Why does Gerstley Borate melt in two stages? Because it is two minerals.

Monday 11th August 2014

The ulexite in Gerstley Borate melts first, producing an opaque fired glass having the unmelted (and still gassing) particles of colemanite suspended in it. By 1750F the colemanite is almost melted also. Boron-containing frits, by contrast, begin softening at a much lower temperature and gradually spread and melt gradually. Not surprisingly they produce a more stable glaze (albeit often less interesting visually).

Pages that reference this post in the Digitalfire Reference Database:

Gerstley Borate, Colemanite, Ulexite, B2O3, Reducing the Firing Temperature of a Glaze From Cone 10 to 6

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.