Nanometer-Scale Imaging Used to Study Coating of Stradivari's Violins

Antonio Stradivari (1644 – 1737) crafted string instruments—violins, in particular—that are considered masterpieces with a special sound quality and high value as collector’s items. Some of their special properties have been attributed to finishing and coating treatments. Thus, there have been research efforts to try and characterize the coating of Stradivari’s instruments. While the varnishes used have been well-explored, less is known about any treatments applied before varnishing. There have been indications that there is protein-based material on the wood surface—possibly from a ground coat, but there is no consensus on this coating.

Lisa Vaccari, Elettra Sincrotrone Trieste S.C.p.A, Trieste, Italy, Marco Malagodi, Università degli Studi di Pavia, Cremona, Italy, and colleagues have investigated the material used for the preparation of the ground layer in Stradivari violins. The team used microsamples from two precious violins, named the “Toscano” (1690) and the “San Lorenzo” (1718). In both samples, the team first identified three layers based on their intrinsic fluorescence: the varnish, a preparation layer, and the wood. However, the coating layers are very thin and challenging to distinguish.

The researchers then used infrared scattering-type scanning near-field optical microscopy (IR s-SNOM) to further analyze the samples and obtain a better understanding of the chemical composition of the layers. This method provides a high resolution. It uses an atomic force microscopy (AFM) probe tip, which can read the sample topography, and can measure light scattered by nanometer-sized regions directly under the probe tip.

The results of this method, compared with known spectra, showed that the layer between the wood and varnish in both violins contains protein-based compounds. According to the researchers, the results for these two violins cannot be generalized, but the work shows that IR s-SNOM could be useful in future studies of very small samples from historical objects.