I'm hoping to make up a glaze (eggplant) that I stumbled upon a few months ago, but don't have the main element on hand.  I hate to buy a supply of it until I know if I really need it.  Is there someone that could tell me if I might possibly be able to substitute 457 grams of nepheline syenite for 457 grams of soda feldspar (Kona F4, Minspar 4)?  I use nepheline syenite in many of my glazes so already have that in my studio.  I would greatly appreciate guidance on this question.

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Nepheline Syenite has less silica, more alumina and far more sodium than Minspar 200.

http://digitalfire.com/4sight/material/minspar_200_1031.html

http://digitalfire.com/4sight/material/nepheline_syenite_a270_2963....

To keep the chemistry the same you would need to reduce alumina by reducing other alumina containing ingredients like kaolin and add extra silica. But without seeing the recipe my guess is reducing the excess sodium nepheline syenite adds is going to be difficult.

You can use the Insight-Live software by joining George Lewter's Insight-Live Users Group http://cone6pots.ning.com/group/insight-live-users-group

It's sometimes interesting to make a glaze simply replacing one ingredient with a similar one without making any adjustment, as you're in a position to do.  Sometimes you get a winner, but I suspect at a minimum you'll need to add the silica Nepheline Syenite is missing to make it something you'd want.

An alternate approach is to use the color mechanism used by your eggplant recipe in a different glaze.  Is this magnesium and cobalt?  If so I like this Pike Purple Oatmeal.

102.0%   Pike Purple Oatmeal
37.3%   Feldspar Custer
22.7%   Silica
13.6%   Talc (magnesium)
12.7%   Gerstley Borate
6.4%   Dolomite
4.5%   Kaolin
2.7%   Zinc Oxide
2.0%   Cobalt Carbonate
An extra 1.5% Cobalt makes Darker Purple

If you look closely at the photo of this glaze you'll see the glaze has crazed (crackled).

Wow!  So much information and a beautiful purple recipe to try.  thank you so much!

I'll paste in the eggplant recipe below.  The first time I made it up, I did so in a very small batch and subbed Custer Feldspar. Believe it or not, it came out quite nice used over black, but not so great on it's own.

EGGPLANT (^6 OX)

 

457 soda feldspar (Kona F4, Minspar 4) 

152 flint

127 talc

129 gerstley borate

 90 dolomite

 45 epk

10 bentonite

20 cobalt carbonate

50 manganese carbonate

Looking at your recipe, I'd substitute any feldspar you have for the soda feldspar and see how it turns out. Potash Feldspar often makes colors brighter and more vivid than other feldspars or fluxes like nepheline syenite or wollastonite.

Alternately by looking at your recipe in Insight-Live - you can see that eliminating the 4.5% Kaolin (EPK) and adding an extra 9% Silica gets you to the same place, except with more sodium for which there is no solution - which increases the COE (thermal expansion) from 6.4 to 6.8 which may or may not make the Eggplant glaze craze. 

The 12.9% Gerstley Borate plus the 1% Bentonite should still suspend the glaze fully without the EP kaolin. You could also increase the amount of Dolomite by a bit to supply the slightly reduced level of calcium and magnesium, but this is unlikely to make a noticeable difference.

Compared with Pike Purple Oatmeal with 3.3% Cobalt Oxide interacting with magnesium primarily in the talc, the Eggplant recipe uses only 2% cobalt carbonate (1.26% cobalt oxide equivalent), relying more on the interaction between the magnesium and manganese which also produces a light purple.

The http://cone6pots.ning.com/group/insight-live-users-group is free for members of Cone6Pots.



Carole Evans said:

Wow!  So much information and a beautiful purple recipe to try.  thank you so much!

I'll paste in the eggplant recipe below.  The first time I made it up, I did so in a very small batch and subbed Custer Feldspar. Believe it or not, it came out quite nice used over black, but not so great on it's own.

EGGPLANT (^6 OX)

 

457 soda feldspar (Kona F4, Minspar 4) 

152 flint

127 talc

129 gerstley borate

 90 dolomite

 45 epk

10 bentonite

20 cobalt carbonate

50 manganese carbonate

Thank you so much, Norm.  I will be making up a few small batches of this glaze trying out the tweaks that you've recommended.  I did get some crazing with my first batch when I subbed Custer Feldspar for the soda feldspar and your research explains why to me now.  You've been so very helpful and generous with your time helping me with this.  Thank you so much!

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