Ron Roy on sun 9 aug 09
Hi Miri,
The never g200 is called G200 HP (for high potassium) - your dealer my have
a stock of the old
g200 - if it's the new stuff there should be some kind of marking on
the bag if it's in the original container.
If there is a lot of G200 in a recipe you may see some small changes - I
don't think it's going to be much of an issue except for some colour change=
s
in some specialty glazes. I'll be happy to revise any of our glazes for you=
.
East to so it with the new g200 and Neph sy for instance.
You should not sub equal amounts of Neph Sy for soda spar however - the is =
a
lot more KNaO in neph Sy and not as much silica - best to do that with
calculation software - let me know if I can help.
On Sat, Aug 8, 2009 at 3:26 PM, Miri Hardy wrote:
> Hi and thanks so much for posting your response to the list.
>
> I have been using G-200 in most of the MC6 glazes that I recently tested
> (Variegated Blue, Bright Sky Blue, Licorice and variations on Glossy Base
> Glaze
> 2) and I had no idea there was a change in the composition (it is still
> listed
> simply as G-200 by my supplier). I'm not sure if this can explain some o=
f
> the
> results I noted in my testing (slightly dull variegated blue, licorice a
> tad
> brownish, pinholing) or if those are simply down to glaze
> application--images of
> my results can be seen at
> http://nickandmiri.wordpress.com/2009/07/06/glaze-
> testing-1-0/
>
> I was about to mix up some larger batches to further test the glazes. No=
w,
> I
> guess I really should mix up small batches again, adding a soda feldspar
> where I
> previously only used the G-200HP.
>
> My supplier tells me I can use Nepheline Syenite as a Soda Feldspar. Wil=
l
> this
> work using the 0.7 to 0.3 ratio or will I need to completely adjust the
> glaze
> recipes?
>
>
> Thanks again,
>
> All the best,
> Miri in Puerto Rico
>
--
Ron Roy
15084 Little Lake Road
Brighton, Ontario, Canada
K0K 1H0
Richard White on sun 9 aug 09
Miri, and all:
This G-200 change is tricky to navigate. The manufacturer just made the
change at the beginning of 2009, so there may be significant quantities o=
=3D
f
the old product still in the distribution channels. Some suppliers still
have the old product while others with a higher turnover of their stock m=
=3D
ay
have only the new version. Some suppliers will be more punctilious than
others about informing their customers of the change. If you are buying a=
=3D
full bag, the new product will be labeled G-200HP, but you need to look f=
=3D
or
that. If you are buying smaller quantities repackaged by your supplier, y=
=3D
ou
need to ask them to look at the source bag. Whether the change will
seriously affect the final results of your glaze is something only testin=
=3D
g
will determine. As with so many things in life, the devil is in the detai=
=3D
ls.
So here are the gory details (aren't you glad you asked... lol).
When dealing with the chemical analysis of glazes, I find it useful to
convert everything to a Seger Unity Analysis so that I can compare the
relative proportions of the various oxides in a consistent way. (Glaze
calculation software like Glazemaster or Insight makes this easy.) I also=
=3D
do
this for the complex minerals by themselves so that I can see at a glance=
=3D
their relative oxide contents and thus, the likely effect of adding or
subtracting one or another in a glaze recipe.
The original G-200 had a unity analysis of:
Na2O-0.275; K2O-0.643; CaO-0.082; Al2O2-1.024; SiO2-6.315
The new G-200HP has a unity analysis of:
Na2O-0.138; K2O-0.787; CaO-0.075; Al2O2-1.003; SiO2-6.163
Note that the sodium level is half the original and the potassium is much=
=3D
higher (and the other oxides are about the same). The HP added to the nam=
=3D
e
stands for High Potassium to differentiate it from the original.
After the mine for the original G-200 was depleted, the Imerys Corporatio=
=3D
n
(and its predecessor, The Feldspar Corporation) had for several years bee=
=3D
n
blending the higher potash feldspar from a new mine 70:30 with a soda
feldspar trucked in from another mine several hundred miles away so as to=
=3D
achieve a similar chemical content as the original. This was prohibitivel=
=3D
y
costly, so they stopped doing that. But you can make a similar adjustment=
=3D
in
your recipes.
A 70:30 blend of new G-200HP and Kona F4 soda spar yields a unity analysi=
=3D
s of:
Na2O-0.277; K2O-0.622; CaO-0.101; Al2O2-1.001; SiO2-6.039
This is pretty close to the original.
As an alternative, Custer feldspar alone has a unity analysis of:
Na2O-0.302; K2O-0.664; CaO-0.033; Al2O2-1.043; SiO2-7.134
The sodium and potassium levels are a bit higher, the calcium is lower, a=
=3D
nd
the silica is higher, but overall it isn't that far off.
Miri, you asked about the nepheline syenite as a soda spar:
For reference, the Kona F4 alone has a unity analysis of:
Na2O-0.578; K2O-0.265; CaO-0.157 Al2O2-0.998; SiO2-5.773
and neph sy has a unity analysis of:
Na2O-0.712; K2O-0.220; MgO-0.011; CaO-0.056; Al2O2-1.030; SiO2-4.555
Note the much higher sodium and lower potassium, calcium, and silica. So
yes, while neph sy is not a true feldspar in geologic terms, it is a
feldspathic mineral akin to a soda spar. Be careful, however, don't just
directly substitute neph sy in a recipe calling for a soda spar; test it
first and adjust as needed until you get the result you want.
As for blending G-200HP with neph sy to come up with a substitute for the=
=3D
old G-200, the glaze analysis program suggests that will work at an 80:20=
=3D
ratio:
Na2O-0.274; K2O-0.652; MgO-0.003; CaO-0.071; Al2O2-1.009; SiO2-5.781
Note that with this substitution, the silica level is lower, so you will
need to bump that up in your recipe.
cheers
dw
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