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Home | Overview | Description | Substitutes | Boraq | Glaze Recipes | Learning | Contact

INSIGHT Software, the best way to deal with this: Watch the Gerstley Borate video here.

Substitutes Available at One Time or Another

It is amazing how many of these appeared (there are others not listed here also). Remember, this site is about the history of a collective substitution effort for a very common boron material. This effort largely died over time but it taught us many lessons. We are leaving this site in tact for the lessons. In addition, many people still struggle with substitution of GB and need background information. Each manufacurer had to consider cost and material availability to arrive at a compromize recipe. As you will see, this challenge is best met on a glaze-by-glaze basis. No one material or approach was or is going to be a miracle substitute, that is guaranteed. Our goal is to provide everything you need in this web site to tackle any glaze that contains GB. We will give you more than just a substitute, as you will see.

The best way to manage this situation is to use your account at to handle the chemistry, testing and documentation needed on a case-by-case substitution basis.

Laguna Borate, Murray's Borate, Cadycal, Frit CC-298C, Frit 439, Other frits, Gillespie Borate, Ulex Empresa Minera, Imco Borate, US Pigment Borate, Ulexite, Colemanite

Laguna Borate

This is the most popular product so far. We have dedicated a special page to it because of the many things it teaches us about this whole affair. We have shown a method for deriving its recipe using INSIGHT software and how you can improve on it. Click here to learn about Laguna borate.

Murray's Borate

Kickwheel Pottery Supply introduced this material and offered support to users at their web site at They had a page with quite a bit of information about how to use it. They claim it is a frit that has been specially manufactured for this purpose and they assure its continued availability. They recommend the use of bentonite to match the plasticity and outline how to decide how much to use on their website. They cite a melting temperature of 1987F (Cone 03) although our flow tests indicate that it actually flows freely as an amber glass at cone 06 and melts lower than this.

They do not publish the chemistry on the information page. This was made available during introduction and we are assuming this is still valid. The chemistry is more different from GB than Laguna Borate, it contains a lot more silica but balances this with more boron to melt it. The result may be a similar glaze but you should keep in mind that the greater boron content will have implications (i.e. lower hardness, less resistance to leaching, color response problems with certain oxides or stains, earlier melting). It could also mean that glaze will devitrify (crystallize) more. For some glazes there may be merit in mixing MB and LB since this will correct both the CaO and B2O3 (although SiO2 will still be high).

Surprisingly, this material produces a lot of gases on decomposition, a bar of it will expand to a bubbly mass many times original size if heated anywhere from 1400-1700F; this is unlike a frit and very unusual. We have had reports that this expansion behavior causes some glazes to shed off the ware. This material was analyzed by Mary Simmons who came as follows: SiO2 27.5, Al2O3 1.6, MgO 1.8, CaO 23.1, Na2O 4.8, K2O 0.1, TiO2 0.05, B2O3 28.6, LOI 12.4. This is quite different from the analysis Kickwheel first cited and the 5% LOI they claim. This material is a blend of a base frit (prepared by Fusion Ceramics) and likely a significant amount of calcite (the material fizzes when subjected to acid). This also explains why the material bubbles the way it does and why it has an LOI of 12%. 

Many testers have reported on MB and comments from some have indicated good success. Comments relating to its lack of plasticity (and therefore ability to suspend) were common, some users reported rock-like settling of the slurry. Kickwheel recommends the addition of 6% bentonite to the MB portion of the recipe. Others reported the MB is a stronger flux and that glazes tended to run more.


Cadycal was a calcium borate mineral produced by Fort Cady Minerals Corp of Newberry Springs, CA ( Their plant was in the same area as US Borax and other boron producers. The ore body from which Cadycal is made is 1400 feet below the surface. The mining was done by drilling wells into the ore body and pumping down a solution of diluted sulfuric acid. The resulting boric acid was pumped to the surface and reacted with lime to produce a pure calcium borate that was dried and packaged. The approximate chemical analysis was given on the chart below. Like Gerstley Borate, Cadycal does have some particulate lime material (depends on the quality of the lime used in the process).

Cadycal contained much more boron than Gerstley Borate and has a different mix of auxiliary oxides so it was not a chemical equivalent. It was not plastic and did not impart the creamy consistency, dry hardness, or suspension properties. It was a great candidate to make up part of the recipe for a substitution for GB because its chemistry was so similar to Colemanite, a fundamental component of GB. Cadycal was an exceptionally good melter at low temperatures, it was already very fluid by cone 06. Amazingly, although Cadycal has a higher boron content than Ulexite, the latter melts one or two cones earlier.

