Understanding the Influence of Copper on the Color of Glasses and Glazes: Copper Environment and Redox

This study explores the influence of copper on the color properties of lead and aluminosilicate glasses by using optical and electron paramagnetic resonance (EPR) spectroscopies. Optical absorption spectra unveil distinct UV absorption characteristics in blue and green compounds, attributed to Cu+ ions, with notable variations depending on glass composition. EPR quantification of copper oxidation states reveals correlations with color variations, particularly evident in UV absorption shifts towards green colors at lower Cu2+ ratios. Redox analysis elucidates color differences in identical compositions subjected to different melting temperatures. Additionally, XANES spectroscopy highlights the role of copper proportions in modulating redox balance. Cu2+ site distortions, assessed through Gaussian fitting of optical absorption spectra and EPR simulations, present challenges in determining the impact of site geometry on color. The presence of copper clusters, inferred from broad absorption bands possibly due to Cu2+-Cu+ intervalence charge transfer (IVCT), suggests their contribution to coloration. The apparition of this IVCT contribution appears as the main reason for the sift in coloration from blue to green as the CuO content increases. Overall, the study emphasizes the multifaceted relationships between copper redox state, site distortions, and clustering phenomena in influencing color perceptions in silicate glasses.