Generally, a cone 6 fire is considered food safe for many commercially available glazes. You can often fire up to cone 10, but some glazes may lose their metallic qualities at that high of a temperature.
Firing ceramics to the correct temperature is crucial for ensuring food safety. Underfired glazes may not fully vitrify, leaving them porous and susceptible to harboring bacteria.
Cone Temperature and Food Safety
The cone number refers to the temperature at which a glaze matures. Different glazes have different maturing temperatures, so it's important to choose a glaze that is rated for the cone you plan to fire to.
- Cone 6: A mid-range temperature commonly used by potters. Many commercially available glazes are designed to be food-safe when fired to cone 6.
- Cone 10: A high-fire temperature. Some glazes formulated for cone 6 can be fired to cone 10, but you should test them thoroughly for leaching and durability. As the reference mentions, some may lose desired qualities.
Ensuring Food Safety
To ensure your pottery is food safe, consider the following:
- Use Food-Safe Glazes: Specifically look for glazes labeled as "food safe."
- Fire to the Correct Temperature: Follow the manufacturer's instructions for the correct firing temperature.
- Test for Leaching: After firing, test your pottery for leaching. You can use a commercially available lead test kit. While lead isn't a common ingredient in modern glazes, other materials can leach if the glaze isn't properly fired.
- Avoid Cracks and Crazing: Crazing (fine cracks in the glaze) can provide a haven for bacteria. Choose glazes that are compatible with your clay body to minimize crazing.
