Copper in glazes of Renaissance luster pottery: Nanoparticles, ions, and local environment

TLDR

Recent studies show that luster decorations in historical pottery glazes contain copper and silver nanoparticles dispersed in glass, and the local environment of copper ions resembles that in copper–alkali ion‑exchanged silicate glass, indicating a similar ion‑exchange process. The study was performed nondestructively using Rutherford backscattering spectrometry, ultraviolet and visible spectroscopy, X‑ray fluorescence, and extended X‑ray absorption fine structure (EXAFS). Elemental analyses reveal that gold lusters contain silver and copper while red lusters contain only copper, with color determined mainly by metal nanoparticles—silver for gold and copper for red—and EXAFS shows copper in both types is mostly oxidized (Cu⁺ and Cu²⁺ with a predominance of Cu⁺), supporting a copper‑ and silver‑alkali ion exchange followed by nucleation and growth of metal nanoparticles.

Abstract

Following the recent finding that luster decorations in glazes of historical pottery consist of copper and silver nanoparticles dispersed in a glassy medium, the glaze in-depth composition and distribution of copper nanoparticles, copper ions, and their local environment have been studied in original samples of gold and red luster. The study has been fully carried out by nondestructive techniques such as Rutherford backscattering spectrometry, ultraviolet and visible spectroscopy, x-ray fluorescence, and extended x-ray absorption fine structure (EXAFS). Elemental analyses indicate that gold decorations are characterized by silver and copper, while red decorations by copper only. The color is determined mainly by metal nanoparticles. Specifically, silver nanoparticles determine the gold color, while the red color is determined by nanoparticles of copper. EXAFS measurements, carried out at the Cu K edge, indicate that in both gold and red luster copper is mostly the oxidized form (Cu+ and Cu2+) with a large prevalence of Cu+. States and local environment of copper ions are similar to those found in copper–alkali ion-exchanged silicate glass samples. This strongly supports the view that luster formation is mediated by a copper– and silver–alkali ion exchange as a first step, followed by nucleation and growth of metal nanoparticles.