Tom Buck on thu 2 apr 98
Every so often, someone raises the question of the effects on pots of the
formation of cristabolite during a firing.
Anyone interested in this topic will find it thoroughly, and
lucidly, covered by Michael Cardew in his book "Pioneer Pottery", pages
36-43. Others could rehash his words but he does it so well, with
illustrations, that such a rehash would seem pointless.
Tom Buck ) tel: 905-389-2339
& snailmail: 373 East 43rd St. Hamilton ON L8T 3E1 Canada
(westend Lake Ontario, province of Ontario, Canada).
Bill Merrill on tue 14 aug 07
Everyone talks about crystobalite. Here's my experience on
crystobalite. . When fluxing clay bodies, feldspar is
one of the traditional fluxes. I have used Talc in my clay body as a
flux for two reasons. One, the Talc has MGO and silica in it. Talc
helps cone 10 reduction stoneware bodies promote a more orange color.
The other reason Talc helps with thermal shock when cooling your kiln.
Remember too, slow down your cooling around 1000 degrees F. during
quartz inversion. A greater danger lurks around 500 degrees Fahrenheit
when cooling whatever crystobalite exists will contract about 2%, This
can cause the clay body to crack. Be patient with cooling. After I
shut off my kiln, I leave it alone and resist opening the kiln for
several days. Slow cooling promotea a crystal growth and matt glazes
have a crystalline structure. I have used a clay body using talc as a
flux(7%), have made large baking dishes and casseroles. If you are using
matt glazes they will have a richer surface with slow cooling. I have
never had one break in 40 years. I use spar as a flux instead of Talc
when using the clay body when I am going to fire in a salt kiln. The
silica in a salt body promotes the sodium vapor glaze associated with
salt fired pots. The MGO body in salt firing goes more matt and doesn't
have the traditional orange peel effect a spar body gives. Since I cool
the kiln slowly, I just plan in advance when I need to fire.
=20
Regards to all,
=20
Bill
=20
billm@pcadmin.ctc.edu=20
=20
Keep making pots!!!
=20
-----Original Message-----
From: Clayart [mailto:CLAYART@LSV.CERAMICS.ORG] On Behalf Of Dan Semler
Sent: Monday, August 13, 2007 5:57 PM
To: CLAYART@LSV.CERAMICS.ORG
Subject: Re: Phase Diagrams!!
=20
Hi Linda,
=20
Cristobalite is one of the 20 or so forms of silica - its a =20
crystalline form. It has its uses as it increases the expansion of a =20
body which is useful to help fit of higher expansion glazes, but ... =20
it has a nasty side effect which is that the body is more suspectible =20
to failure due to thermal shock. I don't know about the effect of =20
lower alumina but crsitobalite will form at high temps on its own =20
(stoneware and porcelain temps), though it will form faster in the =20
presence of iron. Higher quantities of feldspar will help dissolve it =20
in bodies.
=20
On the glaze front, I'm not sure that you'd get any in a glaze if =20
its well fluxed but I've not studied it.
=20
When I studied this in some detail last year the only forms that =20
seemed significant to us are, alpha- and beta- quartz, alpha and beta- =20
cristobalite, and the various tridymite forms, plus the =20
non-crystalline form, silica glass. Silica is a fascintaing material =20
on which no doubt entire books could be written.
=20
Thx
D
=20
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Send postings to clayart@lsv.ceramics.org
=20
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Ivor and Olive Lewis on wed 15 aug 07
Dear Bill Merrill,
Adding Magnesium Oxide, as Magnesite, as Dolomite or as Talc, to a clay =
body promotes the formation of Cordierite. This is a component which =
promotes thermal shock resistance to mature clay bodies, making them =
useful as Kiln Shelving.
Talc carvings are made durable by firing them to about 900 deg C. This =
increases the hardness by creating a new substance known as Enstatite.
Best regards,
Ivor Lewis.
Redhill,
South Australia.
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