Formula: 2CaO.3B2O3.5H2O or CaB3O4(OH)3·H2O
|If this formula is not unified correctly please contact us.|
|DENS - Density (Specific Gravity)||2.95|
|HMOH - Hardness (Moh)||4.5|
|GSPT - Frit Softening Point||1050C|
|SLBY - Solubility||in HCl but not in water|
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.
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). Ulexite, a similar mineral, is far less likely to do this. Courtesy of Nigel Hicken.
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.
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).
Out Bound Links
This term is very generic, referring of course to ...
In Bound Links
Colemanite, Calcium Borate, Borocalcite
PotteryCrafts Frit 2954
The major borate minerals are Colemanite and Ulexi...