Ferro Frit CC298-C

This was announced around the middle of June, 2000 and they claim it gives similar effects. They recommend substituting as follows: "For every 2 parts Gerstley Borate, substitute 3 parts CC298-2 and remove 1 part feldspathic material (feldpar/nepheline syenite). In other words, if there are currently 20 parts Gerstley Borate in the glaze formula, add 30 parts CC298-2 and remove 10 parts feldspar from the formula. Slight adjustments to the formula may be necessary due to variation in body, firing cycle, or specific glaze formulation." They claim the frit has given excellent results from cone 07 to 4 as a replacement and shows promise as an all fritted glaze for use with oxides at cone 07.

Chemistry information from Ferro is approximate only, however we can deduce that it has a chemistry equivalent to 3 parts Gerstley Borate and 1 part feldspar. This material has no plasticity, so glazes that depended on GB for suspension may be difficult to use (they recommend an addition of 1-2% Veegum or bentonite if needed for suspension however we suspect more might be needed).

Other Frits

There are lots of boron frits, but very few have anywhere near the borate content of GB. However frits are consistent and there are some calcium borate frits that could definitely be used in the mix by people with ceramic calculation and recipe adjustment skills. But there is no straight substitute. Remember that a frit with the same chemistry as GB would be soluble and possibly rehydrate, thus all will have a much higher silica and/or alumina content. Also remember that many of the effects that Gerstley Borate produced were a direct result of its mineralogy (or mix of mineralogies). Of all the tester feedback received, frits got the most 'bad press'.

Gillespie Borate

Hammill and Gillespie announced its availability, beginning March 1, 2001. Although they went through a period of problems with their initial formulation it now appears that they well understand the issues involved. They promote the fact that they have achieved a chemistry, mineralogical and physical properties match. Early promotion suggested that it was 'a mixture of ulexite (Na2O.2CaO.5B2O3.16H2O) blended with various clay minerals and alkaline earth carbonates and silicates'. At first they did not release the chemistry however it is now published openly on the web site. They have a link to a Powerpoint presentation on iron reds to demonstrate the equivalency. (  

Product From South America

Ulex Empresa Minera in Buenos Aires, Argentina have a product that is a blend of Ulexite, Colemanite, (and maybe clay?). The American Importer is Miller & Co. in Pittsburgh, Pa. at (412) 787 9680 and Ceramic Supply of NY/NJ. Hank Saxe tells us that they have arranged sample to be sent directly from Argentina, which he now has. Like Gillespie, these people have the right idea about what Gerstley Borate really was mineralogically.

Dal Tile Frit 439 From Mexico

This is a high lime boro-silicate with a melting temperature of 1328F. We heard one good report that it worked well in a raku glaze. MileHi Ceramics reported that it is more fluid that GB. They had pretty good success with this in Floating Blue, better success a Cream glaze, but it did not work in Randys Red (produced a green color instead, maybe due to iron silicate crystallization). Rumors say that will sub well for GB however this is doubtful because the GB chemistry is soluble in frit form and GB has an LOI of 25% so a 1 for 1 by weight substitution cannot work. Maybe the substitution should be 3:4 instead of 1:1? One customer contacted us in Aug 2002 saying "It is indeed a wonderful really low fire frit. I got excellent results in raku using it will 20% ball clay. Good smooth melt, good crackle, stayed suspended in the bucket..... BUT IT'S SOLUBLE! After a couple weeks in the bucket, just crystallized out. Useless! I contacted Dal Tile via email and they confirmed it is soluble.

IMCO Borate

IMCO Borate is a blended product that is a direct substitute for Gerstley Borate. They say it "has the same physical properties as Gerstley Borate. It mixes with water to form a creamy slurry that gels and keeps the glaze from settling. It melts at cone 08 and becomes fluid at cone 06 just like Gerstley Borate. Use it as a one to one substitute in your glaze formulas for glazes that behave just like you're accustomed to". IMCO Borate is available at or (916) 383-2811.

They only provide one paragraph of information on their website and the chemistry is not shown. We were able to get it in Feb 2001 (shown below). We have tested this material and verified the LOI. We used it in a transparent raku glaze (80% GB, 20% Nepheline Syenite). It fired very similar to the GB version, a crazed crystal clear.

