When creating functional ceramics, particularly pieces designed to hold or serve food and drink, choosing the right glaze is only part of the story. How the glaze is applied and fired, the clay body beneath it and even the kiln it is fired in can all affect the suitability of the finished piece for food contact.
So, what should ceramic makers consider?
Follow the Manufacturer’s Firing Instructions
The first step is always to follow the glaze manufacturer’s instructions for application, firing temperature and intended use.
A glaze should be applied at the recommended thickness. For example, the specified two or three coats for a brush-on glaze and fired within its recommended firing range.
It is important not to assume that reaching a particular cone or temperature automatically makes every glaze food safe. Although a glaze may physically melt at a lower temperature, firing below its intended maturation range can result in an underfired surface with different durability and chemical-resistance characteristics.
For functional ware, the aim should be a properly matured glaze and clay body, fired according to the manufacturer's recommendations.
Gloss, Matt and Surface Texture
A smooth, well-fired glaze surface is generally easier to clean than a rough, porous or heavily textured one.
Matt glazes aren't automatically unsuitable for food-contact surfaces. A correctly formulated and properly fired matt glaze can be suitable for functional ware. However, matt, textured or underfired surfaces can have more complex microscopic structures, so formulation, firing and testing become particularly important.
The appearance of a glaze alone whether glossy or matt should therefore not be used as proof that it is food safe.
Don't Forget the Clay Body
Food safety isn't just about the glaze. The clay body beneath it also matters, particularly its water absorption and degree of vitrification.
A well-vitrified body has very low porosity, making it considerably less absorbent than an underfired or porous body.
In the unfortunate event that the glaze surface becomes cracked or chipped, the clay body beneath may become exposed. If that body is porous, moisture and food residues can potentially penetrate into its microstructure, creating areas that are more difficult to clean effectively and where bacteria may develop.
A properly vitrified clay body provides an additional level of protection by significantly reducing water absorption and the ability of food residues to penetrate the ceramic body.
For functional ceramics, it is therefore important to consider both the glaze surface and the maturity of the clay body beneath it when assessing the finished piece for food contact.
Your Kiln Matters Too
Another factor that can sometimes be overlooked is the firing history of the kiln itself.
Modern glazes may be formulated without intentionally added lead, but studios should still consider what other materials have previously been fired in the kiln.
During firing, some glaze and decorating materials can release volatile compounds. Kiln refractories and furniture are porous and can retain material from previous firings, creating the potential for contamination in subsequent firings.
This is particularly worth considering with shared, educational and community kilns, where makers may have less control over the materials being fired. Particular care should be taken where a kiln has historically 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 is about the finished ceramic piece, not simply whether an unfired glaze contains a particular ingredient.
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.
This is an important distinction. A glaze described as unleaded or formulated without intentionally added lead isn't automatically guaranteed to meet all requirements once it has been applied to a particular clay body and fired under a particular set of conditions.
Clay, glaze, application, firing and kiln conditions all form part of the finished result.
The Best Way to Know? Test.
If you're producing functional ceramics particularly if you're making pieces commercially, independent testing of the finished ware is the most reliable way to verify its suitability for food contact.
Laboratories such as Staffordshire-based Lucideon offer testing of ceramic ware for the release of lead and cadmium under recognised test methods.
Testing the finished piece is important because it evaluates the combination that actually matters:
your clay + your glaze + your application + your firing + your kiln.
In Summary
For functional ceramics:
- Follow the glaze manufacturer's recommended application and firing instructions.
- Make sure both the glaze and clay body are properly matured.
- Don't assume that a particular firing temperature, a glossy finish or an "unleaded" glaze automatically makes a finished piece food safe.
- Pay particular attention to rough, porous or underfired surfaces.
- Consider the firing history of shared or community kilns.
- Where food-contact safety needs to be verified, have representative finished ware independently tested.
Ultimately, food safety isn't determined by the glaze in the pot alone.
It's the finished, fired ceramic piece that matters.