How many times have I already read in the literature: "The maximum firing temperatures for glazes made with Lead Bisilicate frit is 1950 °F (≈ cone 04)”.
Or: "There is never lead in a high temperature vitrified ceramic glaze (stoneware, porcelain): it is quite simply chemically impossible because the melting point of lead is much too low to be used as a high temperature flux (stoneware, porcelain)".
However, I note that several renowned international ceramists use the lead bisilicate frit in glazes recipes (mainly based on feldspars) for T° of 2000°F (≈ cone 5, 6...) and above!!
MY QUESTION : Could someone explain to me when and for what reason(s) it would be wise to use lead bisilicate at T° 2000°F and above?