Kiln over-firing due to cold weather?

[Ceramics.np.04]

 

 

This is only about the sixth firing I've done with my kiln but it is verrrrrry old and a little worn out HOWEVER recently the temperatures where I live have been around -9°C (16°F ish) and my kiln has obviously taken a little longer to reach the desired temperature out in the shed/studio.

I forgot to put a cone pack in this firing but it's obvious from the results that something has gone a bit awry. Glazes that are normally very stable have become a lot more fluid (and ran off), underglazes have completely burnt away etc etc.

I don't really know what I'm talking about but is it possible that because it took so long for the final section of the firing to reach the desired temperature (1240°C/2264°F) that it was too hot for too long? And if so, is there anything I can do to counteract this? I stupidly decided to make all my christmas presents this year and, after the disaster of the last firing, I'm scared to do my final glazing with the crazy winter weather.

Thanks in advance! I hope at least some of this made sense.

[Hulk]

Hi np.04,

Is your firing governed by a controller?
If you are able to set cones where they can be watched*, that might be a real help - you could start your cool down when the heat work is completed.

*be sure to wear kiln glasses against the harmful rays and any flying bits!

Edited December 18, 2022 by Hulk
oops

[PeterH]

19 minutes ago, Ceramics.np.04 said:

I don't really know what I'm talking about but is it possible that because it took so long for the final section of the firing to reach the desired temperature (1240°C/2264°F) that it was too hot for too long?

A cone temperature chart confirms that this is a real effect. A cone number relates to an amount of heat-work and the maximum temperature needed to achieve this depends on how fast the kiln is heating towards the end of the firing.
https://www.overglazes.com/PDF/Orton-Cone-Chart-C.pdf

Did you previously fire by observing the cone drop or just use it as record of the firing?

[Bill Kielb]

3 hours ago, Ceramics.np.04 said:

don't really know what I'm talking about but is it possible that because it took so long for the final section of the firing to reach the desired temperature (1240°C/2264°F) that it was too hot for too long?

Yes definitely possible. So silica and alumina do not melt basically on their own below about 1600c/3000f. So they need flux to help melt at a lower temperature The fluxed reaction pretty much takes place in the last 100c of the firing, prior there is just not enough energy to get things going. So temperature and firing speed in the last 100c is super critical, to which PeterH has posted the Orton cone chart above.
As kiln elements age their power decreases and eventually they can’t make a reasonable final firing speed and things begin to over fire. The same goes for anything that slows down the firing such as worn elements AND a cold external firing temperature. In the end though -9c is cold but not that cold so likely the elements are fairly worn. How many firings are on them and to what cone do you routinely fire?

 

 

 

 

Edited December 18, 2022 by Bill Kielb

[Callie Beller Diesel]

As someone who’s been firing an analog kiln outdoors in the Canadian winter for the last few years, I can confirm that -9 is not enough to make a difference in your firing speed. I don’t start getting much slow down until about -25 or so, and even then, it’s only about a 15 minute difference.

Since your elements are likely worn, you can still continue, but definitely include the cone packs this time, and make sure you’re checking the firing every 15 minutes for the last 2 hours or so. You want to be able to turn the kiln off yourself once the cones fall.  If you fire overnight to avoid peak electricity rates, maybe start it about 2-3 hours later than you would normally, so you’re awake for the end. 

[PeterH]

9 hours ago, Callie Beller Diesel said:

Since your elements are likely worn, you can still continue, but definitely include the cone packs this time, and make sure you’re checking the firing every 15 minutes for the last 2 hours or so. You want to be able to turn the kiln off yourself once the cones fall.  If you fire overnight to avoid peak electricity rates, maybe start it about 2-3 hours later than you would normally, so you’re awake for the end.

18 hours ago, Hulk said:

*be sure to wear kiln glasses against the harmful rays and any flying bits!

Both seem very sound advice (while the elements are worn).

19 hours ago, Ceramics.np.04 said:

I stupidly decided to make all my christmas presents this year and, after the disaster of the last firing, I'm scared to do my final glazing with the crazy winter weather.

