Gillespie Borate is doing something very strange at 1700F

On the left is G2826A3, a cone 6 transparent glaze (an improvement on the 50:30:20 classic Gerstley Borate base transparent recipe, substituting Gillespie Borate, reducing its percentage and increasing SiO₂). Despite the improvements it exhibits this strange cracking and crawling. The G2826A1 on the right uses a frit to source the boron instead, clearly a better idea. These tiles were fired to 1700F. The problem is likely the ulexite mineral in the Gillespie Borate - it is known for this behavior of suddenly shrinking and then suddenly melting (the latter of which is just starting). Since Gillespie Borate is plastic and suspends slurries well, I thought calcined kaolin would be better than raw kaolin in the G2826A3 recipe (to minimize drying shrinkage). However, it did not improve the situation. All of this being said, this recipe is still working reasonably well at cone 6 (stopping and holding it at 1700F may exaggerate the problem).

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Cat litter, is it really just clay?

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We ran the tests, assigning our code number of L4567 to the project. Of course, putting some in water was the first obvious test. As expected, it swelled in a manner similar to bentonite. Then we sieved 100g through a 50 sieve mesh removing almost exactly 50% aggregate, which looks similar to grog. We fired the aggregate to 2200F (upper left) and it is refractory, with few particles melting. Is it brick grog? No, we poured some water on the fired aggregate and it hydrated the same way limestone does (upper right). The 50% finer material was slurried and allowed to settle. It did not do so, even after a week (bottom), showing it is clearly colloidal, likely sodium bentonite. The bentonite portion fired dark brown and melting, the same behaviour as typical raw bentonite. So what is this cat litter? Dry raw bentonite grains sized to ~30 mesh mixed 50:50 with limestone aggregate of the same size.

The Gerstley Borate 50:30:20 glaze was a bummer

Is the Gillespie Borate version any better?

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This recipe, G2826A, a base transparent recipe having 50% Gerstley Borate plus 20% kaolin, is "jelly city". Although a low temperature base, this was much more commonly used at cone 5-6. This recipe, G2826A, was at the limit of the slurry properties that could be tolerated with this material. In this test, even with 2.5g of Darvan deflocculant in this jar, it was still thick enough to require pushing this tile down into it! It still needed 5 seconds to build up enough thickness. And did not cover the recesses properly. Yet people have used this popular fluid-melt recipe for 50+ years to get the surface variegation its high melt fluidity produces (because it is so high in boron). They added all manner of colorants and opacifiers and it generally performed without blistering. This was a "Dr. Jekyll and Mr. Hyde" of ceramic materials!

Potters are using Gillespie Borate in this recipe (with issues), see the G2826A2 recipe. Other approaches are to source the boron (B₂O₃) from a frit (or mix of frits). An example is G2826A1, it does not variegate as much but added titanium or rutile can emulate that. Another hybrid option is the G2826A3 that employs both Gillespie Borate, nepheline and talc.

Gerstley Borate 50:30:20 glaze using Gillespie Borate instead

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This is the G2826A 50:30:20 GB:kaolin:silica base clear recipe. It is been used for decades as a base for all kinds of glazes. It starts melting early enough for use on low-temperature earthenware and is widely used in the raku process. Yet it is also common at middle temperatures (obviously care must be taken or it will run off ware onto kiln shelves when fired to cone 5-6). These tests were fired to cone 6 using the PLC6DS schedule.
The samples on the left use Gerstley Borate, on the right Gillespie Borate. The GBMF test tiles (lower left and right) reveal how much off-gassing is still happening on both when melting starts (they are full of bubbles). The GLFL test (centre) shows the melt flow of the two glazes, it is very similar (normal glazes do not run off the end of the runway like this). The two porcelain test tiles show it to fire crystal clear (there is some pooling since these were applied too thick). There is thus good reason to believe that Gillespie Borate will work well in this class of recipes.

A pottery glaze that can eat through a firebrick!

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The melt fluidity tester was fired at cone 6. The glaze on the left is G2826A2, the 50:30:20 historically popular Gerstley Borate base recipe used for reactive and transparent glazes. The B₂O₃ in that glaze was 3x the normal amount in cone 6 glazes, making it so melt fluid it can eat through a firebrick!

The glaze on the right is G2926A3, an adjusted version that cuts the B₂O₃ level and adds SiO₂ (from silica and nepheline). The result is more sane, although still very melt-fluid glaze. This is also a lesson in the chemistry that produces boron-blue: High B₂O₃ is not the key; my adjustment lowers it significantly. CaO/MgO is also lower, so that is not the key. The SiO₂ appears to be the enabler here; it is much higher (from 2.6 to 3.48, a hugh increase). And, I am using 325 mesh silica, so it dissolves in the melt better, delivering even more SiO₂. Boron blue thus seems to thrive on enough SiO₂ coupled with high B₂O₃ and low Al₂O₃ with some MgO/CaO.

Here is why Gillespie Borate is crawling some glazes

Gillespie Borate crawls glazes because of this

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This is a variation on the 50:30:20 cone 6 pottery glaze recipe, it contains 22% Gillespie Borate (GB) and 12% calcined kaolin. Our objective was to reduce its melt fluidity. But the crawling is so bad that it is almost unusable. The reason was not obvious until we fired a sample to 1550F and 1650F. At the former the integrity of the glaze layer is great, but by 1650F it does this (many of the edges of these are curling upward). Ulexite, which GB contains, is known for the behaviour of suddenly shrinking and then suddenly melting over a narrow range of temperatures. Since GB is plastic and suspends slurries well we thought calcined kaolin would be better than raw kaolin (to minimize drying shrinkage). However, the improvement is minimal.

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Gillespie Borate
A Gerstley Borate substitute that became available during the early 2000s and is still available in 2023.

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