Limit Formulas
Glaze Limit Formulas
Glaze Limit Formulas are broad guidelines for stable, functional base glazes. There are many published versions—and plenty of debate. Decorative glazes often exceed these ranges. If your functional glaze lands outside a range, that’s a signal to pay attention to those oxides and consider tweaks; if it falls inside a range, it isn’t automatically stable or food-safe.
Glazy’s patron features allow layering various limit formulas like Cushing, Hesselberth & Roy, and Montmollin Fuse Diagrams.
Screenshot of the Glazy Stull chart showing various limits: Δ5-6 Cushing Glossy, Δ6 Hesselberth & Roy, and Δ6 Green & Cooper. Even though they’re for the same temperature, there are big differences between them. Also note that in this search for cone 6 glazes, a number of the recipes fall outside of all the limits.
| RO/R₂O (Fluxes) | R₂O₃ (Stabilizers) | RO₂ (Glass Formers) | |||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|
| Name | KNaO | Li₂O | PbO | ZnO | CaO | MgO | BaO | SrO | Al₂O₃ | B₂O₃ | SiO₂ |
| Δ5-6 Val Cushing Glossy | 0.05 - 0.6 | 0 - 0.5 | 0 - 0.6 | 0 - 0.15 | 0.05 - 0.6 | 0 - 0.1 | 0 - 0.15 | 0 - 0.15 | 0.1 - 0.3 | 0 - 1.0 | 1.5 - 4.0 |
| Δ5-6 Val Cushing Satin | 0.05 - 0.35 | 0 - 0.15 | 0 - 0.4 | 0 - 0.3 | 0.05 - 0.7 | 0 - 0.35 | 0 - 0.35 | 0 - 0.35 | 0.2 - 0.4 | 0 - 0.5 | 2.0 - 3.5 |
| Δ5-6 Val Cushing Matte | 0.05 - 0.3 | 0 - 0.1 | 0 - 0.2 | 0 - 0.4 | 0.05 - 0.8 | 0 - 0.45 | 0 - 0.5 | 0 - 0.5 | 0.2 - 0.5 | 0 - 0.5 | 2.0 - 3.0 |
| Δ5-6 Hesselberth & Roy | 0.01 - 0.03 | 0 - 0.2 | 0.2 - 0.6 | 0 - 0.3 | 0 - 0.2 | 0.25 - 0.4 | 0.15 - 0.35 | 2.5 - 4.0 | |||
| Δ9-10 Val Cushing Glossy | 0.05 - 0.5 | 0 - 0.4 | 0 - 0.15 | 0.05 - 0.8 | 0 - 0.15 | 0 - 0.15 | 0 - 0.15 | 0.2 - 0.5 | 0 - 0.5 | 2.0 - 6.0 | |
| Δ9-10 Val Cushing Satin | 0.05 - 0.4 | 0 - 0.2 | 0 - 0.4 | 0.05 - 0.8 | 0 - 0.5 | 0 - 0.5 | 0 - 0.5 | 0.25 - 0.6 | 0 - 0.4 | 2.0 - 5.0 | |
| Δ9-10 Val Cushing Matte | 0.05 - 0.3 | 0 - 0.1 | 0 - 0.5 | 0.05 - 0.9 | 0 - 0.6 | 0 - 0.6 | 0 - 0.6 | 0.25 - 0.8 | 0 - 0.2 | 2.0 - 5.0 | |
| Δ9-10 Hesselberth & Roy | 0.1 - 0.3 | 0.3 - 0.7 | 0 - 0.4 | 0 - 0.3 | 0.3 - 0.6 | 0 - 0.3 | 3.0 - 5.0 | ||||
There are many articles concering glaze limits, some notable ones are:
- Digitalfire’s Limit Formulas and Target Formulas
- Frog Pond Pottery Glaze Stability Literature
Montmollin Fuse Diagrams
Daniel De Montmollin’s (Wikipedia ) book “Pratique des Emaux de grès” (“The Practice of Stoneware Glazes”) introduces the concept of melting “fusion” or “fuse” diagrams. These diagrams provide a simplified way to understand the melting behavior of glaze components, illustrating how different oxide combinations interact at specific temperatures. In Glazy, Montmollin Fuse Diagrams can be layered on top of each other as well as with the limit formulas and Stull Chart to further help visualize the melting behavior of the glaze.
Left: Example Fuse Diagram of 0.3 KNaO 0.7 CaO from Montmollin’s book.
Right: Screenshot of the Glazy Stull chart showing two of Montmollin’s Fuse
Diagrams: 0.3 KNaO 0.7 CaO and 0.2 KNaO 0.8 CaO. Note that Montmollin’s
diagrams have SiO₂ as the Y-axis.