Somehow the Gerstley Borate 50:30:20 glaze worked.

But does it work using Gillespie Borate?

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 how melt fluid a glaze could be. And 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 it produces (because of boron blue) and the fluid melt (because it is so high in boron). They added all manner of colorants and opacifiers and it generally performed without blistering. The melt fluidity required careful control of thickness (to avoid it running onto shelves). 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.

Related Pictures

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.

Gillespie Borate is doing something very strange at 1700F

Gillespie Borate suddenly shrinking at 1700F

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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 we 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 (likely stopping and holding it at 1700F exaggerates the problem).

Videos

Links

URLs

https://insight-live.com/insight/share.php?z=HzyNzj9ELs
Replacing the Gerstley Borate in recipes containing 50% or more of it
Many recipes were built on bases employing exceptionally high percentages of Gerstley Borate. At medium temperatures these melt fluid transparents hosted additions of colorants, variegators and opacifiers. At low fire they were used as transparents over underglaze decoration.

Materials

Gerstley Borate
Gerstley Borate was a natural source of boron for ceramic glazes. It was plastic and melted clear at 1750F. Now we need to replace it. How?

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