Reflectance Spectroscopy as a Novel Tool for Thickness Measurements of Paint Layers.

A major challenge in heritage science is the non-invasive cross-sectional analysis of paintings. When low-energy probes are used, the presence of opaque media can significantly hinder the penetration of incident radiation, as well as the collection of the backscattered signal. Currently, no technique is capable of uniquely and noninvasively measuring the micrometric thickness of heterogeneous materials, such as pictorial layers, for any painting material. The aim of this work was to explore the possibility of extracting stratigraphic information from reflectance spectra obtained by diffuse reflectance spectroscopy (DRS). We tested the proposed approach on single layers of ten pure acrylic paints. The chemical composition of each paint was first characterised by micro-Raman and laser-induced breakdown spectroscopies. The spectral behaviour was analysed by both Fibre Optics Reflectance Spectroscopy (FORS) and Vis-NIR multispectral reflectance imaging. We showed that there is a clear correlation between the spectral response of acrylic paint layers and their micrometric thickness, which was previously measured by Optical Coherence Tomography (OCT). Based on significant spectral features, exponential functions of reflectance vs. thickness were obtained for each paint, which can be used as calibration curves for thickness measurements. To the best of our knowledge, similar approaches for cross-sectional measurements of paint layers have never been tested.

UV laser removal of varnish on tempera paints with nanosecond and femtosecond pulses.

Two laser cleaning approaches based on ablation by ultraviolet laser pulses of femtosecond (fs) and nanosecond (ns) durations for the removal of shellac varnish from egg-yolk based tempera paints are investigated. Laser irradiation effects, induced on the varnish layer and on the underlying temperas by multiple pulses in the fs domain at 398 and 265 nm and single pulses in the ns domain at 213 nm, were examined following a spectroanalytical approach. By using optical microscopy, colorimetry and laser induced fluorescence it was found that irradiation of the varnished temperas with fs pulses changes the texture of the varnish surface and results in degradation of the underlying coloured paint. In contrast, operating with pulses of 15 ns at the highly absorbed wavelength of 213 nm, controlled micrometric layer removal of the varnish is possible without noticeable modification of the coloured temperas. These results widen the choice of laser conditions for painting restoration.

Multianalytical non-invasive characterization of phthalocyanine acrylic paints through spectroscopic and non-linear optical techniques.

The documentation and monitoring of cleaning operations on paintings benefit from the identification and determination of thickness of the materials to be selectively removed. Since in artworks diagnosis the preservation of the object's integrity is a priority, the application of non-invasive techniques is commonly preferred. In this work, we present the results obtained with a set of non-invasive optical techniques for the chemical and physical characterization of six copper-phthalocyanine (Cu-Pc) acrylic paints. Cu-Pc pigments have been extensively used by artists over the past century, thanks to their properties and low cost of manufacture. They can also be found in historical paintings in the form of overpaints/retouchings, providing evidence of recent conservation treatments. The optical behaviour and the chemical composition of Cu-Pc paints were investigated through a multi-analytical approach involving micro-Raman spectroscopy, Fibre Optics Reflectance Spectroscopy (FORS) and Laser Induced Fluorescence (LIF), enabling the differentiation among pigments and highlighting discrepancies with the composition declared by the manufacturer. The applicability of Non Linear Optical Microscopy (NLOM) for the evaluation of paint layer thickness was assessed using the modality of Multi-photon Excitation Fluorescence (MPEF). Thickness values measured with MPEF were compared with those retrieved through Optical Coherence Tomography (OCT), showing significant consistency and paving the way for further non-linear stratigraphic investigations on painting materials.

Evidence of anomalous switching of the in-plane magnetic easy axis with temperature in Fe3O4 film on SrTiO3:Nb by v-MOKE and ferromagnetic resonance.

The evolution of the magnetic anisotropy directions has been studied in a magnetite (Fe3O4) thin film grown by infrared pulsed-laser deposition on SrTiO3(100):Nb substrate. The magnetic easy axes at room temperature are found along the in-plane 〈100〉 film directions, which means a rotation of the easy axis by 45° with respect to the directions typically reported for bulk magnetite and films grown on single-crystal substrates. Moreover, when undergoing the Verwey transition temperature, TV, the easy axis orientation evolves to the 〈110〉 film directions. This anomalous behavior has been demonstrated by measuring first the angular dependence of coercivity and remanence well above and below TV by high-resolution vectorial magneto-optical Kerr effect (v-MOKE). Ferromagnetic resonance (FMR) measurements have additionally proven a well-defined fourfold magnetic anisotropy induced during growth with confirmed easy axis directions along 〈100〉 for T > TV and 〈110〉 for T < TV. These results provide a clear proof of the possibility of tuning magnetic anisotropy in Fe3O4 thin films by proper control on the growth parameters and substrate choice.

