Résumé
Since 2004, the wooden panel used by Leonardo da Vinci to paint his Mona Lisa has been studied by a specially established international research team formed by specialists in wood science, technology and engineering, and in optical measurements, interacting closely with curators and restorers, with the following main objectives: (a) to evaluate the climatic specifications adopted for the then new climate-controlled display case, (b) to assess the risk of propagation of a historical crack affecting the Panel, (c) to suggest possible improvements to the Panel’s framing, and (d) to improve the technological aspects of the current monitoring procedure. The study has been carried out mainly 1) by direct observation and measurements, during the yearly opening of the display case, 2) by an automatic continuous monitoring of the Panel’s mechanical behaviour, and 3) by developing a predictive numerical finite element model (FEM). This paper deals with the following topics: (i) a technological description of the Panel and of its framing, (ii) the measurement and monitoring non-invasive methods and equipment developed (including miniaturized force and displacement transducers, optical methods based on Fringe pattern profilometry, pressure sensitive film, and methods and equipment for data collection, storage and wireless transmission), (iii) the predictive FEM developed to accurately simulate the Panel’s hygro-mechanical behaviour and its internal strains and stresses, (iv) the most significant results obtained in the course of the now nineteen years of work, including the derivation of the elastic parameters of the wooden Panel, the description of the contact zones between Panel and framing, the evaluation of the amount and location of the contact forces, possible actions for improving the framing, the implementation of the FEM to show a correspondence between principal strains in the Panel and craquelure patterns in the paint layers, and to assess the risk of propagation of the historical crack.<br />****<br /><b>Plan</b><br /><i>Introduction: objectives and outline of the Project</i><br /><i>Brief technological description of the </i>Mona Lisa<br />— The Panel and its framing<br />— The Panel’s shape and the crack<br />— The crossbeams<br />— The auxiliary frame and the gilded external frame<br /><i>Measurements and monitoring: methods, equipment and main results</i><br />— Orientation of the wooden panel inside the original log<br />— Panel’s weight<br />— Manual measurements of Panel’s profiles<br />— Optical measurements of Panel’s shape<br />— Monitoring forces, deformations and climate<br /> * The load cells mounting system installed in 2013<br /> * The displacement transducers<br /> * The aluminium case<br /> * The influence of gravity<br /> * The main results from the monitoring<br /> * The Panel’s stiffness<br />— Contacts between Panel and auxiliary frame<br />— Improvement of the framing conditions<br /> * Interaction between the two frames<br /> * A load-limiting device<br /> * Securing the grip of the screws fixing the crossbeams against the auxiliary frame<br />— Data collection, storage and wireless transmission<br /><i>Numerical simulations</i><br />— Development of a predictive finite-element model <br />— Elastic properties of the Panel<br />— Implementation of the FEM model, and correspondence between principal strains and craquelure patterns<br />— Assessing the risk of crack propagation and the contribution of the butterfly to prevent it<br />— Evaluation of the effects of interposing a material between Panel and auxiliary frame<br /><i>Conclusions and perspectives</i>