Jana Striova

Colored grounds of gilt stucco surfaces as analyzed by a combined microscopic, spectroscopic and elemental analytical approach.

A survey of gilts applied to stucco surfaces that specifically focuses on the compositions of their colored grounds is reported. Gilt samples of a common geographical (Lombardy in Italy) and temporal provenance (17th–18th century) were studied in the form of polished cross-sections by optical and electron microscopy (SEM-EDS), micro-Raman (μRaman) spectroscopy and Fourier-transform infrared microspectroscopy (μFTIR). Comparing samples with superimposed grounds and gilts enabled light to be shed on the choice of specific materials, their stratigraphic functions, decorative effects, and technological performances. Iron oxide pigments were found in the older grounds, sometimes in the presence of lead white (2PbCO3·Pb(OH)2) or minium (Pb3O4). In more recent grounds, chrome yellow (PbCrO4), chrome orange (PbCrO4·PbO), cinnabar (α-HgS) and barium white (BaSO4), invariably mixed with lead white, were encountered. Evidence for the use of organic mordants (colophony and wax, or siccative oil) was obtained by μFTIR. This combined μFTIR and μRaman spectroscopic and elemental (SEM-EDS) analytical approach enhances knowledge of the composition of gold grounds, their variability and their chronological evolution.

Terracotta polychrome sculptures examined before and after their conservation work: contributions from non-invasive in situ analytical techniques.

The potential of non-invasive in situ analytical techniques such as portable Raman, portable X-ray fluorescence, portable optical microscope and fibre optics reflectance spectroscopy has been shown studying painted layers of Renaissance terracotta polychrome sculptures belonging to the statuary of Santo Sepolcro Church in Milan. The results obtained allowed pointing out the contribution of these techniques to the compositional diagnostic, providing complete information, in some cases, better than micro-destructive techniques, on the kind of pigments used on the external painted layers. Moreover, a comparison with the results obtained before the last conservation work (2009) with micro-destructive techniques allowed ascertaining the removal of the external painted layers during the conservation operations.

Non-destructive and non-invasive analyses shed light on the realization technique of ancient polychrome prints.

Five polychrome prints representing famous painters, such as Albrecht Dürer, were analyzed using a non-destructive and non-invasive methodology as required by the artwork typology. The diagnostic strategy includes X-ray fluorescence (XRF), reflectance micro-infrared (microFTIR) and micro-Raman (microRaman) spectroscopy. These prints were realized with a la poupée method that involves application of the polychrome inks on a single copper plate, before the printing process. A broad range of compounds (i.e., cinnabar, red lead, white lead, umber earth, hydrated calcium sulfate, calcium carbonate, amorphous carbon, and Prussian blue) was employed as chalcographic inks, using linseed oil as a binding medium. Gamboge was identified in the delicate finishing brush touches realized in watercolor.

Determination of thickness of thin turbid painted over-layers using micro-scale spatially offset Raman spectroscopy

We present a method for estimating the thickness of thin turbid layers using defocusing micro-spatially offset Raman spectroscopy (micro-SORS). The approach, applicable to highly turbid systems, enables one to predict depths in excess of those accessible with conventional Raman microscopy. The technique can be used, for example, to establish the paint layer thickness on cultural heritage objects, such as panel canvases, mural paintings, painted statues and decorated objects. Other applications include analysis in polymer, biological and biomedical disciplines, catalytic and forensics sciences where highly turbid overlayers are often present and where invasive probing may not be possible or is undesirable. The method comprises two stages: (i) a calibration step for training the method on a well characterized sample set with a known thickness, and (ii) a prediction step where the prediction of layer thickness is carried out non-invasively on samples of unknown thickness of the same chemical and physical make up as the calibration set. An illustrative example of a practical deployment of this method is the analysis of larger areas of paintings. In this case, first, a calibration would be performed on a fragment of painting of a known thickness (e.g. derived from cross-sectional analysis) and subsequently the analysis of thickness across larger areas of painting could then be carried out non-invasively. The performance of the method is compared with that of the more established optical coherence tomography (OCT) technique on identical sample set. This article is part of the themed issue ‘Raman spectroscopy in art and archaeology’.

