Pure MinSpar feldspar fired at cone 6 on Plainsman M370 porcelain. Although it is melting, the crazing is extreme! And expected. Feldspars contain a high percentage of K2O and Na2O (KNaO), these two oxides have the highest thermal expansion of any oxide. By far! Thus, glazes high in feldspar (e.g. 50%) are likely to craze. Using a little glaze chemistry, it is often possible to substitute some of the KNaO for another fluxing oxide having a lower thermal expansion.
Feldspar is a natural mineral that, by itself, is the most similar to a high temperature stoneware glaze. Thus it is common to see alot of it in glaze recipes. Actually, too much.
|Na2O - Sodium Oxide, Soda
Ask the right questions to analyse the real cause of glaze crazing. Do not just treat the symptoms, the real cause is thermal expansion mismatch with the body.
In ceramics, feldspars are used in glazes and clay bodies. They vitrify stonewares and porcelains. They supply KNaO flux to glazes to help them melt.