Search

In questo lavoro viene descritta una metodologia non invasiva ne, tanto meno, distruttiva per studiare il degrado degli strati superficiali dei manufatti lapidei, correlandolo ai parametri di resistenza meccanica. La metodologia adopera come strumento diagnostico la propagazione delle onde elastiche nel mezzo in esame. In particolare, considerando che l'ordine di grandezza degli spessori ricercati è compreso tra le frazioni di millimetro e pochi centimetri, le frequenze delle onde elastiche utilizzabili sono quelle ultrasoniche, dalle decine di kHz ai MHz (e quindi, con velocità di propagazione dell'ordine di 4000 m/s, con lunghezze d'onda dai centimetri alle frazioni di millimetro). Questa metodologia, chiamata GTT (Global Tomographic Traveltime, e in italiano dromocrona tomografica globale), prevede di effettuare sul manufatto una serie di misure di velocità di propagazione di onde ultrasoniche su percorsi appositamente organizzati, in modo da valutare il livello di degrado e lo spessore dello strato superficiale alterato. Le misure dei tempi di propagazione infatti vengono riportate su opportuno grafico unidimensionale Tempo-Distanze (cioè la cosiddetta "dromocrona") e l'andamento dei punti sperimentali in detto grafico consente la valutazione dei parametri ricercati. La metodologia, che può essere considerata una semplificazione unidimensionale di un rilievo tomografico tridimensionale, e relativamente rapida per i tempi di esecuzione ed ha dato risultati interessanti, alcuni dei quali vengono presentati. Tuttavia rimangono alcune criticità, in ispecie ne]l'acquisizione dei dati dei cosiddetti "percorsi vicini".

Non invasive method used to detect a "sonic imprint" of three-dimensional objects, particularly suitable for the identification and monitoring of artworks, consisting in acquiring the vibrations caused by a source of elastic waves and using a set of detectors fixed in various predetermined points of the external surface of the object. An apparatus, cheap and simple to utilize, suitable to execute this method, is also described.

The “Villa del Casale” is a wonderful Roman Villa excavated in the archaeological site of Piazza Armerina (Sicily) in 1929. Its interest is mainly to be referred to the floor mosaics, among the largest and most beautiful ones of Roman times, of superlative quality. The floor of the Corridor is covered by a wonderful mosaic depicting the hunt and capture of wild animals, so that it is called the “Corridor of the Great Hunting Scene”. The Villa is now under restoration, and during the planning of the restoration a 3D electrical resistivity tomography was performed on the corridor by using an acquisition grid composed by 704 electrodes. Furthermore 25 GPR profiles with 400 MHz antenna were acquired. The 3D ERT survey was carried out by utilizing a new 3D array, named “Maximum Yield Grid” (MYG), that allows a great decrease of the acquisition time and of the invasivity of the ERT investigation in respect of classical arrays. The results show shallow anomalies, that correspond to the canalizations for water drainage and to the concrete basement built below the corridor during a previous restoration intervention, and a diminution of resistivity with depth, related to the aquifer below the Villa.

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.

Identification of artworks is mainly based on a few characteristics which can be observed using non-invasive tools (sight, touch, simple instruments), the investigated properties being geometry, weight, colours, texture, etc. Nowadays, technology allows reproducing all these characteristics to such an extent that even expert conservators can be deceived: in particular at the present time even the geometry of an artwork can be easily reproduced with the help of laser scanner analysis and with a rapid prototyping machine or a computer numerical control (CNC) milling machine. We propose a new tool, the Sonic Imprint, producing a code capable of identifying a rigid artefact from its vibrational resonance frequencies beyond doubt. In fact the vibration modes of an artefact strongly depend on the spatial distributions of its density and elastic parameters, as well as on its internal defects, definable in terms of abrupt changes of elastic properties in a small portion of the object. Then even small differences of these properties (differences usually present even among “identical” objects produced with industrial methods, at least in terms of defects) give appreciable variations of the Sonic Imprint codes, allowing secure identification of artworks, prevention of clonation and even damage monitoring. Moreover the procedure is really robust, rapid, inexpensive and not invasive. We tested it on a large number of commercial objects with the same shape and dimension and on many artworks in archaeological museums: an example is described. The application of this methodology to small-size artefacts (from small stones, vessels, pottery to medium-large coins) involves some problems in the detection of the Sonic Imprint. The problems, just due to the smaller sizes of this kind of objects, arise from the presence of higher resonance frequencies and larger damping of the induced vibrations. This implies that probes and instrumentation should be replaced to be adapted to the new experimental conditions.