The Sonic Imprint to identify and monitor precious artefacts: further developments

Abstract

A new non-invasive technique, developed to univocally identify and monitor the integrity of precious artefacts, like potteries, statues and objects, generally made of stone, metal or wood, has been already presented (2). This technique responds to the demand, especially felt among museum managers, of an art collection cataloguing which allows the identification of artefacts and the assessment of their physical conditions. This is also linked to the international loan of precious artefacts, which are then subjected to transport stresses and to the connected risks (damages, substitutions, physical deterioration, etc.). Furthermore, in these days many art exhibitions are itinerant, and the artefacts are continuously subjected to transport and/or displacement. Nowadays, the general characteristics for the identification of an object - such as its shape, dimension, aspect, colours and texture of the surface, etc. - are well imitable with use of laser technologies so that it is possible to obtain quasi - perfect copies of any artefact. Nevertheless, other physical parameters, dependent on internal non-observable properties (chemical composition, atomic structure, internal defects, etc.), are practically impossible to be cloned: for instance the distribution of resistivity, permittivity, dielectric constant, hydraulic permeability, etc. However, some mechanical parameters (e.g. density, elastic moduli, damping parameters) are very suitable for the unique identification of the objects and the monitoring of their integrity: therefore, the analysis of the propagation of elastic waves in the artefacts may allow the construction of a sort of DNA, called "Sonic Imprint", which identifies the objects and, eventually, establishes their mechanical consistency (or integrity). A way for studying the resonance and the damping of an artefact is to generate in it elastic waves by means of a suitable source (first tests have been done through a little gummed hammer) and then to measure the induced free damped oscillations by means of piezoelectric transducers put on the surface of the object. The vibrations are closely linked to the geometry of the artefacts as well as to the spatial distribution of its elastic parameters; then they present a significant variability even in presence of small structural modification of the object. Practically, any object presents a continuous distribution of vibrational modes and decay times, and then it oscillates with a superposition of vibrational modes of different frequencies: the distribution of the frequencies shows resonance peaks which are linked to the shape, dimensions and elastic parameters of the object itself. Furthermore, the eventual deterioration of the handmade artefact, e.g. caused by a crack, generates significant variations in its vibrational modes: these variations are easy to check comparing the Sonic Imprints acquired before and after the deterioration. Many experimental tests have been carried out demonstrating that the methodology is reliable and it is very suitable to distinguish artefacts which are practically indistinguishable to one from the other using only sizes, weight, colours, texture, etc. as identifying parameters. The late developments of this methodology include the construction of a new dedicated and standardized instrumentation.