Invasive and non-invasive analyses for knowledge and conservation of Roman wall paintings of the Villa of the Papyri in Herculaneum

Abstract The Villa of the Papyri is one of the most imposing architectural examples of Herculaneum dated before the eruption occurred on 79 A.D. During the earliest years of the excavation of the site (18th century), the Villa has been explored through a thick network of tunnels dug into the hard bank of tuff rock. Recent excavations went on discontinuously between 1990s and 2008, until the discovery of a room located in the lower floor of the building, showing serious phenomena of colour change of some areas of the painted walls. Some dark green parts of mural paintings after volcanic mud removal became white and powdered, detaching from the plaster. During the last restoration works a scientific approach became necessary in order to acquire detailed information about the wall painting technique and the degradation phenomena occurring in some decorative elements in the vault of the Basis Villa (a room in the lower floor of the building); it was established to perform a scientific recognition. Integrated investigations – carried out by optical microscopy, scanning electron microscopy with X-ray microanalysis (EDS), X-ray diffractometry (XRD), ion chromatography (IC), FT-IR spectroscopy, gas chromatography mass spectrometry (GC-MS) – had primarily the goal to characterize the materials used for the realization of the these impressive wall paintings, as well as to assess their state of conservation. A second aim of the research was to test the reliability of non-invasive analyses that are sometimes performed in situ to study mural paintings, like reflectance spectrometry in the visible range (vis-RS), X-ray fluorescence (ED-XRF) and micro-Raman spectroscopy, as well as imaging analyses like false colour IR (IRC) and visible induced IR luminescence (VIL). The whole set of analyses allowed to identify a typical Vitruvian plaster with the following pigments: Egyptian blue, red and yellow ochre, goethite and hematite particles, green earth and carbon black particles. The high content of soluble salts, which is related to the sulphur-rich water in the site located few metres above water level in the walls and floors, caused a worsening in the state of conservation of mural paintings.