Hygromechanical behaviour of wooden panel paintings: classification of their deformation tendencies based on numerical modelling and experimental results

AbstractWooden panel paintings are among the most important historical and artistic artworks from the Middle Ages and the Renaissance period. Currently, they represent a challenge for conservators and scientists who face complex issues related to their conservation. Panel paintings can be considered multilayer objects, that for brevity can be considered to consist of a wooden support and various paint layers. The wooden support is known to be hygroscopic and is continuously seeking hygroscopic equilibrium with the humidity of the environment, thus it tends to deform. Based on various hygroscopic tests carried out on 6 real panel paintings chosen by expert restorers to represent different periods and construction techniques, this paper describes the deformation tendencies of the selected panel paintings. Among possible variables, three most important variables were identified: (a) tree ring orientation of the wooden support, (b) stiffness and (c) emissivity of the paint layers. The internal equilibrium of the forces, governed by the moisture gradients across the thickness of the wood, changes drastically according to the varying characterisation of these factors. To observe their individual contributions, the 6 panel paintings underwent various humidity cycles, were completely free to deform and were always in complete safety. To characterise the stiffness and emissivity of the paint layers, the 6 panel paintings underwent a few humidity cycles with the front face totally waterproofed; thus, the moisture exchange was forced from the back only, and one of the three variables was eliminated. A complex system emerges where the tree ring orientation of the wooden support, the stiffness and emissivity of the paint layers are strongly coupled and determine the deformation modes of the panel paintings. A numerical analysis was conducted to classify the various general deformation modes of panel paintings and the specific classification of the 6 real panel paintings analysed experimentally. The complexity of the interaction of the variables studied suggests that experimental procedures must be conducted in preparation for numerical analyses of real panel paintings

Learning from Objects: the use of advanced numerical methods to exploit a complete set of information from experimental data, for the Mona Lisa’s Digital-Twin

International audienceThe 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

Strengthening of Ancient Timber Beams during Restoration Operations: Effect of Wood Cutting on Modulus of Elasticity

In ancient buildings, timber members may require specific on-site interventions, including reinforcement or repair and sometimes the insertion of reinforcing material into grooves routed in the original sound wood. The required number, positions, and dimensions of grooves and the strengthening materials may differ according to the desired increases in bending stiffness and strength. The modulus of elasticity (MOE) of each beam is of key importance: the MOE is typically a point of weakness for wood and is considered a constant characteristic of each beam. However, taking into account the wood lost for the groove, whether the needed incisions affect the stiffness is unknown. As such, in this study, 12 old beams were accurately measured, and their static and dynamic MOEs were calculated before and after groove formation to simulate the typical processes of reinforcing interventions. One groove was routed along the length of each beam and progressively deepened in three steps. The results of tests showed how the MOE is affected by the groove depth, decreasing by up to one-third (34.1%), and that beam stiffness cannot be regarded as constant. Beam stiffness depends on the features of the beam, mainly the pith, fissures, and slope of the grain, as well as its structural integrity. Beam stiffness is strongly influenced by the cuts upon it. This study proves how the weakening effect of grooves created on-site can be assessed using the dynamic MOE and roughly predicted with a visual survey. The grooves required for strengthening beams must be thoroughly evaluated, considering the potential reduction in the mechanical properties of the beam, which goes well beyond the wood lost during cutting

Integration of experimental and numerical methods to study the mechanical risk conditions for the conservation of unique art pieces: the case of the Mona Lisa

International audienceThe 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

Suivi du comportement mécanique d’une peinture sur bois soumise à des variations d’hygrométrie

International audienceCe travail collaboratif (présenté lors de la précédente journée) regroupe le musée Fabre, des restaurateurs mobilier et couche picturale ainsi que des chercheurs du GESAAF (Université de Florence), de l’institut PPRIME (Université de Poitiers) et du LMGC (Université de Montpellier). Le but de cette étude est la caractérisation du comportement mécanique d’un panneau de bois peint du patrimoine soumis à des variations d’environnement hygrothermique contrôlées. Dans la perspective d’une exposition prévue en 2018 et destinée à destinée à faire connaître les pratiques de la restauration des œuvres d’art au public, la communication sur cette interaction entre acteurs patrimoniaux et académique fait l’objet d’une attention particulière