Two centuries of painted plasters from the Lateran suburban villa (Rome): investigating supply routes and manufacturing of pigments

Abstract

• Combined non-destructive analytical approach using portable instrumentation • Complementary techniques and multivariate analysis allow characterization of pigments • Improved knowledge on pigments production, sourcing and use in 27 BC – 193 AD Rome Polychrome painted plasters from a suburban villa of Imperial epoch were recovered during the 1930s, and are currently exhibited in the galleries underneath the St. John in Lateran Church, Rome. As wall paintings from the Rome area are a rare find, and as the fragments were never subjected to conservation treatments, it was chosen to non-invasively characterize 113 coloured areas, representative of colours and decorative systems. The chemical and mineralogical information collected from this measurement campaign were interpreted in the light of archaeological context (i.e. the high status of the family who owned the villa , the different renovation works that affected it, the identification of decorative systems linked to such works), as well as of published data from contemporary wall paintings in the Rome area. A multi-analytical, non-destructive characterization of the painted surfaces was carried out in situ by means of portable Raman spectroscopy (pRS) and X-ray fluorescence (pXRF), in the framework of a wider project concerning the study of raw materials and paintings of archaeological interest. The combination of pXRF and pRS allowed to characterize the whole palette, as well as to discriminate among different pigments sharing the same key element, and to identify degradation issues. In this context, the use of principal component analysis gave an important contribution to the interpretation of data ant to the individuation and characterization of pigments such as ochres, green earths, lead based pigments, Egyptian blue, cinnabar, Tyrian purple. Calcium and magnesium carbonates are present, as expected, in the plaster, and are often associated with their sulphates and nitrates, and Ca-oxalates indicating degradation. Lead white and bone white have been used. Some peculiar yellow pigments have been identified, such as realgar/pararealgar and Naples’ yellow, in addition to ochres. Hematite, minium and cinnabar were all identified, alone or in mixtures. The presence of the precious dye Tyrian purple is very likely, but could not be confirmed univocally. Green earths and complex mixtures of iron and copper compounds were detected. Egyptian blue was used, highlighting different production processes for this artificial glassy material. For the blacks, we hypothesize the use of carbon-based pigments. The obtained data confirms the general palette used by Roman painters, and sheds new light on the materials used in the Lateran area across two centuries, including their provenance and manufacturing. It appears that different earths and ochres were used diachronically, and that the owners could afford very expensive pigments, such as cinnabar and Tyrian purple.