Raman focal point on Roman Egyptian blue elucidates disordered cuprorivaite, green glass phase and trace compounds

The discussed comparative analyses of Roman Imperial pigment balls and fragmentary murals unearthed in the ancient cities of Aventicum and Augusta Raurica (Switzerland) by means of Raman microspectroscopy pertain to a predecessor study on trace compounds in Early Medieval Egyptian blue (St. Peter, Gratsch, South Tyrol, Northern Italy). The plethora of newly detected associated minerals of the raw materials surviving the synthesis procedure validate the use of quartz sand matching the composition of sediments transported by the Volturno river into the Gulf of Gaeta (Campania, Southern Italy) with a roasted sulphidic copper ore and a mixed-alkaline plant ash as fluxing agent. Thus, the results corroborate a monopolised pigment production site located in the northern Phlegrean Fields persisting over the first centuries A.D., this in line with statements of the antique Roman writers Vitruvius and Pliny the Elder and recent archaeological evidences. Beyond that, Raman spectra reveal through gradual peak shifts and changes of band width locally divergent process conditions and compositional inhomogeneities provoking crystal lattice disorder in the chromophoric cuprorivaite as well as the formation of a copper-bearing green glass phase, the latter probably in dependency of the concentration of alkali flux, notwithstanding that otherwise solid-state reactions predominate the synthesis.

Automated summary

The comparative analysis of Roman Imperial pigment balls and fragmentary murals unearthed in ancient cities in Switzerland reveals the presence of associated minerals in the raw materials and confirms the use of quartz sand sourced from the Volturno river and roasted sulphidic copper ore with mixed-alkaline plant ash as a fluxing agent. These findings support the existence of a monopolized pigment production site in northern Phlegrean Fields, as mentioned by ancient Roman writers and supported by archaeological evidence. Raman spectra also provide insights into the process conditions and compositional inhomogeneities, resulting in crystal lattice disorder and the formation of a copper-bearing green glass phase.