Is your glaze food-safe? Limit formulas and leaching
Why a glossy glaze isn't automatically safe — and the numbers that actually predict durability.
The limit formulas below predict whether a glaze is likely stable and durable. The only way to confirm a specific glaze leaches within safe limits is laboratory testing on your fired piece. Treat the numbers as a strong screen, not a guarantee.
A glaze can look perfect and still leach metals into food or wear away over time. Whether it's durable comes down to its unity molecular formula sitting inside known "limit formula" ranges — the bands of oxide levels that experienced chemists associate with stable, well-fitted glazes.
General limit ranges (cone 6, as a starting screen)
Per Digitalfire's general ranges (tighten per cone and per your sources):
| Oxide (UMF) | Typical range | Out-of-range hint |
|---|---|---|
| SiO₂ (silica) | 2.4–4.7 | Too low → soft, leaches; too high → may craze |
| Al₂O₃ (alumina) | 0.285–0.64 | Too low → runny; too high → matte, underfired |
| CaO | 0–0.55 | Too high → crawling |
| K₂O + Na₂O (alkalis) | 0–0.375 | High → crazing (high thermal expansion) |
The two big durability levers
- Enough silica + alumina. Silica is the glass; alumina stiffens and toughens it. A glaze low in both is soft and prone to leaching and cutlery marking. The fix for a leachy glaze is usually more silica and alumina (kept in ratio), not less flux.
- No crazing. Crazing is a network of fine cracks from a glaze that shrinks more than the clay. Beyond looking bad, the cracks harbor bacteria and weaken the surface. High alkali fluxes (K₂O + Na₂O) are the usual culprit — they have high thermal expansion.
Some fluxes are hazardous regardless of how well the glaze melts. Barium (BaO) and lead are the classic ones; cadmium and some colorants (chrome, cobalt, manganese, copper in acidic foods) also warrant caution. If a recipe relies on barium or lead for its surface, it should not be used on food surfaces without verified leach testing — and many potters avoid them entirely for functional ware.
Gear this guide uses
Weighing glaze materials accurately and not breathing the dry powders are the two basics behind every food-safe recipe.
- A 0.1 g digital scale so batch weights match the recipe you ran through the UMF.
- A NIOSH P100 dust respirator for mixing silica and other fine glaze powders.
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Sources
General education, not a safety certification. Confirm food-contact safety of any specific glaze with proper leach testing.