|Monthly Tech-Tip |
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.
Crazed ceramic glazes have a network of cracks. Understanding the causes is the most practical way to solve it. 95% of the time the solution is to adjust the thermal expansion of the glaze.
|Oxides||Na2O - Sodium Oxide, Soda|
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.