Influence of composition and roughness on the pigment mapping of paintings using mid-infrared fiberoptics reflectance spectroscopy (mid-IR FORS) and multivariate calibration

Mid-infrared fiberoptics reflectance spectroscopy (mid-IR FORS) is a very interesting technique for artwork characterization purposes. However, the fact that the spectra obtained are a mixture of surface (specular) and volume (diffuse) reflection is a significant drawback. The physical and chemical features of the artwork surface may produce distortions in the spectra that hinder comparison with reference databases acquired in transmission mode. Several studies attempted to understand the influence of the different variables and propose procedures to improve the interpretation of the spectra. This article is focused on the application of mid-IR FORS and multivariate calibration to the analysis of easel paintings. The objectives are the evaluation of the influence of the surface roughness on the spectra, the influence of the matrix composition for the classification of unknown spectra, and the capability of obtaining pigment composition mappings. A first evaluation of a fast procedure for spectra management and pigment discrimination is discussed. The results demonstrate the capability of multivariate methods, principal component analysis (PCA), and partial least squares discrimination analysis (PLS-DA), to model the distortions of the reflectance spectra and to delimitate and discriminate areas of uniform composition. The roughness of the painting surface is found to be an important factor affecting the shape and relative intensity of the spectra. A mapping of the major pigments of a painting is possible using mid-IR FORS and PLS-DA when the calibration set is a palette that includes the potential pigments present in the artwork mixed with the appropriate binder and that shows the different paint textures. Graphical Abstract

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Pigment identification in artwork using graphite furnace atomic absorption spectrometry

The use of a sampling technique is described for the identification of metals from inorganic pigments in paint. The sampling technique involves gently contacting a cotton swab with the painted surface to physically remove a minute quantity (∼1-2 μg) of pigment. The amount of material removed from the painted surface is invisible to the unaided eye and does not cause any visible effect to the painted surface. The cotton swab was then placed in a 1.5 ml polystyrene beaker containing HNO₃ to extract pigment metals prior to analysis using graphite furnace atomic absorption spectrometry (GFAAS). GFAAS is well suited for identifying pigment metals since it requires small samples and many pigments consist of main group elements (e.g. Al) as well as transition metals (e.g. Zn, Fe and Cd). Using Cd (cadmium red) as the test element, the reproducibility of sampling a paint surface with the cotton swab was approximately 13% in either a water or oil medium. To test the feasibility of cotton sampling for pigment identification, samples were obtained from paintings (watercolour and oil) of a local collection. Raman spectra provided complementary information to the GFAAS, which together are essential for positive identification of some pigments. For example, GFAAS indicated the presence of Cu, but the Raman spectra positively identified the modern copper pigment phthalocyanine green (Cu(C₃₂Cl₁₆N₈). Both Raman spectroscopy and GFAAS were useful for identifying ZnO as a white pigment.     

Benefits of applying combined diffuse reflectance FTIR spectroscopy and principal component analysis for the study of blue tempera historical painting

This paper explores the application of diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) to the examination of historic blue pigments and blue tempera paintings commonly found on works of art. The discussion is mainly focused on the practical benefits of using this technique joined to principal component analysis (PCA), a powerful multivariate analysis tool. Thanks to the study of several replica samples that contain either pure blue pigments (azurite, lapis lazuli and smalt), or pure binder (rabbit glue) and mixtures of each of the pigments with the binder (tempera samples), different aspects of these benefits are highlighted. Comparative results of direct spectra and multivariate analysis using transmittance-Fourier transform infrared spectroscopy (T-FTIR) are discussed throughout this study. Results showed an excellent ability of PCA on DRIFT spectra for discriminating replica samples according to differing composition. Several IR regions were tested with this aim; the fingerprint IR region exhibited the best ability for successfully clustering the samples. The presence of the binder was also discriminated. Only using this approach it was possible to completely separate all the studied replica samples. This demonstrates the potential benefits of this approach in identifying historical pigments and binders for conservation and restoration purposes in the field of Cultural Heritage.     