So does this mean waiting for appropriate safety glasses? Are we talking about welders glasses for IR & UV protection?
In the Studio: Eye Health for Potters  https://tinyurl.com/3tcnc8tp

PS Or -- in the absence of goggles -- could you measure the rate of temperature rise towards the end and stop the kiln closer to the right heat-work. Although you only have figures for 15C/h, 60C/h & 150C/h.  BTW -- assuming you have a controller -- what is the rate of your last ramp?

Edited December 19, 2022 by PeterH

[Ceramics.np.04]

Thank you all SO much for your responses. All ridiculously helpful!

11 hours ago, Callie Beller Diesel said:

 I don’t start getting much slow down until about -25 or so, and even then, it’s only about a 15 minute difference.

My kiln has been about five hours slower so I think we might have found the issue! (About 16 hours to 21 hours)
I suspect the elements were already very worn when I bought it. I was just thrilled it switched on once I realised just how 'second-hand' it was. 

 

21 hours ago, PeterH said:

Did you previously fire by observing the cone drop or just use it as record of the firing?

 

I normally just use cones as a record of the firing. My kiln has one hole in the door and a bung for said hole which obviously expands and gets stuck once things start to heat up in there. If I leave the bung out to monitor the cones it takes even longer to get to temperature. 
(I posted a while ago about my kiln not reaching temperature and then realised I wasn't putting the bung in the door...oops!)

3 hours ago, PeterH said:

BTW -- assuming you have a controller -- what is the rate of your last ramp?

I normally put my kiln on full-whack for the final ramp. I do 150C up to 600 and then 999C (top temp on the controller) up to 1240. I'm aiming for a cone six but I still find the cone thing a bit confusing.

 

I'm starting to think I maybe better off replacing the elements before I even bother trying to fire again? It sounds like they might be beyond help.

Which leads me to my next issue: I don't think the manufacturers of my kiln exist anymore and no one seems to have heard of them anyway. What would I need to look into to choose replacement elements?

Thanks again! 

[Bill Kielb]

1 hour ago, Ceramics.np.04 said:

normally put my kiln on full-whack for the final ramp. I do 150C up to 600 and then 999C (top temp on the controller) up to 1240. I'm aiming for a cone six but I still find the cone thing a bit confusing.

 

Cones are easy,
If you want to follow the cone chart then for your last segment go 60c/hr in the last 100c of the firing to 1222c. So

• pick a cone, let’s say 6.
• find the temperature in the 60c per hour column = 1222
• subtract 100 from it, 1222-100 =1122
• program your final segment to start at 1122, go 60c per hour and end at 1222.

the last 100c is super important to the heatwork done. From the times you state though it does not appear your kiln is keeping up with the program and therefore likely worn elements.

For a worn kiln you may be able to use the 15c column which means for cone 6,  your ending temperature is 1185c, final segment is 1085c => 1185c and final segment firing speed is 15c per hour.

There is a limit to how slow you can go and how glazes perform.  

As far as cones and many other things, In the final segment It’s rarely only about top temperature. As an example, just like baking something, it’s not simply 170c, it’s 170c for 30 minutes ………. For kilns we depend on rate per hour though.

If you cannot find elements Euclids.com can likely wind new and they ship worldwide.

Edited December 19, 2022 by Bill Kielb

[PeterH]

1 hour ago, Ceramics.np.04 said:

Which leads me to my next issue: I don't think the manufacturers of my kiln exist anymore and no one seems to have heard of them anyway.

I don't know the best place in the UK to get elements, but some of them will probably try to reproduce elements for kilns from defunct manufacturers.

For example:
Kiln Elements, Repair, Servicing and Support
https://www.potterycrafts.co.uk/Products/kiln-elements-servicing

Can you post the information from the plate here as well.

PS The plate probably looks something like this, partially printed and partially stamped.

[Ceramics.np.04]

Thank you, @Bill Kielb! I think I actually understand that 

@PeterH My kilns plate: 

 

[Hulk]

Looking at the cones, that's direct feedback.

With the bung having to be in, then getting stuck, relating the pyrometer readings and clock times (for temperature rate of change) against the cones might be the best you can do.
That's assuming you have a pyrometer - from there, take notes on the readings (time and temperature), then compare against the cones afterward.

