Pottery glazes and firing explained: bisque, cones, clay types and food safety

TUREFF• •0

For many beginners, glazing and firing are the most mysterious part of pottery. You hand over a dull, chalky piece and get back something glossy and colourful, or something cracked and stuck to a shelf. This guide explains what happens in the kiln and what decides whether a pot is safe to eat from.

Two firings

Most studio pottery is fired twice.

Bisque firing: the first firing, of bone-dry clay. It turns clay into a hard but porous material called bisque, which no longer dissolves in water and absorbs glaze well.
Glaze firing: the glazed bisque is fired again, usually hotter, so the glaze melts into a glassy coating that bonds to the clay.

Clay must be completely dry before the bisque firing. Moisture or trapped air in thick sections can turn to steam and crack or burst a piece.

Earthenware, stoneware and porcelain

These are families of clay, defined largely by how hot they are fired.

Earthenware is low-fired and stays somewhat porous. It relies on glaze to hold water and tends to be less chip-resistant.
Stoneware is fired at higher temperatures, becoming dense and strong. It is popular for everyday tableware.
Porcelain is fired high as well, becoming very dense and often translucent where thin.

Glazes are formulated for a firing range, so a glaze must match the clay body and the kiln temperature. A low-fire glaze in a high-fire kiln can run off the pot; a high-fire glaze in a low-fire kiln will not melt properly.

What cones are

Potters describe firing temperatures in cones. A pyrometric cone is a small cone of ceramic material designed to bend when it has received a certain amount of heat. Cones measure heat work, which is temperature combined with time, so the same cone bends at a lower temperature in a slow firing than in a fast one. Cone numbers can be confusing: those with a leading zero, such as 06 or 04, are low-fire, and they get cooler as the number after the zero gets bigger. Cone 6 and cone 10 are common mid- and high-fire targets. According to Orton's chart, cone 6 is about 1,222 degrees C and cone 10 about 1,285 degrees C at a medium heating rate. Your clay and glaze packaging will state the cone they are meant for.

Applying glaze

Clean bisque with a damp sponge to remove dust.
Wax the base and the lowest part of the foot so glaze does not touch the kiln shelf. Molten glaze will glue a pot to the shelf permanently.
Apply by dipping, pouring or brushing, following the glaze's instructions for thickness.
Thick glaze runs; thin glaze looks patchy. Test tiles help you learn before risking a favourite pot.

Are glazes food safe?

Food safety depends on several things, not only the glaze recipe.

Ingredients: some glaze materials can leach into food, with lead being the best-known concern. Many glazes sold for studio use are labelled for whether they are intended for food surfaces; check the label and the maker's guidance.
Correct firing: according to the US FDA, when pottery is fired at the proper temperature for the proper time, lead in the glaze is essentially bound into it. Underfired glaze can let metals leach. A food-safe glaze fired to the wrong cone may not be food safe.
Clay body: a fully fired, well-matched clay body is less likely to crack the glaze over time.
Surface condition: crazing (fine cracks in the glaze), pitting and heavy wear can harbour bacteria and affect leaching.

For anything you plan to sell, give away for food use or use daily, follow your studio's guidance and consider lab leach testing. Treat old, worn or unknown handmade or imported dishes with caution, especially bright red, orange or yellow glazes, which the FDA flags among higher-risk pieces. If in doubt, keep a piece decorative.

Safety in the glaze room

Dry glaze materials are dust you should not breathe. Mix powdered glazes with a suitable respirator and good ventilation, clean with water rather than sweeping, and keep food and drink out of the glaze area. What glaze results have surprised you, for better or worse? Share your questions and kiln stories in the comments.