Impact of structural features on pigment properties of α-Fe2O3 haematite

Various α-Fe₂O₃ haematite samples were synthesized by precipitation routes (under standard or hydrothermal conditions) followed by thermal treatments under air. The trigonal distortion (C_3v point group) of the Fe³⁺ octahedral sites, which depends on the synthesis route and thermal treatment, was investigated by X-ray diffraction, Mössbauer spectroscopy and visible-near infrared (Vis-NIR) spectroscopy. The correlation between diffuse reflectance spectra and structural features of the haematite samples is reported and discussed herein. The slight increase of the average distortion of the Fe³⁺ octahedral sites, which depends on the annealing temperature of the precipitated sample, directly linked to the crystallite size, contrasts with the larger reduction of the sites distortion for the compound prepared by hydrothermal route due to the occurrence of hydroxyl groups substituted for O²⁻ anions as well as Fe³⁺ cationic vacancies. On a local point of view, as shown by Mössbauer spectroscopy, the Fe³⁺ octahedral sites distortion decreases from the centre towards the surface of the grains. Then the smaller the grain size, the lower the average site distortion. Finally, the reduction of the octahedral distortion was directly correlated to the two FeO charge transfer bands in the visible range and the colour of as-prepared haematites.     

Classification and quantification analysis of Radix scutellariae from different origins with near infrared diffuse reflection spectroscopy

A rapid near infrared spectroscopy analysis method was developed for the geographical origin discrimination and content determination of Radix scutellariae, a kind of Traditional Chinese Medicine (TCM). 81 R. scutellariae samples from six different origins were analyzed with HPLC-UV as reference method. The NIR spectra were collected in integrating-sphere diffused reflection mode and processed with different spectra pretreated methods. Discriminant analysis (DA) and discriminant partial least squares (DPLS) were applied to classify the geographical origins of those samples, and the latter had a better predictive ability with 100% accuracy after two exceptional samples eliminated from the calibration set. For the quantitative calibration, the samples were divided into calibration set and validation set by Kennard-Stone algorithm. The models of baicalin, wogonoside, baicalein, wogonin were established with partial least squares (PLS) algorithm and the optimal principal component (PC) numbers were selected with Leave-One-Out (LOO) cross-validation. The established models were evaluated with the root mean square error of prediction (RMSEP) and corresponding correlation coefficients. The correlation coefficients of all the four calibration models are above 0.920, and the RMSEPs of baicalin, wogonoside, baicalein and wogonin are 0.752%, 0.094%, 0.418% and 0.139%, respectively. This research indicated that the NIR diffuse reflection spectroscopy could be used for the rapid analysis of R. scutellariae, which is beneficial to the quality control of this raw material in TCM pharmaceutical factory, and will also help to solve analogous problems.     

Integrated analytical techniques for the study of ancient Greek polychromy

The materials used in the decoration of three painted astragaloi (knucklebones) from the Koroneia cave (Greece) were investigated by means of sequential application of non-destructive and destructive techniques: optical microscopy, environmental scanning electron microscopy coupled with X-ray microanalysis (ESEM-EDX), Fourier transform infrared spectroscopy (FTIR) with micro-attenuated total reflection (μ-ATR) technique, high performance liquid chromatography (HPLC) coupled with UV-fluorescence and gas chromatography-mass spectrometry (GC-MS) were used.The main results highlighted that the three astragaloi were prepared with a ground of ochre or iron clay and painted with a proteinaceous matter such as binder egg tempera. Both FTIR and GC-MS agree in the detection of lipids that can be related to egg. Organic dyestuffs identified as madder lake and shellfish purple were used together with inorganic pigments.     

Nonlinear Unmixing of Hyperspectral Datasets for the Study of Painted Works of Art

Nonlinear unmixing of hyperspectral reflectance data is one of the key problems in quantitative imaging of painted works of art. The approach presented is to interrogate a hyperspectral image cube by first decomposing it into a set of reflectance curves representing pure basis pigments and second to estimate the scattering and absorption coefficients of each pigment in a given pixel to produce estimates of the component fractions. This two‐step algorithm uses a deep neural network to qualitatively identify the constituent pigments in any unknown spectrum and, based on the pigment(s) present and Kubelka–Munk theory to estimate the pigment concentration on a per‐pixel basis. Using hyperspectral data acquired on a set of mock‐up paintings and a well‐characterized illuminated folio from the 15th century, the performance of the proposed algorithm is demonstrated for pigment recognition and quantitative estimation of concentration.     

