A pot of chili left in a bowl overnight is, chemically speaking, an experiment. Tomatoes are acidic. Ceramic glazes are thin, glassy films that can give up small amounts of the metals that color and melt them. Most of the time nothing happens. But a slightly underfired glaze, or one that has crazed into a web of hairline cracks, can release enough material to matter — especially if that bowl holds tomato sauce every week for ten years.

Food safety in ceramics is not a checkbox on a recipe card. It is a property of the finished, fired piece: its chemistry, how well the glaze fits the clay, and how hot and how long it was fired. Get those three things right and you can make glazes that are genuinely safe for daily use. Get them wrong and even a lovely glossy surface can be a problem.

Choose a Chemistry Built for Durability

A fired glaze is a glass. Like any glass, it can be attacked slowly by acid, and the more flux it holds relative to silica and alumina, the more easily that happens. Silica builds the network. Alumina stiffens it and makes it considerably more resistant to chemical attack. Fluxes — boron, sodium, potassium, lithium, calcium, magnesium, zinc, barium — lower the melting point, and without them your kiln could never melt the glaze at all. The trick is using just enough.

In practice, that means starting from a known, durable base rather than inventing one. Glaze chemistry software and the tested recipes in standard ceramics references usually include stable base glazes and "limit formulas" — ranges of oxide ratios that tend to produce hard, acid-resistant surfaces. If you are new to formulating, borrow a proven clear or white liner glaze and adjust the colorants, not the underlying chemistry.

Materials to Treat With Suspicion

Not every oxide is equally risky, and risk depends on how much is present and how well the glaze is fired. These are the ones worth thinking hard about before they touch a food surface:

• Lead. There is no reason for a studio potter to use it. Even industrial lead glazes require precise firing and routine laboratory verification. Skip it.
• Cadmium. The source of brilliant reds, oranges, and yellows. Cadmium release into acidic food is a real concern. Encapsulated pigments are more durable, but they still belong on the outside of a pot, not the inside.
• Barium. Common in satin and matte surfaces. Barium carbonate is toxic to handle in the raw state, and barium can leach from the fired glaze as well.
• Lithium. An efficient flux, but one that can reduce a glaze's resistance to acid.
• Manganese. Browns and purples. It leaches readily and is a neurotoxin. Keep it away from food-contact surfaces.
• Copper, cobalt, chromium, nickel, antimony. Usable in small amounts inside a hard, well-fired glaze, and risky in a soft or underfired one.
• Metallic lusters. Gold, platinum, and mother-of-pearl finishes are decorative only. They do not belong on or near a food-contact surface, and they should never go into a microwave.

Soluble materials cause quiet problems

Some raw materials — borax, soda ash, certain frits and sulfates — dissolve in the glaze bucket. That can pull a recipe off balance, create scum on the surface, and leave patches that melt differently from the rest of the pot. Slightly soluble materials are also the easiest for acid to attack later. When a recipe calls for a soluble ingredient, consider whether a frit can do the same job.

Glaze Fit: Crazing, Shivering, and Liners

Crazing is the web of fine cracks that shows up when a glaze shrinks more than the clay body as it cools. It looks charming on a raku vessel. On a dinner plate it is a liability: those cracks give liquid a route into the body, and a damp body can harbor bacteria while both the clay and the glaze come under attack. Shivering is the opposite problem — a glaze under so much compression that it flakes off in sharp chips that can end up in someone's food.

Fixing fit means adjusting the glaze or changing the clay body, and it is worth the effort. Crazing is determined by the fired materials, not by how thickly you applied them, though an overly thick application makes it worse. If a glaze crazes on your stoneware but not on your porcelain, the body is the variable.

The most reliable studio strategy is a liner glaze: one simple, glossy, thoroughly tested glaze used on every interior and rim, with the more experimental surfaces kept on the outside. Glossy surfaces are generally more resistant to acid than dry matte ones, simply because they present less surface area to the food.

Firing Is Not a Formality

The most common cause of a leaching glaze is a glaze that never fully melted. A cone 6 recipe fired to cone 5 may look smooth and still be porous, with a surface that gives up material far more readily than it should. A glaze that is properly melted forms a continuous glass; one that is underfired is part glass, part sponge.

Use witness cones on multiple shelves rather than trusting the kiln's controller alone. Consider a short hold at the top temperature to heal pinholes and blisters, which trap food and expose extra surface. A slow cool can smooth a surface, though on some recipes it also encourages crystal growth, which cuts gloss and adds surface area. Look at every load with a critical eye: pinholes, blistering, and crawling are all signals that the surface is not what you want touching someone's soup. Refire, reglaze, or set the piece aside for dry goods.

Testing for Leaching at Home

Laboratory methods expose a glazed surface to dilute acetic acid and measure what comes off. You can approximate the idea at your kitchen counter. It will not give you numbers and it will not catch everything, but it flags the worst offenders and it beats assuming.

1. Fire a test cup from the same clay, the same glaze batch, and the same kiln load as the pots you plan to use or sell. Label it: recipe, cone, date, shelf position.
2. Fill it with plain white vinegar, cover it loosely, and leave it at room temperature for 24 hours.
3. Pour the vinegar into a clean glass dish and let it evaporate. A ring, film, or colored residue is a warning sign — that is material the glaze gave up.
4. Rinse and dry the cup. Look for dulled patches, color shifts, or a chalky bloom where the acid sat.
5. Run a control cup filled with water at the same time, so you are comparing against the glaze's normal appearance rather than your memory of it.
6. Repeat with hot lemon juice or a stronger acid if you want a harsher screen, and repeat again after a few dishwasher cycles to see whether crazing develops.

Lead swab kits detect lead on a surface, and they detect only lead. A clean swab does not clear a

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