Optical and spectroscopic tools for evaluating Er:YAG laser removal of shellac varnish

Abstract We report on tests to remove naturally and artificially aged shellac varnish by laser and traditional chemical cleaning from the substrates of mural paintings. Optical tools were used for the evaluation of cleaning processes, in particular laser microprofilometry to assess the changes in the surface morphology and time-domain confocal optical coherence tomography (OCT) to evaluate varnish thickness. The cleaning assessment was integrated with molecular characterization provided by portable Fourier transform infrared spectroscopy operating in reflectance mode, and colorimetric measurements. This complete analytical approach led to optimized laser-based cleaning tests at 1.9 and 2.6 J/cm2 in the presence of isopropanol, which ensured controllable and safe partial removal of shellac varnish without alteration to the substrate morphology.

Thin layered drawing media probed by THz time-domain spectroscopy

Dry and wet drawing materials were investigated by THz time-domain spectroscopy in transmission mode. Carbon-based and iron-gall inks have been studied, some prepared following ancient recipes and others using current synthetic materials; a commercial ink was studied as well. We measured the THz signals on the thin films of liquid inks deposited on polyethylene pellicles, comparing the results with the thick pellets of dried inks blended with polyethylene powder. This study required the implementation of an accurate experimental method and data analysis procedure able to provide a reliable extraction of the material transmission parameters from a structured sample composed of thin layers, down to a thickness of a few tens of micrometers. THz measurements on thin ink layers enabled the determination of both the absorption and the refractive index in an absolute scale in the 0.1-3 THz range, as well as the layer thickness. THz spectroscopic features of a paper sheet dyed by using one of the iron-gall inks were also investigated. Our results showed that THz time-domain spectroscopy enables the discrimination of various inks on different supports, including the application on paper, together with the proper determination of the absorption coefficients and indices of refraction.

Portable Raman versus portable mid-FTIR reflectance instruments to monitor synthetic treatments used for the conservation of monument surfaces

This study aims to evaluate the relevance of portable Raman and portable mid-Fourier transform infrared (FTIR) reflectance instruments in monitoring the synthetic treatments applied on plaster substrates, a crucial issue in a conservation work. Some polymeric consolidants and protectives have a relatively short life owing to their degradation, and after some years the surface should be retreated. It follows that any information about the presence and composition of the products applied, their chemical transformations and their distribution on the surfaces is essential. For these purposes, conservation scientists should seek and test new in situ methods, and this is of utmost importance especially in the case of buildings, considering their large dimensions and consequent extensive mapping. The effectiveness of portable Raman and portable mid-FTIR reflectance instruments has been compared by analysing a set of laboratory specimens prepared and treated with variable amounts of products belonging to three classes of polymers; the spectroscopic investigation highlighted, for the first time, the limits and the advantages of portable Raman and portable mid-FTIR reflectance instruments in the detection of small amounts of products commonly employed for the conservation of plasters.

Laser cleaning of a nineteenth-century bronze sculpture: In situ multi-analytical evaluation