Unilateral NMR, 13C CPMAS NMR spectroscopy and micro-analytical techniques for studying the materials and state of conservation of an ancient Egyptian wooden sarcophagus

A multi-technique approach was employed to study a decorated Egyptian wooden sarcophagus (XXV–XXVI dynasty, Third Intermediate Period), belonging to the Museo del Vicino Oriente of the Sapienza University of Rome. Portable non-invasive unilateral NMR was applied to evaluate the conservation state of the sarcophagus. Moreover, using unilateral NMR, a non-invasive analytical protocol was established to detect the presence of organic substances on the surface and/or embedded in the wooden matrix. This protocol allowed for an educated sampling campaign aimed at further investigating the state of degradation of the wood and the presence of organic substances by ¹³C cross polarization magic angle spinning (CPMAS) NMR spectroscopy. The composition of the painted layer was analysed by optical microscopy (OM), scanning electron microscopy–energy dispersive spectroscopy (SEM–EDS), Raman and surface enhanced (resonance) Raman spectroscopy (SERS/SERRS), infrared and GC–MS techniques, evidencing original components such as clay minerals, Egyptian green, indigo, natural gums, and also highlighting restoration pigments and alteration compounds. The identification of the wood, of great value for the reconstruction of the history of the artwork, was achieved by means of optical microscopy.     

Effect of the Nature of the Precursor on the Activity of Copper-Containing Catalysts Based on Erionite in Oxidation of CO

We have studied the catalytic activity of copper-containing zeolite catalysts based on erionite (ERI) in oxidation of CO. We have established that the activity of Cu-ERI systems is due to isolated coordination unsaturated Cu²⁺ cations which are stabilized in the catalyst on sites with strong tetragonal distortion and are reduced to Cu⁺ during catalysis. According to X-ray photoelectron spectroscopy (XPS), diffuse reflection electronic spectra, and temperature programmed reduction by hydrogen (TPR-H₂) spectra, activity differences between 3% Cu-ERI catalysts obtained from different precursors are determined by the different numbers of Cu²⁺ cations capable of being reduced during the reaction at T < 400 °C: the higher the content of such cations in the samples, the higher the activity of the Cu-ERI systems. __________ Translated from Teoreticheskaya i Eksperimental'naya Khimiya, Vol. 41, No. 5, pp. 317–322, September–October, 2005.     

External reflection FTIR spectroscopy of the cationic surfactant hexadecyltrimethylammonium bromide (CTAB) on an overflowing cylinder

External reflection Fourier transform infrared spectroscopy (ER-FTIRS) has been used to study the adsorption of the cationic surfactant hexadecyltrimethylammonium bromide (CTAB) at the air-water interface under nonequilibrium conditions. An overflowing cylinder (OFC) was used to generate a continually expanding liquid surface with a surface age of 0.1-1 s. ER-FTIR spectra were acquired by a single bounce of p- or s-polarized radiation from the flowing surface of the OFC. The C-H stretching region of CTAB spectra was analyzed both by subtraction of a reference spectrum of pure water and by a chemometric technique known as target factor analysis (TFA). The TFA method is shown to give lower scatter in the weight of the component assignable to the adsorbed CTAB monolayer and to permit analysis of spectra at lower bulk surfactant concentrations. The surface sensitivity of ER-FTIRS is demonstrated both experimentally and by theoretical modeling. Modeling shows that surfactant adsorbed at the surface and dissolved in the bulk solution can be distinguished by reflection spectroscopy but also highlights potential errors that can arise from the neglect of the bulk surfactant contribution to the ER-FTIR spectra. Polarized spectra are consistent with an isotropic distribution of transition dipole moments of the hydrocarbon chains in CTAB. Component weights of the CTAB monolayer determined by TFA are compared with an independent determination of values of the dynamic surface excess, Gammadyn, by neutron reflection and ellipsometry. The relationship between the component weights and Gammadyn shows a small but significant deviation from linearity. Possible explanations for this deviation are discussed. The feasibility of using TFA to deconvolute ER-FTIR spectra of mixtures of hydrocarbon surfactants is demonstrated.     

Hydrothermal synthesis and characterization of In<Superscript>3+</Superscript> modified BiVO<Subscript>4</Subscript> nanoparticles with enhanced photocatalytic activity

In³⁺-doped BiVO₄ nanoparticles with enhanced visible light activity have been successfully synthesized by a hydrothermal method. The synthesized materials were characterized by X-ray diffraction, Raman, X-ray photoelectron spectroscopy, scanning electron microscopy, BET surface areas analysis, and ultraviolet–visible diffuse reflectance spectra. In comparison with pure BiVO₄, the In³⁺-doped BiVO₄ displayed greater photocatalytic activity in the degradation of methyl blue under visible light illumination. All samples possessed a single monoclinic structure. The introduction of In ions resulted in structural distortion and the decreased band gap energy, producing more electrons and holes for photocatalytic reaction. In the meantime, the doping In ions entails a red shift in the absorption edge and an increase in the intensity of light absorption. The best photocatalytic performance was obtained with the BiVO₄ sample containing 5.0 mol% In ions.     

