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

GB Available Again?

May 7, 2001: Laguna Clay negotiated to have a remaining stockpile processed at the quarry site. They have stated that chemistry and melt properties are the same. This material has been in the sales channel for some time now and is expected to last through 2003 (according to estimates from Laguna production department Dec 2002). While it lasts the interest surrounding substitutes has waned. It has also provided more time for suppliers to study the situation more.

Feb 2, 2004: Laguna Clay said this: "Gerstley Borate is no longer being mined, has not been for a number of years now. There are no plans that Laguna is privy to, of US Borax or anyone else, to reopen the mine. There is however a stockpile of crude GB in existence that was mined prior to US Borax shutting down their operation. Laguna has been having GB ground from that stock pile since we became aware of it's existence, a couple of years ago. The estimates for how much is really there have varied widely, but the most recent estimate I have goes like this, 'If current levels of usage are maintained, the stock pile will last about 10 more years'."

Jan 2009: Laguna Clay has apparently bought the site and claim a 20 year supply! But we still do not have a definitive chemistry from the source. How consistent it will be?

Oct 2010: Laguna did not provide any technical information but said this: "At the current rate of sales we should still have at least a five year or more supply left. The material was of course all taken from the same deposit for the past 50 years, but has always varied slightly from stock pile to stock pile. What we have been getting for the last 8-10 years is from the last pile, and we have not seen any real inconsistency."

July 2011: says "There have been rumors that this product would become unavailable very soon. Those reports are wholly inaccurate. The supply will last for many years, and because it is a stockpile (not a changing source from an active mine) the chemical analysis will remain quite consistent. This product has also been tested in the traditional Floating Blue glaze and the results were excellent." We have added their new analysis to the chart below.

2012: They have a page about the chemistry of Gerstley Borate here: However many are awaiting information about the consistency of the stockpike.

What Was Gerstley Borate?

For technical details please visit the Gerstley Borate page at the Digitalfire Reference Library, there are lots of pictures and you can see the exact melting behavior visually. There are links to videos that show you how to use Insight software to substitute this material for a frit.

Gerstley Borate (GB) was mined near Boron, CA and ground by Hammill & Gillespie and Luguna Clay Co. (in fact it was named after a former president of The Borax Company*). In addition to its use in glazes as a secondary flux at mid range and often as primary one at low fire and raku (this material was also employed in ceramic bodies, as a bonding agent in grinding wheels, and a little was even used as a fire retardant). GB was popular in middle temperature glazes because its high melt fluidity produced 'hare's fur', textured and variegated visual effects that were remnants of the interactions between the more and less viscous phases of the melt. In low fire glazes GB was an excellent base that made for slurries with good application and melt properties.

It is not a mineral, but an aggregate of minerals and therefore it has qualities that cannot be duplicated by frits. GB form in evaporite basins where water and sediments collected. The ore mainly contained the minerals Colemanite (Ca2B6O11 : 5H2O), Ulexite (NaCaB5)9 : 8H2O) and Hectorite. It fizzes with HCL so we know it also contains limestone (although borates dissolve in HCL they don't form a gas the way carbonates do).

This shot of the valley shows the head frame for one of the vertical shafts (foreground to the left). The adit opening is not visible but is in the distance near the building immediately to the left of the yellow structure.

Mixing and bagging equipment

It was discovered in the 1920's as an outcropping of colemanite in a stream located on a sloping alluvial fan of basically surface gravel. A shaft was excavated some 100 ft below the surface where the ore was found. To extract ore an adit was excavated from the bottom of the shaft horizontally out to the surface. The deposit was mined via the adit (not the shaft). This was an underground mine and the ore was found to be in four sloping beds. Most of bed 4 (the upper most bed) and parts of beds 3 and 2 have been extracted and there is still some 100 k-tons of ore left in the deposit today. The age of the deposit is dated as ca 12m years whereas most of the Death Valley mines are from 4-6m. Because of its geologic setting, some have postulated the mineralization occurred from a lake in that was inside a volcano.

The mine was recently closed because the cost to meet all of today's regulatory requirements now far out strips the material value. For example, the support timber in the mine is very old and cannot be fire proofed and must be replaced.

On a material level Gerstley Borate is a light tan powder that mixes with water to produce a creamy slurry that gels and is very slow to settle out. Gerstley Borate is actually quite plastic (due to its hectorite content) and if its considerable appetite for water is satisfied it can be thrown on the potter’s wheel to potentially make vitrified stoneware at temperatures lower than typical bisque firings! A thick layer of pure material will run right off the ware at cone 06 producing a glossy completely transparent glaze. A solid specimen fired to cone 010 (about 1600F) transforms into a molten mass.

