The original Floating Blue recipe, our code number G2826R, has been popular for 50 years. But also troublesome (because of a fragile mechanism, poor slurry properties and inconsistencies in Gerstley Borate and rutile). Gillespie Borate, it's 2023 apparent successor, appears to solve most of its issues. These specimens of the recipe were fired using the cone 6 C6DHSC schedule. We have "vintage" Gerstley Borate from the 1990s, that is what was used here.
Top left: Floating Blue using Gerstley Borate (GB) (top) and Gillespie Borate bottom on a buff burning body.
Top right: Same but on a red burning body.
Centre: Melt fluidity GLFL test of the two glazes (GB) on the left.
Bottom: The two recipes and their calculated chemistries.
Clearly, the Floating Blue itself is firing greener than usual. And the Gillespie Borate version is much bluer. You may be used to something in between these two. The green tones could likely be restored by a reduction in the cobalt and increase in the iron oxide. The best news is that at 1.47 specific gravity, Gillespie Borate produces a far better slurry, there is no gelling. And no sign of settling into a hard layer.
The chemistry comparison at the bottom highlights some concerns, the difference is not insignificant. B2O3, Al2O3 and SiO2 are all lower (this could be part of the reason for the differences in color also). For better or worse, the melt fluidity is the same: Very high. This is likely because the percentage of Ulexite is higher (that melts better than Colemanite).
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.
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?
A Gerstley Borate substitute that became available during the early 2000s and is still available in 2023.
GerstleyBorate.com - The best place for info on Gerstley Borate
Rutile Blue Glazes
A type of ceramic glaze in which the surface variegates and crystallizes on cooling in the presence of titanium and iron (usually sourced by rutile)