Kiln Firing Too Hot

[yarddog]

I have an old Gare that I picked second-hand.

I'm on maybe my fifth glaze firing, and after some unexpected drama with flowing glazes etc, I finally took the sensible step of using bracketed witness cones.

After a rocky cone 6 firing last time, I tried it yesterday set to cone 5, with witness cones 4, 5, and 6.

Whoops. Although the electronic display indicated that it fired to an appropriate temp of 2155, the witness cones--both in the top and bottom of the kiln-- all slumped into puddles, as if it had fired hotter than 6.

Is it time for a new thermo-couple? Nobody around here works on these things...

Any advise much much appreciated.

[Marcia Selsor]

you could just program it down since you know the reading is so far off. Maybe next time you fire watch the witness cones to see when they start dropping. That is why they are called "witness" cones. So you can witness what is going on in the kiln. Then match the thermo couple reading and set to that temp.

Marcia

[Bob Coyle]

A new thermcouple might help but you would be better off following Marcia's advice. You would end up doing it the same even with a new thermocouple.

[yarddog]

This is my first kiln without a gold ol' kiln sitter, and I guess I was too trusting.

Although watching the witness cones is clearly sensible, I guess I still am uncertain:

Would a faulty/failing thermocouple be consistent one way or the other--under firing or over firing?

[neilestrick]

Before you start making adjustments to the firing program based on the visual cones, put in a new thermocouple. A bad thermo can cause problems in either direction. Start fresh.

[Bob Coyle]

 

Would a faulty/failing thermocouple be consistent one way or the other--under firing or over firing?

Neil thinks you should get a new thermocouple. I don't trust thermocuples old or new. A cone bends based on "heat work" This means both temperature and time come into play. A thermocuple measures only temperature.  What your cones tell you about your glaze is more impotant than what your thermocouple tells you about your temperature. You can run your temperature up and stop 100F below your "cone" reading and still the cone will eventually bend over if you hold temperature long enough.

 

I bought three thermocoulpes from three different sources and shoved them into my test kiln and at cone 6 temperature I got three different readings that were more than 100 degrees off based on millivolts at the thermocouple leads. This was measured with an accurate voltmeter.

 

No matter what you do you will need to run some tests and come up with a ramp that works for you... even if the temperature on your controller seems way high or way low. Once you do, stick with that ramp and thermocouple until you see some problems. A thermocuple will slowly degrade with time but you should be able to get a good number of runs before it drifts enough to effect a normally stable glaze.

 

So get  a new thermocuple if you wish but ALWAYS use three cones to bracket your run to see what really happened and don't expect it to solve your problems immediatly.

[neilestrick]

Yes, thermocouples measure temperature, not heat work. But the kiln's controller does the calculations to determine the what work based on the temperature and rate of climb. So if you want the controller to be able to do its job, then a new thermocouple is in order.

 

I personally don't see the sense in using cones in a digital kiln once you have calibrated it. That's the whole reason to get a digital kiln. If your glazes are formulated well, they will be able to deal with the slight variations in heat work that may occur. I have only put cones in my kiln once in the last five years. I keep an eye on my thermocouples and change them regularly.

[Bob Coyle]

 

I personally don't see the sense in using cones in a digital kiln once you have calibrated it. That's the whole reason to get a digital kiln. If your glazes are formulated well, they will be able to deal with the slight variations in heat work that may occur. I have only put cones in my kiln once in the last five years. I keep an eye on my thermocouples and change them regularly.

 

I agree with you here Neil. Once you have established a stable ramp and have stable glazes you can back off of the cones. But if you open the kiln and your glazes look bad, you have no way of knowing whether there was a problem with the heat unless you have some record, and cones give you that record. To me, it is worth the little extra time and expense to stick a set of cones in front of the peep hole. Besides it makes me feel good to open the kiln and see the middle cone just touching the base

[neilestrick]

 

 

I personally don't see the sense in using cones in a digital kiln once you have calibrated it. That's the whole reason to get a digital kiln. If your glazes are formulated well, they will be able to deal with the slight variations in heat work that may occur. I have only put cones in my kiln once in the last five years. I keep an eye on my thermocouples and change them regularly.

 

I agree with you here Neil. Once you have established a stable ramp and have stable glazes you can back off of the cones. But if you open the kiln and your glazes look bad, you have no way of knowing whether there was a problem with the heat unless you have some record, and cones give you that record. To me, it is worth the little extra time and expense to stick a set of cones in front of the peep hole. Besides it makes me feel good to open the kiln and see the middle cone just touching the base

 

 

I do understand what you're saying, but if my glazes look bad, I can tell if they were under fired or over fired without using cones. And from a repair standpoint it won't make a difference how much it went either way. Half a cone cold or two cones cold will result in the same repair. So that's why I don't use them. Good discussion!