Effect of molecular weight on the morphological modifications induced by UV laser ablation of doped polymers.

This work investigates the effect of polymer molecular weight (M(W)) on the surface morphology of poly(methyl methacrylate) (PMMA) and polystyrene (PS) films doped with iodonaphthalene (NapI) and iodophenanthrene (PhenI) following irradiation in air at 248 nm. In agreement with previous studies, irradiation of PMMA at 248 nm results in surface swelling and bubble formation within the irradiated bulk. Most importantly, the size of bubbles varies sensitively for the different M(W) values, with larger bubbles being formed for the low M(W) systems. Nevertheless, the maximum swelling attains higher values for the high M(W) values (when compared at the corresponding ablation threshold of the systems). Real-time monitoring of transmission of a probing beam shows that morphological changes last longer in the low M(W) polymer. Melting, consistent with a thermal mechanism, occurs, and enough evidence is gathered to provide direct support for the bulk photothermal model, according to which ejection requires that a critical number of bonds is broken. In particular, the observed different morphological effects can be ascribed to the interplay of two factors, namely, of the much more efficient decomposition of the low M(W) polymer to gaseous products and of the dependence of the mechanical polymer properties on M(W). For PS at 308 nm, the changes parallel the ones for PMMA at 248 nm. In contrast, at the strongly absorbed 248 nm, the morphological processes in PS show a less dramatic dependence on M(W). In all, these results are of direct importance for the optimization of laser processing schemes and applications (e.g., tissue processing, laser deposition, laser restoration, etc.).

Influence of polymer molecular weight on the chemical modifications induced by UV laser ablation.

This paper investigates the influence of polymer molecular weight (M(W)) on the chemical modifications of poly(methyl methacrylate), PMMA, and polystyrene, PS, films doped with iodonaphthalene (NapI) and iodophenanthrene (PhenI), following irradiation at 248 nm (KrF excimer laser, 20 ns fwhm and hybrid excimer-dye laser, 500 fs fwhm) and at 308 nm (XeCl excimer laser, 30 ns fwhm). The changes of intensity and position of the polymer Raman bands upon irradiation provide information on cleavage of the polymer bonds. Degradation of PMMA, which is a weak absorbing system at 248 nm, occurs to a higher extent in the case of a larger M(W), giving rise to the creation of unsaturation centers and to degradation products. For highly absorbing PS, no degradation is observed upon irradiation with a KrF laser. Consistently irradiating doped PS at 308 nm, where the absorption is low, induces degradation of the polymer. Results provide direct support for the bulk photothermal model, according to which ejection requires a critical number of broken bonds. In the case of irradiation of doped PMMA with pulses of 248 nm and 500 fs, neither degradation nor dependence with polymer M(W) are observed, indicating that mechanisms involved in the femtosecond laser ablation differ from those operating in the case of nanosecond laser ablation. Participation of multiphoton/avalanche processes is proposed.

Analytical spectroscopic investigation of wavelength and pulse duration effects on laser-induced changes of egg-yolk-based tempera paints.

The application of laser cleaning methodologies to light-sensitive substrates such as those encountered in artistic paintings is an extremely delicate issue. The cleaning of paintings and polychromes is an irreversibly invasive intervention; therefore, prior to the implementation of laser cleaning methodologies, a thorough characterization of the interaction between laser pulses and painting components is required. In this work, the modifications induced by irradiation with pulses of 150 picoseconds (at 1064 and 213 nm) and 15 nanoseconds (at 213 nm) on unvarnished aged model egg-yolk-based paints were examined following a spectroanalytical approach. Laser-induced chemical changes on samples of unpigmented and widely used artist's pigment temperas were investigated by spectrofluorimetry and Fourier transform Raman spectroscopy, while color changes were quantified by colorimetry. Noticeable modifications of the Raman and fluorescence bands attributed to pigments are absent except for vermillion, for which the pigment bands tend to disappear upon irradiation at 1064 nm. Interestingly, no discoloration was observed on most of the pigments upon irradiation at 213 nm (150 ps), including the light-sensitive vermillion, while no indications of carbonization or charring of the paint layers, which could give rise to amorphous carbon bands, were obtained at any of the irradiation conditions explored. Comparison of the results using the two different pulse durations and wavelengths illustrates the participation of mechanisms of diverse origin according to the chemical composition of the pigment and highlights the importance of the optimization of the laser parameters, specifically fluence, pulse duration, and wavelength, in conservation treatments of paintings.