• Materials for Ceramics

Colemanite

Description: Calcium borate mineral

Notes

Colemanite has been a popular natural source of insoluble boron for many decades. It is similar to Ulexite in its oxide contribution to glazes (although the latter sources Na₂O also). Frits are used as boron sources in industry whereas potters and smaller companies have used colemanite.

Colemanite does not melt as low or as uniformly as Ulexite. Gerstley Borate contains significant amounts of colemanite. Pure colemanite, however, is much higher in B₂O₃ than Gerstley Borate.

Higher percentages of colemanite in a glaze can result in wrinkling of the fired surface, likely due to to a phenomenon called 'decrepitation' (very active decomposition) that occurs when colemanite is heated. The glaze layer can actually delayer, even disintegrate if sufficient colemanite is present (pieces of glaze can be spit off the ware onto other ware or the kiln shelf. For many applications Ulexite is thus a better choice. If you must use Colemanite, be sure to screen out any materials coarser than 200 mesh, or ball mill the glaze. Gum or other binders also help.

Colemanite is available from Turkey, Chile and California. The chemistry of these varies quite a bit, and of course, none of the available materials have the theoretical chemistry shown here.

Related Information

This high-colemanite underglaze has decrepitated, ruining the overglaze

The Colemanite-based black underglaze over which a raspberry non-colemanite glaze was poured resulting in severe crawling as the Colemanite exfoliated and detached the overglaze. Courtesy of Nigel Hicken.

Colemanite and what its decrepitation does in glazes

Decrepitation refers to a decomposition accompanied by scaling, delayering, even disintegration of the glaze layer. Moving rightward these glazes have increasing percentages of colemanite. At its worst (far right) the glaze is spattering off the sample and onto the kiln shelf. The others are crawling, first pulling away from the corners (far left) moving toward pulling away on the flat surfaces (center). Gerstley Borate and Ulexite, similar minerals, are far less likely to do this (but they have other serious issues also). A much better solution is to use frits to source the oxide B₂O₃ (easy to do in your account at Insight-live.com). Photos courtesy of Nigel Hicken.

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

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, soften slowly over a wide temperature range and gradually spread and melt. Not surprisingly they produce a more stable glaze (albeit often less interesting visually without additives e.g. titanium, rutile).

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