmm, I wonder if the elements are worn, as I set up a camera and checked the temp every hour against what it should have been. I found that the kiln follows the ramp spot on, until it gets to about 1170C and then it slows down, and added about an extra 15 mins to the total time”

So it looks like she did the hard work and figured out the kiln runs out of speed at the top. I think worn or bad element connection, relay etc… is really likely. Recalibrating around one of those conditions is not very likely.

Edited by Bill Kielb
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Ok but if goes from 190, 160, 15oC/hr to 60C/hr for last 80 or 60 degrees it will slow down but it still falls within the time expected to reach opt. Temp albeit actually way too high etc.

It did not add extra time except in test 4.

only 1 deg/ minute after all. 

Can you guess the geatwork,temp required to puddle those cones.

Loose connection thermocouple to controller?

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I don't think the kiln slowing down is going to cause over firing of 3-4 cones. If the elements are worn enough that they're slowing down at the end, it's not going to be able to slow down enough to get that kind of heatwork without the controller putting up an error code. I think this is a calibration issue or a thermocouple issue. I replace elements in dozens of kilns every year and over firing has never been a result with worn elements. Or if it was it was so little that it wasn't an issue. One time I even intentionally fired one of my kilns until the elements were so worn that they couldn't reach cone 6, and it never over fired. The cost of firing doubled, but it had no issues with accuracy.

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We don't know what the temp is when it's raising at 60C/hr do we as the cones are indicating a much higher temp was reached that what the controller is showing. Therefore rate of climb at  60C/hr doesn't really indicate worn elements.  What we do know is the kiln is far exceeding the programming and displayed temps. Simple solution would be to do as Babs and Neil suggested and ascertain the temp when target cone drops then go from there.

How many thermocouples does your kiln have? What type of controller? Can you input a target cone rather than a target (top) temperature?

edit: I noticed you are using a broad firing range clay, 1140-1280C and it looks like you are making functional pots. For a clay to be mature at 1280C it can't also be mature at the lower temps. For functional pots that don't weep it's best to keep the absorption down to around 1.5% or lower, might want to check what your clay is once you get the firing sorted out as the clay might still be fairly porous at cone 6. 

Edited by Min
added a thought
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1 hour ago, Min said:

Therefore rate of climb at  60C/hr doesn't really indicate worn elements.  W

She programmed the last segment for 60c per hour but timing it realized it was off trac, not going 60c per hour and taking longer. So what we know is it could not achieve 60 c per hour once it got above 1170C, according to the OP tracking it. The kiln only exceeded time, not temps so far as we know. The cones reflect the heatwork which at this point appears to be the extra time spent during the final segment.

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22 hours ago, Bill Kielb said:

 

Hello everyone and thank you for your responses!

Hopefully I'll answer all of your questions.

The controller is an Ipco Studio 3000, it only lets you input a target temp and not a target cone.

Unfortunately, my kiln doesn't have a peep hole, so I'm not able to watch until the cones bend.

The kiln can fire to 1260C and it has one thermocouple.

I wasn't able to get a multimeter from my local store yesterday, so I've ordered one online instead. I thought I'd do a bisque firing while I wait for that to arrive, as I had lots of pots ready to go. And I recorded the temperatures again. Weirdly, the controller shows that the temperatures rises at 100C/hr no problem, until it gets to about 800C and then it starts to slow down again, by 35C/hr, and then 50C/hr for the last 100C. Which confuses me, as when doing the glaze firing it was able to get to 1170C before slowing down. I didn't have any cones for this temp, but the pots came out fine, nice and pink.

When we got the kiln, we noticed one of the wires connected to the thermocouple had snapped off, so we removed the bit stuck in the hole and wired it back in again. Perhaps it has something to do with this? (Sorry, should probably have mentioned this earlier!) I'll get my partner to check again later for any loose wires, and report back.

What I don't understand though, when the kiln is cooling down, I wait for the controller to get below 50C before I start taking stuff out, if the temperature reading was incorrect, wouldn't everything inside be way too hot to touch?

Thanks for your help everyone.

