Glaze

In ceramics, glaze is a thin glass coating that seals the piece against air and water. The beauty it adds is really secondary, but for the artist it’s integral.

As with window glass, ceramic glaze begins with silica as the glass forming agent. But because silica requires much hotter temperatures to melt than the clay itself, glaze also contains chemicals such as boron or lithium to act as a flux to trigger melting at a lower temperature. Unfortunately for potters, the demand for lithium-ion batteries has made raw lithium so expensive that many are have abandoned some of their favourite glaze recipes.

Next, glazes call for small additions of clay to stabilize it and prevent it from sliding off the pottery. Finally come mineral colourants, which can be the most interesting components of the final product. If you see blue, cobalt is often responsible. Chrome and copper give us green. Iron for brown, manganese for black and metallic sheens, vanadium for yellow and tin for white.

Glaze chemistry can be fun (or maddening). For example, tin exposed to chrome vapours blushes pink, as does lithium to cobalt, while copper fired in the absence of oxygen turns blood red. Magnesium can make a glossy surface matte, and titanium can make a transparent glaze opaque.

The takeaway is knowing that the colours of your ceramics are neither a mystery, nor the result of inscrutable laboratory chemistry. Pottery glaze is made from relatively simple, common minerals dug from the ground, as it has been for thousands of years. And now you can recognize many of these minerals in the pottery you use.

Detail of matte glaze with dolomite speckles

Detail of flowing pink and blue glazes.

Detail of glossy red and white crater glaze.