Digitalfire Ceramic Glossary

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SiB:Al Ratio

Conceptually we consider fired glazes as being composed of 'oxides'. The ten major ones likely make up 98% of all base glazes. By evaluating their absolute values and balance we can predict many fired properties and move individual properties in a specific direction. Degree of matteness is a good example.

INSIGHT software reports the SiB:Al ratio as part of its chemistry calculation of a batch recipe. This ratio refers to the number of SiO2:B2O3 molecules compared to the number of Al2O3 and is often an indicator of glaze matteness. Since B2O3 also acts as a glass former (in addition to SiO2) it is logical to group the two and compare that to the Al2O3 content in low and medium fire glazes (since they almost always contain B2O3). This ratio is independent of the Si:Al ratio, this is because glazes that contain significant boron can dissolve a more Al2O3 into solution and still stay glossy, so a matte glaze would not a lower ratio. While it is not typical to formulate high alumina mattes at low fire, the other mechanisms (like high CaO or MgO, underfiring a boron frit or creating a fluid melt that encourages micro crystal growth) all perform best with specific SiB:Al ratios.

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By Tony Hansen

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