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How To Heat Kiln To Complete Bisque Firing Which Was Interrupted?

kiln underfired bisque

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#1 MizzVincent

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Posted 17 July 2014 - 01:42 PM

Hello, all,

 

I'm new to this forum, and have enjoyed reading and learning from this forum. I haven't seen my situation addressed in another thread, so I'll start this one.

 

I'm wondering how to handle the kiln as I refire a load of underfired bisque.

 

A little background: I am re-acquainting myself with pottery, which I loved when I was introduced to it in high school. Back then (25 years ago?!), I threw some pots and had a grand time, then moved on to other classes. M oldest daughter, in high school herself, loved the ceramics class she took this year, too. I think she used the same kickwheels I did!

 

We bought a kickwheel from a school that had closed, and found an small used kiln (electric, Evenheat) on Craigslist. We've found a great clay and supplies store near us and have been having so much fun. Yesterday I fired a load of greenware using the kiln sitter with mini-cones that I used successfully the one other time I've used the kiln before this time. Because it was a much larger load than I've fired before, the kiln was still on when I went to bed at midnight (it had been about 10 hours by then). I'd set the default "off" switch to twelve hours, thinking that it was overkill since the other very small load I'd done was finished very quickly, only about 4 hours or so.  The default switch turned off the kiln while I slept, and the cone in the kiln sitter didn't melt too much, and the kiln sitter was still engaged. I'm not sure how hot it actually got, as I didn't put any witness cones in the kiln. I hadn't planned ahead and made a clay plaque (is this the correct term?) for the cones and I didn't want wet clay to explode in there.

 

I wonder, as I refire the load, do I need to ramp up like usual, with one switch at a time each hour, and the lid propped for a couple of hours? I looked at the pots when the kiln was cool, and while the clay has changed color, it hasn't changed from the dirty gray to bright white that the first load did, so I know it's not "done," and needs to get hotter before I glaze. 

 

Thanks for your patience and sensitivity with an enthusiastic dilettante. 



#2 Benzine

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Posted 17 July 2014 - 04:23 PM

Welcome to the forums.

It could be a bad cone, or it could have been in the sitter incorrectly, things like that can happen. They do make pyrometric "bars" that go in kiln sitters, and I believe I've seen people here state, they work better in the sitters.
Several cones held together, to show the heat work, is known as a "Cone Pack". You were right to be worried about it exploding, if you just made it, I've heard of potters, who were in a rush, and had just that happen.

You can do less than scientific test of the wares. Give them a tap, and see if they sound more like glass, or more like wood. Alternately, you can scratch the wares with your fingernail. If it is bisqued, nothing will happen, other than a nails on a chalkboard sound. if it didn't reach bisque, then you will be able to scrape some off. You can also just try to sponge on some water, to see if anything comes off. Once again with bisqueware, nothing will happen.
Slightly underfired bisquewae, generally isn't an issue. It is more absorbent, so glazing will be. little different, than you may be used to. But other than that, a couple cones lower won't hurt anything.

You can refire the wares, if you are concerned. Odds are, you don't need to be concerned with anything exploding, so a slow ramp won't be necessary. The two temperatures you need to worry about, are around 212 F, when any remaining physical water leaves the clay, and around 1000 F where the chemical water is driven out.
Of course, with nothing to tell you how hot the kiln got, you don't REALLY know if you got past those temps. BUUUUUT, odds are you did. Because after ten to twelve hours, if you didn't hit either of those temps, then that kiln has issues.

With that said, your kiln might have issues anyway. If the cone was in the sitter correctly, then the kiln simply didn't reach temperature. So an element might be going out, or a relay, or something else.

So a good first step would be to just refire the wares, with the cone pack included. See what happens with the cones, but definitely set the timer again, and babysit that kiln until it has really cooled down, no going to bed!
"Anything worth believing, is worth questioning"

#3 Chris Campbell

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Posted 17 July 2014 - 04:47 PM

I agree with Benzine  ... I think you probably do not need to re-fire them before glazing. Bisque is a step to make wares easier to glaze and supposedly to burn out impurities and gasses ... I would suspect those are long gone after 10 hours.

You should order some witness cones so you know what is happening from now on.

 

I don't think the kiln as is will get up to glaze temperatures anyhow.

Did the kiln by any chance come with the directions book that would tell you how to trouble shoot and what the readings should be?

If not, you might be able to download one from the kiln manufacturers website.


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#4 MizzVincent

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Posted 17 July 2014 - 05:44 PM

Thanks for the answers! You've given me a lot to research and consider.

 

I fired the load again this afternoon, adding one switch every half hour. I figured that since I'd already closed the lid on the first run, I could leave it closed from the start this time. This time it went a lot faster: the kiln sitter turned it off after about 3 hours. I'll post tomorrow when it's cool and et you know the results. I'll try that scratch/ tap test and the water test.

 

Witness cones next time, for sure. :) I'm using student- grade clay and glazes; as I understand it, the kiln doesn't need to go past 05/04 until I try different clay bodies.

 

Until then, I'm having fun re-learning the movements and rhythms of throwing. 







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