Comparing glaze melt fluidity balls with their chemistries

Sunday 14th June 2015

Ten-gram GBMF test balls of these three glazes were fired to cone 6 on insight-live.com/glossary/71">porcelain tiles. Notice the difference in the degree of melt? Why? You could just say glaze 2 has more frit and feldspar. But we can dig deeper. Compare the yellow and blue numbers: Glaze 2 and 3 have much more B2O3 (boron, the key flux for cone 6 glazes) and lower SiO2 (silica, it is refractory). That is a better explanation for the much greater melting. But notice that glaze 2 and 3 have the same chemistry, but 3 is melting more? Why? Because of the mineralogy of Gerstley Borate. It yields its boron earlier in the firing, getting the melting started sooner. Notice it also stains the glaze amber, it is not as clean as the frit. Notice the calculated thermal expansions: The greater melting or #2 and #3 comes at a cost, their thermal expansions are considerably higher, so they will be more likely to craze. Which of these is the best for functional ware? #1, G2926B. Its high SiO2 and enough-but-not-too-much B2O3 make it more durable. And it runs less during firing. And does not craze.

Pages that reference this post in the Digitalfire Reference Library:

Gerstley Borate, GBMF Glaze Melt Fluidity - Ball Test, Does adding boron alone always increase glaze melt?, Click here for case-studies of Insight-Live fixing problems, Glaze Chemistry, Mineralogy, Melt Fluidity, Calculated Thermal Expansion

This post is one of thousands found in the Digitalfire Reference Database. Most are part of a timeline maintained by Tony Hansen. You can search that timeline on the home page of digitalfire.com.