Why Food Safety Starts at the Glaze Bucket

A mug is not just a mug. It is a food contact surface, and that surface will spend its life holding hot tea, tomato soup, wine, lemon juice and whatever else your household throws at it. Glazes are glass, but glass is not automatically inert, and the difference between a safe piece and a questionable one often comes down to glaze chemistry, firing and a bit of patient testing.

The good news is that food-safe glazing is not mysterious. It follows three practical rules: the glaze must be fired within its stated limits, the fired surface must be durable enough to resist scratching and thermal shock, and the finished glass must not release harmful amounts of metals into food. Get all three right and your tableware will serve for decades.

Firing Limits: Match the Glaze to Your Kiln and Clay

Every glaze has a maturity range. Under-fire it and the glass never forms properly, leaving soluble materials that can be drawn out by acidic foods. Over-fire it and the surface can boil, blister or run off the rim, sometimes volatilising the very ingredients that make it stable.

• Follow the stated cone. A glaze rated for cone 6 will not behave at cone 06, and a low-fire glaze pushed to stoneware temperatures may lose its colour or become brittle.
• Check clay and glaze compatibility. Thermal expansion mismatches cause crazing or shivering. Crazing leaves a network of cracks that traps liquids; shivering can flake glass into food, which is worse.
• Confirm the clay body is vitrified. Porous earthenware absorbs moisture through any unglazed foot or crack. Choose a body suited to tableware or accept that the piece needs careful handling.
• Beware of unknown or inherited glazes. Old studio recipes and imported bargain buckets may contain lead or high barium and are not worth the risk on a plate.

Durability: A Surface That Survives Real Life

Durability comes largely from a well-formed silica network reinforced with alumina. Glazes high in alkalis, or those that are simply under-fired, tend to be soft and prone to cutlery marks. You can often feel the difference: a good tableware glaze is smooth, tight and resistant to a fingernail or a spoon edge.

• Test with metal. Drag a spoon across a fired test tile. Pale grey scratch marks that wash off are usually metal transfer, but deep grooves mean a soft glaze.
• Consider dishwasher and microwave use. Many glazes survive both, but metallic lustres, decals and some copper reds do not.
• Watch rims and feet. Thin glaze on a rim wears through quickly. A slightly thicker, well-rounded rim lasts far longer.

If you sell your work, state clearly whether a piece is dishwasher safe, hand wash only, or decorative. Customers appreciate honesty far more than a chipped mug.

Leaching: What Might Come Out of the Surface

Leaching is the migration of metals from the glaze into food, and acid is the usual trigger. Vinegar, citrus, tomatoes and wine are the toughest tests. Copper, cobalt, manganese, barium, lithium and zinc are all useful in glazes, but in a poorly formulated or under-fired glass they can be released in unwanted quantities.

• High-metal decorative glazes such as saturated copper greens, manganese purples and barium-rich mattes are best kept off food contact surfaces.
• Lustres, gold and platinum finishes should never touch food or drink; use them on the outside of a vessel only.
• Raku, pit-fired and saggar-fired wares are porous, often crazed, and not suitable for eating or drinking from.
• Crazed surfaces are a hygiene concern as well as a leaching one, since they hold residue and moisture.

In the UK, commercial tableware must meet strict limits on lead and cadmium release, so commercial glazes sold for tableware are formulated with this in mind. That reassurance disappears the moment you mix your own from raw materials without checking the recipe.

Testing Before You Sell or Serve

Testing takes a few days and costs very little. It is the single most useful habit a potter can build.

• Acid test. Fill a fired sample with hot vinegar or lemon juice, cover, and leave for 24 hours. Repeat with a fresh sample several times. Look for dulling, colour change or a metallic taste.
• Boiling water and thermal shock. Pour boiling water into a chilled mug, then plunge it into cold water. Crazing that appears here will appear in a customer's kitchen too.
• Microwave and dishwasher cycles. Run a test piece through ten cycles and inspect the surface again.
• Leach test kits. Proprietary kits give a quick indication of lead and cadmium release if you are working with an unfamiliar recipe.
• Keep records. Note glaze name, batch, cone, clay body and test results. A tile in a drawer is worth nothing if you cannot remember how it was fired.

Building a Reliable Food-Safe Palette

Most working potters end up with two clearly separated sets of glazes: a small, thoroughly tested food-safe range, and a larger experimental range for decorative work. Label everything, keep food-safe buckets on their own shelf, and never dip a tableware piece into a glaze you cannot account for.

Aim for a handful of dependable recipes — a clear, a satin white, two or three reliable colours — and learn them intimately. Repeatable, well-fired, durable glazes will do more for your tableware than any exotic surface that fails the vinegar test. That quiet reliability is what makes a handmade bowl worth reaching for every morning.