As many of you know I am a retired potter. I had extensive experience in glaze calculation from one of the best known potters of his day, Val Cushing. I was fortunate enough to partake of his master glaze/clay calculation classes during my junior year at NY State College of Ceramics @Alfred.
Today I rediscovered what was a simple experiment posted to me to change the melting level of a glaze to work on a lower firing clay. Its amazing what I remember after all these years. But here is an article that actually outlines how to do it for a celadon glaze.