Gene Arnold on tue 12 aug 08
After Ron Roy tested some of my clay (standard 119 fired to cone 10
reduction) he said it had a large amount of cristobalite in the fired clay.
I looked up cristobalite in my Hammer pottery dictionary and now I'm more
confused than when I started. So I have a few questions.
What is cristobalite? Where does it come from? How can I get rid of it? Is
the way I fire or cool causing it to appear in the clay? Or is it something
that is just common to Standard 119 clay?
I bisque fire to cone 06 in 10 hrs with a 10 minute hold at the end.
The gas kiln is fired is to cone 10 in 101/2 hrs. Reduction is started at
1600F degrees and held for 1 hour, then light reduction to the end of the
firing. Then the kiln is closed up tight and takes two days to cool.
Thanks!!!!
Gene & Latonna
mudduck@mudduckpottery.net
www.mudduckpottery.net
June on tue 12 aug 08
Check the absorption rate of the clay. If it's high, you can wedge some extra spar in the clay. The extra spar will help to deal with the free silica which is the culprit. You'd have to do a few tests with varying amounts of spar and then test the clay for absorption before you know how much to add to satisfactorily solve the problem. Check with your supplier, tell them about the cristobalite test and ask them how much spar is in the body. To start, if it's under 10%, add whatever the difference is to get it to 10% of the dry body weight, which is about 25% less than the wet body.
If only your plates/platters are cracking, you might just use the clay for forms without a wide base. Frankly, you're better off replacing that clay body and with tests in hands, you might be able to get a refund for what you have on hand.
Regards,
June
http://www.shambhalapottery.com
http://shambhalapottery.blogspot.com
http://groups.yahoo.com/group/sodasaltfiring/
----- Original Message ----
From: Gene Arnold
To: CLAYART@LSV.CERAMICS.ORG
Sent: Tuesday, August 12, 2008 7:24:43 AM
Subject: cristobalite???
After Ron Roy tested some of my clay (standard 119 fired to cone 10
reduction) he said it had a large amount of cristobalite in the fired clay.
I looked up cristobalite in my Hammer pottery dictionary and now I'm more
confused than when I started. So I have a few questions.
What is cristobalite? Where does it come from? How can I get rid of it? Is
the way I fire or cool causing it to appear in the clay? Or is it something
that is just common to Standard 119 clay?
I bisque fire to cone 06 in 10 hrs with a 10 minute hold at the end.
The gas kiln is fired is to cone 10 in 101/2 hrs. Reduction is started at
1600F degrees and held for 1 hour, then light reduction to the end of the
firing. Then the kiln is closed up tight and takes two days to cool.
Thanks!!!!
Gene & Latonna
mudduck@mudduckpottery.net
www.mudduckpottery.net
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