What to look for in a ceramic glaze for daily use
Most people choose a ceramic piece for how it looks and think about the glaze only if something goes wrong. That's understandable. A well-made bowl is primarily a visual and tactile object. But the glaze is also a functional layer, and understanding a few basics about it will help you buy more confidently and look after what you own.
What food-safe actually means
In Canada, food-safe ceramics are tested for lead and cadmium leaching under Health Canada guidelines. A glaze can be food-safe at room temperature and still leach trace amounts of certain compounds when subjected to repeated high-heat cycles. Which is why 'dishwasher-safe' and 'food-safe' are not always the same claim.
When we source ceramics for the shop, we ask for batch-level test documentation. For the Okanagan stoneware line, the ceramicist fires to cone 10 in reduction, which produces a fully vitrified body with very low porosity. The glaze is also fully matured at that temperature, meaning there are no partially dissolved compounds sitting at the surface.
Ash glazes: what makes them different
Ash glazes have been used in East Asian ceramics for centuries and have been adopted by a growing number of North American studio potters. The ash acts as a flux. It lowers the melting point of the silica in the glaze and creates a surface that's simultaneously glassy and slightly textured. The result is a finish that catches light differently depending on the angle.
Because the ash composition varies between batches (see our earlier journal post on this), the colour range of an ash glaze is wider than a commercial glaze. This is not a defect. It's the material responding honestly to its inputs.
Signs of a well-made glaze
- Even coverage with no bare patches or crawling (where the glaze has pulled away from the clay body during firing)
- No crazing (fine cracks in the glaze surface) unless it's intentional and the piece is decorative only
- A foot ring that's unglazed or wiped clean. Glaze on the foot causes pieces to stick to kiln shelves and often indicates rushed production
- Consistent wall thickness when you hold the piece up to light
- A sound that rings cleanly when tapped, not a dull thud. The latter suggests underfiring
Caring for glazed stoneware
Stoneware is. Dishwasher, microwave, oven. Most fully vitrified stoneware handles all three. The exception is thermal shock: don't move a cold piece directly into a very hot oven, or pour boiling water into a piece that's been in the refrigerator. The clay body can absorb that kind of stress occasionally, but repeated thermal shock will eventually cause cracking along any existing micro-stress lines.
For ash-glazed pieces specifically, the surface will develop a slight sheen over time from contact with oils in food. This is normal and doesn't indicate any problem with the glaze.
The Okanagan stoneware we carry is currently the only ceramics line in the shop. We're looking at a second maker for next spring, but we haven't committed to anyone yet. It takes time to verify the process properly.