Comparison of historical violins by non-destructive MRI depth profiling

Bernhard Blümich,Maria Baias,Christian Rehorn,Valeria Gabrielli,Denis Jaschtschuk,Colin Harrison,Claudia Invernizzi,Marco Malagodi

doi:10.1016/j.microc.2020.105219

Abstract

The back plates of master violins at the Ashmolean Museum in Oxford and the Museo del Violino in Cremona were analyzed by portable magnetic resonance imaging with the NMR-MOUSE in terms of depth profiles. Distributions of transverse NMR relaxation times were recorded versus depth at points of small curvature across the violin back plates providing insight into the physical states of the materials. Their analysis reveals the effects of surface treatments, wood treatment, and possible wood degradation. While conclusions about the type of treatment cannot be drawn, the profiles report surprising differences between the violins from the museums in Oxford and Cremona.

Highlights

The art of making todays violins is largely defined by the violin makers of Cremona beginning in the first half of the 16th century [1,2]
Moisture affects the hydrogen bonds connecting the hemicellulose of the wood to the lignin, so that deformation and ambient humidity are coupled on the time scale of hours leading to mechano-sorptive creep to which an instrument is exposed during concerts, practice and travel [8,9,10]
While the profiles of the violins studied from the Hill collection are inconspicuously straight and report mostly pristine wood apart from the signature of outer surface treatments in some cases, the profiles from the analyzed Cremona violins are wrinkled, bearing the signature of surface treatments, degradation of amorphous cellulose and treatments of the outer and of the inner surfaces

Summary

Introduction

The art of making todays violins is largely defined by the violin makers of Cremona beginning in the first half of the 16th century [1,2]. Non-destructive analyses of violin materials often focus on analyzing the composition and thickness of the varnish and ground layers [13,14,15,16,17,18,19,20], and the way they affect the sound properties while the state of the wood and how its material properties are affected by age, use, maintenance and restoration are more difficult to characterize nondestructively [8,9,10] This is believed to be a consequence of the fact, that common non-destructive analytical techniques like ultra-violet (UV) induced fluorescence, optical coherence tomography (OCT), and infrared (IR) reflectance spectroscopy are surface techniques, which typically can penetrate only up to 100 μm of the instrument surface [13,19], while 3D techniques with larger fields of view like neutron imaging (NI) [21] and micro-computer tomography (μCT) [20,22,23] primarily provide dimensional and density information, even though NI is sensitive to moisture and resins [23] and CT to glue [22,24], and, in principle, to crystallinity. Their origin cannot be explained by the NMR measurements alone and warrants further studies by different techniques to correlate these findings with the individual histories of these instruments

Materials

NMR measurements

Comparison of depth profiles from the Cremona and Oxford violins

Summary and conclusions