It is not just a powder you buy at your supplier. Gerstley Borate was a very unique material, knowing more about it is a key to being able to substitute it in recipes. Digitalfire has always been the best place to learn about it.
During the early 2000s, the demise of Gerstley Borate appeared imminent. Multiple companies developed substitutes, including Laguna Clays. But after they began processing another stockpile at the mine interest in interest in substitutes waned. In 2023 the cycle could repeat.
Gerstley Borate is passing on to a better place. With a 300% price hike.
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 glazeflux, 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
This frit, or any of similar chemistry (e.g. Fusion F-12) IS NOT A SUBSTITUTE for Gerstley Borate, their chemistries are totally different. 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 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
Gerstley Borate is being compared to Ferro Frit 3134 in a GLFL test to compare melting behavior. Clearly, these are two very different materials, the frit 3134 is obviously not a substitute for the Gerstley Borate (GB). Notice the GB shrinks to about half its original size by 1600F and then suddenly by 1650 it has exploded out of the starting gate and crossed the finish line! The frit, conversely, slowly softens through the entire 1350-1650 range and then starts down the runway between 1650 and 1700F.
Why does Gerstley Borate melt in two stages? Because it is two minerals.
The ulexite in Gerstley Borate melts first, producing an opaque fired glass having the unmelted (and still gassing) particles of colemanite suspended in it. By 1750F the colemanite is almost melted also. Boron-containing frits, by contrast, soften slowly over a wide temperature range and gradually spread and melt. Not surprisingly they produce a more stable glaze (albeit often less interesting visually without additives e.g. titanium, rutile).
Can you actually throw a Gerstley Borate glaze? Yes!
G2931 Worthington Clear is a popular low to medium-fire transparent glaze recipe. It contains 55% Gerstley Borate (GB) plus 30% kaolin (GB melts at a very low temperature). GB is also very plastic, like a clay. I have thrown a pot from this glaze recipe! This explains why high Gerstley Borate glazes often dry so slowly and shrink and crack during drying. When recipes also contain a plastic clay the shrinkage is even worse. GB is also slightly soluble, over time it gels glaze slurries even in smaller percentages. Countless potters struggle with Gerstley Borate recipes.
These GBMF test balls were fired at 1550F and were the same size to start. The Gerstley Borate has suddenly shrunk dramatically in the last 40 degrees (and will shrink even more and then suddenly melt down flat within the next 50). The talc is still refractory, the Ferro Frit 3124 slowly softens across a wide temperature range. The frit and Gerstley Borate are always fluxes, the talc is a flux under certain circumstances.
Gerstley Borate (compared with Ferro frit 3124) from 1600-1750F. At 1550F (not shown) it suddenly shrinks to a small ball and then by 1600F it has expanded to double its size. By 1650 it is well melted, but still gassing and bubbling.
These are various different terra cotta clays fired to cone 04 with a recipe I developed that sources the same chemistry as the popular G2931 Worthington clear (50:30:20 GB:Kaolin:Silica) but from a different set of materials. The key change was that instead of getting the B2O3 from Gerstley Borate I sourced it first from Ulexite (G2931B) and then from a mix of frits (G2931K). All pieces were fired with a drop-and-holdfiring schedule (like C03DRH). Fit was good on many terra cottas I tried (pieces even surviving boiling:icewater stressing). Where it did not fit I had thermal expansion adjustability because more than one frit was sourcing the boron. Frits are so much better for sourcing B2O3 than Gerstley Borate for functional ware (the later is notorious for turning glaze slurries into jelly!). Of course, if you want to make artware then the issues that come with glazes employing high percentages of Gerstley Borate might just need to be endured!
Gerstley Borate 50:30:20 glaze. Let's just kill it!
This recipe, G2826A, a base transparent recipe having 50% Gerstley Borate plus 20% kaolin, is "jelly city". Even with 2.5g of Darvan deflocculant in this jar it is still thick enough to require pushing this tile down into it! Even then, it needs 5 seconds to build up enough thickness. And then does not even cover properly. People have suffered with this popular recipe for 50 years or more just to get the surface variegation it produces. They add all manner of colorants and opacifiers to it. And endure its incessant running on to kiln shelves, bubbling and clouding. It is time to just stop this "Dr. Jekyll and Mr. Hyde" of ceramic materials! And use a different base transparents that employ frits to source the boron (B2O3). Same chemistry, just a better recipe: G2826A1. Then just add rutile or titanium to restore the variegation.
This chart compares the decompositional gassing (Loss on Ignition) behavior of six materials as they are heated through the range 500-1700F. These materials are common in ceramic glazes, it is amazing that some can lose 40%, or even 50%, of their weight on firing. For example, 100 grams of calcium carbonate will generate 45 grams of CO2! This chart is a reminder that some late gassers overlap early melters. That is a problem. The LOI (% weight loss) of these materials can affect your glazes (causing bubbles, blisters, pinholes, crawling). Notice talc: It is not finished gassing until 1650F, yet many glazes have already begun melting by then (especially fritted ones). Even Gerstley Borate, a raw material, is beginning to melt while talc is barely finished gassing. And, there are lots of others that also create gases as they decompose during glaze melting (e.g. clays, carbonates, dioxides).
