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Si:Al Ratio

Conceptually we consider fired glazes as having a structure of oxides held together by molecular bonds. Ten major oxides likely make up 98% of all base glazes. Each oxide contributes specific characteristics to the glass and they interact in predictable ways. By rationalizing their absolute values and balance with how glazes fire we can move individual properties in a specific direction (e.g. melting temperature, thermal expansion, degree of matteness or gloss).

Digitalfire Insight-live shows the Si:Al ratio as part of its chemistry calculation of a batch recipe. This ratio refers to the number of SiO2 molecules compared to the number of Al2O3 (in the fired formula). It is an indicator of glaze matteness (where the mattness mechanism is a low Si:Al ratio and the glaze is melted well enough that sources of SiO2 have all melted or dissolved in).

Consider an example: There is 5.0 SiO2 and 0.5 Al2O3 in a formula. The ratio is thus 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. As noted, there are other matteness mechanisms (like high CaO or MgO, under firing a boron frit or creating a fluid melt that encourages micro crystal growth) that are not so directly related to this ratio. Still, since 80-90% of most glazes is made up of SiO2 and Al2O3, the silica and alumina balance is a valuable piece of information about any glaze.

Since Al2O3 adds toughness and durability to glazes it is often advisable to have more alumina in the formula. Glazes do not necessarily need a 12:1 ratio to be glossy, many are very glossy even at an 8:1 ratio. And any boron present will help dissolve the Al2O3.

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.

A secret to an ultra clear at low fire. Magnesia-alkali, low Si:Al ratio, more boron.

On the left is G2931J, a zinc alkali fluxed and high Si:Al ratio glaze. Those look like micro-bubbles but they are much more likely to be micro-crystals. High-zinc and high-silica is the mechanism for crystalline glazes, so it appears that is what they are. G2931K on the right has much more boron, double the Al2O3, less SiO2 and is magnesia-alkali instead of zinc-alkali. It is the product of dozens of tests to find an ultra-clear having a glassy smooth surface. This particular chemistry, although having only a 6:1 SiO2:Al2O3 ratio is ultra-gloss. In addition is has low expansion, will fast fire and the boron is not high enough to compromise the hardness.

Out Bound Links

In Bound Links

  • (Glossary) SiB:Al Ratio

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

By Tony Hansen

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