Synchrotron radiation micro-computed tomography for the investigation of finishing treatments in historical bowed string instruments: Issues and perspectives

Coating systems in historical bowed string instruments are often multi-layered structures, where several inorganic and organic materials are variously combined. The methods to characterise such complex systems normally require samples to be detached from the artwork, whereas Synchrotron Radiation micro-Computed Tomography (SR-micro-CT) may reveal --through a non-invasive approach-- procedures and materials employed by the ancient violin makers. Since the application of SR-micro-CT for the investigation of the finishing layers in historical bowed string instruments is still unexplored, the experimental settings were optimized for the detection of the main features expected in the finishing layers of a historical instrument. In this work, two sets of mock-ups mimicking the finishing layers of historical instruments and a large fragment removed from a damaged cello by Andrea Guarneri were scanned. By considering SR-micro-CT data, and data obtained by previous micro-invasive analyses, the merits of reconstructed volumes and virtual slicing in investigating the layered structures have been highlighted and discussed. The developed procedures enabled the detection of the main morphological features of the overlapping layers, producing a valuable, non-invasive insight into the structure of the coating systems.