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

What it Teaches Us

There is a great educational opportunity here. Why not learn and teach your students how to deal with this situation using the resources on this web site and others. Teach them that glaze recipes are made of materials that we need to understand. Glazes have a chemistry that is very important.

For many years North American students have formulated glazes using a Gerstley Borate base. It is literally a "natural plastic low fire frit", you can mix almost anything with GB and it will produce an active low melting glaze with good application properties and fit to most bodies. In medium fire cone 3-6 glazes its use has been very common because when otherwise benign raw glazes are mixed augmented with GB its aggressive melting and non-homogeneous mineral mix produce visually striking effects that surprise most 'high fire snobs' who maintain that only temperature can do these things. Industry has to admit that fritted glazes often look sterile beside GB ones. When we consider that the main alternative, feldspar, does not begin to flux for another 200 degrees it is easy to see why GB was so popular! In fact, very few frits fuse as well and the only other common material that melts better is lead oxide.

But the fusibility of Gerstley Borate was actually a curse as well as a blessing. Overuse of GB has been partly responsible for a generation of flux-saturated glazes that look interesting but are lacking in key areas of hardness, leach resistance and consistency. This "if-it-looks-good-use-it" attitude has often been paired with a curious high tolerance toward firing and application problems that this temperamental material has subjected us to.

Gerstley Borate has demonstrated something else about the ceramic community that is interesting. We thrive on the trade in undocumented glaze recipes and pay little regard to understanding them or the materials in them. All the trouble that GB gave us was not incentive enough to study its properties. How many people were aware that it melted so actively? How many realized that GB was so plastic, suspended so well, and had such an affinity for water?

Some technicians have recommended straight substitutions of frits and ceramic materials that are completely different physically and chemically. Some of the suggestions and substitutions that have cropped up make one wonder what sort of tests were done to determine equivalency. This harkens back to the same "easy-fix" thinking that brought us the trade in undocumented glaze recipes. If you have mixed hundreds of recipes that don't work you already know that this roulette wheel approach is not productive. Understanding and adjusting your own glazes is faster and easier. The same goes for materials, we need to understand them and do it right.

Some will still want to take a substitute and continue doing things the same old way. But one substitute will not work in all situations, customer support from each company is going to be required for success. But can all these companies give the needed support when there is such a small market? No. But if you understand how your recipes work you will be able to adjust each as appropriate. However the 'Nirvana' of glazing is not here.

We suggest you start by studying the common glaze recipes that Gerstley Borate was used in. There are many things to be learned. Some remarkable recipes are available on this site, we think you will agree that it is not time to throw away the idea of a plastic borate, it is time to embrace it using Boraq and take it to the next step. For example, a very common base recipe is 50 GB, 20 kaolin and 30 silica. With the high kaolin and silica content this looks much like a cone 10 stoneware recipe, but it isn't. It has a low thermal expansion (unlike most cone 6 bases) and it flows so well it can be used well below cone 6. The high alumina and silica content of this glaze potentially give it excellent hardness. And it works well using Boraq.

Some of the glazes are provided in on this site show the formula only. However in each case enough clues are given that you can deduce the recipe or calculate it using INSIGHT software. Visit the Boraq main page to start.

Nirvana 1

With Ceramic Calculations You Don't Need a Substitute!

You can do what we did. Glazes can be viewed as mixtures of ceramic minerals and manufactured ceramic powders. However, as already noted, 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. Boron is a low melting glass, thus it reacts well with other oxides like SiO2, Al2O3, CaO, etc. to produce ceramic glazes that melt at low 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. Thus, rather than create your own GB replacement, it is better to remove it from each glaze, add boron and calcium sources and juggle the recipe to restore its chemistry.

Ceramic calculation software enables you can juggle the amounts of materials in the recipe and watch the formula as you do so.

Consider an example, Ron Roy's Clear Glaze.

F-4 FELDSPAR      31.00  CaO    0.66*
SILICA            21.00  MgO    0.12*
FRIT 3134          5.00  K2O    0.05*
GERSTLEY BORATE   10.00  Na2O   0.17*
EPK KAOLIN        17.00  Al2O3  0.41
WHITING           13.00  B2O3   0.20
TALC               3.00  SiO2   3.05

This contains a smaller amount of GB and part of the boron is sourced from a frit. In addition, the 17% kaolin is adequate to suspend this glaze so removing the GB will not be as big an issue for the in-the-bucket properties. Using INSIGHT software I first removed the GB and increased the frit to bring the B2O3 back into line. Finally I had to increase the talc slightly to make up for a loss in MgO and the kaolin to restore the Al2O3, and reduce the silica to bring the SiO2 down to match. Here is the adjusted recipe:

F-4 FELDSPAR   31.00  CaO    0.63*
SILICA         18.50  MgO    0.12*
FRIT 3134      19.50  K2O    0.05*
EPK KAOLIN     21.00  Na2O   0.20*
WHITING        13.00  Al2O3  0.41
TALC            4.50  B2O3   0.20
            ========  SiO2   3.06

I have not re-totaled it to 100 so you can see the juggling I have done in the recipe (in INSIGHT it is easy to nudge the amount of a recipe material up or down and watch the formula). I have not taken the trouble to match all of the oxides exactly. This is an inexact science anyway. This approach to removing GB from this particular glaze has worked very well.