This is one of the things Gerstley Borate does to glazes (when its percentage is high enough). This slurry has a high water content and should be far more watery. It is also highly thixotropic - this can be stirred vigorously to thin it and yet within seconds it turns back to jelly again. This was deflocculated with Darvan yesterday and it was stable enough to dip bisque ware - but overnight salts have gone into solution and it is no longer dispersed. It also dries very slowly on bisque ware. What can be done with a mess like this? Replace the Gerstley Borate with something else. Gerstley Borate sources B2O3, it can be supplied using frits or Ulexite (glaze chemistry calculations are needed to juggle the recipe), this can be done in an account at insight-live.com.
What to do about lithium carbonate and Gerstley Borate in glazes
Lithium is getting really expensive. These are four recipes submitted by a customer who wonders if there is a substitute. The answer is not simple, each glaze is a unique situation. Fortunately, lithium carbonate is almost always a minor addition (in the first two recipes it is 1% and 3%). Lithium is a powerful low expansionflux, in some cases, a low melting low expansion frit could perform the same function (e.g. Ferro 3249). Even for the 6.5%, as in the third one, this could still work. But in these cases wouldn't it be better to continue using lithium? Even for the last one that has 9%? It's only expensive if you make glazes and don't use them. Perhaps a solution is to make them as brushing glazes, a 1-pint jar only needs about 350g of powder (that is only about 30g for recipe 4).
This situation can also be considered as an opportunity to rationalize the recipes you use. Let's pretend that each of these might be used on functional ware and should measure up to common sense recipe limits. The Gerstley Borate in three of these is also a red flag, that won't be available shortly (calculating how to source B2O3 from frits is better anyway). During that process, you might find that lithium is not even needed. Another issue is thermal expansion. Notice that one of these calculates to 8.5 and another to 9.6! Those are virtually certain to craze. Why not lower that number while removing the Gerstley Borate? Notice that two of these have clay percentages over 70% (Alberta Slip and Gerstley Borate), these are virtually certain to crack on drying (and crawl on firing), that can also be fixed. The percentage of titanium, Zircopax and rutile in the fourth one are guaranteed to make it crystallize heavily on cooling produce big problems with cutlery marking and staining.
An example of how much Gerstley Borate LOI can affect a glaze
Fired at cone 6. The samples on the bottom tiles are from ten-gram balls that have melted down (in our GBMF test). These glazes have the same chemistry, but the one of the left sources its B2O3 from Gerstley Borate (which has a high LOI). The one on the right gets it from a frit. Because the fritted version has less gases of decomposition to expel, the glass is much smoother. Curiously, the fritted version is flowing less and the red color has been lost. Why? This could be because the Al2O3, which stabilizes glazes against excessive fluidity, is being dissolved into the melt better and thus is more available for glass building.
Original glaze with Gerstley Borate vs. improved version with frit
These pieces are fired at the same temperature. The glaze on the left is a popular recipe found online, Worthington Clear (our code numberG2931). The Gerstley Borate in it is "farting" as the glaze is melting (the calculated LOI of the glaze as a whole is 15% mainly from that one material). Unless applied very thinly tons of micro-bubbles appear (this example is fired to cone 03). And strange, it is crazing badly also despite the low calculated thermal expansion. Using my account at insight-live.com I was able to source the B2O3 and MgO from a frit (actually two frits) to create the G2931K recipe. Although the thermal expansion calculates higher it is strangely fitting better (albeit not firing as white). As you can see, the new fritted glaze is very glassy and clear (thick or thin).
Example of how bubbles dissipate in a glaze with increasing temperature
This is a Gerstley Borate based recipe (45%) melted in crucibles at increasing temperatures. Although the recipe is well melted at cone 2, it is still not fluid enough to enable their migration in the time available. By contrast, the melt at the upper temperature is much less viscous, enabling all bubbles to completely clear on the thinner sections. If this glaze were applied to ware it would be in a thin layer and the bubbles would likely clear at cone 6. Not to be ignored is the degree to which the thousands of bubbles passing upward through the melt have helped to mix the melt and remove discontinuities in the cone 7 and 8 specimens.
One reason why Gerstley Borate can make glaze difficult to use
This glaze slurry contains 30% Gerstley Borate. I poured it onto a plaster table and it can take five or ten minutes to dewater enough to form it in test balls. A typical glaze would dewater twenty times faster! Gerstley Borate contains microfine clay (e.g. hectorite, bentonite), the percentage is high enough that it voraciously hangs on to the water it has. This is the reason that many GB glazes take a long time to dry on bisque ware. And why they require more water and gel the slurry. Generally, slow drying also means cracking, that in turn can lead to crawling.