How can copper oxide produce turquoise-blue enamel?

In enamelwork, the creation of specific colors relies on the chemical interaction between metallic oxides and the base flux—a clear glass compound of flint or sand, red lead, and soda or potash.

To produce a turquoise-blue enamel using cupric oxide (the black oxide of copper), a craftsman must adjust the chemical composition of the flux rather than simply changing the quantity of the oxide itself. Specifically, turquoise-blue is achieved by incorporating a comparatively high proportion of carbonate of soda into the mixture.1

This process highlights how the final appearance of enamel is determined by the precise balance of components within the molten flux during the fusion process.1