iandol on mon 7 may 01
Dear Stephen=3D20
I have never had the chance to fire up to cone eleven but I accept your =3D
observations as valuable evidence.
In these circumstances, Potash felspar may be superfluous.Your mixture =3D
would be approaching maturity as a ceramic at that temperature (Cone =3D
11). The solid state reaction of Kaolin to Mullite and Silica will have =3D
occurred. Even if the mass has not sintered, Soda will react with new =3D
and raw silica once the temperature passes 1150 C. Melting will start =3D
and proceed as the temperature continues to rise.
I might ask, if the reaction takes place so freely without the =3D
intervention of Potash Felspar, why do so may recipes which are provided =
=3D
for, or suggested as being useful for salt glazing contain Potash =3D
felspar?
Thank you for putting this information forward.
Best regards,
Ivor Lewis.
Stephen Grimmer on wed 9 may 01
Ivor,
Only a very few clays have the combined qualities of workability and
plasticity, good behavior in drying, maturity at the desired firing
temperature, glaze fit, fired color, atmospheric sensitivity, et cetera. So
potters usually combine several clays together in a body to enhance
desirable characteristics and minimize less wanted traits. In a stoneware
body, these might include fireclays, kaolins, stoneware clays, ball clays,
earthenware clays, flint for color, as a filler and for glaze fit, and
feldspar for maturity and glaze fit. In a porcelain body, one might choose
from several kaolins, a white burning ball clay or two, flint, feldspar, and
bentonite or macaloid. Often, it is impossible to fit glazes to a clay body
containing no feldspar, as the expansion of the body will be very high,
leading to shivering, dunting, and shattering. Additionally, many bodies
containing no feldspar are underfired at stoneware temperatures and
consequently are weak and porous. A shining example would be the 1:1 mix of
kaolin and flint, which takes soda very well but remains powdery and easily
scratched with a fingernail.
In choosing a feldspar, many potters gravitate toward potash varieties
because, while beginning to melt at lower temperatures, K-spars work their
fluxing magic slowly and produce a more viscous glass than soda spars. This
quality helps to extend the useful firing range of the body and minimize
warping. Soda feldspars tend to melt later, but more abruptly, producing a
more fluid glass. They also are known to exhibit slight solubility and can
deflocculate a clay body after a long aging period. Soda spars do tend to be
more sensitive to slight variations in atmosphere in soda/salt/wood kilns,
and are sometimes used when the potter wants work that captures every nuance
in such a firing. Spar mixes such as Nepheline Syenite can bring in
sufficient flux to mature the body while adding more alumina than other
spars, further heightening the body's color response to atmospheric
variation.
Best,
Steve
Seen over the Glory Hole over in the Glass Dept.: "HOT GOOD, COLD BAD!"
--
Stephen Grimmer
Southern Illinois University at Carbondale
> From: iandol
> Reply-To: Ceramic Arts Discussion List
> Date: Mon, 7 May 2001 17:07:58 +0930
> To: CLAYART@LSV.CERAMICS.ORG
> Subject: : chlorine gas and salt firing
>
> I might ask, if the reaction takes place so freely without the intervention of
> Potash Felspar, why do so may recipes which are provided for, or suggested as
> being useful for salt glazing contain Potash felspar?
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