Late production of Egyptian blue: synthesis from brass and its characteristics

Abstract

This work follows the recent discovery of a zinc-bearing Egyptian blue (EB) pigment widely used for the production of the early medieval mural paintings cycle in Santa Maria foris portas Church at Castelseprio (Lombardy Region, Italy). The inclusion of zinc in the synthesis of EB has been studied for the first time trying to evaluate whether its addition could be casual or deliberate. Historical reconstructions of the pigment have been carried out with a special focus on the use of zinc besides copper, using the different production methods. The influence of zinc on the pigment’s NIR photoluminescence and VIS-NIR reflectance has been characterized using FORS spectroscopy, X-ray diffraction, optical microscopy, and scanning electron microscopy-energy dispersive X-ray spectroscopy. A comparison of the production methods including salt-flux, solid-state, and Zn-rich syntheses showed that the solid-state synthesis results in particularly efficient NIR photoluminescence and VIS-NIR reflectance. Modern replicas were compared with an ancient sample in order to understand the zinc environment inside the structure of the Zn-enriched EB. Zn was found to be concentrated in a glass-based matrix surrounding cuprorivaite crystals, the main mineral associated with the EB pigment, and not included in a hypothetical Zn-doped cuprorivaite with formula CaCu1−xZnxSi4O10. The Zn-rich synthesis opens up the possibility of producing EB from brass and demonstrates that EB used in Castelseprio’s mural paintings could have been produced in this way. The relationship between the microstructure and the NIR photoluminescence of cuprorivaite-like pigments is of interest also for applications in modern and future technologies.