Characterization of ultramarine blue in Roman wall paintings: case study from Volsinii (Bolsena, Italy)

The identification of blue pigments in Roman wall paintings provides crucial insights into the materials, artistic techniques, and trade routes in the antiquity. The present study focuses on the analysis of a blue pigment lump discovered at Ponticello (Bolsena, Italy) initially thought to be Egyptian blue. The lump pigment is compared with two synthetic ultramarine pigments (Poggi and Divolo), "Herculaneum blue," Egyptian blue, and natural lapis lazuli from Monte Somma, Italy. Through a multi-analytical approach that comprehends energy-dispersive X-ray fluorescence (ED-XRF) spectroscopy, micro-Raman spectroscopy, and X-ray powder diffraction (XRPD), the pigment was univocally identified as natural lazurite, the primary component of lapis lazuli. Chemometrics, including principal component analysis (PCA), was applied to the ED-XRF data to reveal patterns in the elemental composition of the samples. This finding is significant as ultramarine blue was a rare and expensive material in Roman times, which was traditionally sourced from distant regions like Afghanistan. While this study does not aim to determine the geographical provenance of the pigment, it offers important insights into Roman artistic practices, economic resources, and trade networks. Future research will explore the chemical composition of the pigments present in the frescoes from the archaeological area of Volsinii to provide important further insights into ancient material exchanges.