Why do ceramic bowls and plates test positive for heavy metals?

First, what was found

In September 2024, HK's Consumer Council tested 30 ceramic items (15 cups, 15 bowls), simulating room-temperature and microwave use. 11 released lead (6 cups, 5 bowls) and 4 released cadmium — over 40% of samples showed lead or cadmium migration, but all release levels were below international and mainland standard limits ("detected, but not over the limit"). Microwave-safe samples released none under a hot-microwave simulation.¹

In other words: proper, standard-compliant ceramic tableware is low-risk in everyday use. The trouble comes from the other end — cheap, uncertified, brightly-decorated ware. A mainland report described a family of five with elevated blood lead after long-term use of low-quality colourful ceramics; the expert's one-liner: "the brighter the colour, the higher the risk."²

The key question: why are heavy metals in ceramic dishes?

The ceramic body (fired clay) isn't the problem. Almost all the heavy metal comes from two things on the surface — the glaze and the decorative paint.³

• Lead in the glaze: lead is used as a flux — it makes the glaze melt evenly, look glossy, and fire at a lower temperature. So cheap factories use leaded glaze because it's easier and looks good.
• Lead and cadmium in the pigments: bright colours — especially yellow, orange, red and green — often rely on cadmium and lead compounds. So "brighter" frequently means "more lead and cadmium."

On-glaze vs under-glaze — position decides the risk

For the same patterned bowl, safety depends on where the decoration sits:

• On-glaze (higher risk): the pattern/pigment is printed on top of the glaze and re-fired at a lower temperature. The pigment sits on the surface, in direct contact with food; heat, acidity or abrasion can leach the lead/cadmium out — or flake it off.
• Under-glaze / in-glaze (safer): the pigment is applied before the glaze, then sealed under a high-fired glassy glaze layer, separated from food, so migration is far lower.

So a bright pattern on the food-contact surface (inside the bowl, the rim of a cup) deserves the most caution; a plain interior or under-glaze decoration is generally much safer.

Also: the firing

Even with acceptable materials, if the firing temperature is too low or the time too short, the glaze doesn't fully vitrify and the metals aren't locked in — so they leach more easily. Some small factories don't control lead and cadmium well, and that's how limits get exceeded.³

When does it leach into food?

Even if a dish contains lead or cadmium, it won't necessarily migrate into food. What drives it out (the more factors, and the stronger, the higher the risk):

• Acidity: vinegar, lemon, tomato, fruit juice, yoghurt, soda — acid leaches more lead/cadmium.
• Heat: hot soup, hot drinks, microwaving.
• Long contact: storing food (especially acidic) in ceramic for extended periods.
• Scratches and wear: once the surface is scratched, worn or crazed, the protective layer is broken and more leaches out.¹

What does it do if ingested?

Both lead and cadmium are cumulative — the concern is long-term, low-level intake:

• Lead: accumulates in the nervous system and interferes with children's brain and cognitive development — kids and pregnant women are most vulnerable.
• Cadmium: accumulates in the kidneys, harming kidney and bone over time; the International Agency for Research on Cancer (IARC) classifies cadmium and its compounds as a Group 1 carcinogen.⁴

How parents can choose and use ceramics

1. Plain food-contact surfaces: for the inside of bowls and cups, prefer white/undecorated; if you want a pattern, choose under-glaze (design sealed beneath the glaze, smooth to the touch).
2. The brighter, the warier: especially bright yellow/orange/red/green decoration on the interior or cup rim of a rock-bottom-price item.
3. Prep new ware: before first use, boil for 5 minutes or soak in white vinegar for 2–3 hours to remove most surface residue.³
4. Don't store acidic food long-term in ceramic; replace pieces that are scratched, faded or crazed.
5. Microwave only what's marked microwave-safe; don't microwave undecorated-status-unknown painted ceramics.
6. Buy certified, reputable brands — products meeting food-contact standards (e.g. China's GB 4806.4, EU 84/500/EEC) have regulated lead/cadmium migration.

A related story: cheap "melamine" bowls — formaldehyde, not metal

While we're on heavy metals, one mainland report recently resurfaced and spread widely: China's CCTV exposed how cheap "imitation-porcelain" bowls (melamine ware) release formaldehyde when heated. Note the key point — melamine ware isn't ceramic. It's a plastic (melamine-formaldehyde resin, made by polymerising melamine and formaldehyde), an entirely different material and hazard from the lead and cadmium above.⁵

CCTV's testing found that a low-quality bowl costing about 3 yuan released nearly 8× the formaldehyde limit once hot oil was added, while better bowls (~10 yuan) with a stated heat-resistance rating released very little. The problem ware often uses cheap urea-formaldehyde resin that starts breaking down at around 100°C — enough for hot soup alone to release formaldehyde.⁵

So the same caution applies to cheap, unlabelled, brightly-coloured "shatterproof melamine bowls" (often the kind given to kids): don't use them for hot soup or hot oil, don't microwave them, and pick reputable brands with a stated heat rating. The same Consumer Council round that tested ceramic cups and bowls rated 17 items 5-star with no detectable lead or cadmium — a useful reference list.¹

The bottom line

Trace lead or cadmium in ceramics doesn't mean "unusable" — proper, plain, standard-compliant dishes are safe for daily use. What to remember: the metals come from the glaze and the paint, and the risk rises with brighter colour, lower price, and decoration closer to the food. Choose plain interiors, boil/soak new pieces, and don't store acidic food — a few simple moves clear most of the risk.

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