[Clayart] Clayart Digest, Vol 100, Issue 12 firing closed forms

Mon Apr 1 20:57:29 UTC 2024

I also have a problem with the idea of outgassing of chemically bound water causing explosions. I'm not saying it cannot since any generation of steam generates pressure over time.

It seems to me that the volume of water around a given clay particle, as an aggregate effect, would have to outgas at a very high rate in order to generate enough pressure to overwhelm resistance, and that of course translates to a very rapid RoR.

I will soak up to about 350, but I don't jump my way there. By the point chemically bound water is coming off I'm in a RoR that is very regular, and the only way that could be otherwise would be a malfunction.

That said, I recommend a lower soak temp since as the speed of outgassing is increased, which is a function of heat increase, the occurrence of flaws increases, and if you push too hard you can get some interesting, but unsellable cracking.

Joel.
________________________________
From: Clayart <clayart-bounces at lists.clayartforum.com> on behalf of Paul Randall <paul at plrandall.com>
Sent: Monday, April 1, 2024 7:23 AM
To: Clayart international pottery discussion forum <clayart at lists.clayartforum.com>; vincepitelka at gmail.com <vincepitelka at gmail.com>
Cc: 'Clayart international pottery discussion forum' <clayart at lists.clayartforum.com>
Subject: Re: [Clayart] Clayart Digest, Vol 100, Issue 12 firing closed forms

Ron, Vince,
I always do a 250-degree soak for one or two hours before each bisque fire, no matter how long the ware has dried on the shelf. I have the kiln vent running to vent out moisture. I also do a slow bisque. Never have problems.

I did have a pot blow up once in a glaze fire, evenn though I did a three hour pre-soak. The pot that blew was a lidded pot, the interior glazed just before the fire. The lid was raised on wadding. My theory is that super-heated steam from the damp glaze got trapped under the lid and flashed at some temperature, causing the pot to explode. My experience seems to support Vince’s experience.

There is a thermodynamic principle by which steam gives off additional heat through a process, “latent heat of fusion.” The steam gives of additional heat as the water molecule's turn to steam. It turns out ,that heat is given off suddenly and rapidly, enough to cause a pot to explode.
Paul

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________________________________
From: Clayart <clayart-bounces at lists.clayartforum.com> on behalf of ronroy at ca.inter.net <ronroy at ca.inter.net>
Sent: Sunday, March 31, 2024 10:44:01 AM
To: Clayart international pottery discussion forum <clayart at lists.clayartforum.com>; vincepitelka at gmail.com <vincepitelka at gmail.com>
Cc: 'Clayart international pottery discussion forum' <clayart at lists.clayartforum.com>
Subject: Re: [Clayart] Clayart Digest, Vol 100, Issue 12 firing closed forms

Hi Vince,

My experience says that ware does not blow up during water smoking
when clay gives up it's chemically combined water.

I once mistakenly programed a bisque firing so that the kiln went from
100C to 650C in an hour - it was stacked tightly and nothing blew up.

Needless to say the ware was completely dry before the fast firing.

RR

Perhaps an experiment is in order to see which is reality.

Quoting vincepitelka at gmail.com:

> Hi Joel -
> It's not expanding air that causes explosion in the kiln.  It's water
> turning to steam.  Even if the piece is very dry, explosions can occur
> because of the release of chemically-combined water as the kiln goes through
> the "water-smoking" period.  I saw a lot of closed forms explode over my 35
> years in academia, but I also know that it is possible to fire closed forms
> as long as they are bone dry and the kiln is fired properly.
>
> One of the things that is not clearly understood by a lot of people working
> in clay is that bone-dry wares are far less porous than bisque-fired wares,
> so if there are air bubbles or other closed spaces and steam pressure builds
> up, it can overcome the mechanical strength of the clay and cause it to pop
> apart.  The worst explosions in a bisque-firing happen at higher
> temperatures after the clay has developed some mechanical strength, and
> those are from the escape of chemically-combined water.  Those still are not
> serious enough to damage the kiln, but they can wreck other wares nearby.
> - Vince
>
> Vince Pitelka
> Potter, Writer, Teacher
> Chapel Hill, NC
> vpitelka at dtccom.net
> www.vincepitelka.com<http://www.vincepitelka.com><http://www.vincepitelka.com<http://www.vincepitelka.com>>
> https://chathamartistsguild.org/
>
> -----Original Message-----
> From: Clayart <clayart-bounces at lists.clayartforum.com> On Behalf Of joel
> joelfink.net
> Sent: Tuesday, March 26, 2024 9:33 AM
> To: stevj09 at mail.ru; Clayart international pottery discussion forum
> <clayart at lists.clayartforum.com>
> Subject: Re: [Clayart] Clayart Digest, Vol 100, Issue 12 firing closed forms
>
> I've been a production potter for about 20 years, as my sole source of
> income. During that time I had one closed form explode, it was a type of
> handle I was experimenting with for a mixing bowl idea.
>
> I don't know much definitively, as to why it exploded. One problem is that
> it simply could have been wet. I will still get an explosion or two over the
> course of about a year because I rush things.
>
> Another thing that occurs to me is that the volume being contained could
> matter if there is a kind of square-cube problem where the volume of gas, as
> a ratio to the surface area of the container, could expand too much after
> the surface seals over. That seems like I'm torturing the physics of this
> too much, which means I probably am. But that is my only guess as to why
> that one handle exploded.
>
> Joel.
> ________________________________
> From: Clayart <clayart-bounces at lists.clayartforum.com> on behalf of CONPOINT
> <dannon at ccrtc.com>
> Sent: Monday, March 25, 2024 8:30 PM
> To: clayart at lists.clayartforum.com <clayart at lists.clayartforum.com>
> Subject: Re: [Clayart] Clayart Digest, Vol 100, Issue 12 firing closed forms
>
>
>
> ---
> T
>
> O
>>
>> You can indeed fire completely closed forms without a pinhole
> Just be sure it is dry, and fire a bit more slowly, perhaps.
>>
>>
>>
>> Today's Topics:
>>
>>    1. Toshiko Takaezu (Kathleen Schultz)
>>    2. Fwd: kiln chugging (gerrg42 at gmail.com)
>>    3. Re: Fwd: kiln chugging (vincepitelka at gmail.com)
>>    4. Re: Greg Lindsey,  kiln chugging (David Woof)
>>
>>
>> ----------------------------------------------------------------------
>>
>> Message: 1
>> Date: Sun, 17 Mar 2024 11:41:10 -0400
>> From: Kathleen Schultz <k1claylady at aol.com>
>> To: clayart at lists.clayartforum.com
>> Subject: [Clayart] Toshiko Takaezu
>> Message-ID: <C16F3072-8CCE-4505-9A8C-65F0E9BF7913 at aol.com>
>> Content-Type: text/plain; charset=utf-8
>>
>> There is a wonderful article in the New York Times art section about
>> this fabulous ceramicist. Her work is getting a posthumous reappraisal
>> and they are planning an online exhibition. It said when working on a
>> closed form she would drop clay pebbles wrapped in paper with secret
>> messages into  the form before sealing them up. After firing the
>> sculptures would rattle when moved. My question is I thought you could
>> not fire a completely closed form because it would explode? She must
>> have had a pinhole?
>> Kathy Schultz ?
>> Sent from my iPad
>>
>>
>> ------------------------------
>>
>>
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Ron Roy
ronroy at ca.inter.net
Web page ronroy.net

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