Copper Oxide

Copper oxide (CuO, the black form) is one of the most magical colorants in a potter’s palette because of its dramatic response to kiln atmosphere. The same copper can produce completely different colors depending on whether the kiln burns clean or reduces.

In an oxidation firing, copper produces greens: bright emerald in alkaline glazes, muted olive in high-calcium glazes, and striking turquoise in sodium or lithium-rich glazes. Historic Egyptian faience and Islamic lusterware made extensive use of copper for these turquoise effects.

In a reduction firing, copper shifts dramatically into the reds: the famous “copper reds” of Chinese Jun ware and sang de boeuf (ox-blood) glazes. These range from pale peach-blossom pinks to deep oxblood reds, often with blue, purple, or green flashes. Copper reds are notoriously difficult to produce reliably, which is part of their mystique.

Copper is a strong flux as well as a colorant, so copper-rich glazes melt more fluidly than their colorless counterparts.

Safety note: Copper is a leaching concern in acidic food-contact glazes. Keep copper under 2 percent for food-safe surfaces, or use it only on decorative work.

Typical usage: 1 to 3 percent for greens in oxidation. 0.5 to 1 percent for copper reds in reduction. 3 to 5 percent for saturated metallic surfaces (non-food use).