[Clayart] re-firing

ronroy at ca.inter.net ronroy at ca.inter.net
Tue Apr 26 15:55:32 UTC 2022

Hi Barry, Hi David,

When clay is refired to it's top temperature - depending on how fast  
the firing is - it's like firing the original clay about a cone  
higher. If the firing is abnormally fast perhaps a half cone higher.  
If abnormally slow perhaps a cone and a half more. If you still have  
the cones from the first firing or a similar firing you could include  
them in the second firing to check that.

The same thing can happen in a bisque firing. The ware will be less  
absorbent. Easy to test for using a standard absorbency test. I can  
provide my testing procedure if anyone is interested.

More free silica would be converted to liquid form in a double firing  
so the beta to alpha conversion of quartz would be less. It means you  
have less anti craze when the quartz gets smaller at 573C. This could  
make a glaze start to craze if it were close to doing so.

At cone 6 there is not much cristobalite generated if any. My  
dilatometer experiments never addressed double firing at cone 6 do I  
can't say if there is a chance. Longer firings and slower cooling  
above 1100C promote cristobalite formation.

Cristobalite goes through it's conversion at about 200C (ovenware  
temperature) so any ware that has to heat or cool quickly is affected.  
This can be a major problem at cone 10 if there is not enough KNaO  
(sodium and potassium oxide oxide) in the body to control the  
cristobalite as it is generated.

There is a definitive article on the subject of cristobalite in the  
December 1999 issue of The Studio Potter by Peter Sohngen which I did  
the dilatometery for. The issue is available digitally on line from  
Studio Potter for $5. That issue also has a wonderful articles on  
Davis Shaner, M. C. Richards and flexible mold making and many more.

I do agree with David that the clay would be more brittle and more  
susceptible to failure from knocks and temperature change especially  
in larger vessels.


Quoting David Hendley <farmpots at eastex.net>:

> On 4/24/2022 1:02 PM, Barry Salaberry wrote:
>> Re-firing
>> I wonder if re-firing makes the pots more brittle that convention would
>> serve?
>> Would a dilatometer measurement of both provide less anecdotal survey?
>> Does cristobalite increase or decrease with that re-fire?
>> Is the pot which has been re-fired more subject to thermal shock?
>> Does it affect glaze fit?
>> Questions surface when we enter a new cave, but light does provide clarity
>> one hopes.
> Those are all good questions, and I think re-firing does, indeed,  
> change things.
> I don't have any answers, but I think cristobalite is increased in a  
> re-fire, and the
> piece is probably more sensitive to thermal shock and more brittle.
> Just my hunch.
> In my experience glaze fit is not noticeably changed.
> David Hendley
> david at farmpots.com
> www.farmpots.com

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

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