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).
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