Behaviour of copper and lead as chromophore elements in sodium silicate glasses

Analytical investigations of blue/green glasses used in Middle Age wall mosaics in Venice have shown that the role of lead concentration is fundamental to the green tonality in the tesserae. In this work, copper containing lead silicate glass samples have been studied by changing lead (II) oxide concentration in order to highlight the role of lead (II) in creating changes of hue in glass samples containing copper, to understand the behaviour of copper and lead as glass chromophore elements and, finally, to define the mechanism driving the phenomenon (chromatic synthesis, variation of the copper chemical coordination or production of mixed-valence ions). Our data show that glass samples having only lead oxide or only copper oxide appear, respectively, transparent white and intense blue, while glasses containing low percentages of copper oxide are turquoise-blue with a concentration of lead oxide below 40% and emerald-green with a higher percentage of lead oxide (about 60%). Nine coloured glass samples, prepared by mixing finely ground sodium silicate glass with the same copper (II) oxide weight percentage (2%) and with variable concentrations of lead oxide (4, 8, 12, 20, 40, 60, 70, 90%), were examined using several analytical methods.