From tiny to immense: Geological spotlight on the Alexander Mosaic (National Archaeological Museum of Naples, Italy) using non-invasive in situ analyses.

A key challenge in the art and archaeological field is the instrumental analysis of objects and materials while preserving their integrity. In this study, the world-renowned artwork Alexander Mosaic (The Issus Battle, collection of the National Archaeological Museum of Naples, IT), the most iconic representation of the face of the Macedonian king Alexander the Great coming from a Pompeii domus, was thoroughly analyzed with mobile and non-invasive methods, within a great project of restoration started in 2020. Representative areas of the Mosaic, overall consisting of ca. two million of tesserae, was studied by in situ videomicroscopy, infrared thermography (IRT), multispectral imaging, portable X-ray fluorescence (pXRF), Fourier transform infrared (FTIR) and Raman spectroscopy. Ten tesserae colors were discriminated, and hypotheses on their geological provenances are proposed. Plasters, mineral components and other substances of old protective materials were characterized. The information obtained with this approach paved the way to knowledgeable restoration.