Glaze

Silica:Alumina Ratio Calculator

Calculate the SiO₂:Al₂O₃ ratio from a glaze UMF to predict glossy, satin, or matte surface quality.

Updated

Enter SiO₂ and Al₂O₃ values from a Unity Molecular Formula (UMF) analysis. These are mole amounts, not percentages.
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Enter your measurements above and click Calculate.

The Seger Unity Molecular Formula

Hermann Seger developed a method of expressing glaze chemistry as a unity molecular formula (UMF), normalizing the flux oxides to 1. The ratio of silica (SiO₂) to alumina (Al₂O₃) is one of the most important indicators of glaze surface quality — whether it will be glossy, matte, or crystalline.

Si:Al Ratio and Surface Prediction

Firing Range Matte Si:Al Glossy Si:Al
Low-fire (cone 06–1)3.5–5.05.0–8.0
Mid-fire (cone 2–7)4.0–6.06.0–10.0
High-fire (cone 8+)5.0–8.08.0–12.0

Role of Alumina (Al₂O₃)

Alumina is a glass stiffener — it raises viscosity and prevents the glaze from running. Low alumina (below 0.3 moles in UMF) produces soft, scratch-prone surfaces. High alumina glazes are very durable but may appear dull or underfired. The typical functional range is 0.3–0.6 moles Al₂O₃ in UMF for mid- and high-fire work.

Frequently Asked Questions

What Si:Al ratio produces a glossy glaze? expand_more
At mid-fire (Cone 6), a Si:Al ratio of 6–10 produces a glossy surface. Below 4 tends toward matte/dry; above 12 may run. These are guidelines — base fluxes also play a role.
How do I produce a true matte glaze? expand_more
True matte glazes use a low Si:Al ratio (below 5 at mid-fire) combined with adequate calcium (CaO) or barium (BaO) flux. Cooling rate also matters — slower cooling grows larger crystals and increases matteness.