Cindy Strnad on tue 25 jul 00
Alisa,
Since I live in Custer, I always use Custer spar. Just makes sense. So if a
recipe calls for soda spar, I try it with soda spar and Custer spar, and
I've never seen much of a difference. I'm sure there are some glazes which
will be more sensitive to this change in fluxes, though.
Cindy Strnad
Earthen Vessels Pottery
RR 1, Box 51
Custer, SD 57730
earthenv@gwtc.net
www.earthenvesselssd.com
Alisa and Claus Clausen on tue 25 jul 00
Dear Clayart,
In all the receipes that I see Custer feldspar, I have substituted with =
one I have called Forshammer.
It is described as a feldspar to use when a receipe calls for just =
"feldspar". =20
On reading further, I see now that it is mixture of sodaspar and potash.
In Marcia's recent post about basic studio materials, she sites Custer =
feldspar as potash.
If I reformulate the tests I made where I substituted Custer with =
Forshammer, but this time
use postash instead of the Forshammer, will I expect to see a =
significant difference in the glaze?
All the glazes worked fine with this combination spar instead of potash, =
but I am curious. I will
make the tests anyway, but wondering in advance.
Simpler question
is this general feldspar (Forshammer)
(K,Na)2OAl2O3.6SiO2 significantly different from Potash K2O Al2O3 6SiO2.
I can see it has salt in it, but do not know how that differently =
influences the final glaze.
Would anyone be kind to send me a list of the commonly used feldspars in =
America with their category types so
perhaps I do not make this error again.
Custer Potash base
F-4 soda base
kona?
G200?
etc?
Thanks and best regards,
Alisa in Denmark
Michael Banks on wed 26 jul 00
I'll leave the listing of North American feldspars to people in that part of
the world.
But a general comment here: Do not be concerned that Forshammer has a
significant soda content. All commercial potash feldspars do -including
Custer. Theoretically pure (at 16% potash), end member, soda-free potash
feldspar is rare and not a useful flux because it only melts at over 1500
degrees C. Potash feldspars with a few percent soda are the norm and melt
200 to 300 degrees lower.
Potash feldspars (usually containing 10 to !2% potash and 3% or more soda)
confer lower thermal expansion, greater chemical stability and wider firing
ranges to glazes, than soda feldspar.
Soda feldspar (commonly containing 7 to 9% soda, <1 to 7% potash) confers a
somewhat brighter colour response with some oxides, begins to flux at a
lower temperature, but produces softer, more craze-prone and volatile glaze
than potash feldspar and is slightly soluble in water which can lead to
deflocculation of clay in the glaze bucket.
If you are working with cone 9-10 stonewares, potash feldspar may be a
better choice, while soda feldspar can be useful in the mid-fire and
low-fire glazes.
Michael Banks,
Nelson,
NZ
----- Original Message -----
Alisa in Denmark wrote (snipped):
In all the receipes that I see Custer feldspar, I have substituted with one
I have called Forshammer.
It is described as a feldspar to use when a receipe calls for just
"feldspar".
On reading further, I see now that it is mixture of sodaspar and potash.
In Marcia's recent post about basic studio materials, she sites Custer
feldspar as potash.
Sharon31 on wed 26 jul 00
Hello Alisa!
what about the Norwegian feldspar?(potash)
I have the analysis from Lasse ?stman's site, the way it is kept on Insight:
NR POTASH FELDSPAR
===================
MgO................. 0.14 0.14%
K2O................. 10.74 10.74%
Na2O................ 3.43 3.43%
Al2O3............... 18.32 18.32%
SiO2................ 67.37 67.37%
========
100.00
MgO 0.02* 0.14%
K2O 0.66* 10.74%
Na2O 0.32* 3.43%
Al2O3 1.04 18.32%
SiO2 6.49 67.37%
Si:Al 6.24
SiB:Al 6.24
Expan 8.40
Good luck
Ababi
sharon@shoval.org.il
http://www.israelceramics.org/main.asp?what=gallery
http://www.milkywayceramics.com/cgallery/asharon.htm
----- Original Message -----
From: Cindy Strnad
To:
Sent: Wednesday, July 26, 2000 05:14
Subject: Re: Feldspar types
> Alisa,
>
> Since I live in Custer, I always use Custer spar. Just makes sense. So if
a
> recipe calls for soda spar, I try it with soda spar and Custer spar, and
> I've never seen much of a difference. I'm sure there are some glazes which
> will be more sensitive to this change in fluxes, though.
>
> Cindy Strnad
> Earthen Vessels Pottery
> RR 1, Box 51
> Custer, SD 57730
> earthenv@gwtc.net
> www.earthenvesselssd.com
>
>
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