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UMF glaze chemistry explained
A glaze recipe by weight ("40 Custer feldspar, 20 silica, 20 whiting…") tells you what to weigh out, but almost nothing about how the glaze will behave. The Unity Molecular Formula (UMF, also called the Seger formula) converts that recipe into the ratio of oxides the melt actually forms — and that's what predicts gloss, durability, and faults.
The three oxide groups
UMF sorts every oxide in a glaze into three columns by their role in the melt:
| Group | Role | Examples |
|---|---|---|
| Fluxes (RO / R₂O) | Make the glaze melt | CaO, MgO, K₂O, Na₂O, ZnO, BaO, SrO, Li₂O |
| Stabilizer (R₂O₃) | Stiffens the melt, adds durability | Al₂O₃ (alumina) |
| Glass-formers (RO₂) | Form the glass itself | SiO₂ (silica), B₂O₃ (boron) |
Why "unity"?
The formula is normalized so that the flux group sums to 1.0. Everything else — alumina, silica — is then expressed relative to that. This is the whole point: it lets you compare any two glazes on the same footing, regardless of batch size or which materials supplied the oxides. Two recipes with totally different ingredient lists can have nearly identical UMFs — and they'll fire nearly identically.
The number that matters most: the silica-to-alumina ratio (Si:Al). It's the single best predictor of surface. Roughly, a ratio around 5:1 sits near the matte boundary; as it climbs past 7:1, glazes get increasingly glossy. This is the horizontal axis of the Stull chart.
What UMF lets you do
- Predict the surface before you fire — matte, satin, or gloss — from the Si:Al ratio and alumina/silica levels.
- Diagnose faults — high alkali fluxes (K₂O + Na₂O) raise thermal expansion and cause crazing; too little silica leaves a soft, leachable surface.
- Substitute materials safely — swap one feldspar for another and keep the same UMF, and the glaze stays the same even though the recipe changed.
Flux computes the UMF as you type
Enter a recipe and Flux gives the live unity formula, the Si:Al ratio, and plain-English warnings when an oxide falls outside the limit ranges — cited to named sources (Digitalfire, Sue McLeod, Hesselberth–Roy). Cone 06 / 6 / 10, materials library, share as PDF. Pay once, no subscription, works offline.
Sources
- Digitalfire — unity formula (Tony Hansen)
- Glazy — chemistry concepts · Digitalfire — silica:alumina ratio
General education. Always test-fire your own glazes — chemistry predicts behavior, but your materials, kiln, and firing schedule have the final say.