After a few trials, you will be able to repeat your heat work by watching the pyrometer and the clock!
...that's assuming that your pyrometer is consistent and reasonably accurate.

Heat work
Take a long look at a cone chart, noting the relationship between time and temperature.
In short, it takes a higher temperature to slump the cone when the rate of change is faster; the cone will slump at a lower temperature when the rate of change is slower.
 

"... Are we talking about welders glasses for IR & UV protection?"

Yes, also protection against any flying bits! The odds that something would go ping and fly into one's eye are minimal, yes, but not worth chancing.
For sure, don't look into the yellow hot kiln without protection!
I'm seeing several welding supply shops in northern UK, where a decent pair of Fronius UV wraparound glasses are about fifteen pounds.

Added: or, as Bill points out, these (IR and UV)
Warrior Safety Spectacles Smoked (weldsafe.co.uk)

Edited December 20, 2022 by Hulk
spelling, IR specific glasses...

[Callie Beller Diesel]

Even if you just get the little glass plate that goes in the welding visor, that’s better than going without. 

If you’ve got to have the pieces for Christmas,  it is possible to do it, but it’ll be under less than ideal conditions for your kiln. The good news is, once you replace your elements, and likely the relays too, you’ll have essentially a brand new kiln. Insulating bricks don’t expire. 

 

[PeterH]

9 hours ago, Ceramics.np.04 said:

I'm aiming for a cone six but I still find the cone thing a bit confusing.

You may find this of interest, especially the sections What is Heatwork?  and  Put Witness Cones in Every Firing.
Temperature vs Heatwork – Why We Use Witness Cones
https://suemcleodceramics.com/do-you-put-witness-cones-in-every-firing/

[Bill Kielb]

Hey just a late add here, you will be looking to protect your eyes from infrared light, not necessarily ultra violet. The lowest shading coefficient I have seen is 3 with a range of 3 to 6. If you buy reputable welding goggles or even buy from a reputable kiln supply house then this should not be an issue. I have on occasion met folks who bought UV glasses, more sunglasses however so worth mentioning that you take care and purchase proper lenses. For kilns you will be looking into an extreme infrared environment. 

Edited December 19, 2022 by Bill Kielb

[PeterH]

On 12/18/2022 at 12:56 PM, Ceramics.np.04 said:

it took so long for the final section of the firing to reach the desired temperature (1240°C/2264°F)

Thinks for posting the kiln plate. I hope that the experts will chime in, but I suspect that the max temperature of 1300C means that the element life - when firing to your desired temperature -- is likely to be quite short.

For example see the first answer in

PS I'm not certain if the figure of 50 firings includes the lower-temperature bisque firings.

[Hulk]

Looks like 1300 °C is well over Cone 10, excepting particularly (very) "fast" heating rates...

[Ceramics.np.04]

On 12/20/2022 at 2:11 PM, Hulk said:

Looks like 1300 °C is well over Cone 10, excepting particularly (very) "fast" heating rates...

Am I missing something? I don't (aim to) fire to cone 10 or 1300C, that's just the top temperature on my kiln. I aim for a cone 6/1240C.

[Ceramics.np.04]

On 12/20/2022 at 11:28 AM, PeterH said:

I suspect that the max temperature of 1300C means that the element life - when firing to your desired temperature -- is likely to be quite short.

This makes a lot of sense. I hadn't heard of this but, judging by the age of my kiln (and it looked pretty well used when I got it), it must have done a fair few firings. 

I knew this day was coming...I was just hoping I'd be able to get christmas out of the way first 

I'm going to give it a go, monitor the temperature and the timings and hopefully end up with something giftable... 

Thanks again for all the help!

[Callie Beller Diesel]

1300*C is roughly cone 12, if we’re going by the slower rate of 60C/hour in the last hour. Cone 10 is 1285* C, and and and cone 6 is 1201*C.

