Last reviewed: April 2026
Why Glaze Safety Matters
Ceramic glazes are glass-forming mixtures applied to pottery before a high-temperature firing. They create a smooth, impervious surface that makes the ware functional for holding food and liquids. However, certain glaze ingredients — particularly compounds containing lead or cadmium — can migrate into food and drink if the glaze is improperly formulated or under-fired. Long-term exposure to elevated levels of these metals has been linked to kidney damage, high blood pressure, and developmental issues in children.
Regulatory Framework in Singapore
The Singapore Food Agency (SFA) requires that all ceramic tableware imported, sold, or used in the country complies with specific migration limits (SMLs) for heavy metals under the Food Regulations. These limits align with international standards such as ISO 6486, which specifies permissible lead and cadmium release from ceramic and glass dinnerware intended for food contact.
SFA conducts periodic testing of ceramic tableware on the market. Manufacturers and importers are expected to ensure compliance before their products reach consumers. For hobbyist potters producing functional ware, the same principles apply: pieces intended for food use should be glazed and fired in a way that meets these safety thresholds.
Types of Glazes and Their Risk Profiles
Underglaze Decoration
Underglaze involves applying coloured pigments directly onto bisque-fired clay, which is then covered with a transparent glaze and fired again. Because the pigment layer sits beneath a sealed glass surface, migration risk is minimal. Underglaze-decorated ware is widely considered the safest option for everyday tableware.
In-Glaze Decoration
In-glaze application places pigments on top of an unfired glaze layer. During firing, the pigments sink into the glaze and become partially encapsulated. Safety levels are comparable to underglaze in most cases, though vibrantly coloured patterns fired at lower temperatures may not fully encapsulate the pigments.
Overglaze Decoration
Overglaze enamels are applied on top of an already-fired glaze and re-fired at a lower temperature (typically 700 – 850 °C). Gold and silver lustre bands on teacups are a common example. Because overglaze sits on the surface and is fired at lower temperatures, it has a higher risk of leaching. The SFA recommends avoiding overglaze decoration on surfaces that come into direct contact with food or lips.
Practical Notes for Hobbyist Potters
- Use commercially tested glazes that are explicitly labelled as food-safe and lead-free. Many Singapore studios supply house glazes that meet these criteria.
- Fire to the full temperature range recommended by the glaze manufacturer. Under-firing leaves the glaze incompletely melted, increasing the likelihood of leaching.
- Avoid mixing glaze recipes without understanding the chemistry involved. Some colorants (copper, manganese, barium) can be problematic if present in high concentrations or if the glaze matrix does not properly contain them.
- Test functional pieces by filling them with a mild acid (lemon juice or vinegar) overnight. Visible colour change in the liquid suggests migration that warrants further investigation.
- Decorative or sculptural pieces that are not intended for food contact do not need to meet food-safety standards, but should still be handled sensibly (e.g. not used for storing drinking water).
Kiln Atmosphere and Glaze Behaviour
Glazes behave differently depending on whether the kiln fires in an oxidation or reduction atmosphere. In oxidation (the standard for electric kilns), the kiln has a steady supply of oxygen. In reduction (used in gas kilns), oxygen is restricted partway through the firing, which alters the colour development of certain glazes, especially those containing iron and copper.
Reduction firing can produce distinctive effects — celadon greens, iron-red tenmoku, copper-red flashing — but it also makes glaze behaviour less predictable. Potters experimenting with reduction should test each batch for food safety separately, as the altered atmosphere can change the way metals are held within the glaze matrix.