We are all familiar with ultrasound, the ultrasound imaging technique commonly used for monitoring pregnancies and medical diagnosis, an application in which our research team has been involved since its creation.
Several members of my family having exercised the profession of craftsman, having practiced painting, sculpture or woodturning, it is quite naturally that I sought to bring my profession of scientist closer to the artistic field. By discussing this with Faddoul Khallouf, a painting restorer friend, the idea was born to consider the use of ultrasound to explore the state of pictorial works. In particular, it can be expected that certain degradations such as detachments between internal layers (between the canvas and the preparation layer, between the latter and the layers of paint) will greatly reduce the transmission of ultrasound through the work. , whereas such defects are very difficult to detect by the techniques currently available.
Moreover, a bibliographical search showed that practically no study in this direction had been published. It was enough to decide to explore this path.
From the outset, we are confronted with a difficulty: a painted work must not be brought into contact with either a liquid or with another object, whereas conventional ultrasonic measurements require either the use of a coupling gel between the transducers ( devices that convert electrical signals into ultrasonic waves and vice versa) and the object being explored, i.e. immersion in a liquid, in order to promote the transfer of acoustic energy.
A first device
However, for several years the ultrasonic transducers dedicated to applications in the air have seen their performance increase, so that it is possible to produce transmission images without contact through thin structures such as a painted canvas: an ultrasound emitter is placed on one side of the canvas, a receiver placed in front of the other side, the two are then scanned so as to cover the entire surface of the work. The transmitted wave is recorded at each point, making it possible to obtain an image.
Author provided
A doctoral thesis was initiated in the fall of 2019 by the University of Tours in order to develop a contactless ultrasound imaging system, then to explore its potential for characterizing painted works. The doctoral student, Victor Takahashi, supervised by Jérôme Fortineau, Michaël Lemâtre and myself, first designed and implemented the system using elements available in the laboratory as well as transducers, electronic modules and motorized movement axes. specially acquired.
The results obtained on known materials were first compared with acoustic simulations and measurements taken from the scientific literature in order to validate the methods. An article detailing simulations and a procedure for tracing material properties has been published.
The ultrasonic frequencies chosen, in the range of 300 to 400 kHz, i.e. 15 to 20 times the highest frequency that the human ear can hear, and a focusing of the waves made it possible to achieve an image resolution of the order of 2 millimeters (it is possible to differentiate two points of the table at this distance).
Ultrasound brings rich information
After tests on painted canvases made by us, a work dating from 1742 which I had inherited a few years ago was imaged. The results and the work were presented to painting restoration professionals who were able to relate certain areas of the ultrasound image corresponding to strong variations in wave transmission to inhomogeneities visible to the naked eye on the surface. some cobweb.
Provided by the author
These results were presented to the scientific community of acousticians. To confirm these encouraging results, a painting requiring restoration was partially imaged. Then, during the work, the restorer Stéphanie Teyssier established correlations between the ultrasound images and the defects or irregularities she observed: variations in thickness, cracks, small perforations, wear, impasto of the shoulder pad, etc.
Provided by the author
To go further, we contacted the Granville Museum of Art and History, via its assistant curator Alexandra Jalaber, as well as with Antoine Cazin of the Fabrique de patrimoines en Normandie in Caen, a specialist in painting imagery. Two works belonging to the Musée de Granville have been imaged by Antoine Cazin on the one hand by X-rays and, on the other hand, by several photographic techniques in the visible spectrum (direct or grazing light) and at the margins of the visible (infrared, ultraviolet). The paintings having returned to the museum, we went there and installed our ultrasound imaging system. Our images have been compared to those obtained by other techniques and this analysis has already made it possible to draw some conclusions:
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Transmission ultrasound makes it possible to obtain images of paintings on canvas with a resolution of the order of 2mm, with a possible improvement to reach mm by increasing the frequency;
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Inhomogeneities, i.e. areas where the amplitude of the transmitted waves is significantly higher or lower than in the whole image, can be attributed to anomalies on the work, some of which are also observable by others techniques ;
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Ultrasound images reveal certain inhomogeneities that are not visible by other techniques.
Thus, thanks to its low cost compared to that of other techniques such as X-rays and to the fact that the waves used are not harmful, non-contact ultrasound imaging of painted works has the potential to become a complementary modality to community service for the preservation and restoration of works of artistic heritage.
We hope to be able to continue this study beyond the doctoral thesis in order to obtain new results, to improve the resolution of the images and to derive quantitative information from them. For this we wish to establish partnerships with other institutions involved in the conservation of works and identify potential future users of the ultrasound technique.
