Kiln Over firing

[Cherry]

My Paragon shut off early.  Turned out that the top element wasn't working.  My DH tested the other two and their resistance was with in the limits so I only replaced the top one.  It fired over.  The thermocouple was old and had some cracks, so I replaced it.  Then I fired it with just shelves, to cone 6.  These pictures show cones 5-9.  I don't know what else to do.  Any suggestions?

 

[JohnnyK]

HI Cherry,

I had a similar problem with my Cress kiln and a new digital controller over-firing, using the built-in ^6 program. What I did was set up a user-defined program after offsetting my thermocouple by +45*, lowering the end temp to 2200* with an end soak of 10 minutes. I forgot to reset the timer on the kiln sitter (which I have since taken out of the circuit) and it shut off at about 1925*.

I restarted the kiln, with the same end temp of 2200* with a 15 minute end soak. Everything worked perfectly...I was really excited. I did another firing with the same parameters as above and the timer adjusted properly and I got a good ^5read on the witness cones, which was still within the tolerance of the glazes I was using, but below where I wanted to be. The temp difference between top and bottom was about 1/2 cone.

My next go-around will be with an end temp of 2225* with a 15 minute soak to see where everything falls.

In your case, I think the new top element is burning significantly hotter than the older elements. The difference between the top and the bottom is about 2 cones. Would it be too inconvenient to reverse the top and bottom elements? I think it would give a better overall balance of the heat distribution. Then, if possible, do a TC offset. Do another firing as you did above and see where the witness cones fall. If you use a preset cone program you may have to drop tp a ^5 or even a ^4 end point to compensate for the current situation. You will probably also see a difference between a full kiln and the empty kiln you used for your test above. Testing is the only way to get it figured out...

JohnnyK

[Cherry]

Thanks, I'll try that.

[neilestrick]

Do not try to swap your elements. You'll end up breaking the old ones. They're too brittle to move like that.

The overfiring and unevenness issues are two different issues. I'm assuming this is a single zone kiln (1 thermocouple)? You'll need to get the middle area accurate first, then adjust the top and bottom by how you load the kiln. It looks like the middle shelf is almost 2 cones off. Did you confirm its accuracy before changing the element? That is, are you sure you were firing to an accurate cone 6 before? If not, it could be that you've been off by a cone or two all along and just didn't know it. If you're sure you were accurate before, then you need to either do a thermocouple adjustment or a cone offset. If it's firing hot at all temperatures, then you need to do a thermocouple offset. If it's only off at the higher temps, then you should just do a cone offset so you don't affect the lower temps. I would start with 30 degrees and go from there. It won't necessarily need to be changed as much as the cone charts say, because the inaccuracy may be different at different temps. Once you get that set, you shouldn't need to do anything with a custom program.

Once you get it accurate in the middle where the thermocouple is, then you need to pack the kiln looser at the bottom and tighter at the top to help even it out.

[Cherry]

Thanks for the warning, I had forgotten about how brittle the elements get. Yes, it was firing accurately before.

My studio was a single room with a screened porch. Then, we had windows installed, and insulated and finished the walls, specifically so the kiln would have a separate room. I did one bisque firing, then the cone 6 that failed. When we opened the controller box, a wire between the top element connector and the thermocouple had come loose. It had touched the kiln jacket and melted a hole it.

It is an old kiln, I've had very little trouble with it.

[Magnolia Mud Research]

Keep in mind that an 'empty' kiln fires differently than does a properly loaded kiln.   I am using 'empty' to mean only shelves,  shelf spacers, and cone packs.   

LT

[GEP]

22 hours ago, Cherry said:

My Paragon shut off early.  Turned out that the top element wasn't working.  

 

2 hours ago, Cherry said:

When we opened the controller box, a wire between the top element connector and the thermocouple had come loose. It had touched the kiln jacket and melted a hole it.

 

It sounds to me like you didn’t need to replace the element, but instead just fix the loose connection. Maybe replace the connector wire if it looked fried.

I’m a firm believer in having all of the elements and TCs at the same age. This is even more important with a 1 zone kiln. It only makes sense to replace only one element when they are all new or almost new, and one fails due to an incident, like a glaze spill or a pot explosion. 

What you’re experiencing now is the reason why your parts should all be the same age. New elements fire hotter with the same amount of juice. Although the older elements aren’t finished yet, I think the best solution is to replace them. And I would also follow Neil’s advice about checking if your new TC needs an offset. 

[JohnnyK]

13 hours ago, neilestrick said:

Do not try to swap your elements. You'll end up breaking the old ones. They're too brittle to move like that.

The overfiring and unevenness issues are two different issues. I'm assuming this is a single zone kiln (1 thermocouple)? You'll need to get the middle area accurate first, then adjust the top and bottom by how you load the kiln. It looks like the middle shelf is almost 2 cones off. Did you confirm its accuracy before changing the element? That is, are you sure you were firing to an accurate cone 6 before? If not, it could be that you've been off by a cone or two all along and just didn't know it. If you're sure you were accurate before, then you need to either do a thermocouple adjustment or a cone offset. If it's firing hot at all temperatures, then you need to do a thermocouple offset. If it's only off at the higher temps, then you should just do a cone offset so you don't affect the lower temps. I would start with 30 degrees and go from there. It won't necessarily need to be changed as much as the cone charts say, because the inaccuracy may be different at different temps. Once you get that set, you shouldn't need to do anything with a custom program.

Once you get it accurate in the middle where the thermocouple is, then you need to pack the kiln looser at the bottom and tighter at the top to help even it out.

Thanks for your input, Neil. I was hoping you would jump in on this...

JohnnyK