Now consider a much more complicated GB removal challenge.

Nephy Syenite       47.30  CaO    0.41*
Gerstly borate      27.50  MgO    0.10*
Silica              22.30  K2O    0.09*
EPK                  5.40  Na2O   0.40*
                 ========  Al2O3  0.51
                   102.50  B2O3   0.44
                           SiO2   3.55

This recipe relies almost entirely on GB to suspend it. If we try to substitute Cadycal or frit the resultant mix won't suspend. There is no combination of the two dominant materials, Nepheline Syenite and CadyCal that gives anywhere close to the chemistry needed. However Ferro Frit 3134 is a different story. Much more material by weight is needed to supply the same amount of boron. The side effect of this is that the Nepheline Syenite proportion is reduced (and therefore the alumina it contributes). After supplying lost MgO from talc I am able to add plenty of kaolin to both source Al2O3 and suspend the slurry. It all works out surprisingly well.

Nephy Syenite.......   47.30  46.15%     25.43  25.43%
Gerstly borate......   27.50  26.83%
Silica..............   22.30  21.76%     14.68  14.68%
EPK.................    5.40   5.27%     20.65  20.65%
FRIT 3134...........                     36.02  36.02%
TALC................                      3.23   3.23%
                    ========          ========
                      102.50            100.00

                   CaO  0.41*  6.47%      0.48*  7.52%
                   MgO  0.10*  1.08%      0.10*  1.12%
                   K2O  0.09*  2.43%      0.05*  1.26%
                  Na2O  0.40*  6.98%      0.38*  6.59%
                 Al2O3  0.51  14.54%      0.51  14.68%
                  B2O3  0.44   8.55%      0.44   8.54%
                  SiO2  3.55  59.84%      3.56  60.27%

Notice that the CaO and KNaO do not match exactly. This is as close as I can get them using this selection of materials. Try using Ulexite, Cadycal, and Nepheline to get an even better match. After seeing these two demonstrations I am sure you will agree that substituting GB out of your glazes using this technique on a case-by-case basis has the most potential.

Nirvana 2: Improve the Glaze While You Are At It

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 plenty of alumina-sourcing materials (i.e. clay and feldspar). There is a problem here: If GB and feldspar predominate in a glaze you'll end up with a flux-saturate that lacks hardness and resistance to leaching. If you employ kaolin or ball clay to source the alumina you may have a glaze with excessive plastic ingredients, one that shrinks and cracks off during drying and gels excessively.

The answer is simple: Low fire glazes can have high proportions of GB and use feldspar to source alumina. Higher temperature glazes (i.e. cone 6) should employ only minor amounts of GB since feldspars are effective melters at middle temperatures. Since GB is lacking in silica also, glazes employing significant amounts of it should also contain other materials to source SiO2. But the truth is that many GB recipes do not contribute adequate Al2O3 and SiO2. While these flux saturated glazes might dance in the light, remember they can be quite soluble and lacking in strength and hardness. It thus seems appropriate, not only to substitute the GB in your functional glazes, but to rationalize their chemistry as a whole to make sure they are durable and functionally safe to eat and drink from. It is logical to improve the "balance" of the glaze (the overall fluxes:silica:alumina proportions). In addition, the plastic content of the raw mix can be optimized during the GB substitution process.

Consider the Ron Roy clear glaze I adjusted previous. While it is not an example of the above, I can demonstrate a couple of changes I might make. I prefer to use Wollastonite rather than whiting to source CaO. Further, Nepheline Syenite is a more convenient source of alkalis for me. Here is the original recipe and the new adjustment. Notice that the kaolin is higher so this new recipe will be an even better duplicate of the suspension and hardening properties of the original kaolin/GB combination.

F-4 FELDSPAR........   31.00  31.00%
SILICA..............   21.00  21.00%     17.09  17.09%
FRIT 3134...........    5.00   5.00%     19.23  19.23%
EPK KAOLIN..........   17.00  17.00%     23.93  23.93%
WHITING.............   13.00  13.00%
TALC................    3.00   3.00%      4.27   4.27%
GERSTLEY BORATE.....   10.00  10.00%
WOLLASTONITE........                     16.67  16.67%
NEPHELINE SYENITE...                     18.80  18.80%
                    ========          ========
                      100.00            100.00

                    CaO  0.66* 12.37%      0.66* 12.44%
                    MgO  0.12*  1.57%      0.11*  1.51%
                    K2O  0.05*  1.73%      0.03*  1.02%
                   Na2O  0.17*  3.61%      0.20*  4.13%
                   TiO2  0.00   0.07%      0.00   0.09%
                  Al2O3  0.41  14.17%      0.41  13.94%
                   B2O3  0.20   4.58%      0.20   4.60%
                   P2O5  0.00   0.05%      0.00   0.06%
                   SiO2  3.05  61.66%      3.06  62.00%

To sum up, here are some things to keep in mind when rationalizing your glaze during the GB substitution process.


Perhaps you wanted a simple answer. No such animal exists for this one. There is so much to be learned and so many changes to be made so let us get started!

Tony Hansen

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