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temp fluctuations in old electric kiln


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I'm still fairly inexperienced and used to always fire my pottery at a studio where I went to classes etc, so the adventure of firing in my own kiln is still pretty new to me. 

I was given an old electric kiln, which is now connected and running, any dangling heating coils were fixed, and the whole thing was given a once-over by a kiln engineer. The kiln has an electric controller and I've set a number of different programmes (biscuit, cone 6, cone 8, etc). I've done a few firings so far, 3 biscuit firings and one glaze firing to cone 8. While none of the firings have gone wrong and everything seems to be working fine, I noticed that despite the temp controller there seem to be fluctuations in the temperature. I was using orton cones to check the kiln was firing up to temp and I find that for a biscuit firing it consistently seems to fire to slightly below temp (so first cone will just start to dip rather than lying flat, the other two are not touched).

For the higher glaze firing the opposite seemed to be true - when I checked all the cones had dipped - the first two had completely keeled over, the last one (for the highest temp) was about half-way. 

I realise that these are fairly minor temp fluctuations, but is this a normal thing? I imagine with glaze firings it could make all the difference? Could outside temp account for it? The kiln is in a shed so in winter it does get pretty cold. 

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Perfectly normal. Controllers need to be calibrated. Check your manual, but there should be a way to change the temperature setting for any given cone. We call that a cone offset. You can set your bisque cone to go a little hotter, and your glaze cone to go a little cooler. I would test 10 degrees at a time. There are also settings called thermocouple offsets, where you can adjust the thermocouple reading, but you only want to do that if the temp is off the same at all cones.

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I've been where you are now with the firing "inconsistencies" on an old kiln. The witness cones are a big help in getting things straight. What you'll have to do is compensate in the case of the bisque firing by either setting a higher finish temp on the controller or setting a longer soak time at the end of your current program. As for the ^8 firing, a lower finish temp and/or a shorter soak time at the end, will be the order of the day.

Neil just beat me to what I just said. :-)


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