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Question about Kiln heating/cone temps


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Hello! I have a question regarding what cones my kiln is able to fire to. I’m using a manual skutt 1227 that hasn’t been used in approximately 4yrs. I am a school teacher and have been told this kiln has been at the school since 1996. 
 

I have successfully fired the kiln up to cone 04 with low fire clay and glazes. Today I tried to fire some cone 6 clay/glazes, but after about 9.5 hrs on high the kiln still had not reached cone 6 temps. Could this be a problem of needing new elements? Or something more complex? 

Edited by RebeccaGood
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  • RebeccaGood changed the title to Question about Kiln heating/cone temps

Welcome to the ceramics zoo, glad to have you join us.

We need a bit more information about the electrical specifications of the kiln. These should be on a metal plate attached to the front of the control column, usually near the kiln sitter. Specifically, what is the rated voltage and phase of the kiln? For voltage, there are 2 possibilities - 208 volts and 240 volts. For the phase, again 2 possibilities - single phase (probably shown as 1 PH) and three-phase (probably shown as 3 PH). There will also be numbers for the wattage and amperage, but the voltage and phase will tell us enough to start.

(Just to jump to the worst case scenario, if it is 208 volts single phase, it is rated to a maximum of cone 5, thus cone 6 is out of range. For the other 3 possible combinations of voltage and phase, the kiln is rated to cone 8, but cone 6 is the best it will do in the long term, and after awhile it won't do that unless you replace the elements).

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6 minutes ago, Dick White said:

Welcome to the ceramics zoo, glad to have you join us.

We need a bit more information about the electrical specifications of the kiln. These should be on a metal plate attached to the front of the control column, usually near the kiln sitter. Specifically, what is the rated voltage and phase of the kiln? For voltage, there are 2 possibilities - 208 volts and 240 volts. For the phase, again 2 possibilities - single phase (probably shown as 1 PH) and three-phase (probably shown as 3 PH). There will also be numbers for the wattage and amperage, but the voltage and phase will tell us enough to start.

(Just to jump to the worst case scenario, if it is 208 volts single phase, it is rated to a maximum of cone 5, thus cone 6 is out of range. For the other 3 possible combinations of voltage and phase, the kiln is rated to cone 8, but cone 6 is the best it will do in the long term, and after awhile it won't do that unless you replace the elements).

Thanks for the welcome! I’m not new to ceramics, just new to manual kilns and figuring out all their needs! The kiln is 240 V and 1 phase 48 amps 

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Ok, good news there. It is rated to cone 8 - it has the maximum allowable amperage for a plug-in kiln, but it is larger than average so the heat-to-volume ratio is low, so cone 6 is about the best you can expect in the long term. I have 2 of those in the community studio, and they typically take 14-16 hours to reach cone 6. When things start dragging to 18 or 19 hours, I know it is about time for new elements. As for a firing schedule, we usually run it with all 3 switches on low for an hour, then medium for an hour, and finally turn all up to high until the sitter drops. Note that sitters can drift out of calibration, so just because the sitter dropped doesn't always mean you got to cone. An immediate visual check that the sitter is close to calibration is the final bend of the sitter mini-cone or pyrometric bar. It should be bent to a 90° angle.

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9.5 hours isn't necessarily too long for a cone 6 firing in that kiln. What is your firing schedule- how often do you turn it up? I would give it a good 10 hours before worrying. Do you know how hot it actually got? Did you have witness cones in the kiln/ are you using a pyrometer?

Have you verified that the electrical service in the school is 240 volts? It would be much more common for it to be 208 volts. Check with your maintenance people. If they say '220 volts', that's not correct. It's either 208 or 240. Ask them to measure the voltage at the kiln if they aren't sure. If the service is actually 208 volts and the kiln has 240 volt elements, it won't get to cone 5. 240 volts is the default setup when ordering a kiln from a web site, and I've been in many schools where they ordered the wrong kiln.

Next check that all the elements are working. Put a little piece of paper on each element, start the kiln for a minute or two on high, shut it off, and check if the papers have burned. Alternatively, put the kiln on high, let it run for 5-10 minutes, then carefully crack the lid and see if all the elements are glowing. The very top and very bottom elements will glow hotter than the middle 4.

If the voltage is correct and all the elements are working, then you probably need new elements.

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1 hour ago, Dick White said:

Ok, good news there. It is rated to cone 8 - it has the maximum allowable amperage for a plug-in kiln, but it is larger than average so the heat-to-volume ratio is low, so cone 6 is about the best you can expect in the long term. I have 2 of those in the community studio, and they typically take 14-16 hours to reach cone 6. When things start dragging to 18 or 19 hours, I know it is about time for new elements. As for a firing schedule, we usually run it with all 3 switches on low for an hour, then medium for an hour, and finally turn all up to high until the sitter drops. Note that sitters can drift out of calibration, so just because the sitter dropped doesn't always mean you got to cone. An immediate visual check that the sitter is close to calibration is the final bend of the sitter mini-cone or pyrometric bar. It should be bent to a 90° angle.

Ok, thanks for the help! I don’t think I can let it go for 14+ hrs because I don’t want to run it over night or when I won’t be there, and it doesn’t have a timer. I’d be worried the sitter might not drop or something would go wrong and my biggest fear is burning down the school! I might just stick to low fire clay and glaze for now, since I know it can do that in a timely manner! 

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  • 3 months later...
On 1/13/2021 at 7:46 AM, Chilly said:

Are you sure?

Yes, I am sure. Just the sitter, no timer or dial on the panel at all. Whoever installed it clearly did not know what they were doing, I can tell that by several different things! 

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30 minutes ago, RebeccaGood said:

Yes, I am sure. Just the sitter, no timer or dial on the panel at all. Whoever installed it clearly did not know what they were doing, I can tell that by several different things! 

There are a lot of old Kiln Sitters out there that don't have timers.

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