All Glossary

Formula Ratios

The ratios of individual or group oxide molecule numbers are indicators of things like fired gloss, durability, melting temperature, balance, tendency to craze, etc.

Key phrases linking here: formula ratios, si:al - Learn more

Details

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 balancing that with how glazes fire we can tune individual properties (e.g. melting temperature, thermal expansion, degree of matteness or gloss).

Digitalfire Insight-live shows several ratios as part of its chemistry calculation of a batch recipe.

Si:Al Ratio: The ratio of silicon dioxide to alumina oxide (SiO₂:Al₂O₃ ratio) is often used as an indicator of glaze matteness. A glaze with high alumina thus has a low silica:alumina ratio. This ratio has some value because alumina stiffens the glaze melt (stiffer melts do not smooth out as well on cooling thus creating a fired surface that scatters light). There are obvious limitations to using this indicator since there are other mechanisms and influencers of glaze matteness (e.g. crystallization, phase change). Generally, the mechanism of matteness is never purely the alumina:silica ratio. The presence of boron can gloss even a high alumina mix. Of course firing temperature affects gloss (a glossy glaze will be matte if under fired). Firing range is another variable: at low temperatures high temperature fluxing oxides (e.g. CaO, MgO) turn into matting agents (via melt stiffening). In addition, the presence of zinc can cause crystallization and matteness in an otherwise glossy glaze with high ratio. CaO in large amounts is a matting agent in high and medium temperature glazes also (via crystallization). Opacifers will affect matteness.

SiB:Al Ratio: 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, other matteness mechanisms perform best with higher SiB:Al ratios.

R2O:RO Ratio: Alkalis:Alkaline earths or (Li₂O+Na₂O+K₂O : MgO+CaO+SrO+BaO). See the link below for more information on the R2O:RO Ratio.

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 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 lower the Si:Al ratio is, the more matte a glaze will be (in the absence of other matting mechanisms). Since Al₂O₃ adds toughness and durability to glazes, it is often advisable to have more alumina. Glazes can still be glossy even at an 8:1 or lower (especially if boron is present).

Related Information

Insight-Live comparing two recipes:

A glossy and matte cone 6 bases

This picture has its own page with more detail, click here to see it.

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.

One way for an ultra clear at low fire:

Magnesia-alkali, low Si:Al ratio, more boron.

This picture has its own page with more detail, click here to see it.

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). 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, it has low expansion, will fast fire and the boron is not high enough to compromise the hardness.

If your glaze can handle more silica and melt just as well then add it!

This picture has its own page with more detail, click here to see it.

The cone 6 G1214M glaze on the left melts well. Can it benefit from a silica addition? Yes. The right adds 20% yet still melts as well, covers better, is more glossy, more resistant to leaching, harder and has a lower thermal expansion.

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