Replacing 10% Gerstley Borate in a clear glaze

Video coming soon.

Video Unavailable
Please follow transcript instead.

Glazes can be viewed as mixtures of ceramic minerals and manufactured ceramic powders. Each material contributes its chemistry to melt. The overall chemistry determines fired properties. Boron, or B₂O₃, is an oxide, it is sourced by 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. Reducing and increasing individual oxides has much more predictable effects than manipulating the amounts of the materials. Thus, rather than use a GB substitute, it is better to remove it from each glaze, add boron and calcium sources and juggle the remain materials to restore any disruption in other oxides.

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 the slurry, 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 increased 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
                  107.50

I have not re-totaled to 100. 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 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 a frit the resultant mix won't suspend. Much more Ferro Frit 3134 is needed to supply the same amount of boron as the GB. Nepheline Syenite was reduced (and therefore the alumina it contributed). After supplying lost MgO from talc I increased the 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 we can get them using this selection of materials. However we can make another couple of changes, using Wollastonite rather than whiting to source CaO and Nepheline Syenite to source of alkalis. 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.

Compare the glaze against limits and speculate on any shortcomings.
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 and use as little as 10-15%, or less plastic ones like EPK at 15-25%. If necessary you can augment with some bentonite or ball clay.
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, contribute boron from a material like low-alumina Ferro Frit 3134 so you can source Al₂O₃ from a clay.
Employ high-boron Cadycal if you can, but remember that it contributes fewer oxides than GB so a lot of juggling of other materials in the recipe will be required to source the other oxides. If the color of your glaze depends partly on the KNaO, Cadycal is less attractive since it does not contain these.
Have the glaze tested for leaching. Find information about this at the bottom of the web page here. If the glaze leaches excessively use other information on the above web page to deal with the situation.

By Tony Hansen
Follow me on

Got a Question?

Buy me a coffee and we can talk