• Sciences de l'ingénieur/Mécanique
  
• Sciences de l'ingénieur/ photonique
  
• Sciences de l'ingénieur/Matériaux
  
• Sciences de l'ingénieur/Mécanique/Matériaux et structures en mécanique
  
• Sciences de l'ingénieur/Mécanique/Mécanique des matériaux
  
• Sciences de l'ingénieur/Mécanique/Mécanique des solides
  
• Sciences de l'Homme et Société/Art et histoire de l'art
  
• Sciences de l'Homme et Société/Héritage culturel et muséologie
  
• Physique/Mécanique/Mécanique des matériaux
  
• Sciences de l'ingénieur/Traitement du signal et de l'image [eess.SP]
  

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).

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.

Ce papier présente le développement d'une méthode expérimentale pour quantifier la réponse des couches préparatoires liée à l'humidité, dans le comportement global des panneaux peints.