[Clayart] Thanks all for bisque info

ronroy at ca.inter.net ronroy at ca.inter.net
Thu Feb 13 12:09:44 EST 2020

Hi Edouard,

The information I have and my experience says that melting high fired  
clay using iron will produce cristobalite. My dilatometry work  
confirms that.

There were some references that say iron works like a catalyst to  
produce cristobalite.

Again I urge anyone interested in this subject to read Peter Sohngens  
article on this subject. The only way to prevent the formation of  
cristbalite in high fired stoneware is to make sure there is enough  
KNaO present. That ensures that the fine silica ejected at about 1100C  
is melted before it can be the seed for cristobalite production.

Peter's article (Cristobalite: The Hump - New Data on Silica at Cone  
Ten) is available from studio potter in digital form - volume #28 #1.


Quoting Edouard Bastarache <edouardb at colba.net>:

> Hello al,
> If you fire cone 10 an iron bearing clay body in reduction, the  
> Fe2o3 will transform to FeO a very strong melter that will
> Join to free silica a make fayalite, it will eliminate the  
> production of cristobalite and the cristobalite squeeze.
> See also
> http://cristobalite.blogspot.com/
> Edouard
> -----Message d'origine-----
> De : Clayart [mailto:clayart-bounces at lists.clayartworld.com] De la  
> part de Snail Scott
> Envoyé : 11 février 2020 10:07
> À : Carol Marians; Clayart international pottery discussion forum
> Objet : Re: [Clayart] Thanks all for bisque info
>> On Feb 10, 2020, at 1:13 PM, carol at knighten.org wrote:
>> ...Specially thanks to Snail on the lack of need for the slow down
>> during the quartz/cristobalite inversions.  That had always seemed
>> like voodoo to me, yet I hadn't sufficient data to ignore it?
> In the name of strict accuracy, I don?t really have data in any  
> legitimate statistical sense, either; just a lot of anecdotal data  
> points assembled into a working hypothesis in my squishy brain, with  
> no identifiable analytical methodology.  It does seem to dovetail  
> with what I read about fired silicates, though - crystobalite seems  
> to be a non-issue unless you are firing around ^10 with a highly  
> vitrified clay body.  So, firing up = no issue at any temp.  Cooling  
> = possible issues with highly vitrified high-fire work, but not with  
> lower firings such as bisque, earthenware, or even mid-range  
> stoneware and so-called ?mid-range porcelains?.
> My basic philosophy of oxidation firing: Candling is critical,  
> soaking is frequently useful, and everything in between is actually  
> pretty adaptable.
> I agree wth Mel?s view that an extended time frame in the  
> early-to-middle phases of firing is mostly a waste of fuel, but I  
> don?t think that the minimal heat output of a long candling is a big  
> issue - it?s only 190F/90C, after all.
> I will happily yield this mental model of the process to any  
> better-substantiated version.  Carol, your research on ceramic  
> chemistry is a delight, and a pillar of this list!
> -Snail

Ron Roy
ronroy at ca.inter.net
Web page ronroy.net

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