Food-safe glazes, leaching tests, and Prop 65 compliance

What makes a glaze food-safe. ASTM testing. Lead/cadmium leaching. The compliance that protects you and your customers.

Image: Glaze leaching test

Most potters making functional pottery don't formally test their glazes for food safety. They use "commercial food-safe glazes" and assume that's enough. Sometimes it is. Sometimes it isn't.

What makes a glaze food-safe

A food-safe glaze must:

• Be fully matured at firing temperature (not under-fired)
• Have a glass-formation that doesn't leach toxic metals at use temperature/pH
• Pass ASTM C-738 (lead leaching) and ASTM C-927 (cadmium leaching) at acceptable levels
• Not crack, craze, or pinhole (which trap bacteria)

Commercial food-safe glazes

Brands like Amaco Potter's Choice, Coyote, Mayco Stroke & Coat: most are labeled food-safe BUT only when applied and fired correctly. The food-safety claim assumes:

• Glaze is fired to manufacturer's recommended cone (usually cone 5 or 6)
• Glaze is fully matured (not under-fired or over-fired)
• Glaze is applied at manufacturer-recommended thickness
• The glaze is the same recipe as tested (not modified)

If you under-fire, the glaze may not vitrify completely, leaving micro-pores that leach. If you mix glazes, the food-safety claim doesn't carry over.

DIY glaze recipes and the food-safe trap

Home-mixed glaze recipes (from Ceramic Arts Daily, John Britt, etc.) are NOT food-safety tested unless you test them yourself. A "Mike Wynn red" glaze recipe may be beautiful but might leach copper in acidic food contact.

If you mix your own glazes, you need to:

• Submit samples to a testing lab (Brandywine Ceramic Materials, Alfred University) — about $80-150 per glaze
• Test for lead, cadmium, copper, and barium leaching specifically
• Document the results for liability protection

California Prop 65 for pottery

If you sell into California (including Etsy/Shopify shipping there), Prop 65 may apply. The relevant chemicals for pottery:

• Lead (in some commercial glazes, more in DIY)
• Cadmium (in some red/yellow/orange glazes)
• Crystalline silica (in clay dust)

For most modern commercial glazes (post-2010), lead and cadmium are below Prop 65 reporting thresholds. For DIY glazes, you need to test or warn.

The honest compliance approach for small potters

1. Use commercial food-safe glazes. Don't mix your own unless you're willing to test.
2. Follow firing schedules exactly. Under-fired glazes are the biggest food-safety risk.
3. Test your kiln cone accuracy with cones every 10-20 firings. Kilns drift.
4. For any non-standard color (deep reds, oranges, bright yellows), be cautious about food contact.
5. Label "decorative use only" for any pottery you're uncertain about. This is legitimate for pieces with risky glazes.