[Clayart] re-firing

vpitelka at dtccom.net vpitelka at dtccom.net
Tue Apr 26 16:56:58 UTC 2022


Hi Paul - 
In the field of midrange and high-fired utilitarian ceramics, I would say that ideal vitrification means a point where cristobalite formation is kept to a minimum and there is an ideal balance of strength and porosity.  Firing longer and/or higher than the target cone will reduce porosity and encourage more cristobalite formation, both of which will make the wares more brittle and reduce thermal-shock resistance.
- Vince

Vince Pitelka
Potter, Writer, Teacher
Chapel Hill, NC
vpitelka at dtccom.net
www.vincepitelka.com 
https://chathamartistsguild.org/ 

-----Original Message-----
From: Clayart <clayart-bounces at lists.clayartworld.com> On Behalf Of paul gerhold
Sent: Tuesday, April 26, 2022 7:48 AM
To: Clayart international pottery discussion forum <clayart at lists.clayartworld.com>
Subject: Re: [Clayart] re-firing

That hypothesis should be very easy to test. Just reuse a cone pack and see if there is any significant change in the cones.  Or you could fire an unglazed piece repeatedly and see if there is any significant change in absorption. 

Also, what exactly is over- vitrified?  I don't think we are talking firing to the point of slumping.

Paul

Sent from my iPad

> On Apr 25, 2022, at 10:29 PM, vpitelka at dtccom.net wrote:
> 
> David Hendley gave a good answer.  To me, the most important consideration is that maturation of clay and glazes depends on both temperature and duration.  So if you glaze-fire again to the same cone, it is the like increasing the duration of the firing beyond the normal maturation point.  Thus, you end up with slightly overfired wares, possibly over-vitrified, more brittle, with lower thermal-shock-resistance, as David said.
> 
> A cone 8-11 claybody fired to cone-9 both times would stand up to refiring just fine.  A cone 8-11 claybody fired to cone-11 both times would almost certainly be over-vitrified.  
> - Vince
> 
> Vince Pitelka
> Potter, Writer, Teacher
> Chapel Hill, NC
> vpitelka at dtccom.net
> www.vincepitelka.com
> https://chathamartistsguild.org/
> 
> -----Original Message-----
> From: Clayart <clayart-bounces at lists.clayartworld.com> On Behalf Of 
> Barry Salaberry
> Sent: Sunday, April 24, 2022 2:03 PM
> To: clayart at lists.clayartworld.com
> Subject: [Clayart] re-firing
> 
> 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.
> 
> --
> Barry Salaberry
> 
> "Hope is the thing with feathers
> That perches in the soul
> And sings tunes without the words
> And never stops - at all!"
>                      -Emily Dickinson
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