How Can I Substitute G200 hp for Custer Feldspar

[juli long]

Hey all,

I need some help adjusting this glaze, because I need to substitute G200 hp for the custer feldspar. Would I adjust by adding +3 silica?

Please chime in with opinions.

Thanks,

juli

Maggie Moo's Blue-Green glaze Cone 6

From http://umtclayandglaze.blogspot.nl

Maggie Moo's Blue-Green glaze #5

This glaze recipe is adapted from a combination of a Mastering Cone 6 Glazes base glaze and Maggie Schlebecker's blue-green glaze recipe. It is cone 6 oxidation.

You can adjust the percentage of copper, tin, and zinc to produce a range of color from a pale mint to a deep sea green. Varying the amount of tin and zinc will affect the opacity of the glaze and make it milkier the more you add. Overdoing it on the copper could cause leaching, so going over 5% is probably not a good idea, but otherwise this base is extremely stable and food safe.

Custer Spar 20.0 (kaliveldspaat)
Ferro Frit 3134 20.0
Wollastonite 10.0
EPK 21.0 (kaoline)
Talc 11.5
Silica 17.5 (kwarts SiO₂)
------------
Total 100.0

Add 
Tin Oxide 3.96
Zinc Oxide 2.45
Copper Carb 3.21 (or Black Copper Oxide 2.14)

[Donna Kat]

My analysis shows them so close that it does not matter.  3% would be too much - 1% more would be good enough.

[Norm Stuart]

Using G-200HP feldspar, instead of Custer, replaces 2.1% Na flux with K flux, and increases total Na/K flux by 8%. I've found this direct replacement usually leads to brighter colors with less expansion.

Custer feldspar does have 3% more silica, but I wonder if you could tell the difference between a batch without an extra 3% silica and a batch with 3% extra silica.

[juli long]

Thanks for the helpful info all, I will test it this week.

juli

[George Lewter]

FYI - Feldspar comparison from Insight-Live