Abstract Laser cleaning of bronze artefacts is a very promising conservation approach and its wider distribution would benefit from further studies of its advantages and drawbacks. Some important references are available, yet laser cleaning cannot be considered a traditional method for bronze surfaces. In this paper, we report on the evaluation of cleaning tests performed on an important statue of Napoleon, depicted as Mars the Peacemaker, cast by Righetti in 1811 after a model by Antonio Canova. The sculpture is exposed to the outdoor environment in the main courtyard of the Brera Gallery, Milan, Italy. An analytical survey of corrosion products was carried out using X-ray diffraction, micro-Fourier transform infrared spectroscopy (μFTIR), and μRaman spectroscopy; and scanning electron microscopy with energy-dispersive X-ray spectroscopy analysis was applied to cross-sections. The results showed the ubiquitous presence of brochantite Cu4(SO4)(OH)6. Antlerite Cu3SO4(OH)4 is also occasionally present. Residues of an past conservation treatment (Incralac®) and of proteins were detected by μFTIR. Laser cleaning tests were carried out with an El.En. Combo laser, both in long Q-switched and short free-running regimes. Evaluation of the effects of cleaning was carried out with a portable microscope and a reflectance colorimeter. Moreover, a portable FTIR spectrometer was used in order to evaluate the presence of coating residues and corrosion products after cleaning. Electrochemical impedance spectroscopy was used in situ on a selection of the tested areas in order to characterize the effect of laser cleaning on the rate of corrosion. A specially designed contact probe was used for measurements in the frequency range 100 kHz–10 mHz both before and after cleaning.

Improvement of the visibility of concealed features in artwork NIR reflectograms by information separation

Abstract Near Infrared (NIR) reflectography, coupled to visible (VIS) one, is a spectrophotometric imaging technique employed to probe both the inner and the outer layers of artworks. NIR reflectograms may partially contain information pertinent to the visible spectrum (due to the poor pigment transparency in NIR) and this decreases their comprehensibility. This work presents an innovative digital processing methodology for accentuating information contained in the infrared reflectograms. The proposed method consists of inducing minor changes in pixel intensity by suppressing VIS information content from NIR information content. The method creates such enhanced NIR reflectogram by extrapolating VIS reflectogram to a reflectogram recorded in NIR range and by subtracting it from the measured values in the near infrared spectral sub-band. As an extrapolator we suggest a feed forward artificial neural network (ANN). Significant results of improved visualization are exemplified on reflectograms acquired with a VIS-NIR 〈 400 , 2250 〉 nm scanning device on real paintings such as Madonna dei Fusi attributed to Leonardo da Vinci. Parameters of the method, artificial neural network and separability of used pigments are discussed.

Nonlinear optical imaging techniques (NLO) for painting investigation

Nonlinear optical imaging techniques (NLO imaging), i.e. multiphoton excitation fluorescence (MPEF), second and third harmonic generation (SHG, THG), are high-resolution imaging modalities which may provide non-destructive determination of thickness and composition within multi-layer objects as a function of depth. NLO techniques have been recently introduced into artworks analyses, giving promising results. As regards paintings, the guiding idea is to exploit the low absorption in the near-infrared (NIR) of most materials, using a laser light in this spectral region which may generate non-linear interactions within materials. The objective is to make feasible the in-depth profiling of thin films on the basis of refractive index changes, variation of optical activity and presence of fluorophores. Such information is definitely useful for the analysis of painted objects and can be also crucial for the monitoring of restoring operations. In this study, we present the preliminary results obtained through the application of NLO imaging techniques on a series of singleand multi-layers systems simulating real egg-tempera wood panel paintings. This research is part of the wider Iperion CH Project, whose WP6 entails the evaluation of NLO-imaging to obtain highly resolved en-face and 3D images of small areas on paintings. To that purpose, a set of multi-layer painted samples, simulating real paintings, was designed and realized. They are currently under analysis with different operating systems and modalities within the partnership. In order to assess the reliability of NLO measurements, data were complemented with other well-established techniques, such as Fourier Transform Infrared Spectroscopy (FTIR) and X-Ray Fluorescence (XRF) for the chemical characterization of materials, Optical Coherence Tomography (OCT) for the determination of layers thickness and laser scanning micro-profilometry for the study of surface morphology. In addition, colorimetric analyses, i.e. reflectance spectroscopy in the visible region, were performed in order to chromatically characterize pigments and lakes. Finally, the transparency of the different materials was investigated by means of Scanning Multispectral VIS-NIR Reflectography. The preliminary results suggest that the 740 nm wavelength, while enabling the visualization of the painting surfaces micromorphology, is not effective for the determination of thickness through the detection of the non-linear signals.