Complementary mapping techniques to characterize the wood finish of musical instruments

The wood finish of historical bowed string musical instruments made in Cremona (Italy) during the seventeenth and eighteenth century is a complex multi-layered coating system, where varnishes and other organic binders are variously mixed with inorganic fillers and pigments. It consists of several layers with reduced thickness (tens of microns or less), hard to be distinguished due to the similarity of the constituent materials. Nevertheless, the identification of chemical and morphological features (layering and boundaries) is strictly necessary to disclose the traditional manufacturing procedures. In this paper, we propose an innovative protocol to fully characterize such a multi-layered coating system by combining hyperspectral photoluminescence (PL) micro-imaging with μFTIR-ATR mapping and SEM-EDX analysis. The protocol has been employed to study three cross-sectional samples from violins made by Lorenzo Storioni (second half of eighteenth century), whereas a properly reproduced laboratory mock-up was used to set the analytical protocol. The obtained results demonstrate that the combination of these complementary spectroscopy mapping techniques in a high-resolution strategy allows one to clearly identify the morphology of a few microns thin layers, to assess the penetration depth of sizing treatments into the wood and to detect restoration areas.