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potash versus soda

updated tue 19 dec 00

 

Joan Walton on mon 11 dec 00


I was about to mix up a batch of my favorite black
glaze when I remembered that somewhere I had a recipe
for another I received from a Mary Barringer workshop
that I thought was just as beautiful (satiny and deep
with a metallic shimmer). I went to compare recipes
and found to my surprise that they were identical
except that mine uses Kona F-4 and hers contains
Custer feldspar. These recipes must have originated
from the same source - the amounts are oddly measured.
Here are my two questions. Does anyone know the source
of this glaze? More importantly, why would one choose
to use a potash feldspar versus a soda feldspar and
vice versa? When does it make a difference? I canít
find the answer in the books I have.
Here is my recipe:

LEATHER - cone 6 ox.
Kona F-4 4578
Whiting 318
EPK 288
Gerstley Borate 624
--------------
5808
add:
Copper Carb. 240
Manganese diox. 240
Cobalt oxide 120

TIA to anyone who takes this on! and I hope some of
you will enjoy this glaze.
Joan in Brooklyn, where almost everyone has a
relative.


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Wade Blocker on tue 12 dec 00


Joan,
There is a difference between soda and potash spars, though both can be
used interchangeably.


Potash feldspar
(theoretical)K2O.Al2O3.6SiO2


Soda feldspar
(theoretical) Na2O.Al2O3. 6SiO2

Soda spar affects glaze maturity because the fusion point is lowered. It
can also change the colors in a glaze. Mia in cold ABQ

Cindy Strnad on tue 12 dec 00


Hi, Joan.

That is a pretty recipe. It might craze, but it hasn't for me, despite the
fact that the expansion is higher than what I usually use. Actually, I'm
grateful you posted it, as I had the recipe with no colorants and had been
experimenting with my own. It's a very nice dark leather color with 4% red
iron oxide, btw, if you're interested.

Many people substitute feldspars according to what they have on hand. As I
live in Custer, I tend to go with Custer spar. Kind of a local loyalty
thing, you know. And besides, they give it to me free. Sometimes the
choice between soda and potash spar makes a difference. However, I've found,
in my own limited experience, that either of the two will usually work just
fine.

Cindy Strnad
Earthen Vessels Pottery
RR 1, Box 51
Custer, SD 57730
USA
earthenv@gwtc.net
http://www.earthenvesselssd.com

Paul Lewing on tue 12 dec 00


Joan Walton wrote:

> Here are my two questions. Does anyone know the source
> of this glaze? More importantly, why would one choose
> to use a potash feldspar versus a soda feldspar and
> vice versa? When does it make a difference?

Joan, very seldom can you actually track down the real source of a
glaze. Sometimes even when a glaze has someone's name on it, that's not
the person who originally came up with it. So who knows?
As to the feldspar choice, as Cindy Strnad said, it might just be what
someone had on hand, and they noticed no difference, so that's how the
recipe was passed on. Also as Cindy said, I've mixed glazes with both
kinds of spar, and sometimes there's a noticeable difference, and
sometimes there isn't.
But as to why you would deliberately switch, there could be two reasons.
Soda spars tend to melt just a bit more and have just a bit higher
expansion than potash spars. So at some point, some potter decided this
glaze looked just a bit better a bit glossier or fired just a bit lower,
or they needed just a bit higher expansion to fit their clay body and
firing cycle. I would expect color response to be identical in these
two recipes.
Keep testing.
Paul Lewing, Seattle

Ron Roy on fri 15 dec 00


Feldspars vary in analysis depending on where they are mined.

Soda spar is called that because the is more soda than potash by percent.
Sometimes they are almost the same - it's a very imprecise characterization
and not to be trusted - look at the analysis before making a decision.

F4only has about one third more sodium than potash. G200 and Custer have
over three times potash to soda - Cornwall stone is about 50/50. If you
really want sodium check out Neph Sy but be aware - sodium is very soluble
and can deflock a glaze to hard pan.

There is no such thing as a feldspar with only soda or potash - that kind
of theory was used in glaze calculation and was pratically useless. Use
real analysis when calculating - the real analysis for the materials you
are using - every mine has em.

Soda spars are not always easier melting than the potash spars - it all
depends on how much fluxing material is present compared to the
refractories (alumina and silica)

Potash spars are recommended for clay fluxing above cone 6 - thats because
sodium is volatile above cone 6 and will contribute to the glazing of your
kiln and shelves. Those of us who use high soda glazes do so because we
like the look of them so we put up with the fuming - or should I say we
count on it.


RR

Ron Roy
93 Pegasus Trail
Scarborough
Ontario, Canada
M1G 3N8
Evenings 416-439-2621
Fax 416-438-7849

June Perry on fri 15 dec 00


Ron,

There is, or was, such an animal as a spar with only sodium.
I still have a bag of N spar(I think it was also called Cal N spar), which
only has Na. I don't know if it's still available but I got it when I lived
in S. California.

It's formula is Na2O 1 Al2O3 1.33 SiO2 11.9 weight 924

Regards,
June

Khaimraj Seepersad on fri 15 dec 00


Hello June ,

greetings to all ,

Ever thought of trying a Na20 / Ti02 / Si02 , in your glazes ?
Available from enamel suppliers .
Khaimraj


-----Original Message-----
From: June Perry
To: CLAYART@LSV.CERAMICS.ORG
Date: 15 December 2000 8:49
Subject: Re: potash versus soda


Ron,

There is, or was, such an animal as a spar with only sodium.
I still have a bag of N spar(I think it was also called Cal N spar), which
only has Na. I don't know if it's still available but I got it when I lived
in S. California.

It's formula is Na2O 1 Al2O3 1.33 SiO2 11.9 weight 924

Regards,
June

____________________________________________________________________________
__

Ron Roy on mon 18 dec 00


Hi June - thanks for this - it's not an analysis so I poked around and
found one - not sure how accurate it is. You are right - it is mostly
sodium - a very unusual situation - better be careful - high sodium spars
will tend to deflocc glazes because sodium is so soluble.

Calspar N

SiO2 - 77.48
Al2O3 - 13.76
K2O - 0.29
Na2O - 7.10
MgO - 0.12
CaO - 0.49
Fe2O3 - 0.22
LOI - 0.54
Total - 100.0

Notice the total of KNaO in this one 7.39% KNaO

G200 has K2O @10.67 and Na2O @3.01 for a total of 13.68%

What makes the N spar so different - aside from the predominance of soda -
is the small amount of flux compared with other spars. I also notice the
iron is higher as well.

RR


>There is, or was, such an animal as a spar with only sodium.
>I still have a bag of N spar(I think it was also called Cal N spar), which
>only has Na. I don't know if it's still available but I got it when I lived
>in S. California.
>
>It's formula is Na2O 1 Al2O3 1.33 SiO2 11.9 weight 924
>
>Regards,
>June

Ron Roy
93 Pegasus Trail
Scarborough
Ontario, Canada
M1G 3N8
Evenings 416-439-2621
Fax 416-438-7849