Why Food Safety Is Really a Glaze Question

Clay on its own is inert enough that most potters don't give it a second thought. The glaze is what sits between your dinner and your dinner plate, and it's where almost every food-safety problem begins. A glaze is, chemically speaking, a thin glass, and glass can be persuaded to surrender tiny amounts of whatever is dissolved inside it — especially when it meets acid, heat and time.

Coffee, tea, tomato sauce, wine, vinegar dressings and citrus are all quietly aggressive. The three things to watch for are lead, cadmium and the metal oxides used to colour a glaze. Lead made older glazes glossy and forgiving, and it also leaches readily and accumulates in the body. Cadmium produces brilliant reds and oranges and is toxic in very small quantities. Copper, cobalt, manganese, barium and lithium are all perfectly workable within a well-balanced glaze, but a weak or badly fired one will release them. Modern, well-made glazes are entirely safe — the craft lies in knowing which ones you're dealing with.

Reading a Glaze With Food in Mind

Commercial glazes usually declare themselves on the tub: look for the words "food safe", "dinnerware safe", or an explicit statement about lead and cadmium content. If a shop-bought glaze says nothing on the subject, treat it as decorative. If you mix your own, these are the qualities that matter:

• A balanced base — generous silica, adequate alumina, fluxes in sensible proportion. A high-silica, moderate-alumina glaze is far more durable than a soft, heavily fluxed one.
• Stable colorants — iron oxide, chrome-tin pinks and zircon-based stains are generally reliable. Unstable copper greens, heavy manganese browns and some cobalt blues can leach if the base is weak or the firing is cool.
• No lead, no cadmium — request a safety data sheet for anything you buy, and keep it on file.
• A recipe you can repeat — if nobody can tell you what's in the bucket, it doesn't go on a plate.

A useful habit is the liner glaze: choose one glaze you trust completely for the inside of every mug, bowl and plate, then enjoy your more experimental surfaces on the outside, where food rarely lingers.

Firing to Maturity Matters Just as Much

A glaze that hasn't reached maturity is porous, soluble and far more likely to give up its ingredients. Follow the maker's recommended cone rather than a favourite schedule, and record what you actually did. A firing log noting cone, top temperature, soak time and kiln load will tell you more about a failed glaze than any amount of staring at the pot.

The clay body counts too. Mid-fire and high-fire stoneware vitrify to a low absorption rate, so the body itself resists liquid. Earthenware stays porous and relies almost entirely on its glaze for protection — which is fine, provided the glaze is sound and uncrazed. Keep leaded glazes out of any kiln used for foodware, and if you share a kiln, ask what other people are firing in it.

Simple Tests You Can Run at the Bench

Home tests are not laboratory tests, but they will flag a problem before it reaches your table.

• The vinegar soak: fill the piece with white vinegar, leave it for 24 hours, then look for any change in colour, feel or weight. Test the same piece again after a dishwasher cycle.
• Acetic acid: a 4% solution for 24 hours at room temperature is much closer to the standard migration test than malt vinegar will ever be.
• Lead swabs: inexpensive kits detect surface lead. A positive result is a definite no; a negative is not a guarantee.
• The ink test for crazing: rub coloured ink or strong tea over the glaze, wipe it off and look for fine lines holding colour. Crazed glazes let liquid into the body, where it can sit and grow unpleasant.
• Surface inspection: pinholes, blisters, crawling and rough rims are all places where a glaze has failed to do its job.

If you sell your work, you are the responsible person, and professional laboratory testing is money well spent on a production line. UK and retained EU limits sit at roughly 0.8 mg per square decimetre of lead for flatware and 4.0 mg per litre for hollowware under three litres, with cadmium at 0.07 mg per square decimetre and 0.3 mg per litre respectively.

Looking After Handmade Ware in Daily Use

Even a sound glaze can be undone by carelessness. Chips and cracks expose the clay body, so retire or repurpose anything that has lost its surface — a chipped mug makes a fine pen pot. Avoid dramatic temperature change: no boiling water into a mug straight from a cold shelf, and no freezer-to-oven moves. Grey cutlery marks are usually metal transfer rather than a fault, and a paste of bicarbonate of soda will lift them. Check handle joins and rims periodically, since these take the knocks.

If You Also Work in Wood

Wooden spoons, boards and bowls follow the same logic: use a finish that cures hard and is itself food-grade. Pure tung oil, food-grade raw linseed oil, walnut oil, and mineral oil blended with beeswax are all dependable. Avoid the hardware-shop "boiled" linseed oils, which often contain metallic driers, and steer clear of varnish on any surface that will be cut on. Re-oil little and often, and let pieces dry thoroughly between uses.

Quite simply: a glaze you can name, a firing you can trust and a test you've actually carried out will give you ware you can hand to a friend with confidence.