|Monthly Tech-Tip |
Alternate Names: Gilespie Borate
Description: Gerstley Borate Substitute
August 2023: We received our multi-ton shipment and tested it side-by-side with vintage Gerstley Borate (see below).
This substitute became available during the early 2000s and won the attention of many Gerstley Borate (GB) users (it is still available in 2023 and they state readiness to take the market from GB). With Gillespie Borate they claim to achieve chemistry, mineralogy and physical properties very similar to GB. They went well beyond what other companies did in explaining their research, development and testing (however their QuickFacts web page is now gone). Of course, it is assumed this material does not turn glaze slurries into a bucket of jelly the way GB did.
Here is a summary of some of the claims they were making:
"Gillespie Borate is a blended borate mineral for use in glaze formulas replacing Gerstley Borate on a pound-for-pound basis. It has better consistency, gives increased glaze surface smoothness, reduces crawling and pinholes and has fewer impurities so glazes and colors are brighter."
By virtue of having the same mineral profile, their product variegated glaze surfaces the same way. They did x-ray diffraction and physical tests to fully characterize GB and identified three key characteristics: extended particle size distribution including a colloid fraction, the oxide analysis, and firing properties that promote variegation. They identified immiscibility and solubility as key characteristics, the former generating the characteristic variegated texture and the latter the mottled glaze surface (via migration of soluble oxides). They emphasized that theirs was a consistent, reliable raw material with fewer impurities to throw colors off.
Their recommendations for using it were:
- Do not use excess water in mixing the glaze.
- Store the wet glaze in an airtight container.
- Always test first it in a small batch.
- Test on the same clay body you will use in production.
- Test on vertical tiles placed throughout the kiln.
As to the recipe, they hinted it was "comprised principally of ulexite (sodium calcium borate), with small amounts of colemanite (calcium borate) and approximately 10% colloidal clay-like materials". They claimed to add "various clay minerals" (vs Boraq that only used hectorite). In addition, they said the product contained "alkaline earth carbonates and silicates" (like talc and calcium carbonate). We got an exact analysis early but they later said the product was "approximately 25% B2O3, 23% CaO, 4% total alkali and 31% loss on ignition". For some reason they did not state the amount of SiO2, it would have to be at least 10%.
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.
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).
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).
Gerstley Borate has shrunk and vitrified to a porcelain state here at 1550F (the ball has shrunk to half the original size and gets even smaller by 1600F). Gerstley Borate has a significant LOI, it finishes off-gassing at about 1400F, which enables this high shrinkage that happens between 1350 and 1600F. Gillespie Borate, on the other hand, is obviously experiencing an overlap between the gassing and melting phases. What does that mean? It is already melting while gasses of decomposition are being expelled. Glazes having a high percentage of it are going to do this (to a lesser extent of course) as they are heated through this range in a firing. It was not clear at first how this might affect glazes but it became evident later: Crawling.
This is a GBMF test, it compares the melt fluidity of the Gerstley Borate based cone 6 Perkins Studio clear recipe original (left, our code number G2926) and a reformulated version that sources the boron from Ferro Frit 3134 instead (right, our code number G2926A). The latter is less amber in color (indicating less iron). The good news was that it melted so much better that we were able to add significant Al2O3 and SiO2 to really drop the thermal expansion (improving glaze fit on common clay bodies), which produced our G2926B base recipe. Every time I use it I think of how unfortunate we would have been had we continued to use the Gerstley Borate original.
Hamil and Gillespie Website
Gillespie Borate fact sheet from Hammill and Gillespie.
GerstleyBorate.com - The best place for info on 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?
This Gerstley Borate substitute was available during the early 2000s. Its recipe and development are well documented but two materials are no longer available.
A Gerstley Borate substitute that was available during the early 2000s.
Gerstley Borate Substitutes
Many development efforts to create Gerstley Borate substitutes took place during the early 2000s (the initial period when the demise of Gerstley Borate appeared imminent). A number of companies, including Laguna Clays itself, produced and sold these for many years. When Laguna secured another stockpile at the mine and began producing the original material again, interest in substitutes gradually waned. However, the sudden dramatic price increase in 2023 appears to have initiated the process again. Gillespie Borate appears to be the only viable and visible substitute now. Thus, the substitutes listed here are mostly no longer made. Other high-boron materials shown are also no longer available. We continue to recommend sourcing B2O3 from frits instead. Please contact us if you have a specific recipe and we can work with you in your Insight-live account to develop a new recipe that both eliminates the GB and improves overall working and firing properties.
|By Tony Hansen|
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