Digitalfire Ceramic Glossary

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Si: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 (but not always) an indicator of glaze matteness. For example, if there is 5.0 SiO2 and 0.5 Al2O3, then the ratio is 5.0:0.5 or 10:1, or just 10. This ratio is significant in stoneware glazes, for example, because high silica tends to produce glossy glazes when alumina is low and high alumina creates matte glazes when silica is low. It thus follows that the higher the Si:Al ratio the glossier a glaze will be. Of course there are other matteness mechanisms (like high CaO or MgO, underfiring a boron frit or creating a fluid melt that encourages micro crystal growth) that are not so directly related to this ratio, but it is still taken into account since the silica and alumina balance (together they make up the bulk of the glass) cannot be divorced from any phenomena existing within it.

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  • (Glossary) SiB:Al Ratio

    Conceptually we consider fired glazes as being com...

By Tony Hansen

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