Ancient Roman wall paintings mapped nondestructively by portable NMR

The stratigraphies of decorated walls in ancient Herculaneum, Italy, were analyzed by single-sided (1)H NMR. A large version of the NMR-MOUSE® with a maximum penetration depth of 25 mm was used to map proton density profiles at different positions of the Mosaic of Neptune and Amphitrite showing considerable differences between different tesserae and the mortar bed at different times of the year. In the House of the Black Room, different mortar layers were observed on painted walls as well as different proton content in different areas due to different moisture levels and different conservation treatments. The proton density profiles of the differently treated areas indicated that one method leads to higher moisture content than the other. Untreated wall paintings from different times were profiled in a recently excavated room at the Villa of the Papyri showing two different types of mortar layer structures which identify two different techniques of preparing the walls for painting. Reflectance Fourier mid-infrared spectroscopy and in situ X-ray fluorescence measurements complemented the NMR measurements and provided additional insight into the identification of organic coatings as well as the nature of the pigments used, respectively. The information acquired nondestructively by NMR is valued for elaborating conservation strategies and for identifying different schools of craftsmen who prepared the mortar supports of the wall paintings.