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URL: GerstleyBorate.com - The best place for info on Gerstley Borate

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Gerstley Borate is passing on to a better place. With a 300% price hike.

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Gerstley Borate has just become Costly Borate. The supplier, LagunaClay.com, likely raised the pice to wake us all up to take action in substituting it before supplies run out. It is a ceramic glaze flux, sourcing boron to melt far better than any other common raw material. It has been a foundation material in low and middle temperature pottery glaze recipes for many decades. Potters have a love/hate relationship with it: Enjoying its low melting point but enduring its problems (inconsistency, gelling of the slurry, crawling, micro-bubbles, boron-blue discoloration). Strangely most people have used it without knowing what it really was. And few realize how easy it is to replace. Yes, existing substitutes work sometimes - but it is better to adjust each glaze recipe to source boron from a frit (fixing other issues also). Please read that last statement carefully. It did not say that there is any frit that can substitute. It said that frits can source boron.

Ferro Frit 3134 is NOT A SUBSTITUTE for Gerstley Borate

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This frit, or any of similar chemistry (e.g. Fusion F-12) IS NOT A 1:1 SUBSTITUTE for Gerstley Borate, their chemistries are too different. That being said, the frit sources lots of B₂O₃, that makes it a candidate to weave into recipes as the source of boron. To show the difference I have put 100 parts of each in side-by-side recipes in my Insight-live.com account, set the calculation type to non-unity formula and increased the frit until the B₂O₃ in the two match. Notice it takes 118g of the frit to source the same amount of B₂O₃ as 100 GB. Notice also that the frit sources almost triple the amount of Na₂O per weight unit (that is a big deal because it means the recipe containing the Gerstley Borate to be substituted needs an Na₂O-sourcing material that can be reduced to compensate). And the frit sources 3.5 times the SiO₂ (other SiO₂-sourcing materials in the recipe will need to be cut to compensate). And the Gerstley Borate has significant MgO while the frit has none (so an MgO-sourcing material like talc will be needed). Minor tweaks will also be needed to reduce other sources of CaO (since the frit has quite a bit more). The recipe will also need enough flexibility to do the final matching of Al₂O₃ and SiO₂. The GBMF test confirms the difference at 1700F, these 10-gram balls melted down onto the tiles very differently.

Gerstley Borate vs Frit 3134 melt fluidity comparison

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This GLFL test compares the melting behavior of Gerstley Borate (GB) and Ferro Frit 3134 across a wide temperature range. Although both supply boron, they behave very differently because Gerstley Borate is a natural mineral blend while Frit 3134 is a premelted glass. Gerstley Borate shrinks dramatically to about half its original size by 1600°F, then suddenly becomes highly fluid between 1600–1650°F as reactive borate phases form and viscosity collapses. Frit 3134, by contrast, softens gradually, reflecting the progressive viscosity decline typical of a glass. It has no crystal decomposition or raw material phase changes, no sudden liberation of fluxes. It is softening while gases are still escaping (the GB was also foaming at 1650F).

So, if GB delivers boron through mineral breakdown and 3134 delivers it through viscosity-controlled glass softening, then why does a zero LOI frit have bubbles at 1700F? The dried ball is a compacted powder; it has plenty of voids. This melt begins extremely early, at 1350F, easily trapping air in the voids and gases generated by the binder holding the frit ball together. While bubbles have cleared this fluid melt by 1800F, a bubble-free glaze is much more challenging because there are more sources of decomposition gases and a stiffer, more viscous melt.

How does Gillespie Borate compare with the original Floating Blue recipe?

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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).

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