[Clayart] Returning back from La Meridiana in Italy - interesting breakage....

[Robert Harris]

Antoinette, you are absolutely correct.
It is fairly well established that with stonewares and even more with
porcelain, the crazing of the glaze extends through the clay/glaze boundary
(which with porcelain is very mixed together anyway) and therefore, when
force is applied, the crack can easily propagate through the body. (It is
like a tear in a piece of paper).
I have seen studies which show that an unglazed bar of porcelain is
stronger than a bar of porcelain that has been glazes with a crazed glaze.
As I recall Ron Roy has mentioned these experiments on this forum before,
maybe he has more details!

Robert

On Wed, 18 May 2022 at 19:36, Antoinette Badenhorst <
porcelainbyantoinette at gmail.com> wrote:

> Hi Guys. I can report that I had another very successful opportunity to
> teach a 2 week porcelain workshop at La Meridiana. Normally it is fun to
> teach in more than one country and use the opportunity to travel and see a
> little bit of the area.  I am thankful that I only chose to return to Italy
> this time! It was a good experience and opportunity to have a full class.
> Travel however was not that much fun. There is still too much tension, due
> to COVID in the airports and a visible shortage of personnel. The fact that
> I was traveling solo, did not make anything better!
>
> I brought most of the demonstration pieces back home. As many of you
> workshop presenters will know, no perfectly good work comes from teaching a
> class, but this time I had something happening that I did not see before.
> The pieces were glazed with available celadon glazes from the school and
> fired by the school's technicians. I think the firing processes were too
> fast ( black spotting in most pieces), but the glazes crazed heavily, but
> nothing cracked due to the firing process.
>
> One of my very thin pieces did not make it in my luggage. This was no
> surprise, but by closeup examination, the breaking lines were rugged,
> similar to breaking glass of a car window.
>
> If I must guess, I'd say it broke according to the crazing lines. That
> means, if I am correct, the crazing helped to weaken the claybody itself
> and it only needed that rough handling of the luggage to break the way it
> did.  Am I right about that, or is it just the rough handling that caused
> the rugged edge?
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