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timbo_heff

Hold Limitations on Electric Kiln ?

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timbo_heff    37

Question probably for Neil.

 

Someone was told by one of the kiln companies not to do a hold for more than 30 minutes at high temperature... that it would destroy elements or some such thing.

Sounds like misinformation to me. I could imagine a long hold being extra work for the relays but does anyone know what the kiln maker might have been talking about?

 

Thanks!

Tim

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neilestrick    1,380

I've never heard of a long hold damaging the elements. But it depends on what they mean by high temperature. I've done holds of 45 minutes at cone 8 with no problem. That said, here's a little tidbit from the L&L site: The kiln would be ruined if you held for 10 hours, even for just 1 hour, on a cone 10 firing. This makes sense to me, because an hour hold might push the heat work to cone 13 or beyond, which the bricks, element holders, shelves, etc. are not rated for. Cone 10 with a hold is not cone 10. What your source should have said is the kiln should not be held at the highest temperature the kiln is rated to.

 

Somewhat related, there are two things that lessen element life: high temperatures and cycling (turning on/off). So holding at high temperatures will give you both. However, in my own not-so-scientific study, I found that holding temperature (cycling) to achieve a higher cone did not shorten the element life as much as firing up to that higher cone. Specifically, my elements lasted longer doing cone 6 with a 40 minute hold to achieve cone 8, rather than increasing temperature until cone 8 was reached.

 

 

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Stephen    139

I've never heard of a long hold damaging the elements. But it depends on what they mean by high temperature. I've done holds of 45 minutes at cone 8 with no problem. That said, here's a little tidbit from the L&L site: The kiln would be ruined if you held for 10 hours, even for just 1 hour, on a cone 10 firing. This makes sense to me, because an hour hold might push the heat work to cone 13 or beyond, which the bricks, element holders, shelves, etc. are not rated for. Cone 10 with a hold is not cone 10. What your source should have said is the kiln should not be held at the highest temperature the kiln is rated to.

 

Somewhat related, there are two things that lessen element life: high temperatures and cycling (turning on/off). So holding at high temperatures will give you both. However, in my own not-so-scientific study, I found that holding temperature (cycling) to achieve a higher cone did not shorten the element life as much as firing up to that higher cone. Specifically, my elements lasted longer doing cone 6 with a 40 minute hold to achieve cone 8, rather than increasing temperature until cone 8 was reached.

 

 

 

Neil, I don't mean to be dim on this but I don't understand. I have a glaze firing schedule programmed in my kiln controller (Bartlett) that goes to cone 6 and holds for 15 minutes and then ramps down. I just assumed that the electronics monitors the temp and then once cone 6 is reached the elements are turned on and off to achieve a hold at that cone 6 temp for 15 minutes. Are you saying that instead it just continues to whatever temp the elements are capable of achieving for 15 additional minutes and then starts the ramp down? I'm not worried about the kilns as they are rated to 10 but rather the glazes and clay are formulated specifically for cone 6 .

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Question probably for Neil.

 

Someone was told by one of the kiln companies not to do a hold for more than 30 minutes at high temperature... that it would destroy elements or some such thing.

Sounds like misinformation to me. I could imagine a long hold being extra work for the relays but does anyone know what the kiln maker might have been talking about?

 

You can hold at lower temperatures for extended periods without harming the kiln. However, during a long hold, you may reach a higher cone than you had programmed. That is probably the idea behind the above advice.

 

Sincerely,

 

Arnold Howard

Paragon Industries, L.P., Mesquite, Texas USA

ahoward@paragonweb.com / www.paragonweb.com

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I have a glaze firing schedule programmed in my kiln controller (Bartlett) that goes to cone 6 and holds for 15 minutes and then ramps down. I just assumed that the electronics monitors the temp and then once cone 6 is reached the elements are turned on and off to achieve a hold at that cone 6 temp for 15 minutes. Are you saying that instead it just continues to whatever temp the elements are capable of achieving for 15 additional minutes and then starts the ramp down?

 

The temperature during a hold remains steady. It shouldn't increase. However, the clay will continue to absorb heat work during the hold. This can actually fire the clay to a higher cone than the one that you have programmed. Typically, 45 - 60 minutes of hold = one additional cone of heat work.

 

Sincerely,

 

Arnold Howard

Paragon Industries, L.P., Mesquite, Texas USA

ahoward@paragonweb.com / www.paragonweb.com

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neilestrick    1,380

Cones measure heat work, not temperature. Heat work is a function of temperature over time. Holding temperature has the same effect as firing hotter. A 20 minute hold roughly equals 1 cone. So when you hold at cone 6, the temperature stays the same, but the heat work is increasing. I used to fire to cone 8 by firing to cone 6 then holding temperature for 40 minutes. I once helped fire a large wood burning kiln. When we got to 1850 degrees, cone 08 went down. We then held at 1850 for 5 days, at which point cone 5 went down. Same temperature, more heat work. It's kind of like cooking a roast. You can cook it at 350 for an hour, or at 275 for 4 hours. Both ways give you a cooked roast. Holding at either temperature too long will over cook it.

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Stephen    139

Cones measure heat work, not temperature. Heat work is a function of temperature over time. Holding temperature has the same effect as firing hotter. A 20 minute hold roughly equals 1 cone. So when you hold at cone 6, the temperature stays the same, but the heat work is increasing. I used to fire to cone 8 by firing to cone 6 then holding temperature for 40 minutes. I once helped fire a large wood burning kiln. When we got to 1850 degrees, cone 08 went down. We then held at 1850 for 5 days, at which point cone 5 went down. Same temperature, more heat work. It's kind of like cooking a roast. You can cook it at 350 for an hour, or at 275 for 4 hours. Both ways give you a cooked roast. Holding at either temperature too long will over cook it.

 

Thank you for explaining that to me, I did not get this about holding temps and it is obviously something I need to pay close attention to.

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Wyndham    98

You should also have cones in all your firings to validate the final temp that the kiln heat work reaches.

 

Pyrometers/digital controllers are not measuring heat work, as others have said, and can, over a period of time, degrade and give false readings.

 

Cones are a great back up because as your elements age the firing takes longer so you may get to cone 6 with an older set of elements according to the digital controller but a cone may show you passed cone 6 and heading to cone 7 with glazes that may have blistered.

 

Elements wear more the closer you are to the peak firing temp of those element, so it's better to fire at cone 9 than cone 10 for the sake of element life and thermocouple alike( type k thermocouples can get erratic above cone 6 as they age)

 

Keep records of firing times along with cones from each firing to plot the life of the elements and the quality of the glaze firings. Cones are great , cheap insurance.

Wyndham

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