Search

International audience; This paper describes an innovative method, and related equipment, developed by the authors to monitor non-invasively historic panel paintings under museum display conditions. This method permits in-depth knowledge about such artworks, allowing us to understand their reactions to climatic variations, and pro- vides objective data on which conservation decisions can be confidently based, since the data are directly obtained from the individual artwork. Since 2004, following the invitation from the Louvre Museum and the C2RMF (National Centre for Research and Restoration of French Museums), the wooden panel on which Leonardo da Vinci painted his Mona Lisa has been studied by an international research team of wood technologists and engineers, including researchers from French and Italian universities and related scientific institutions (Montpellier, Clermont Auvergne, Poitiers, Florence), to understand its mechanical, hygroscopic and shape characteristics and behaviour, to evaluate its present state, and to provide sug- gestions for optimizing its conservation conditions. Non-invasive methods and equipment were therefore devised and implemented to measure (during the annual opening day of its display case) and automat- ically monitor (during the time the display case remains closed) both the deformations that the panel undergoes (mainly produced by the inevitable small climatic fluctuations within the case) and the con- straining forces acting on the panel itself. The method and the related equipment, improved over the years, are based on miniature load cells and displacement transducers, whose outputs are automatically logged at desired time intervals, typically ranging between 30 min for monitoring during the whole year, and a few seconds for manual measurements, calibrations, and other selected events; the stored data can be downloaded both through a cable connection and wirelessly, by means of a specially developed con- nection apparatus. The panel is confined in a climate-controlled display case, which typically is opened only for a few hours once a year. Additionally, close restrictions must be respected, including absolute non-invasiveness, non-interference with the enjoyment of the artwork by the public, and compliance with strict procedures for safe and secure conservation. The implementation of this method has provided significant information about the actual behaviour of the panel during the whole year. Comparing several annual force-deformation curves, their good linearity suggests that no unacceptable stress or deformation has taken place in it, showing that the climatic conditions (air temperature and relative humidity) main- tained in the display case can be considered favourable to the conservation of the artwork. Moreover, based on the collected data, reliable Finite Element Method (FEM) models are being developed and cali- brated, with the aim of describing the mechanical behaviour of the panel and virtually evaluating the risk of damage (including the propagation of an ancient crack) deriving from external conditions or actions to which it would be unthinkable to submit the original historic artwork.

International audience; The engineering approach to analyse the hygromechanical behaviour of a panel painting requires a deep knowledge of its actual physical structure, including size, shape, wooden material's anatomy and character- istics, and responses to extemal actions such as forces or hygrothermal fluctuations. In fact, each panel is unique, different from all the others, because of numerous factors, in- cluding the intrinsic variability of the wood, the "cut" of the boards, the construction technique of the artwork, the succession of environments in which it has been main- tained, the irreversible processes developed during its existence, and the conservation interventions it has undergone over the centuries. Based on our experience, only a systematic set of experimental investiga- tions, conceived and implemented specifi- cally for the individuai artwork, can provide the information necessary to understand its conservation needs. However, experimen- tal investigations alone are not sufficient for a complete assessment of the artwork: numerical simulations integrated with the experimental tests can greatly improve the level of knowledge and provide more ex- tensive and deeper information. Moreover, following such an approach, the simulation can make it possible to explore situations of risk or modifications of the conservation situation in a totally non-invasive and risk- free way. The wooden panel ofLeonardo da Vinci's Mona Lisa has been studied by an intemational team with the aim of analysing and possibly improving its conservation conditions by combining a comprehensive set ofnon-invasively obtained experimental data with an advanced numerical modelling. The development of a Digital Twin of the pane I has hence provided a sound knowl- edge of the mechanical characteristics and behaviour ofthe panel, including the assess- ment of the risk of propagation of an ancient crack, and the evaluation of a new framing configuration.

International audience; The approach to wooden artefacts of historical importance, and panel paintings in particular, is a task that requires a multidisciplinary approach based on experimental observation of the artwork and advanced techniques to make these data actually useful for the knowledge and preservation of the object. This study illustrates how a series of scientific observations and instrumental analyses can be used to construct a numerical simulation that allows a deeper understanding of the physical structure and behaviour of the object itself, namely to construct a hygro-mechanical predictive model (a “Digital-Twin”) of Leonardo da Vinci's Mona Lisa panel. Based on specific request from the Louvre Museum, a group of experts with different and complementary skills cooperated and are still cooperating to construct a complete set of experimental observation and non-invasive tests; so, the integration of the collected data made the construction possible of the panel’s Digital-Twin. This paper also specifically examines how the Digital-Twin can be used to compare two framing conditions of the panel; although the two experimental configurations are not inherently comparable, the comparison is made possible by the introduction of a technique of projection of the fields obtained as results of the two analyses, named the Projected Model Comparison (PMC), which has been developed specifically for this research.

International audience; This article describes the experimental devices and the processes used to study the hygromechanical behaviour of a historic painted wooden panel (La Sainte Trinité couronnant la vierge, 1516, anonymous, Fabre Museum of Montpellier). A climate showcase was designed for in-museum use, with two glasses allowing visitors as well as scientists to observe both sides simultaneously. The evaluation of the hygroscopic behaviour was done by measuring relative humidity (RH), temperature and variations of panel weight. Variations of panel shape, strain and curvature were observed by two methods: locally with three pairs of displacement transducers fixed on the rear face, and over the entire panel surface by a non-contact optical technique, stereo mark tracking, used simultaneously on both faces. The correlation between local and whole-field measurements was very good. Continuous monitoring (several data per hour over three years) was required to observe the behaviour of the panel during imposed climate variations. The first test, detailed in this paper, was performed before frame removal and panel restoration. It consisted in a stabilization of the panel at 52% RH during 2 months and an increase to 63% RH during 3 months. The small amount of total mass variations, from 31.980 kg to 31.910 kg at 52% RH and 32.088 kg at 63% RH, could not be explained without taking into account the phenomenon of sorption hysteresis. The whole field relief measurement exhibited a concave shape with a maximum amplitude of 16 mm. It showed that shape deformation, ie out-of-plane displacement, was convex bending with a maximum deflection of 1.5 mm and a maximum strain of 0.1 %. Comparable local data had been obtained by the optical and DKs methods. DKs results were ten times more accurate than optical ones. The horizontal strain exhibited a global shrinkage during initial stabilization and a global swelling during RH increase. Heterogeneity of the strain field can be related to the cracks observed on the back face, and to the glued crossbeams. Assuming a linear relationship between swelling and moisture content, we expected, for a 0.6% increase of moisture content, a radial strain of 0.10% and a tangential strain of 0.22%. These results are higher than the ones measured, which can be explained by the cross bars glued on the boards and the cracks. This comparison will be continued for the next stages (crossbars removal, panel restoration).

International audience

International audience; Since 2004, the “Mona Lisa” painting by Leonardo da Vinci has been studied by an international research group of wood scientists and several experimental campaigns have been carried out to understand its characteristics and provide indications for its conservation. Based on the collected data, a numerical model of the wooden panel has been developed to simulate the mechanical interaction with the framing system. The main objective of this modelling work, described in this paper, is to extract as much information as possible from the experimental tests carried out and, thus, reach a sufficient level of scientific knowledge of the mechanical properties of the panel to build a predictive model. It will be used to predict the effect of modified boundary conditions and as a tool of preventive conservation.