Violin varnish induced changes in the vibro-mechanical properties of spruce and maple wood

Abstract Violin varnishes are known to alter the longitudinal and radial material properties of violin tonewood. Here, the varnish-induced changes in the vibrational and elastic properties of spruce and maple wood were studied by modal analysis of rectangular plates. This contact-free investigation yields the eigenfrequencies and corresponding mode shapes and modal damping ratios of multiple out-of-plane modes. The longitudinal and radial E-modulus and the in-plane shear modulus were determined by an inverse material property determination process. The influence of traditional European varnish materials and different coating build-ups, comprising alcohol-based as well as oil-based varnishes, was studied during their application and for several months during drying and aging of the multi-layered systems. The evolutions of the changes induced by the different varnishes were rather similar and, except for modal damping, less pronounced for maple than for spruce. For the latter, the longitudinal E-modulus was slightly decreased while the shear modulus and radial E-modulus were increased after 18 months. The strong increases in modal damping ratios shortly after applying the coating systems (up to 150%) were reduced over time but remained significant for the time-span studied. However, also small differences between the influence of different coating systems and varnish materials on the amplitude of the induced changes became apparent.