Before I go on brace yourself for a switch to the ‘oxide viewpoint’. When I look at a fired glaze I see it as being composed of an oxide structure. The materials in the original recipe ‘source’ these oxides. There is a direct relationship between the oxide makeup of a glaze and the way it fires. B2O3, or boron, is one oxide of about ten we need to learn about.
There is no denying that people knowingly or unknowingly employed GB for its chemistry, it is a source of boric oxide (or boron). Few relatively insoluble natural materials contain significant boron (soluble materials don’t work well in glazes because they affect glaze rheology (fluid properties) and either soak into the clay body or are left on the surface during evaporation.
As you might appreciate it is a little tricky to propose a chemical equivalent for GB given its variable nature. However I have attempted to do exactly that, see the chart above. Since GB contributes mainly CaO and B2O3 to your glaze, theoretically you need to start with another boron sourcing material and adjust other ingredients so that the amount of CaO and B2O3 are duplicated (and to a lesser extent the minor oxides in GB).
Used alone Cadycal contains much more boron than Gerstley Borate. Introducing it to source the same amount of B2O3 will undersupply CaO. CaO will thus 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.
Following is an example of a calculated substitute for the chemistry of GB. I made this using glaze calculation software to juggle the recipe ingredients to ‘zero in’ on the formula I wanted. I have tolerated a small oversupply of Al2O3 and SiO2 to match the fluxes as closely 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. Following is another calculation where I have matched the SiO2 and Al2O3 closely and tolerated an oversupply in CaO and MgO and an undersupply of Na2O.
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 calcia and the other minor oxides in GB. In general though, there is not as much boron in frits as in GB.