The role of iron oxide in the formation of silver and copper luster layers

Luster is a composite material consisting of one or several micrometric layers of silver, cuprite and/or copper nanoparticles embedded in a glassy matrix. Its colorful, metallic and iridescent appearance is due to its quantum-confined optical response. Luster is produced by the ionic exchange of Ag + and Cu + ions from a precursor for the alkali ions from the glaze and reduction to their metallic state. Luster precursor-paints are composed of copper and silver salts, as well as sulfur compounds. It has long been described that benefits of incorporating Fe 2 O 3 into the precursor-paint. While the beneficial role of Fe 2 O 3 has not been verified nor understood. By producing and analyzing a series of luster layers obtained with various copper/silver ratios, with/without Fe 2 O 3 , and with/without a reducing stage during firing, we have revealed the role of Fe 2 O 3 . Our results show that it produces an iron sulfate melt which increases the silver uptake but not the copper uptake. Copper facilitates the nucleation and growth of silver nanoparticles, as copper is oxidized while reducing silver to metal. Furthermore, Fe(II), which is present in the glass substrate, acts as an additional reducing agent, facilitating the formation of the nanoparticles. Finally, copper nanoparticles only form in pure copper lusters.