[Clayart] Cristobalite
Antoinette Badenhorst via Clayart
appear to be many opinions about cristobalite. Some say it needs to
dissolve completely in fluxes while others say some cristobalite helps with
crazing in glazes.
It is also being said that too fast firing around 1000C will allow for
crystal growth.
When I fire porcelain I go very slow to about 800C, speed it to 1250 C (
sometimes 1280C, )and allow for about a 1 hour soak. At around 800C I
begin to slow cool all the way down to 200C.
I was always under the impression that I controlled it well enough to have
very little to no cristobalite left in the clay.
That being said, I also read that glazes have a better fit when some
cristobalite is present in the clay and I wonder if that is true for
porcelain. I think in all these years working with porcelain I might have
had one piece that crazed.
Silica is a huge topic to take on, but I guess a little more knowledge will
not hurt.
Best wishes,
Antoinette Badenhorst
*PorcelainByAntoinette <https://www.porcelainbyantoinette.com/#/>*
*TeachinArt* <https://www.teachinart.com/antoinette-badenhorst.html>
*International Academy of Ceramics*
<https://www.aic-iac.org/en/member/antoinette-badenhorst/>
*Mississippi Arts Commission
<https://arts.ms.gov/artist/antoinette-badenhorst/>*
*MSClayworks <https://www.msclayworks.com/#/>*
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://lists.clayartforum.com/pipermail/clayart/attachments/20251023/521d8c6a/attachment.htm>
ronroy--- via Clayart
When I had my dilatometer I got to understand cristobalite in a visual way.
The key to controlling cristobalite formation in clay is more
important in stoneware clays - especially those low in sodium and
potassium flux. Not enough of KNaO to melt cristobalite as it forms
above 1100C and cracking problems are a good possibility. Or should I
say a bad possibility.
Slow firing means more cristobalite forming on the way up and on the
way down above 1100C
Because having at least 10% feldspar (with at least 12% KNaO) will
take care of any cristobalite that develops you need not worry as
porcelains have way more than 10% spars in them. I must have measured
over 59 different porcelains and never found any cristobalite.
Those brown stoneware clays fired at cone 10 often had too little spar
and when glazed with a low expansion/contraction glaze wound up with a
glaze to big for the pot. To make maters worse potters often made
casseroles and because the cristobalite expansion/contraction happens
at ovenware temperatures they often just ripped apart, often during
the first use.
I'm sure Antoinette knows all this - she just knew this would get me
going. Obviously she was right!
Silica is by far the most used oxide by potters. It is worthy of our
study. The Hamer book has no less than ten pages devoted to it. I'll
say it again - anyone who wants to understand more about clays and
glazes - the Hamer book " The Potters Dictionary of Materials and
techniques" is the ultimate reference book.
RR
RR
ron...@ca.inter.net
Web page ronroy.net
Antoinette Badenhorst via Clayart
you directly, but I thought it would be interesting to other potters too
and was hoping that more of our gurus would chime in.
I agree Hamer and Hamer is the most extensive resource for potters, however
this can be such a confusing topic.
I just finished reading an article by Jim Robinson. His focus was on
stoneware, but he addressed something that I think is often overlooked by
authors. Flint vs. Quartz. Potters use these terms interchangeably, but
according to Jim we should not do that and it makes sense to me since the
various names ( and sources) of silica is something I researched more than
what is good for my limited knowledge and experience.
Anyway, he claims that flint is the main source of cristobalite forming and
that it is not so much from free silica that we add to the clay, but from
the amorphous fine silica found in kaolinite.
He also makes a statement that cristobalite forms in very high firing clay
bodies ( ^12 and up I assume) That make sense to me, since we know the
fluxes in porcelain dissolves silica and if any cristobalite is present,
that will be dissolved as well.
Another curiosity is what else that dilatometer tells you.
Best wishes,
Antoinette Badenhorst
*PorcelainByAntoinette <https://www.porcelainbyantoinette.com/#/>*
*TeachinArt* <https://www.teachinart.com/antoinette-badenhorst.html>
*International Academy of Ceramics*
<https://www.aic-iac.org/en/member/antoinette-badenhorst/>
*Mississippi Arts Commission
<https://arts.ms.gov/artist/antoinette-badenhorst/>*
*MSClayworks <https://www.msclayworks.com/#/>*
URL: <https://lists.clayartforum.com/pipermail/clayart/attachments/20251027/c043bbd6/attachment.htm>
David Woof via Clayart
What you've posted here regarding Cristobalite is the simplest best that you or anyone has ever written for Clayart on this subject.
It is well thought and simple to understand for non techies, yet comprehensive enough for the Studio Potter to apply for everyday needs to know. No fluff, no Puff....... Kudos en Bravo!
And folks Ron has laid down here hints of deeper thought for those who wish to ponder and connect the dots. Perhaps concealed stepping stones, if you will.
Pay attention....impresses yourself!
Joyous firings,
Love to all......
David Woof
________________________________
From: Clayart <clayart...@lists.clayartforum.com> on behalf of ronroy--- via Clayart <cla...@lists.clayartforum.com>
Sent: Friday, October 24, 2025 12:24 PM
To: Clayart international pottery discussion forum <cla...@lists.clayartforum.com>
Cc: ron...@ca.inter.net <ron...@ca.inter.net>
Subject: Re: [Clayart] Cristobalite