Shedding light on the formation mechanisms of Islamic polychrome lusters: A Cu, Fe, Ag micro XANES study

Islamic luster is a ceramic glaze decoration exploiting the quantum-confined optical response of noble metallic nanoparticles to produce a colorful metallic and iridescent appearance. Upon firing, silver and copper initially present in an overpaint diffuse into the glaze and are reduced to Ag⁰ and Cu⁰ nanoparticles. This study examines the relationship between luster colors and paint composition. The six analyzed shards are 9th century Abbasid polychrome lusters exhibiting green, red, ochre, and brown colors. Key chemical factors are revealed by imaging 16 luster cross sections with different micro- and nanoanalytical methods, including UV-Vis spectroscopy, Ag L₃-, Cu-, and Fe K-edge micro x-ray absorption spectroscopy (μ-XAS), micro x-ray diffraction (μ-XRD), and field emission scanning electron microscopy (FIB-FE-SEM). These factors connect together (i) the Cu/Ag ratio; (ii) the Cu, Ag, and Fe speciation; (iii) the nature, size, and concentration of metallic nanoparticles; and (iv) the luster color. Notably, the Cu/Ag ratio balances the competitive reactions between Ag⁺ reduction and Cu⁺ disproportionation.