Low Clear Gerstley Borate Glazes

These pages were a study of how we substituted Boraq 1, Boraq 2 and Boraq 3 into common recipes. Boraq was developed by Plainsman Clays as a substitute for Gerstley Borate (under code number L3127E). The development effort took place during the early 2000s, the initial period when the demise of Gerstley Borate appeared imminent. Other companies, including Laguna Clays, introduced similar products at the time. Later Laguna Clays began processing a last stockpile of the material they found at the mine and interest in substitutes waned. In 2023 the cycle appears set to repeat so these pages are pertinent again.

Since Gerstley Borate has a high LOI it tends to create bubbles and imperfections in low fire glazes. While this is not considered a problem in raku, generally people want a clear bright transparent base for electric fired earthenware. Thus it is generally better to use frit instead of GB in these glazes.

One case where Gerstley Borate is necessary is where frits do not contain enough boron. Gerstley Borate is much higher in boron than almost any common frit available and thus it will make more fluid and earlier melting glazes. We have found that Boraq 1 works well as a substitute for GB is these situations.

Frit GB Low Fire Base

For example, this glaze has a mix of GB and a boron frit along with nepheline syenite, kaolin, silica and a little lithium carbonate. The Boraq 1 version has a smoother surface with better color. The GB version has a bit more of a satin surface, if this is important try Boraq 2. In this sample we have added 8% iron to get an unexpectedly bright maroon color.

2826f1
GB Low Fire Base

2826f2
Boraq1 Low Fire Base

	Unity Formula	Analysis	Mole%
CaO	0.51	9.3	12.3
Li2O	0.15	1.5	3.8
MgO	0.06	0.9	1.6
K2O	0.04	1.2	0.9
Na2O	0.24	4.8	5.8
Al2O3	0.32	10.9	7.9
B2O3	0.47	10.8	11.5
SiO2	2.16	42.5	52.7
Fe2O3	0.14	7.4	3.4
	LOI	10.7

	Unity Formula	Analysis	Mole%
CaO	0.49	8.3	11.0
Li2O	0.17	1.5	3.8
MgO	0.04	0.5	1.0
K2O	0.04	1.3	1.0
Na2O	0.25	4.6	5.5
Al2O3	0.37	11.2	8.2
B2O3	0.58	12.1	12.9
SiO2	2.40	43.1	53.2
Fe2O3	0.15	7.4	3.4
	LOI	9.9

For more information see the page on Raku glazes.

Tony Hansen