Choosing a food safe ceramic glaze is only part of making pottery suitable for food and drink. The clay body, glaze application, firing and kiln conditions all affect the finished piece.
For potters and studios making functional ware, these are the key points to consider.
Follow the Manufacturer's Firing Instructions
Start with the glaze manufacturer's guidance on application, firing temperature and intended use.
Apply the recommended thickness. For a brush-on glaze, this may mean two or three coats, depending on the product. Fire within the recommended range.
Reaching a particular cone or temperature does not automatically make every glaze food safe. A glaze may melt below its intended firing range without fully maturing. This can affect its durability and resistance to chemicals.
For functional ware, both the glaze and clay body should be properly matured. Follow the manufacturer's recommendations for each.
Check the Glaze Surface
A smooth, well-fired glaze is generally easier to clean than a rough, porous or heavily textured surface.
Matt glazes are not automatically unsuitable for food contact. A correctly formulated and properly fired matt glaze can be suitable for functional ware.
However, matt, textured and underfired surfaces can have more complex structures at a microscopic level. The glaze formulation, firing and testing are therefore particularly important.
A glossy or matt finish alone is not proof that a glaze is food safe.
Consider the Clay Body
The clay beneath the glaze matters too. Two important factors are water absorption and vitrification: how dense and non-porous the body becomes during firing.
A well-vitrified clay body absorbs much less water than an underfired or porous body.
If the glaze cracks or chips, it may expose the clay beneath. Moisture and food residues can enter a porous body, making it harder to clean effectively and potentially allowing bacteria to develop.
Proper vitrification reduces water absorption and limits the penetration of food residues. When assessing functional ware, consider both the glaze surface and the maturity of the clay body.
Check Your Kiln's Firing History
What has previously been fired in your kiln can also affect the finished ware.
Even when a glaze is formulated without intentionally added lead, other materials used in earlier firings may still need consideration.
Some glazes and decorating materials release volatile compounds during firing. Porous kiln refractories and furniture can retain material from earlier firings, creating a risk of contamination in later firings.
This is particularly relevant to shared, educational and community kilns, where makers may have less control over the materials used.
Take particular care if a kiln has been used for lead-bearing materials, cadmium-bearing colours, lustres or other specialist ceramic finishes.
"Lead-Free" Doesn't Automatically Mean "Food Safe"
Food-contact safety concerns the finished ceramic piece, not just the ingredients in the unfired glaze.
In the UK, ceramic articles intended to come into contact with food are subject to requirements controlling the migration of lead and cadmium from the finished ware.
A glaze described as unleaded, or formulated without intentionally added lead, does not automatically guarantee that the finished piece meets all requirements.
The result depends on the combination of clay, glaze, application, firing and kiln conditions.
Food Safe Ceramic Glaze: The Importance of Testing
Independent testing of the finished ware is the most reliable way to verify its suitability for food contact. This is particularly important when producing functional ceramics commercially.
Laboratories such as Staffordshire-based Lucideon offer testing for the release of lead and cadmium from ceramic ware using recognised test methods.
Testing the finished piece assesses the combination you actually use: your clay, glaze, application, firing and kiln.
Key Points for Functional Ware
- Follow the manufacturer's application and firing instructions.
- Make sure both the glaze and clay body are properly matured.
- Do not rely on firing temperature, surface appearance or an "unleaded" label alone.
- Pay particular attention to rough, porous or underfired surfaces.
- Consider the firing history of shared or community kilns.
- Have representative finished ware independently tested where food-contact safety needs to be verified.
A food safe ceramic glaze is only one part of the process. It is the finished, fired ceramic piece that matters.
