Linda - Pacifica on mon 21 aug 06
Well I've updated my glaze calculation program and I'm adding cone 5 and 6 recipes for potential adjustment, espcially to eliminate the gerstley borate eventually.
When a recipe calls for a potash feldspar, I have been entering custer feldspar, since I know we have that at school. Does that make sense? Digitalfire lists Kingman spar too, but I'm assuming that Custer will substitute well for either potash feldspar or Kingman.
Wait, I just went to the Digitalfire database and found that they differ a bit:
Custer Kingman Potash
CaO 0.300 0.100
K2O 10.280 12.010 16.920
Na2O 2.910 2.800
Al2O3 17.350 18.720 18.320
SiO2 69.000 66.170 64.760
Fe2O3 0.120 0.110
There's 60% more K2O in Potash Feldspar than in Custer. And Kingman is no lnger available. What would happen using less K2O than called for in a recipe? Do I need to increase the sodium flux to compensate?
I'm trying to hit it right without a whole lot of testing, since we'll need these glazes pretty soon.
Daniel Semler on tue 22 aug 06
Bunch of thoughts on this.
Generally I do the same as you have done. If you've never actually =20
seen the glaze before, or even a photo, it'll have to be tested anyway.
If its an older recipe from around these parts (west coast ish) its =20
possible it used Kingman.
Custer, at least as far as I know, is the common sub since Kingman went o=
You can be sure noone ever used "potash feldspar", its a generic =20
formula which has no impurities in it and I doubt it exists in =20
reality, and not at a price potters would afford.
Potash below has about 17% flux, Kingman about 15%, Custer about =20
over 13.4%. So Custer is the most refractory per gram. Would it be =20
noticeable ? Perhaps not if there is enough other flux in the recipe. =20
Depends how must feldspar is in the recipe, and who knows, maybe it =20
was normally the more refractory one that was used.
If the recipe is too stiff, dry, matte, whatever you could =20
certainly add some flux. Na2O will increase the expansion =20
considerably, depending on how much you add so there might be btter =20
Compensating for the differences is impossible without knowing what =20
is normally used in a recipe unfortunately. I mean it could be G200 =20
which is less refractory also. So if you can find out what is normally =20
used then sure we could sub. Without that some testing is required.
But it is possible to get a feel for the impact of use of any =20
particular feldspar by putting it into the recipe and seeing its =20
impact on the oxide balance.
Let me know if I can be of any help with any of this.
Paul Lewing on tue 22 aug 06
On Aug 21, 2006, at 12:57 PM, Linda - Pacifica wrote:
There's 60% more K2O in Potash Feldspar than in Custer. And Kingman
is no lnger available. What would happen using less K2O than called
for in a recipe? Do I need to increase the sodium flux to compensate?
It's almost impossible to get sodium or potassium without the other
from any natural material. It is possible with a frit, though.
Keep in mind that "potash feldspar", in which all of the alkali comes
from potassium, is a theoretical beast that does not exist in
nature. All feldspars have some sodium, some potassium, and usually
some calcium. They're tagged as either potash or soda depending on
which one predominates.
I remember back in the day, when Kingman went out of business, and
all of us in the West switched to Custer. I, and everybody else I
knew, just made the straight across substitution, and noticed no
Linda - Pacifica on wed 23 aug 06
This is an important bit of information. Thanks Paul. Makes me wonder why Digitalfire even has something called potash spar in its database.
On Tuesday, August 22, 2006, at 06:21PM, Paul Lewing wrote:
>I remember back in the day, when Kingman went out of business, and
>all of us in the West switched to Custer. I, and everybody else I
>knew, just made the straight across substitution, and noticed no