Cobalt Oxide

Cobalt oxide (CoO) is the strongest colorant commonly used in ceramics. Just half a percent produces a visible blue, and 1 to 2 percent produces the deep saturated blues of traditional blue-and-white porcelain. For this reason, cobalt is also one of the most expensive colorants by weight.

Cobalt has been used in porcelain since the Yuan Dynasty (13th century), when Persian cobalt ore was imported along the Silk Road to produce the first underglaze blue decoration. By the Ming Dynasty, blue-and-white porcelain had become a dominant export, influencing ceramic traditions from Delft to Iznik to Mexico.

Cobalt is stable across virtually all firing temperatures, atmospheres, and glaze types, making it one of the most reliable colorants. It can be used as an underglaze paint, a glaze colorant, or a decorative wash over a base glaze.

In magnesium-rich glazes (high dolomite or talc), cobalt shifts from pure blue toward pink or purple, a phenomenon exploited in certain Japanese and Scandinavian traditions.

Typical usage: 0.25 to 0.5 percent for pale sky blues, 1 to 2 percent for deep classic blues, 2 to 4 percent for near-black blues. A little goes a very long way.

Mixing note: Cobalt is so strong that uneven mixing shows up as streaks and spots in the fired glaze. Always sieve cobalt-containing glazes through at least an 80-mesh screen.