•The secret to cool bodies and glazes is a lot of testing.
•The secret to know what to test is material and chemistry knowledge.
•The secret to learning from testing is documentation.
•The place to test, do the chemistry and document is an account at https://insight-live.com
•The place to get the knowledge is https://digitalfire.com
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.
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
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 a...
Calculated Thermal Expansion
Insight-live and desktop Insight calculate the thermal expansion of a glaze from its oxide chemistry. The number it reports is based on the contributing expansion factors and amounts of each oxide in the formula. These numbers are very small and recorded in scientific notation (e.g. 6.5 x 10-7 which...
By Tony Hansen