Combining engineering and wood science for the conservation of artworks
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.
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Plan
Introduction: objectives and outline of the Project
Brief technological description of the Mona Lisa
— The Panel and its framing
— The Panel’s shape and the crack
— The crossbeams
— The auxiliary frame and the gilded external frame
Measurements and monitoring: methods, equipment and main results
— Orientation of the wooden panel inside the original log
— Panel’s weight
— Manual measurements of Panel’s profiles
— Optical measurements of Panel’s shape
— Monitoring forces, deformations and climate
* The load cells mounting system installed in 2013
* The displacement transducers
* The aluminium case
* The influence of gravity
* The main results from the monitoring
* The Panel’s stiffness
— Contacts between Panel and auxiliary frame
— Improvement of the framing conditions
* Interaction between the two frames
* A load-limiting device
* Securing the grip of the screws fixing the crossbeams against the auxiliary frame
— Data collection, storage and wireless transmission
Numerical simulations
— Development of a predictive finite-element model
— Elastic properties of the Panel
— Implementation of the FEM model, and correspondence between principal strains and craquelure patterns
— Assessing the risk of crack propagation and the contribution of the butterfly to prevent it
— Evaluation of the effects of interposing a material between Panel and auxiliary frame
Conclusions and perspectives
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Plan
Introduction: objectives and outline of the Project
Brief technological description of the Mona Lisa
— The Panel and its framing
— The Panel’s shape and the crack
— The crossbeams
— The auxiliary frame and the gilded external frame
Measurements and monitoring: methods, equipment and main results
— Orientation of the wooden panel inside the original log
— Panel’s weight
— Manual measurements of Panel’s profiles
— Optical measurements of Panel’s shape
— Monitoring forces, deformations and climate
* The load cells mounting system installed in 2013
* The displacement transducers
* The aluminium case
* The influence of gravity
* The main results from the monitoring
* The Panel’s stiffness
— Contacts between Panel and auxiliary frame
— Improvement of the framing conditions
* Interaction between the two frames
* A load-limiting device
* Securing the grip of the screws fixing the crossbeams against the auxiliary frame
— Data collection, storage and wireless transmission
Numerical simulations
— Development of a predictive finite-element model
— Elastic properties of the Panel
— Implementation of the FEM model, and correspondence between principal strains and craquelure patterns
— Assessing the risk of crack propagation and the contribution of the butterfly to prevent it
— Evaluation of the effects of interposing a material between Panel and auxiliary frame
Conclusions and perspectives
Depuis 2004, le panneau de bois utilisé par Léonard de Vinci pour peindre sa Joconde a été étudié par une équipe de recherche internationale constituée de spécialistes de science du bois, technologie et ingénierie ainsi que de mesures optiques, en étroite collaboration avec les conservateurs et les restaurateurs, avec les principaux objectifs suivants : (a) évaluer les spécifications climatiques adoptées pour la nouvelle vitrine climatisée, (b) évaluer le risque de propagation d'une fissure historique affectant le panneau, (c) suggérer des améliorations possibles de l'encadrement du panneau, et (d) améliorer les aspects technologiques de la procédure de contrôle actuelle. L'étude a été réalisée principalement 1) par observation et mesures directes, lors de l'ouverture annuelle de la vitrine, 2) par une surveillance continue automatique du comportement mécanique du panneau, et 3) par le développement d'un modèle numérique prédictif par éléments finis (EF). Le présent document traite des sujets suivants : (i) une description technologique du panneau et de son encadrement, (ii) les méthodes et équipements non invasifs de mesure et de surveillance mis au point (notamment des capteurs de force et de déplacement miniaturisés, des méthodes optiques basées sur la profilométrie à franges, des films sensibles à la pression, ainsi que des méthodes et équipements pour la collecte, le stockage et la transmission sans fil des données), (iii) le modèle EF prédictif mis au point pour simuler avec précision le comportement hygro-mécanique du panneau ainsi que ses déformations et contraintes mécaniques, (iv) les résultats les plus significatifs obtenus au cours des dix-neuf années de travail, y compris l’identification des paramètres élastiques du panneau en bois, la description des zones de contact entre le panneau et son encadrement, l'évaluation du niveau et de la localisation des forces de contact, les actions possibles pour améliorer l’encadrement, la mise en œuvre des EF pour montrer une correspondance entre les déformations principales dans le panneau et les motifs de craquelure dans les couches de peinture, et pour évaluer le risque de propagation de la fissure historique.
