Why Food Safety Starts in the Glaze Bucket

Every time someone ladles soup from a bowl you made, the glaze is doing quiet work. A food-safe surface is essentially a very stable glass: high in silica, well melted, and free from anything that can leach into acids or hot liquids. Get the chemistry right and you have a surface that will shrug off lemon juice, vinegar and tomato sauce for decades.

The three pillars of a durable glaze are silica (the glass former), alumina (which stiffens the melt and stops it running) and fluxes (which bring the melting point down to something your kiln can reach). The trick is to use enough flux to melt the glaze properly, but not so much that the resulting surface becomes soft, soluble or prone to scratching.

A few practical rules keep potters out of trouble:

• No lead, ever. Lead glazes are restricted for good reason. Modern lead-free boron frits do the same job far more safely.
• Be cautious with cadmium and barium. Encapsulated cadmium pigments are available for bright reds and oranges, but stick to pigments sold specifically for food-contact surfaces.
• Favour stable colorants such as iron oxide, cobalt, rutile, tin, zirconium silicate and chrome-tin pinks. Copper and manganese can misbehave in acidic conditions.
• Keep glaze recipes tested and documented. A recipe you can repeat is worth more than a one-off success.

Choosing a Recipe You Can Trust

For plates, bowls and mugs, most UK studio potters work in one of two zones: mid-fire stoneware around cone 6 (roughly 1220°C) or high-fire stoneware around cone 10 (roughly 1280°C). Both produce dense, vitrified bodies that pair beautifully with hard, well-melted glazes. Earthenware, fired nearer 1060–1120°C, is perfectly useable too, but the glaze is softer, so it needs to be well formulated and applied with care.

A dependable food-safe glaze generally sits in the region of 60–70% silica and alumina combined, with the balance made up of fluxes such as feldspar, whiting, dolomite, talc and a boron frit. A simple glossy base might use potash feldspar, silica, china clay, whiting and a small percentage of frit — nothing exotic. If you want a white, opaque surface, zirconium silicate or tin oxide will do it without threatening stability.

Testing is not optional. Make up a set of small test tiles, glaze them at your intended thickness, and fire them exactly as you would a dinner service. Then subject them to the sort of abuse a real kitchen delivers. The good news is that a well-formulated glaze is usually obvious from the start: smooth, glossy, no pinholes, no crazing, no shivering.

Thickness: The Detail Most Potters Get Wrong

Application thickness is where beautiful glazes turn into disappointing ones. Too thin and you get a dry, rough, under-fired surface that stains and scratches. Too thick and you risk crawling, running onto the kiln shelf, or a crazed, flaking finish. Aim for a fired thickness of roughly 0.3 to 0.5 mm — about the thickness of a decent coat of paint.

Useful habits to build:

• Check specific gravity. For most dipping glazes, 1.40 to 1.55 is a good working range. A hydrometer takes the guesswork out of it.
• Time your dips. Three to five seconds for stoneware, with a gentle swirl, then a steady lift and a brief pause to let the glaze drain.
• Wax the foot. A clean, dry foot ring stops glaze fusing your pot to the shelf and keeps contact surfaces bare.
• Keep rims a touch thinner. Glaze pools and thins unevenly; a rim that is too thick can chip surprisingly easily.
• Brush glazes need several coats. Three thin layers, dried between each, will usually match a single dip.

If you spray, build up in light passes rather than one heavy cloud. Even coverage matters more than speed, particularly inside mugs where drips collect in the base.

Firing Temperature, Soaks and Cooling

A glaze needs to melt fully, flow just enough to heal pinholes, and then cool into a hard, glassy surface. Dropping the top temperature by even 20°C can leave a glaze under-fired and porous, while overshooting can send it running off the pot. Follow your glaze's recommended cone and give it a short soak of 10–20 minutes at the top to even out the melt.

Cooling matters just as much. A fast cool through the 700–500°C range encourages crazing — those fine hairline cracks that let liquids into the body. A gentle, controlled cool, or even a short hold around 1000°C on the way down, gives the glaze time to fit the clay properly. If your glaze crazes, the fix is usually a fit adjustment: more silica and less flux in the glaze, or a clay body with a closer thermal expansion.

Testing for Real Kitchens

Before any pot goes to a customer, put it through a rough fortnight. Fill a mug with hot tea and leave it for an hour. Pour vinegar or lemon juice into a bowl and leave it overnight. Run a test piece through the dishwasher twenty times. Drag a fork across the surface and see whether it leaves a grey mark — often this is metal from the cutlery rather than damage to the glaze, but repeated marking suggests a soft surface. Finally, brush ink over the surface and wipe it off: if lines of ink remain, the glaze is crazed.

Pots that pass these tests, with a well-melted food-safe glaze applied at the right thickness and fired properly, will serve for years. That is the quiet pleasure of the craft — a bowl that simply works, meal after meal.