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I came across a glaze recipe that used Cornwall stone, and I wanted to substitute for other materials because the cost of Cornwall stone is high here in California. Searching online, I found potash-spar-based recipes that either did a poor job of substituting and recipes that use Nepheline Syenite, a material which adversely affects the glaze gel over time.

Browsing materials to use for substitution, I noticed PV Clay is somewhat similar in composition to Cornwall stone, except lacking in Soda and Calcium. I have come up with this recipe for substitution:

54 PV Clay
44.5 NC4
1.5 Whiting

But I didn't calculate for LOI or expansion.

This yields:
------Substitute--Cornwall Stone
CaO------2.05------2.0
MgO------0.30
K2O------4.64------4.0
Na2O-----3.60------3.5
P2O5-----0.00------0.5
TiO2------0.06------0.1
Al2O3----15.63----16.5
SiO2-----73.42-----73.0
Fe2O3-----0.31-----0.15

Is this close enough? Do you predict the additional .64% K2O would be a big issue? Any suggested alterations?

These materials would yield 86% cost savings here in Southern California, so I'm hoping it works!

Follow-up:

I mixed two ash glazes that call for cornwall stone with three variations each:

1. Cornwall stone
2. PV Clay instead of cornwall stone, gram for gram
3. PV Clay / NC4 / Whiting substitute

I will post the results back here when they're fired.

Another follow-up:

Based on more up-to-date values for PV clay than those listed on Digitalfire, the substitution is more likely:

45 PV Clay
52.5 NC4
2.5 Wollastonite