Gerstley Borate

Date:	2000.12.18
Supplier:	Hammil & Gillespie
Family:	6-Flux Source
Raw Material Weight: Includes LOI	10200.00

Chemistry

Notes

There is alarm across the ceramic community in North America about the demise of Gerstley Borate. US Borax has ceased mining and remaining stocks in the sales channel may last a year.

Gerstley Borate has been mined by US Borax & Chemical Corp near Boron, CA and ground by Hammill & Gillespie and Luguna Clay Co. (in fact it is named after a former president of The Borax Company named Mr. Gerstley). The ore contains the minerals Colemanite (Ca2B6O11 : 5H2O) and Ulexite (NaCaB5)9 : 8H2O). It has been used in ceramic bodies, as a bonding agent in grinding wheels, and as a fire retardant. Most importantly, this versatile material is commonly employed in glazes fired at low, medium and high temperatures. For many years students have formulated glazes using a Gerstley Borate base because there is no other inexpensive and safe natural mineral that melts as readily and vigorously. For example, firing a sample of Gerstley Borate powder to cone 010 (about 1600F) completely transforms it into a molten mass. It is little wonder that so many glazes depend on it when one considers that the main alternative, feldspar, does not begin to flux vigorously for another 400 degrees! Many frits do not even fuse as well. There is one other common inexpensive ceramic mineral that fuses to produce visually superior glazes: lead oxide. Since the demise of lead it is easy to understand why Gerstley Borate has been so important to hobby and educational ceramics and small scale manufacturing.

There are other factors that suggest that Gerstley Borate may not be the dream material many thought it was. For a number of reasons this material is largely unused in industry.

-The sodium and calcium borates found in Gerstley Borate are very slowly soluble in water. The Na+ ions produced tend to gel a normal colloidal suspension. This necessitates additions of water that induce greater shrinkage that compromises bonding to the body, which in turn can cause glaze crawling. In some cases additions of epsom salts (i.e. 0.5%) can help by introducing Mg++ ions that inhibit the agglomeration of particles.

-It is a somewhat temperamental material, with variation in chemistry and physical properties reported by many. Shipments have had sizeable amounts of oversize particlate. H&G have claimed that they cannot guarantee quality from lot to lot as the material is strictly ' run-of-the-mine" (they have to take what they get from the US Borax mine) and variations can and do occur.

-Gerstley Borate can contain gypsum that can cause glaze "pop-outs" if left in the glaze mix.

-The fusibility of Gerstley Borate is actually a curse as well as a blessing. It is often used in excessive amounts or at higher temperatures than is necessary and little regard is paid to its unusual chemistry. The result has been a generation of flux saturated glazes that lack hardness and resistance to leaching in acids.

It is common to blend Gerstley Borate with clays in the proportions necessary to make the mix melt at the desired temperature. Since it is plastic like a clay, blend glazes can shrink excessively during drying on ware and thus tend to crack and then crawl during firing.

Finding a Replacement

Many companies have introduced replacement materials, some of these work well in certain glazes, none of them work in all glazes. Gerstley Borate was a very unique material and it is not easy to believe that another just like it will surface soon. We have set up a web site at http://www.gerstleyborate.com to explore all of the issues surrounding this material and its replacement.

The real secret to replacing Gerstley Borate in your glazes is to understand that GB contributes a chemistry AND physical properties to the glaze slurry. GB glazes have a creamy consistency that will not be duplicated if you simply substitute a 'dead' mineral or frit powder. Replacing GB needs to be done on a case-by-case basis using an appropriate strategy for each.

There is no denying that the principal reason for employing GB was its chemistry. It is a source of boric oxide or boron. Few insoluble natural materials contain significant boron (soluble materials don't work well in glazes because they soak into the clay body and cause it to melt or warp during firing). These soluble forms of boron are typically used to make boron frits. GB has a formula that we would like to think approximates this:

2CaO . 3B2O3 . 5H2O

However, it isn't so simple. GB is actually a lot more complicated that this.

Some Technical Info About Gerstley Borate

Quoted chemistries for GB are very approximate. Tom Buck has researched some history on this material to determine the nature of the "variations that can and have occurred" over the past decade. He said in part: "The colemanite content (calcium borate pentahydrate) can be guesstimated at 30-34%, ulexite (sodium calcium borate) content 26-30%, shale (low-iron clay/feldspar/quartz) 20%, limestone 20%. These proportions would yield a chemical analysis close to H&G's, ignoring of course, the unknown quantities of other tramp minerals in the ore. One could therefore consider that today's Gerstley borate is only two-thirds the equivalent of the once common and familiar colemanite, that is, to get as much boric oxide in a glaze recipe calling for "x" units of colemanite one needs to use 1.5x (and adjust the clay and whiting content downward in some recipes)".

In 1996, Laguna cited this composition (ex-US Borax):
B2O3 28% (+/- 3%)
Na2O 5.3%
CaO 20.6%
Water of crystallization 25.0%
Moisture (average) 0.3%
Insoluble minerals (limestone, dolomite, clay, others) 18.7%

1992 Analysis/Formula From Laguna Clay
SiO2.............. 9.8%
Al2O3............. 1.3%
Na2O.............. 4.7%
K2O............... 0.14%
CaO............... 17.9%
MgO............... 3.5%
Fe2O3............. 0.35%
B2O3.............. 28.3%
*H20 & CO2 IGN LOSS 29.1%
*ASSUMED INERTS.... 4.9%

Notice that GB is about 30% boron and 30% volatiles (if you fire a powdered sample to cone 010 you will find that it weighs 30% less).