Product from U.S. Pigment Corp.

This was announced during January 2001. A phone call to them at 630-893-9217 confirmed that is about $1.00 per pound and "works in some glazes and not in others". They did not have technical information of any kind available. 

Ulexite, Colemanite

Like Cadycal and frits, these materials are not physical or chemical substitutes for GB alone. But you can bet that a number of substitutes on the market contain one or both. The Colemanites we have tested have a strange melting pattern, a more refractory 'skeleton' of material is left after the entrained boron glass has melted and drained out.

Ulexite, on the other hand, exhibits a much more GB-like melting behavior and this is not surprising since GB contained a lot of ulexite. More than any other material, this one deserves our attention. Ulexite mineral is a hydrous sodium calcium borate that develops when lakes of highly concentrated boron solute evaporate to form crystals of various boron minerals. Deposits have accumulated to great thicknesses in Chile, California, Turkey, and Kazakhstan. Deposits of ulexite can contain tufts of acicular crystals (referred to as 'cotton balls'), massive chunks, or veins. It is a fibrous and very soft mineral (hardness of 2 which is softer than fingernail). Gem collectors call it 'Television Stone' because polished surfaces perpendicular to the grain terminate fiber ends that transmit light like fiber optic cables. Granular ulexite is used as a slightly soluble source of boron in the fertilizer industry.

Turkish Ulexite melts remarkably well, at cone 06 it flows like water and is a remarkably clean and crystal clear glass. It melts one to two cones earlier than Colemanite and Cadycal even though the latter are higher in boron. This is probably due to  its sodium content. 

North American borate companies import it from Chile (National Boraxx) and Turkey (American Borates). American Borates now also mines colemanite in California. As of mid-2000 the Chilean material had an uncertain chemistry and is not readily available yet. Turkish ulexite is available in quantity in granular form and works well for ceramic applications. The huge US Borax deposits in Boron, California do not contain a quality ulexite.


Oxide Ave
July 2002
          Practical Sources of Boron
# 1 # 2 Laguna
Frit CC298-2
IMCO ***Calif
B2O3 30.0 26.4 25% 27.5% 38.0% 24.5 20-25 24.4 47.5 41.4 33.2 42.5
Na2O 3.6 3.2 4.0 4.0 6.0 3.8 0-5 3.0 0.1 0.1 5.8 0.1
CaO 21.8 25.4 24.0 18.9 26.2 23.0 10-15 19.2 26.0 26.5 19.7 29.5
MgO 2.3 3.6 4.0 2.3   3.9 0-5 1.8 0.24 1.8 2.0 0.3
K2O 0.6 0.5 0.5 1.4     5-10 1.4 0.01      
Fe2O3 0.1 0.1 0.5 0.1       0.3 0.05 0.1 0.1 0.1
++SrO           .45   2.0 0.03 0.7   1.0
Al2O3 2.8 2.4 2.0 8.1 0.5 1.7 10-15 6.9 0.1 0.1 0.2 0.2
SiO2 13.5 11.9 14.0 18.7 29.2 11.8 35-40 21.1 0.5 5.0 5.2 0.6
LOI 25.4 26.4 26.0 19.0 *0, 5, 12 30.9   19.8 26.5 24.5 33.9 25.5
*Kickwheel has not supplied an LOI figure, we measured this at 5.0 and Mary Simons measured it at 12.0. So that means the rest of their numbers need to be retotaled, we have not done this.
**Ferro does not provide exact numbers, they consider this proprietary information.
***This is was supplied by Fort Cady minerals and is based on thousands of assays done. It differs from analyses we did on three shipments received (these varied considerably but averaged 40.2 boron and  32.3 calcia. That means our analysis work is in question on the other materials. This is under study as of May 2003.)
+This is based on the average analysis of three shipments and this material was very consistent across the three
#Boraq 2 is Boraq 1 with 8% dolomite and 8% whiting added (Boraq 1 deliberately compromises a chemistry match with GB (it has higher boron and lower CaO, MgO) to melt as well as GB at lower temperatures Boraq 2 matches the chemistry and performs like GB at cone 6+).
++There is a small amount of SrO in Boraq but this has been ignored.

For another interesting article on this subject by Jeff Zamek click here.

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