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

Abstract

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.

In-Depth Analysis of Egg-Tempera Paint Layers by Multiphoton Excitation Fluorescence Microscopy

“…The second device used was a nonlinear optical microscope developed at IQFR, which allows for the point-wise collection of MPEF signals in epi-detection mode. A detailed description of this device can be found in [19,27]. Briefly, the excitation light source is a mode-locked Ti:Sapphire femtosecond laser emitting at 800 nm, releasing 70 fs pulses at a repetition rate of 80 MHz.…”

“…For all the profiles, the full width at half maximum (FWHM) was taken as an estimation of the apparent thickness of the paint layer. The real thickness was then calculated by correcting the FWHM values with the apparent depth correction factor (F) [27,51], which considers the refractive index of the analyzed material (n = 1.5-1.6) and the effective numerical aperture (NA) of the focusing objective lens (0. 42    yellow, and blue paints, respectively, which was in good agreement with the paint thickness measurements by UV-Vis cross-section analysis.…”

“…In the last few years, pictorial layers were examined with the nonlinear modality of femtosecond pump-probe microscopy [26]. MPEF was also tested on paint layers [2] for the monitoring of the laser-removal of organic and inorganic acrylic paints [21] and for the evaluation of paint layer thickness with micrometric resolution [27,28]. As pointed out by these previous studies, the presence of highly scattering and/or absorbing media (pigments) in multi-layered painted systems tends to obstruct the detection of the NLOM signal, thus impeding the visualization of the stratigraphy.…”

“…Based on these results, it was hypothesized that a longer excitation wavelength could provide more reliable cross-sectional measurements. Recently, it has been demonstrated that NLOM with excitation of 800 nm in the MPEF modality serves to determine the thickness of single-layered samples (acrylic paints laid on glass support) [27], despite the severe drop in signal intensity with increasing depth. This suggests that the in-depth measurement of paint layers is feasible, provided that paint materials have a sufficient degree of transparency to the excitation wavelength and in the spectral range of the emitted fluorescence.…”

Combined multiphoton fluorescence microscopy and photoacoustic imaging for stratigraphic analysis of paintings

“…More recently, though, nonlinear microscopy approaches were introduced in regard to cultural heritage (CH) diagnostics in an effort to extend the maximum imaging depth when looking into more scattering media, acquiring different kinds of contrast for assessing the composition of the used materials. In this direction, various studies have proven the potential of multiphoton excitation fluorescence (MPEF), second-harmonic generation and third-harmonic generation nonlinear contrast modes for depth-resolved imaging of fresh and aged varnishes [2,3], lining glues [4], historical coatings [5], parchments [6], paints [7,8], and corrosion layers in metal artefacts [9]. Nevertheless, several materials typically met in works of art are optically opaque as a result of their inherently high scattering and absorption properties, not permitting observations beyond a few μm in depth using solely light.…”