Digitalfire Ceramic Materials Database
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Alternate Names: Colemanite, Calcium Borate, Borocalcite
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No common natural material comes anywhere close to melting like Gerstley Borate (GB). It begins to melt between 1550F and 1600F and is a clear amber glass by 1750F and ultraclear and glossy by cone 06 (Ulexite melts better but it is not commonly in use in ceramics). It has thus been a staple among potters for many years. 50% can be found in many cone 06-02 glazes, up to 30% in cone 6 glazes. Gerstley Borate is also very plastic and thus suspends and hardens glazes as they dry. In fact, few clays have the plasticity and the ability to retain water that GB has. A GB slurry can take many hours to dewater on a plaster batt, even in a very think layer. Thus it is common to find Gerstley Borate based recipes having no clay content.
GB natural source of boron that was mined in southern California for many years. Mineralogically it is a combination of colemanite, ulexite and high plasticity clay (likely hectorite). The melting behavior of ulexite and colemanite is quite different, the unusual early melting behavior GB exhibits this, it suddenly implodes to a brown opaque melt (because of the earlier fluxing of ulexite) which later turns transparent (when the colemanite joins in).
Since GB glazes melt well and are so easy to make, most people have overlooked issues surrounding its use. Glazes with high GB content that host potentially toxic metallic colorants or other materials are often assumed to be non-leachable because they melt well (where as, in fact, they may have an unbalanced chemistry). Gerstley Borate has almost no Al2O3, this is a problem because glazes need it and Al2O3 is normally sourced from clays, especially kaolin. But since GB is so plastic, adding more plastic materials to a glaze causes excessive drying shrinkage (producing cracks and ultimately crawling). One solution is to use calcined kaolin. Another option is to source Al2O3 from feldspar, however to get enough to create a stable glass oversupplies KNaO and causes crazing.
High GB glazes often have alot of micro-bubbles in the fired glass and micro-dimples on the fired glaze surface (most visible in transparents). Slurries also tend to flocculate and gel causing problems with glaze application, drying and adhesion. Because this material melts so well, potters who use it have been willing to endure alot of these issues. One common low to middle fire transparent, for example, has 50% GB and adds 30% kaolin to that, producing a slurry the dries even more slowly, gels quite badly and shrinks considerably as it dries.
The mine was closed in 2000 and remaining stocks were to be depleted in 2-3 years. There was alarm across the ceramic community in North America leading up to and after the closure (because Gerstley Borate formed the basis of so many glazes). However in June 2011, the supplier, Lagunaclay.com, announced that there was again a large supply still available. For the best information on substitutes visit gerstleyborate.com. There is a page on the site dedicated to understanding what Gerstley Borate was chemically, physically and mineralogically. There are a number of materials that have been developed as substitutes over the years, these are outlined at the website also.
However the best approach is to finding an alternative is the use of ceramic chemistry on a glaze-by-glaze basis (to substitute other materials). In many cases, it is better to use frits to supply the CaO and B2O3, they are less volatile, more consistent and reliable and do not flocculate or gel the glaze as Gerstley Borate does. In cases where frits cannot deliver the needed chemistry, Ulexite can be employed.
Prior to, and during the decade of uncertainty about the future of this material, the supplier did not provide updated chemistry information. It was during this time that many companies promoted substitutes. We rationalized it (as explained at http://gerstleyborate.com) as 24% CaO, 4% MgO, 0.5% K2O, 4% Na2O, 2% Al2O3, 25% B2O3, 14% SiO2, 0.5% Fe2O3 and 14% 26% LOI. In June 2011 we changed the chemistry provided here to the one provided by Laguna on their website (rounded to 1 decimal). This new chemistry has more B2O3 and less CaO (other oxide amounts are fairly similar).
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Both of these glazes were made as 1000 gram batches and then mixed with the necessary amount of water to produce a slurry of the correct consistency. The one on the left is a fritted glaze with 20% kaolin, the one on the right is a Gerstley Borate based raw glaze (30% GB + feldspar, silica, ball clay). The GB glaze required much more water and gelled shortly after (it also tends to crack during drying). The fritted glaze has very good slurry and application properties.
Here is Cone 6 Perkins Studio Clear and an adjustment open side-by-side in my account at insight-live. The one on the right (G2926A) substitutes Frit 3134 for Gerstley Borate (I did all the juggling of its recipe to match the chemistry of original right within Insight-live). A melt flow of the two is identical (bottom left) except that the GB version has an amber coloration from its iron (the 3134 version actually flows a tiny bit less, the other has already dripped off). Anyway, the flow test on the upper left shows G2926A flowing beside the PGF1 transparent (a tableware glaze used in industry). This extra flow indicates that this glaze is too fluid. That means it can accept some silica (the more silica any glaze can accept the harder, more stable and lower expansion it will be).
The difference between dolomite and calcium carbonate in glazes: These glaze cones are fired at cone 6 and have the same recipe: 20 Frit 3134, 21 EP Kaolin, 27 Calcium Carbonate, 32 silica. The difference: The one on the left uses dolomite. Notice how the MgO from the dolomite completely mattes the surface whereas the CaO from the calcium carbonate produces a brilliant gloss.
Worthington Clear cone 04 glaze left (uses Gerstley Borate to supply the B2O3 and CaO) and a substitute using Ulexite and 12% calcium carbonate) right. This melt flow test demonstrates that the degree of melting is the same but the gassing of the calcium carbonate has disrupted the flow (of the one on the right). However, as a glaze, the 2931A does not gel and produces a clearer glass. A further adjustment to source CaO from non-gassing wollastonite is needed.
Worthington Clear is a popular cone 6 transparent glaze recipe. It has 55% Gerstley Borate (which is quite plastic) and 30% kaolin. That means you can actually throw it as if it were a clay. This explains why it gels almost immediately on slurry mixing and dewaters extremely slowly and shrinks and cracks during drying on the ware. Yet countless potters struggle with this recipe when they could be using frits!
These cone 04 glazes both have 50% Gerstley Borate. The other 50% in the one on the left is PV Clay, a very low melting plastic feldspar. On the right, the other 50% is silica and kaolin, both very refractory materials. Yet the glaze on the right is melting far better. How is that possible? Likely because the silica and kaolin are supplying Al2O3 and SiO2, exactly the oxides that Gerstley Borate needs to form a good glass.
Frit balls fired at 1700F. Frit 3124 and 3195 are base glazes, Frit 3110 and 3249 modify expansion and 3134 is similar to 3124 but without any Al2O3. Gerstley Borate is a raw source of boron, it has a very high LOI.
Gerstley Borate (with a frit) from 1600-1750F. At 1550F it suddenly shrinks to a small ball and then by 1600F it has expanded to double its size.
This chart compares the gassing behavior of 6 materials (5 of which are very common in ceramic glazes) as they are fired from 500-1700F. It is a reminder that some late gassers overlap early melters.
These 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 melt down flat within the next 50). The talc is still refractory, the Ferro Frit 3124 slowly softens across a wide temperature range.
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.
By Tony Hansen
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