Our G2571A and G1947U cone 10R glaze recipes are both in use in Ecuador. One user has been importing Custer feldspar, but cannot get it and needs to substitute a local feldspar. That material is much higher in SiO2 and much lower in Al2O3 and KNaO (so a pound-for-pound substitution is not going to work). Here is how I used my account at insight-live.com to figure out what to do. It turned out to be a matter of increasing the new feldspar to match the KNaO matched (which also matched the Al2O3) and then reducing the silica to match the SiO2 match. The amount by which I had to change the feldspar is a testament to how different the chemistry of these materials can be from country to country.
Substituting Custer Feldspar for Another in a Cone 10R Glaze Recipe
A locally available feldspar contains much less KNaO and Al2O3 and much more SiO2 than Custer Feldspar. Here is a step-by-step method, using my account at insight-live.com, to alter the recipe so that the overall chemistry of the glaze is maintained.
The most common potash feldspar used in ceramics in North America. While having been a standard for many decades its supply appears in doubt in 2024.