Back to Replacing Gerstley Borate in Glazes

Glazes can be viewed as mixtures of ceramic minerals and manufactured ceramic powders. However each of these materials melts in the kiln and contributes its chemistry to the overall chemistry of the glaze. This overall chemistry determines the glazes fired properties. Boron, or B2O3, is an oxide, a part of the chemistry of Gerstley Borate. It is a low melting glass, when it is blended with other oxides like SiO2, Al2O3, CaO, etc. you get a ceramic glaze that melts well at lower temperatures. The real advantage of looking at a glaze as a collection of oxides is that reducing and increasing individual oxides has much more predictable effects that manipulating the amounts of materials in the recipe.

As you might appreciate it is a little tricky to propose a chemical equivalent for GB given its variable nature. Still, in practice it can be done with excellent results. Since GB contributes mainly CaO and B2O3 to your glaze, theoretically you need to introduce another boron sourcing material and adjust other ingredients so that the amount of CaO and B2O3 are maintained in the glaze (and to a lesser extent the minor oxides in GB). Each case presents different challenges in this respect, since substitutes, especially frits, contribute other oxides also. Ceramic calculation software is an ideal way to adjust your glazes when substituting for GB. You can juggle the amounts of materials in the recipe and watch the formula as you do so. Actually it is difficult to see how it could be done any other way as we shall see.

Cadycal at first appears quite attractive as a chemical substitute (it lacks the glaze suspending properties of GB) since it contains only B2O3 and CaO. However, while the CaO content is similar, it contains twice as much B2O3. Thus introducing it to supply the B2O3 will undersupply CaO. CaO will need to be augmented from another material like calcium carbonate. However there are some tradeoffs since you will find that you cannot easily supply all the oxides in GB without oversupplying others. Here is an example of a calculated substitute where I have tolerated a small oversupply of Al2O3 and SiO2 to match the fluxes as close as possible. Notice that the Cadycal recipe does not total 100, don't worry about this.

GERSTLEY BORATE..... 100.00%
Cadycal............. 50.95%
DOLOMITE............ 13.33%
NEPHELINE SYENITE... 33.33%
WHITING............. 2.38%
======== ========
100.00 105.00

CaO 0.68* 0.69*
MgO 0.16* 0.16*
K2O 0.04*
Na2O 0.16* 0.12*
Al2O3 0.02 0.16
B2O3 0.75 0.74
SiO2 0.29 0.71

If your glaze is already melting a little too much then the above substitute would be good since it will introduce a little extra SiO2 and Al2O3 to stabilize the melt. Here is another calculation where I have matched the SiO2 and Al2O3 closely and tolerated an oversupply in CaO and MgO and an undersupply of MgO.

GERSTLEY BORATE..... 100.00%
Cadycal............. 62.79%
WHITING............. 27.30%
SILICA.............. 9.91%
======== ========
100.00 100.00

CaO 0.68* 0.74*
MgO 0.16* 0.26*
Na2O 0.16*
Al2O3 0.02
B2O3 0.75 0.75
SiO2 0.29 0.29

Substituting a calcium borate frit for GB will present similar challenges. However there are lots of other frits that contain boron and many of the other oxides in GB. In general though, there is not as much boron in frits as in GB.

This introduces a third aspect of replacing GB. If you know anything about limit or target formulas for glazes you'll quickly realize when looking at the chemistry of GB that it is severely lacking in alumina and silica and therefore cannot form an insoluble glass by itself. Therefore glazes containing significant GB should also contain more than the normal amount of clay when compared to a feldspar-based glaze to supply the alumina. They should contain more than the normal amount of silica (quartz) than a typical feldspar-based glaze to supply the SiO2 that GB lacks. But the truth is many GB glazes don't have extra clay or silica. While these flux saturated glazes might dance in the light, they can be quite soluble and lacking in strength and hardness.

It thus seems appropriate, not only to substitute the GB in your glazes, but to rationalize their chemistry as a whole to make sure they are durable and functionally safe to eat and drink from. Thus the chemical balance of the glaze can be improved while GB is being substituted out. Here then is an effective approach to finding a substitute. This approach needs to be executed on a glaze-by-glaze basis.

Key your glaze into a glaze software program like INSIGHT and evaluate its formula against limits and speculate on any shortcomings. Rationalize the solution to these against the effects that solution might have on the visual character.

Determine if your glaze has enough clay in it to suspend it properly after the loss of Gerstley Borate. If it does not you must find a way to introduce some without upsetting the chemistry too much. You can employ plastic kaolins like Tile #6, try to get at least 15% in the recipe. If not you can augment with some bentonite. Better yet, try to work some ball clay into the recipe (and bentonite if needed). The INSIGHT manual has examples that show you how to juggle materials while maintaining the chemistry of the whole.

Determine the best material to source boron from. For example, if your glaze already has lots of CaO, then source boron from a frit that has low CaO. If your glaze has little clay, source boron from a material like Cadycal or Ferro Frit 3134 that has almost no alumina so you can source alumina from a clay. If the color of your glaze depends partly on the KNaO, then you'll need to source boron from something other than Cadycal since it does not have these, or you'll need to use a combination of Cadycal and feldspar/Nepheline Syenite.

Fire samples of your new recipe and made adjustments if necessary to maintain the visual character.

Have the glaze tested for leaching. Find information about this at the bottom of this page: http://ceramic-materials.com/cermat/education/12.html. If the glaze fails the test try to rationalize the formula according to the information found on this page, and on those it links to, and adjust and test again.