Infrared microscopy plays first fiddle to solve a violin mystery

Stradivarius violins produce elegant sound with a level of clarity that is unparalleled by modern instruments, according to many musicians. An 18th-century Stradivarius sold for $15 million in June. In the same month, news reports estimated that a 17th-century Stradivarius could fetch $11 million at auction.

Be it the brilliance of the violins’ music, or another allure, experts in the sphere of string instrumentation hold these crown jewels of Italian craftsman Antonio Stradivari’s portfolio in the highest regard. The high quality of a Stradivarius violin’s music has led to various hypotheses regarding the reasons, including a belief that reduced solar activity in the 17th century contributed to slower tree growth that, in turn, resulted in denser wood. Another idea suggests that the distinct elongation of the F-hole carved into the face of the instruments could have a favorable effect on sound.

Courtesy of Analytical Chemistry.

Stradivari also administered a hidden coating beneath the varnish of some of his violins to fill in and smooth out the wood, which would influence the sound, according to yet another theory about how his instruments achieve their excellent sound.

Knowing the components of this coating could help to solve the centuries-old mystery involving the historic instruments, and could even make replicating them a possibility. A group of researchers in Italy endeavored to understand the physical makeup of Stradivarius violins by focusing on the finishing touches that the luthier added to them.

The researchers studied two violins: the San Lorenzo 1718 and the Toscano 1690. Drawing from previously published research results that investigated the violins’ physiochemical characterization, the researchers this time turned to Fourier transform infrared (FTIR) imaging. By using synchrotron radiation FTIR spectromicroscopy, the team found that both violin samples had an intermediary layer between the wood and the varnish. However, the examination method did not enable them to differentiate the layer’s composition from the adjacent wood.

For this analysis, the researchers used infrared scattering-type scanning near-field microscopy (IR s-SNOM). The apparatus includes a microscope that collects images tens of nanometers wide and measures the infrared light scattered from the coating layer and the wood. Information is then collected about the materials’ chemical composition.

The results obtained showed that the layer between the wood and varnish of both instruments contained protein-based compounds that congregated in nano-size patches. Because IR s-SNOM provided a detailed 3D picture of the types of substances on the violin’s surface, the researchers said, the microscopy method could also be used in future studies to identify compounds in complex multilayer cultural heritage samples.

In other words, photonics once again hits all the right notes for scientists who are tasked with deepening our cultural knowledge of the aesthetic and acoustic features of treasured instruments.