On an oxide level, Gerstley Borate is an inexpensive and generous contributor of boron and this is the fundamental reason for its use (boron is the magic behind almost all low fire glazes. At middle temperatures GB made it possible to make the variegated and visually appealing surfaces normally associated with cone 9-10. GB has a formula that we would like to think approximates this (unfortunately reality is quite different):

2CaO . 3B2O3 . 5H2O

GB also contributes a generous amount of calcia. This calcia content completely dissolves in the boron glass at temperatures far lower than it would be possible to get other sources of calcia to go into solution.

Engineers would typically shy away from a material such a Gerstley Borate because of its high LOI (weight loss on firing) because this will contribute to glaze imperfections (e.g. pinholes, blisters). This is true for fast firing, it can make some glazes actually froth so that the surface is completely covered with bubbles. However if firing proceeds more slowly it is surprising how crystal clear some of the Gerstley Borate transparent recipes can be, apparently gases are being expelled either before the melt or are well cleared by the melt. Reduction firing however does tend to bring out blistering problems even if firing is slow. 

The variability in the ore is quite high and it was extracted from the three upper beds sometimes individually and sometimes by mixing the beds. Within a given bed the ore is very inconsistent and attempts were usually made to blend ore on the surface. But, because this was never a very large operation, it was not possible to achieve or maintain uniformity of grade.

Because of its variable chemical and physical nature, quoted chemistries for GB were quite rarely done, even by US Borax. Even H&G, when commenting on the sizeable amounts of oversize particulate in a specific batch (including gypsum that caused glaze "pop-outs") said that they could not guarantee quality from lot to lot as the material is strictly "run-of-the-mine".

The Ore

Here are some analyses:

*3 samples tested 1999/2000 **Control
2001 7/2002
B2O3 26.8% 28% (+/- 3) 28.3 16.13 21.61 23.23 28.63 16.7 29.0 25.0
Na2O 4.0 5.3 4.7 4.09 4.26 3.91 5.15 3.99 4.5 4.0
CaO 19.4 20.6 17.9 26.22 24.29 24.7 24.64 24.96 22.0 24.0
MgO 3.5   3.5 5.59 4.43 4.74 3.55 4.50 3.5 4.0
K2O 0.4   0.14 0.53 0.57 0.55 0.31 0.61 0.25 0.5
Fe2O3 0.4   0.35 0.54 0.60 0.49 0.14 0.36 0.2 0.5
Al2O3 1.0   1.3 1.97 1.89 1.64 0.77 2.18 1.0 2.0
SiO2 14.8   9.8 17.88 15.88 14.67 9.05 17.71 10.0 14.0
H2O, CO2, LOI 29.5 29.1 26.80 26.3 25.9 27.64 29.0 29.0 26.0
Insolubles   18.7 4.9

 limestone, dolomite, clay, others

TiO2, P2O5 and MnO trace amounts have been ignored
*Mary Simmons, Dept of Earth & Planetary Sciences, University of New Mexico.
**Jan 29, 2001: We had Mary Simmons analyze our sample of Gerstley Borate in parallel with multiple tests on Boraq, Ulexite and Cadycal. To check it was done again and we spent some time comparing notes on the proper way to measure boron. The low amount of B2O3 in this analysis is a surprise.
+Analysis rounded to 1 decimal (some of their numbers are to three decimals??).

Rationalizing an Analysis for Gerstley Borate

It is critical to realize that any effort to duplicate the chemistry of GB is only as accurate as the analysis one uses for GB. As you can see from the chart above, boron ranges from 16-28%, SiO2 from 10-18%, CaO from 18-26%. Volatiles are quite consistent from 25-29%. Deciding which analysis to use requires some rationalization, obviously the bag of material we have is not representative and really emphasizes the variation in GB. On one hand, the melt fluidity behavior of typical GB indicates more boron than the lower amounts above suggest. At the same time our analysis indicates that we don't want to estimate boron too high at the expense of silica. After a lot of lab test work and thinking I have rationalized an analysis as shown in the last column. I feel that anything but rounded numbers is a misrepresentation of its consistency and level of understanding of an average analysis.


*Two men played key roles in developing the borate industry. F.M."Borax" Smith and William T. Coleman who developed the 20 mule team to move mineral to the coast.

Interesting Quotes

"I have such fear at this point I'd probably pay for the old GB its weight in gold if I could get it!"

"I am a big fan of GB glazes and have only 4 lbs. left. Help!"

"The studio where I belong has three clay bodies, low fire white clay, terracotta, and a cone 6 stonewhere body.  Gerstley Borate is used in many, if not all, of our considerable glaze palate."

Tony Hansen

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