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

Transparent Bases (For Use With Color)

Bright glossy glazes are very easy to make with Gerstley Borate because it melts so well. Na2O (sodium oxide) is a great melter for glazes (from nepheline syenite, soda felspar, even soda ash) but unfortunately it contributes toward high glaze expansion and therefore crazing. However low expansion boron is a secret to counteracting this and the two oxides work well together in glazes. GB is a great contributor of boron and a surprisingly wide range of proportions work well. In addition it is a significant contributor of Na2O and there are transparent GB glazes in this family that contain no soda-contributing material other than GB (these are possible where GB occurs in high proportions).

This glaze has almost 50% GB, it uses the Butterscotch base (and adds 2% soda ash instead of lithium, likely also for variegation effects). These two cone 5 porcelain tiles show it with a mix of 2% cobalt and 2% chrome. Results are virtually identical. However this base contains a lot of boron and it may have hardness and durability problems. 

2826H with Gerstley Borate 2826H with Boraq 2
Analysis Mole%
CaO 0.69 10.7 14.6
MgO 0.15 1.7 3.2
K2O 0.01 0.2 0.2
Na2O 0.15 2.6 3.2
Al2O3 0.27 7.6 5.7
B2O3 0.73 14.1 15.4
SiO2 2.71 45.1 57.4
Fe2O3 0.01 0.2 0.1
  LOI 17.6  

Analysis Mole%
CaO 0.71 10.8 14.4
MgO 0.16 1.7 3.2
K2O 0.01 0.3 0.2
Na2O 0.12 1.9 2.3
Al2O3 0.30 8.2 6.0
B2O3 0.80 15.0 16.2
SiO2 2.82 45.6 57.2
Fe2O3 0.01 0.2 0.1
  LOI 16.0  

Like the above, this glaze, G2826V, is also based on what appears to be the standard Gerstley Borate 50, Kaolin 20, Silica 30 base and adds 2% soda ash (does anyone know why soda ash is used?).  This variation adds 6 titanium and 2 cobalt oxide. As you can see, the Boraq 2 version (on the right) fires exactly the same.

This glaze demonstrates again how effective titanium (and rutile) are at variegating this base recipe. The butterscotch glaze has a very similar character, although a different color. You would find that an infinite variety of colors could be made using metallic oxides like copper, chrome, cobalt, iron, manganese, etc.

General Purpose Transparent Overglaze

This glaze uses a little more than half as much GB and therefore has a much lower boron content (and is not quite as glossy). It also uses whiting, kaolin and silica but has a higher Na2O content and sources it from nepheline syenite. This glaze was tested over brushed- on metal oxide decoration (using a variety of stains including green, black, blue, brown, red, yellow), all colors fired exactly the same using the Boraq version. The character of the transparent glass is also the same over dark or light clay bodies.

Patti's Clear with Gerstley Borate
Patti's Clear with Boraq 2
Analysis Mole%
CaO 0.62 9.2 11.8
MgO 0.07 0.8 1.3
K2O 0.06 1.5 1.2
Na2O 0.25 4.1 4.8
Al2O3 0.41 11.1 7.8
B2O3 0.33 6.1 6.3
SiO2 3.49 55.9 66.7
  LOI 11.1  
Analysis Mole%
CaO 0.63 9.3 11.8
MgO 0.07 0.8 1.3
K2O 0.06 1.6 1.2
Na2O 0.24 3.8 4.4
Al2O3 0.42 11.3 7.9
B2O3 0.35 6.5 6.6
SiO2 3.55 56.1 66.6
  LOI 10.4  

Base for Use With Chrome Tin Colors

This glaze uses only one quarter as much GB as the first one. Like the second it is made using nepheline syenite, whiting, kaolin and silica; it is a well known cone 6 recipe called Cranberry. This variation employs 7.5 tin oxide and 0.5 chrome oxide (however chrome tin stains are also used). Notice that this one has a lot less boron than the above yet it is still quite glossy and striking and even tends to flow a little if applied thickly. We have not shown the Boraq version because it is identical. Boraq 1 breaks to a lighter color when the glaze thins on edges, Boraq 2 creates a more stable melt. This recipe is interesting in that it creates a variegated surface that does not rely on high melt fluidity.

Cranberry with Gerstley Borate
Cranberry with Boraq 2
Analysis Mole%
CaO 0.76 12.8 16.4
MgO 0.03 0.4 0.7
K2O 0.04 1.2 0.9
Na2O 0.17 3.1 3.6
Al2O3 0.32 9.8 6.9
B2O3 0.15 3.0 3.1
SiO2 3.16 57.2 68.3
  LOI 12.4  

Analysis Mole%
CaO 0.76 12.8 16.3
MgO 0.03 0.4 0.7
K2O 0.04 1.3 1.0
Na2O 0.16 3.0 3.4
Al2O3 0.33 10.0 7.0
B2O3 0.16 3.2 3.3
SiO2 3.19 57.3 68.2
  LOI 12.0  
Tony Hansen

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