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uneven heating in kiln and a multitude of funny glaze problems....



 
 
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  #1  
Old October 11th 04, 09:50 PM
Zander
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Default uneven heating in kiln and a multitude of funny glaze problems....

Hi all,

I just finished a glaze fire yesterday and the results were decidedly mixed.
It is an electric kiln and I am firing to cone6, it also has a downdraft
vent installed and uses a kiln sitter.

The top peep hole is the only one available as the bottom one is sealed up.
The kiln load had 4 shelves, one on the bottom, one 4 inches above that,
one 8 inches above that and the last one 6 inches above that.

I put a cone plaque with a 5,6 and 7 cone on each shelf. But because I'm
limited to the top peep hole I can only see the cones on the very top.

The kiln sitter went off at around 2237 but at that point the cone 6 on the
top shelf hadn't even moved yet.... so I kept the kiln going - roughly
holding that temperature. I watched the cone5 go completely over and kept
watching, at a certain point the cone6 was at about 3 o'clock when I shut
it off. It proceeded to go to about 5 o'clock as it cooled then stopped
moving.

When I unloaded the kiln I was disappointed to see that the shelf
immediately below the top shelf had actually fired to cone 7, The shelf
below that had cone 7 at 3 o'clock and the bottom shelf was cone 6 at about
3 o'clock. So the heating is very uneven. I'm not sure what to do about
that.

Then there is the work itself. One big bowl with a store bought hunter
green glaze has pit marks all over it. A celandine (spelling) glaze that
should be blue is almost a transparent light green color. Another store
bought glaze looks great but is blistered almost as if it was blowing
bubbles but only inside the foot ring on the bottom. Other bowls look
great ones with brushed on clear crackle (even though it didn't crackle)
and others.

I would like to refire and save the pitted green bowl, can I do that?

Any other tips or condolences would be appreciated!!! (really only
marginally bummed out)

Zander


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  #2  
Old October 11th 04, 11:09 PM
Coggo
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Hi again Zander,

The temperature distribution you describe is quite common for your style of
kiln. The top and bottom zones are usually cooler than the middle, because
the lid and floor are absorbing heat rather than radiating it.

Many kilns have hotter elements in the top and bottom to overcome this
problem - some have three controls for top/middle/bottom so you can adjust
the middle one to slightly less than full to compensate. Or maybe you just
have to live with it.

The effects you describe are variously underfired and overfired - when you
know your glazes better, you can put them in the part of the kiln which gets
the best results for each glaze. It's all part of the learning process - you
will always get a few duds.

Pitting generally means underfiring, bubbling and cratering might be
overfiring - but there is a whole range of problems and issues which can
only really be diagnosed by seeing the pieces.

You can refire a glazed pot - it may come out better, the same, or worse -
usually about 50% chance of improvement - nothing to lose anyway!

Don't be discouraged - pottery is always uncertain - there will always be
successes and failures - but the occasional excellent pieces make up for all
the failures.

Dave Coggins


  #3  
Old October 11th 04, 11:12 PM
ShantiP1
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Are you stacking with half shelves or full shelves? Make sure that you leave
good space around the edges and the center to allow the air to flow easily
through the kiln.
With that vent you have, that should actually help you to fire more evenly.
Is this a new kiln? What kin and model is it? Check your elements to make sure
they're all working properly and that there are not bits of brick clogging the
elements or the area where the elements are seated.
Follow your instruction manual for proper shelf positiong.
And make sure those middle space and edge space don't have handles and other
parts of pots going into them. Make sure the pots stay within the confines of
the shelf area itself, so as not to imped the flow of heat.
Fire slowly. Often uneven firings like yours, when all other possibilities have
been excluded, are caused by too rapid firings.
Try firing much slower and you will probably see a big difference in your
results.



  #4  
Old October 12th 04, 07:22 AM
Brad Sondahl
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Default

It also helps to have taller stilts for the bottom and top spaces. I use whole
shelves, usually use 6 inches or more on the bottom, stack the plates and
shorter pieces in the middle, and leave about 6 inches on the top again.
Your results could also be explained if an element burned out in the firing,
near the top peephole, but I'm guessing you haven't fired enough to expect that
yet.
Brad Sondahl
--
For original art, music, pottery, and literature, visit my homepage
http://sondahl.com

To reply to me directly, don't forget to take out the "garbage" from my address.





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  #5  
Old October 12th 04, 03:35 PM
Zander
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I have 3 full shelves and 2 half shelves, so I am usually creating 4 full
layers, I've tried staggering the half shelves for better flow but it never
seems to work out with the size pieces we are firing.

I fired faster this time than before, due to my turning down the flow on my
downdraft vent which I though was drawing too much the temperature climbed
at a faster rate, so instead of 8 hours to cone 6 I was around 7 hours.

I think I will try firing much slower next time, is it possible to fire too
slowly?

Also, I've ordered a bartlett vc-6f controller for this kiln which I'm
imagining will help a lot. Thanks to everyone for all the great advice.

Zander


"ShantiP1" wrote in message
...
Are you stacking with half shelves or full shelves? Make sure that you
leave
good space around the edges and the center to allow the air to flow easily
through the kiln.
With that vent you have, that should actually help you to fire more
evenly.
Is this a new kiln? What kin and model is it? Check your elements to make
sure
they're all working properly and that there are not bits of brick clogging
the
elements or the area where the elements are seated.
Follow your instruction manual for proper shelf positiong.
And make sure those middle space and edge space don't have handles and
other
parts of pots going into them. Make sure the pots stay within the confines
of
the shelf area itself, so as not to imped the flow of heat.
Fire slowly. Often uneven firings like yours, when all other possibilities
have
been excluded, are caused by too rapid firings.
Try firing much slower and you will probably see a big difference in your
results.





  #6  
Old October 13th 04, 03:45 AM
Slgraber
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Default

i also find filling up the kiln with extra furniture if you don't have that
much product helps. i try to cap off the last layer with another kiln shelf.

....but don't use granite rocks....



steve




Subject: uneven heating in kiln and a multitude of funny glaze
problems....
From: Brad Sondahl
Date: 10/11/2004 11:22 PM Pacific Daylight Time
Message-id:

It also helps to have taller stilts for the bottom and top spaces. I use
whole
shelves, usually use 6 inches or more on the bottom, stack the plates and
shorter pieces in the middle, and leave about 6 inches on the top again.
Your results could also be explained if an element burned out in the firing,
near the top peephole, but I'm guessing you haven't fired enough to expect
that
yet.
Brad Sondahl
--
For original art, music, pottery, and literature, visit my homepage
http://sondahl.com

To reply to me directly, don't forget to take out the "garbage" from my
address.





----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet
News==----
http://www.newsfeeds.com The #1 Newsgroup Service in the World! 100,000
Newsgroups
---= East/West-Coast Server Farms - Total Privacy via Encryption =---








steve graber
 




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