In North America, if a kiln is rated to cone 9 or 10, it means it will hit that top temperature for a handful of firings and struggle to do it after that. A cone 10 kiln in all practicality here is a cone 6 kiln. However UK electrical standards are very different, and I believe the standard thermocouples used in the 2 regions are also not the same. But I’d venture that if your kiln is rated for cone 12, with proper replacement parts you ought to be able to reach cone 9 or 10 regularly. Ask some questions of the person you buy your elements from. 

[Hulk]

On 12/20/2022 at 6:11 AM, Hulk said:

Looks like 1300 °C is well over Cone 10, excepting particularly (very) "fast" heating rates...

Hi Ceramics.np.04,

Above I'm disagreeing with PeterH, looks like your kiln is rated Cone 10 (or more), not Cone 6.
I should have been more specific.
A Cone 10 kiln element set should last a good long while when used to fire to cone 5/6.
I'm just saying it looks like a Cone 10 (or more) rated kiln.

[High Bridge Pottery]

On 12/20/2022 at 2:11 PM, Hulk said:

Looks like 1300 °C is well over Cone 10, excepting particularly (very) "fast" heating rates...

 

I would say in the UK 1300c and cone 10 and are pretty much interchangeable even if most kilns wont manage 150c/h for the last 40 min of the firing up to 1300c.

 

On 12/19/2022 at 3:10 PM, Ceramics.np.04 said:

 

I would test what voltage you have in the studio over a few days and get the elements made for that voltage. The UK is supposedly 230v but I get 245V at my house so any elements designed on 230v will draw more power on 245v. Would be worse the other way around running 240v elements on 230v as then you will be down on power.

Just realised it is a front loader which tend to be more complicated than top loaders. Just a guess at the elements inside being 4 in the side walls and two in the floor. The side wall elements may all be the same spec but the floor will possibly be different.

Need to know the length of the tails (probably double what they are now so you have extra to play with/cut off, maybe 9-12") what length each bit of element is in the wall and how long each hairpin is. Also need to know what gauge the wire is, the internal diameter of the coil (maybe 1/2 or 3/4") and how they elements are wired up. Once you know how they are wired up we could possibly figure out what resistance they should be.

A bad drawing to maybe help.

Edited December 23, 2022 by High Bridge Pottery

[PeterH]

On 12/23/2022 at 4:11 AM, Hulk said:

Hi Ceramics.np.04,

Above I'm disagreeing with PeterH, looks like your kiln is rated Cone 10 (or more), not Cone 6.
I should have been more specific.
A Cone 10 kiln element set should last a good long while when used to fire to cone 5/6.
I'm just saying it looks like a Cone 10 (or more) rated kiln.

Apologies to all, a real senior moment.

[Ceramics.np.04]

@High Bridge Pottery Thank you so much for this. So helpful...and makes it all slightly less overwhelming.

My kiln does have four elements up each side and two at the bottom, with the bottom two having a bigger hairpin (...is that right?!) than the side ones. 

I did get in touch with pottery crafts and they told me I would have to find an element winder to make them for me specially. Am I right in thinking I can send the coils to someone for them to make something similar? Would I just need to know the correct voltage, if so? Thanks again!

 

[High Bridge Pottery]

Yes you are right  They don't look in too bad shape but if it is taking ages to fire then worth getting some new ones made up.

 

The trickiest part is working out what resistance each element needs to be as that depend how they are wired up and what KW output you need. If you can get a photo of how they wire at the back then we can go from there. You can try measuring the resistance but it will change as they degrade and you would need to remove the feeder wires first otherwise you could get the wrong reading so probably better to work it out from the wiring.

 

I wouldn't take them out and try posing as they will probably snap in transit. It will be easier just to work it out with them in place. Looking at them all the side wall seem to be the same resistance but the floor coils look like they have bigger gaps in between each coil so will be a lower resistance (if they use the same gauge wire)

You really need to know the resistance of each element, the internal coil diameter, the wire gauge, the length of each coil, the length of the hair pin and the length of the tails, voltage and kw

Edited February 11, 2023 by High Bridge Pottery

[Min]

You could ask Euclid's if they would wind them for you. Send them all the info off the faceplate, kiln measurements, photos and any other info you have and see what they have to say.

https://euclids.com/pages/kiln-elements