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SiB:Al Ratio

For complete info see the Si:Al Ratio topic (link below).

Digitalfire Insight-live shows the SiB:Al ratio as part of its chemistry calculation of a batch recipe. This ratio refers to the number of SiO2 and B2O3 molecules compared to the number of Al2O3 (in the fired formula). The SiB:Al ratio is thus higher than the Si:Al ratio in a glaze. Since middle fire glazes usually have less than 5% molar of B2O3, the difference is not great.

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 these almost always contain B2O3). Glazes that contain significant boron can dissolve more Al2O3 into solution and still stay glossy, so a matte glaze containing boron would not have a higher SiB:Al ratio. While it is not typical to formulate high alumina mattes at low fire, the other mattness mechanisms all perform best having higher SiB:Al ratios.

Insight-Live comparing a glossy and matte cone 6 base glaze recipe

Insight-live is calculating the unity formula and mole% formula for the two glazes. Notice how different the formula and mole% are for each (the former compares relative numbers of molecules, the latter their weights). The predominant oxides are very different. The calculation is accurate because all materials in the recipe are linked (clickable to view to the right). Notice the Si:Al Ratio: The matte is much lower. Notice the calculated thermal expansion: The matte is much lower because of its high levels of MgO (low expansion) and low levels of KNaO (high expansion). Notice the LOI: The matte is much higher because it contains significant dolomite.

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By Tony Hansen

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