Varnish removal from easel paintings is a restoration procedure that is regularly undertaken by cultural heritage conservators. In very few cases, traditional methods (chemical and mechanical) do not allow them to reach the goal of a perfectly controlled and selective cleaning. UV laser ablation has been considered since the 1990s to overcome these limitations, but its application to real cases is far from common practice. This study proposes a calibrated procedure based on ablation by an ns UV Nd:YAG laser at 266 nm combined with optical coherence tomography for micrometric laser varnish removal from inhomogeneous and highly photosensitive pictorial layers. The use of this wavelength for varnish removal, not adapted according to the literature, is discussed again after the beam shaping leading to a homogeneous and controlled intensity distribution. The innocuity is controlled by laser-induced luminescence, and some solutions for the transparency loss of the residual varnish following ablation are proposed.

Understanding soft matter mechanical behavior is of great interest as multiphasic combinations of their composition induce new properties which can be exploited for innovative applications. However, the final features optimization requires a tight multiscale control of the structure, even during the elaboration early stages. This paper presents a multifrequency technique to investigate this evolution at mesoscopic scale and its bonds with other scales. A TMS resonator is used as a discrete spectral ultrasonic microrheometer from 5MHz to 50MHz. Then, soft matter can be described as an elastic structure, due to macromolecular interactions, immersed in an effective viscous fluid. An original model of the measured mechanical impedance is proposed and enables the simultaneous monitoring of the effective viscosity and the internal structure. It is based on fractional calculus. In addition to complex shear modulus, the evolution of a fractional parameter, ranging from zero for solids to one for Newtonian fluids, can be studied. The model and the experimental set-up are validated with various complex materials: Newtonian glycerol mixtures, cosmetic emulsions, and silica gels. Effective viscosity accuracy is demonstrated (less than 5% of error for Newtonian fluids). Structural values of complex fluids range from 0.6 (gels) to 1 (liquids). The extracted structural parameter can be linked to the fractal dimension. Hence, this technique is relevant to describe soft matter structure. Moreover, the structural parameter on-line monitoring can be useful to optimize the elaboration process of new products. Indeed, a microscopic characteristic time can be extracted and correlated to the macroscopic gelation time.

Curators have developed preventive conservation strategies and usually try to control the temperature (T) and relative humidity (RH) variations in the museum rooms to stabilise the artworks. The control systems chosen by museums depend on the size and age of the building, the financial means and the strategies that can be adapted. However, there is a lack of methods that can monitor mechanical changes or chemical reactions of objects in real-time or regularly. It would therefore ideally be preferable to monitor each of them to alert them to preserve them. For this purpose, a non-destructive, non-contact, full-field technique, Digital Holographic Speckle Pattern Interferometry (DHSPI), has already been developed and allows direct tracking of changes on the surface of artworks. This technique is based on phase-shifting speckle interferometry and gives the deformation of the surface below the level of the micro-meter of the analysed object. In order to monitor the deformation continuously, a large number of images are acquired by DHSPI and have to be processed. The existing process consists of removing noise from the interferogram, unwrapping this image, and deriving and displaying a 2D or 3D deformation map. In order to improve the time and accuracy of processing the imaging data, a simpler and faster processing method is developed. Using Matlab^®, a denoising methodology for the interference pattern generated during data acquisition is created, based on a stationary wavelet transform. The unwrapped image is calculated using the CPULSI (Calibrated Phase Unwrapping based on Least-Squares and Iterations) algorithm as it gives the fastest results among the tested methods. The unwrapped phase is then transformed into surface displacement. This process performs these steps for each interferogram automatically. It allows access to 2D or 3D deformation maps.

The investigation of the mechanical properties of skin is of great interest for monitoring physiological and pathological changes in the cutaneous barrier function for dermatological and cosmetic issues. Skin constitutes a complex tissue because of its multi-layered organisation. From a rheological point of view, it can be considered to be a soft tissue with viscoelastic properties. In order to characterise ex vivo mechanical properties of skin on the mesoscopic scale, a biosensor including a thickness shear mode transducer (TSM) in contact with a skin explant was used. A specific experimental set-up was developed to monitor continuously and in real-time human skin explants, including the dermis and the epidermis. These were kept alive for up to 8 days. Skin viscoelastic evolutions can be quantified with a multi-frequency impedance measurement (from 5 MHz to 45 MHz) combined with a dedicated fractional calculus model. Two relevant parameters for the non-destructive mesoscopic characterisation of skin explants were extracted: the structural parameter ${\alpha }_{app}$ and the apparent viscosity ${\eta }_{app}$. In this study, the validity of the biosensor, including repeatability and viability, was controlled. A typical signature of the viscoelastic evolutions of the different cutaneous layers was identified. Finally, monitoring was carried out on stripped explants mimicking a weakened barrier function.

(Text in French)Towards control techniques for the conservation of painted wooden panels. Wood is a hydrophilic material, and wooden panels, which were very common before the advent of easel painting, are sensitive to the changes in humidity that occur in museums, whether in the exhibition hall or in storage. While preventive conservation approaches mostly involve controlling sudden changes in temperature and humidity, it is now possible to monitor the effects of these changes directly on the objects themselves. This article presents an experimental approach to assessing the impact of climatic variations based on actual measurements of humidity and temperature. The results show the real potential of digital holography speckle pattern interferometry (DHSPI) as a measurement and monitoring tool for this type of issue.