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I think smart to check the elements but also, have you checked that they all are glowing  and one set is not out? You did a lot of the hard work by confirming that the kiln was slowing down especially at the top temperatures, you reasonably verified the thermocouple is reflecting room temperature and proven as best you can that even when lowering the endpoint temperature from 1220 to 1190 the kiln still over fires because it takes more time than programmed. That you did these things is quite impressive actually, but they point to not enough power available at the top temperature. You can’t recalibrate or offset your controller to make it fire faster and you have already offset by 25c and only improved things slightly.

All things point to reduced power or or an element not working, it’s logical now to test why you do not have enough power at top temperature. I think everyone understands that elements are expensive and are hoping for another fix including me. I think it’s time to find out why, hopefully you will find a loose connection or come back and tell us one relay was not working.

Definitely make sure all connections are sound and in good order though, thermocouple or other. One thing we did not discuss and has been assumed is if it is operating at it’s required voltage so time to measure all that as well with your new meter.

1260c is cone nine, so this kiln ought to get 100 - 150 firings on an element set before it starts slowing down while reaching cone 6 temperatures and requires new elements. Generally 10% less power and kilns start to take more time until they simply can’t achieve a decent cone 6 glaze firing.

Edited by Bill Kielb
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1 hour ago, ShanRums said:

The controller is an Ipco Studio 3000, it only lets you input a target temp and not a target cone.

Yes it uses a target temperature rather than a target cone. But doesn't it also support program segments and ramp rates?

https://thermaltechnologies.com.au/wp-content/uploads/2020/06/IPCO-Studio-3000-Manual_1_.pdf
image.png.4fcede663ec76b90b7c42ffc91a4734f.png

Edited by PeterH
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On 10/3/2022 at 9:47 AM, ShanRums said:

Test 4: 

1st: 180C/hr to 1110C (6hr06)

2nd: 60C/hr to 1190C (1hr30

Going back to basics here and Bills comments about it taking longer to fire in the final (2nd) segment.

Doing the math, 1190 - 1110 = 80 degrees difference.

Rise of 60/hr to make the 80 degrees = 1 hour and 20 minutes in theory. Actual time spent 1 hr 30 minutes means only took 10 minutes longer than programmed. This in no way would account for your puddled cones. Extra 10 minutes is likely because kiln is far hotter than what the controller is showing, when kilns get up to the higher cones the kiln can struggle to keep up.

Conclusion,  it's not worn elements, it's either the thermocouple or a calibration problem.

3 hours ago, ShanRums said:

When we got the kiln, we noticed one of the wires connected to the thermocouple had snapped off, so we removed the bit stuck in the hole and wired it back in again. Perhaps it has something to do with this? (Sorry, should probably have mentioned this earlier!) I'll get my partner to check again later for any loose wires, and report back.

 

Could you post some images of this? 

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3 hours ago, Min said:

Conclusion,  it's not worn elements, it's either the thermocouple or a calibration problem.

Maybe, but the thermo couple slows down after 1170 and displays room temp ok but fails a 25C correction. Her comment was an observed 15 minute delay which is much like a 15 minute hold even with a 25c depressed final temp.. Firing rates don’t slow down if the thermocouple is miscalibrated. She seams to bisque Ok, not necessarily over fire in the bisque because the controller or thermocouple is off. I think her symptoms point to being under powered so checking would be wise. I have reasons for what I see, I really don’t see anything that contradicts the progression. Under powered can be elements, relay, connection, voltage so I think its wise to check. Not sure why anyone would discourage it actually?

Edited by Bill Kielb
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Posted (edited)

Hello!

Just checked the elements and they are all working. The multimeter has also arrived, and all the elements are reading between 9.2-9.4, I emailed the manufacturer and they said the resistance when new should be 9 - so that all seems good.

Something we noticed was that the other thermocouple wire was damaged (see pictures here https://imgur.com/a/wbN9bBX). The white wire was the one that was completely detached and we rewired it when I first got the kiln, so it's annoying we missed the orange wire! But oh well, we've found it now.

Do you think this could be the culprit? I couldn't find any other loose wires. Am excited to do another test tonight and see if it works! I only have 2 cones left though and they're sold out everywhere, so I hope this works :') 

Thanks for your help everyone!

Edited by ShanRums
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