1 Thermal diffusivity measurement of mural painting by simulated infra-red thermography J.L Bodnar*, J.L. Nicolas*, K. Mouhoubi*, V. Detalle** *GRESPI / ECATHERM, UFR Sciences Exactes et Naturelles, BP 1039, 51687 Reims cedex 02, France ** Laboratoire de Recherche des Monuments Historiques, 29 avenue du Paris, 77420 Champs sur Marne, France Abstract In this work, we present a method of in situ measurement, of longitudinal thermal diffusivity of work of art. We present first the principle of this measurement method. It is based on the temporal analysis of the characteristic radius of the photothermal signature of a laser excitation. We present then the experimental device developed for the study. We show finally, experimentally, that the method allows a good estimation of the longitudinal thermal diffusivity of a sample test and of a partial copy of the “Saint Christophe” of the “Campana” collection of the “Louvre”. 1. Introduction Within the framework of the restoration of work of art, we work, since about ten years, to the detection of detachment and air pockets located in murals paintings. Stimulated infra-red thermography already allowed us to detect detachment located in the “Saint Christophe” of the “Campana” collection of the “Louvre”, in the painted walls of the church “Saint Florentin” of “Bonnet”, in the ceilings painted of the abbey of “Savin Saint sur Gartempe” (classified with the world inheritance of UNESCO) and finally in the “Cocteau” frescos of the church “Saint Pierre” of “Villefranche sur mer” [1-7]. These qualitative studies being positive, we now study the possibilities of the photothermal method for the defects characterization. To this end, the first thermophysical parameter we chose to study is the thermal diffusivity. Indeed, we hope on the one hand that this parameter will enable us to estimate the depth of the studied defect. We hope in addition that this parameter will enable us to characterize the pictorial layer and consequently will give information about the realization of the work of art. We hope finally, that the method will enable us to detect capillary salt ascent, but also to characterize them. To measure these thermal diffusivity parameter, we had two solutions. We could on the one hand, take a sample of work of art and implement a traditional measurement method of this thermophysical parameter (flash method [8-9]). We could in addition, to develop a method usable in situ and being non destructive for the studied mural painting. For reasons of preservation of the works of art, it is this last option which we chose to implement. The thickness of a mural painting being often equal to several tens of centimetres, a transverse thermal diffusivity measurement is often impossible. We thus chose to develop a longitudinal thermal diffusivity measurement of the work of art. It is this method which we present here. We present first the principle of this measurement method. It is based on the temporal analysis of the characteristic radius of the photothermal signature of a laser excitation. We present then the experimental device developed for the study. We show finally, experimentally, that the method allows a good estimation of the longitudinal thermal diffusivity of a sample test and of a partial copy of the “Saint Christophe” of the “Campana” collection of the “Louvre”. 2. The longitudinal thermal diffusivity measurement method developed for the study The principle of the longitudinal thermal diffusivity measurement method developed for the study is the following: A sample is lighted with a localised laser excitation. It is temporally close to a Dirac function δ (t). It is spatially close to a Gaussian shape. We measure then the variations of temperature induced by this excitation. We use an infra-red camera of thermography. From the