Learn · Glaze safety

Lead and Cadmium in Pottery Glazes: What Potters Should Know

By Christina Phillips, safety professional and potter · October 2026

Lead makes glazes brilliantly glossy at low temperatures. Cadmium makes reds, oranges, and yellows pop like nothing else. Both are toxic heavy metals — and both have a long history in ceramics. The question isn't whether they exist in pottery. It's whether they can get from the glaze into your food.

How lead and cadmium get into food

The risk from these metals isn't touching a pot — it's ingestion. When a glaze isn't properly formulated for food surfaces, or isn't fired to maturity, acids in food and drink — orange juice, coffee, tomato sauce, vinegar-based dressings — can attack the glass surface and pull metal ions out. This is called leaching. A well-formulated glaze, fired to the manufacturer's recommended temperature on a compatible clay body, locks those metals into a stable glass matrix. An underfired glaze, a poor clay-glaze fit (think crazing — that network of fine cracks), or a glaze never designed for dinnerware can leach measurably.

What “food safe” actually means on a label

Here's where potters get tripped up: there is no single universal “food safe” stamp on every glaze. What you get instead is manufacturer language. Look for terms like “dinnerware safe” or “food safe when used according to manufacturer's directions” — and then actually follow the directions, especially the firing temperature. A glaze rated dinnerware-safe at cone 6 may not be dinnerware-safe at cone 06.

One label that does not mean food-safe: the ACMI AP (Approved Product) seal. That seal comes from art-material labeling law — it means the product's chronic hazards are labeled appropriately. It's about handling the raw material, not about eating off the fired surface. A glaze can carry the AP seal and still not be rated for food surfaces. Read the food-safety statement specifically.

Why you see California Prop 65 warnings on glaze jars

If you've bought commercial glazes, you've seen the warnings about chemicals known to cause cancer or reproductive harm. California's Proposition 65 requires businesses to warn about significant exposures to listed chemicals — and lead and cadmium compounds are on the list.

The warning usually refers to the raw, unfired material — the powder you're handling — not necessarily the finished pot. Firing transforms glaze chemistry. But don't dismiss the warning either: it's a reminder to handle raw materials with respect (respirator, gloves, no food nearby) and to verify fired ware independently if you're selling food surfaces.

How to know if finished ware is really safe

If you make functional ware — especially to sell — test, don't guess:

• •Home lead swabs. Swab kits detect leachable lead on a surface. They're a rough screen, not a verdict: they can miss lead bound in a glaze matrix and can react to other substances. A negative swab is reassuring but not proof of food safety.
• •Lab leach testing. The standard method is ASTM C738 — a lab soaks the ware in an acid solution and measures what migrates out. If you sell dinnerware, periodic testing of your actual fired pieces (your clay, your glaze, your kiln) is the professional standard.
• •When in doubt, label it “decorative use only.” It's honest, it's safe, and it costs you nothing.

Practical rules for the studio

1. 1.Use glazes rated dinnerware-safe by the manufacturer for anything that touches food — and fire to the rated cone.
2. 2.Keep bright red, orange, and yellow glazes (the colors most associated with cadmium and lead) to decorative pieces, or to outside surfaces only, unless verified.
3. 3.Never assume: test-fire, and lab-test ware you sell.
4. 4.Handle all raw glaze materials with a respirator and gloves — the dust hazard exists regardless of the fired result.