Non-Invasive and Micro-Invasive Characterization of the Tempera Painting Cristo in Trono (Amalfi, SA, Italy) by Domenico Morelli and Paolo Vetri: A Multivariate Statistic Approach Applied to Spectroscopic Data

Non-destructive diagnostic approaches play a crucial role in cultural heritage studies by enabling the assessment of artworks’ conservation conditions without causing damage. This work presents the case study of the tempera painting Cristo in Trono, by Domenico Morelli and Paolo Vetri, located in Amalfi (SA, Italy). A multi-analytical approach combining portable and non-invasive techniques was employed. Hygrometric tomography revealed anomalous surface moisture values (up to 23 units) along the edges and wooden joints, identifying areas requiring particular attention during conservation. Infrared reflectography (IR-R) detected preparatory drawings, compositional changes, and evidence of previous restoration interventions. X-ray fluorescence spectroscopy (XRF) identified the elemental composition of the pigments, indicating the use of cinnabar (or vermilion), ultramarine (or lapis lazuli), ochres, lead white (Biacca), and brass (Orone) for the golden background. Multivariate statistical analysis of the XRF data (PCA and HCA) revealed compositional heterogeneity and chemically distinct areas within apparently similar chromatic regions. Micro-invasive analyses complemented these results by providing information on the stratigraphy and material composition. Fourier-transform infrared (FT-IR) spectroscopy identified polyvinyl acetate (PVA), attributed to previous restoration treatments and associated with the yellowing of lead white, together with protein-based binders. Microscopic observations revealed the woven structure of the support, pigment-layer cracking, and degradation features, while the interaction of the brass with atmospheric agents and marine chlorides was found to be responsible for the alteration of the original golden background to a greenish-gold hue. The lower part of the canvas exhibited the most severe deterioration, resulting from higher moisture levels, unsuitable past restoration treatments, and prolonged exposure to humidity, atmospheric pollutants, and marine aerosols, highlighting the importance of integrated diagnostic investigations for planning effective conservation and restoration strategies.