• Materials for Ceramics

Ulexite

Alternate Names: Television Stone

Description: Sodium calcium borate mineral

Notes

Ulexite is a natural source of boron, it is similar to colemanite mineral. These two minerals are the only practical sources of insoluble boron for glazes (other than frits). Ulexite one of the lowest melting non-lead ceramic minerals, it can form an ultra-gloss transparent glass at cone 06. Strangely this material does not appear to flux bodies nearly as well as one might expect.

Ulexite is a truly uncommon ceramic mineral in that it contains almost no alumina or silica, it is nothing but fluxing oxides. This mineral forms in unusual geologic circumstances and can be found in very few places in the world. The chemistry given here is theoretical, actual deposits will have lower boron content.

The popular mineral Gerstley Borate, used by potters for many decades is, is composed partly of ulexite. It was mined and stockpiled in the California desert many years ago. The last remaining stockpile is being ground, bagged and sold by Laguna Clay.

While Ulexite melts well, it does have a very high LOI (that means that gases are generated during melting). For this reason it is not possible to produce ultra-clear glazes at low temperatures (as with frits). Notwithstanding this, by doing a drop-and-hold firing it is possible to clear many more bubbles. Thinner glaze layering is also much more transparent.

Ulexite is available industrially from Turkey and Chile. It is used in the fiberglass industry as a melter but its potential has never been exploited to any extent in ceramic glazes.

Related Information

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).

The difference between these low fire transparents: Gerstley Borate vs. Ulexite

Left: Worthington Clear cone 04 glaze (A) uses Gerstley Borate to supply the B₂O₃ and CaO. Right: A substitute using Ulexite and 12% calcium carbonate (B). The degree of melting is the same but the gassing of the calcium carbonate has disrupted the flow of B. Gerstley Borate gasses also, but does so at a stage in the firing that does not disrupt this recipe. However, as a glaze, B does not gel and produces a clearer glass. A further adjustment to source CaO from non-gassing wollastonite would likely improve it.

A cure for long-time Gerstley Borate sufferers

These are various different terra cotta clays fired to cone 04 with a recipe I developed that sources the same chemistry as the popular G2931 Worthington clear (50:30:20 GB:Kaolin:Silica) but from a different set of materials. The key change was that instead of getting the B₂O₃ from Gerstley Borate I sourced it first from Ulexite (G2931B) and then from a mix of frits (G2931K). All pieces were fired with a drop-and-hold firing schedule C03DRH. Fit was good on many terra cottas I tried (pieces even surviving boiling:icewater stressing). Where it did not fit I had thermal expansion adjustability because more than one frit was sourcing the boron. Frits are so much better for sourcing B₂O₃ than Gerstley Borate (the latter is notorious for turning glaze slurries into jelly!). Of course, a little glaze chemistry is needed to figure out how to convert a recipe from Gerstley Borate bondage to frit freedom, but there is lots of information here on how to do that.

Alberta slip and Ulexite at cone 6

90% Alberta Slip (which is a mix of half and half raw and calcine) and 10% Ulexite fired at cone 6. A dazzling fluid dark amber transparent. You could also do this using a high-boron frit.

The perfect storm to create boron-blue clouding at low fire

Two clear glazes fired in the same slow-cool kiln on the same body with the same thickness. Why is one suffering boron blue (1916Q) and the other is not? Chemistry and material sourcing. Boron blue crystals grow best when there is plenty of boron (and other power fluxes), alumina is low, adequate silica is available and cooling is slow enough to give them time to grow. In the glaze on the left B₂O₃ is higher, crystal-fighting Al₂O₃ and MgO levels are a lot lower, KNaO fluxing is significantly higher, it has more SiO₂ and the cooling is slow. In addition, it is sourcing B₂O₃ from a frit making the boron even more available for crystal formation (the glaze on the right is G2931F, it sources its boron from Ulexite).

Two transparents having opposite melt fluidity/surface tension balances

Low-fire glazes must be able to pass the bubbles they and the underlying bodies generate (or clouds of micro-bubbles will turn them white). This cone 04 flow tester makes it evident that 3825B has a higher melt fluidity (A has not even dripped onto the tile). And its higher surface tension is demonstrated by how the flow meets the runway at a perpendicular angle (it is also full of entrained micro-bubbles). Notice that A, by contrast, meanders down the runway, a broad, flat and relatively clear river. Low-fire glazes must pass many more bubbles than their high-temperature counterparts, the low surface tension of A aids that. A is Amaco LG-10. B is Crysanthos SG213 (Spectrum 700 behaves similarly, although flowing less). These two represent very different chemistry approaches to making a clear glaze. Which is better? Both have advantages and disadvantages.

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