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


Matte Base Glazes

Many think that the dolomite matte effect only works in reduction firing at cone 10. However these glazes prove that is not true. Gerstley Borate was used to create very nice magnesia matte glazes (using dolomite and talc). This might seem strange considering that GB is so fluid and naturally tends to form runny glossy glazes. However satin matte glazes can be produced by adding moderate to large amounts of magnesia. It acts as a filler and stabilizes the runny melt (since it is not yet an active flux at cone 6). You can also make mattes using high calcia glazes, they tend to form a smoother crystalline matte surface. Magnesia mattes, on the other hand, tend to form a silky 'hare's fur' effect (microscopically wavy surface).

The more magnesia in GB glazes the less stable a glass they will form (i.e. against leaching, cutlery marking). It is thus important to understand the mechanism of the matte and be able to adjust the chemistry as needed. 

An interesting side effect of this mechanism is that calculated thermal expansion values are misleading. We suspect the reason is that the normally low expansion influences of MgO depend on them being active in the glass chemistry. These glazes can thus tend to craze. This problem can be solved by employing more Al2O3 and more B2O3 to dissolve it.

Good Surface But a Color Matching Problem

This glaze takes the magnesia matte concept close to the limit (it uses both dolomite and talc to supply MgO). The amount of MgO in this glaze is double what would be considered normal and there is no question that this glaze fires to a very matte surface.

These cone 5 porcelain tiles show an example of a glaze with a pleasant yet very matte surface (2826G). It has a fairly low B2O3 content and contains less than 25% Gerstley Borate (you'll have to calculate the recipe, we can't release it).

Using the Boraq 1 produces a similar color, but the extra B2O3 tends to cause blistering. Boraq 2 duplicates the satin surface very well, but as you can see the color is way off! This is likely because MgO and cobalt (the colorant used is this sample) react to produce purples! The magnesia tied up in the chemistry of hectorite does not appear to react in the Boraq 1 whereas the magnesia from the added dolomite in Boraq 2 does.

But notice the Boraq 3 specimen. It only contains added whiting. Why does a little extra calcia turn the color like this? If MgO and Cobalt give purple then why isn't the original glaze purple since it contains a very large amount of MgO from talc?

Since this glaze contains so much MgO its resistance to leaching and cultlery marking are in question. In addition it crazes badly on our body. Thus we would not use this glaze before more testing and adjustment.

 2826GFormula and Mole% with GB Boraq1Boraq2Boraq3
CaO 0.38 * 10.18 9.08 10.16 10.62
Li2O 0.03 * 0.88 0.88 0.88 0.88
MgO 0.47 * 12.46 11.95 12.39 11.84
K2O 0.02 * 0.53 0.58 0.57 0.57
Na2O 0.10 * 2.56 2.31 2.21 2.21
TiO2 0.00 0.11 0.11 0.12 0.12
Al2O3 0.18 4.76 4.96 4.90 4.91
B2O3 0.23 6.04 7.17 6.40 6.38
SiO2 2.34 62.42 62.87 62.31 62.39

We have removed a couple of oxides showing trace amounts to reduce clutter. Also note that
the last digit of precision is not significant. 
Click here
to learn about Mole% calculations.


With GB


With  Boraq 1


With Boraq 2


With Boraq 3

INSIGHT 5.3 Report to Compare Boraq 3 Version

2826G1
Lavendar Satin Cone 6 with GB
2826G2
Lavendar Satin Cone 6 with Boraq 3
  Unity
Formula
Analysis Mole%
CaO 0.38 8.3 10.2
Li2O 0.03 0.4 0.9
MgO 0.47 7.3 12.5
K2O 0.02 0.7 0.5
Na2O 0.10 2.3 2.6
Al2O3 0.18 7.1 4.8
B2O3 0.23 6.1 6.0
SiO2 2.34 54.6 62.4
  LOI 12.9  

  Unity
Formula
Analysis Mole%
CaO 0.41 8.6 10.6
Li2O 0.03 0.4 0.9
MgO 0.45 6.9 11.8
K2O 0.02 0.8 0.6
Na2O 0.08 2.0 2.2
Al2O3 0.19 7.3 4.9
B2O3 0.24 6.4 6.4
SiO2 2.39 54.4 62.4
  LOI 12.9  

Possible Solutions

We will test these soon. Do you have experience with this type of glaze?


Oatmeal Glaze

The magnesia in this glaze is much lower than the one above, but there is still enough to form a very silky and pleasant surface (yet it is hard and does not cutlery mark). 10% zircon opacifier and 5% rutile are also added to a standard base of nepheline syenite, GB, silica and kaolin. You can control the amount of color by either applying to a dark body (thin sections of the glaze are darker) or by adding a colorant to the glaze (i.e. a small amount of iron). This glaze fires identical to the GB version using either Boraq 2 or 3.

2826v
Oatmeal with Gerstley Borate
2826v1
Oatmeal with Boraq 2
  Unity
Formula
Analysis Mole%
CaO 0.42 9.0 11.8
MgO 0.29 4.5 8.1
K2O 0.05 1.9 1.5
Na2O 0.21 4.9 5.7
ZnO 0.02 0.7 0.6
Al2O3 0.31 11.9 8.5
B2O3 0.17 4.5 4.7
SiO2 2.13 48.6 59.0
  LOI 13.9  

  Unity
Formula
Analysis Mole%
CaO 0.43 9.1 11.7
MgO 0.30 4.5 8.1
K2O 0.06 2.0 1.5
Na2O 0.20 4.6 5.4
ZnO 0.02 0.7 0.6
Al2O3 0.31 12.1 8.6
B2O3 0.18 4.8 5.0
SiO2 2.15 48.8 59.0
  LOI 13.4  
 

This matte base is suitable for a wide variety of colors. We got identical results using Boraq 2 and GB. Since this glaze is quite stable is acts as a good base on which to apply overlayers of fluid darker colored glossy glazes, these will 'feather into' the matte surface producing a striking contrast of both color and surface.

We have seen variations on this that employ calcium borate frits. However none have had the same pleasant silky matte surface this one has. We suspect that the interplay between the contrasting melting behaviors of colemanite (cadycal), ulexite and feldspar particles in GB coupled with the neighboring influences of refractory magnesia particles creates a surface with more variation than possible using homogeneous frits.

Tony Hansen

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