Extraction of multiple pure component H and C NMR spectra from two mixtures: Novel solution obtained by sparse component analysis-based blind decomposition

Sparse component analysis (SCA) is demonstrated for blind extraction of three pure component spectra from only two measured mixed spectra in ¹³C and ¹H nuclear magnetic resonance (NMR) spectroscopy. This appears to be the first time to report such results and that is the first novelty of the paper. Presented concept is general and directly applicable to experimental scenarios that possibly would require use of more than two mixtures. However, it is important to emphasize that number of required mixtures is always less than number of components present in these mixtures. The second novelty is formulation of blind NMR spectra decomposition exploiting sparseness of the pure components in the wavelet basis defined by either Morlet or Mexican hat wavelet. This enabled accurate estimation of the concentration matrix and number of pure components by means of data clustering algorithm and pure components spectra by means of linear programming with constraints from both ¹H and ¹³C NMR experimental data. The third novelty is capability of proposed method to estimate number of pure components in demanding underdetermined blind source separation (uBSS) scenario. This is in contrast to majority of the BSS algorithms that assume this information to be known in advance. Presented results are important for the NMR spectroscopy-associated data analysis in pharmaceutical industry, medicine diagnostics and natural products research.     

Discrimination of Ephedra plants with diffuse reflectance FT-NIRS and multivariate analysis

The secondary metabolites of different Ephedra plants are various. Therefore, the discrimination of different Ephedra plants is significant. An objective, easy-to-use, rapid and pollution-free approach is proposed for discriminating Ephedra plants of different species, habitats and picking times on the basis of diffuse reflectance Fourier transform near infrared spectroscopy (FT-NIRS) measurements and multivariate analysis. The Fourier transform near infrared diffuse reflectance spectra (NIRDRS) were acquired from 37 pulverized samples of Ephedra plants put in glass vials in the near infrared (NIR) region between 10 000 and 4000 cm⁻¹, averaging 64 scans per spectrum at a resolution of 4 cm⁻¹. After spectra processing and data pre-processing, spectral data were analyzed respectively with three multivariate analysis techniques: discriminant analysis (DA), self-organizing map (SOM) and back-propagation artificial neural network (BP-ANN). The proposed method could distinguish not only the Ephedra plants of three species and two habitats but also the plants picked at different times of day without special sample treatment and the use of chemical reagents. The performance indexes of the DA model were 84.2-91.9% and the prediction accuracies of both the SOM and the BP-ANN models reached 93.3-100.0%.     

Correlation between structural features and vis-NIR spectra of α-Fe2O3 hematite and AFe2O4 spinel oxides (A=Mg, Zn)

Iron (III) rich pigments MgFe₂O₄ and ZnFe₂O₄ spinel ferrites and α-Fe₂O₃ hematite were synthesized by Pechini route and precipitation process, respectively. The compounds were characterized by X-ray diffraction (XRD), Mössbauer spectroscopy and visible-NIR spectroscopy. Diffuse reflectance spectra were interpreted in regard of structural features of the three oxides in order to correlate absorption bands positions with structural parameters. It has been demonstrated that the two main absorption edges occurring in visible range (400-800 nm) can be attributed to two 2p(O²⁻)→3d(Fe³⁺) charge transfers, the energy being directly linked to the distortion degree of the [FeO₆] octahedra.     

Pure Isotropic Proton NMR Spectra in Solids using Deep Learning

The resolution of proton solid-state NMR spectra is usually limited by broadening arising from dipolar interactions between spins. Magic-angle spinning alleviates this broadening by inducing coherent averaging. However, even the highest spinning rates experimentally accessible today are not able to completely remove dipolar interactions. Here, we introduce a deep learning approach to determine pure isotropic proton spectra from a two-dimensional set of magic-angle spinning spectra acquired at different spinning rates. Applying the model to 8 organic solids yields high-resolution ¹H solid-state NMR spectra with isotropic linewidths in the 50–400 Hz range.     

Raman microspectrometric investigation of wall paintings in S. Giovanni Evangelista Abbey in Parma: a comparison between two artists of the 16th century

Micro-Raman spectroscopy, combined with gas chromatography and ultra-violet fluorescence photography, was used to study some wall paintings in the S. Giovanni Evangelista Abbey in Parma, Italy. The restoration of some painted chapels enabled a comparison between two painters of the 16th century: Parmigianino (Girolamo Francesco Maria Mazzola, 1503-1540) and Michelangelo Anselmi (1492?-1556?). Micro-Raman spectroscopy determined the palette used by the artists, leading to the identification of different white, yellow, red, brown, green, blue and black pigments. Some pigments are evidence of later restorations. Gas chromatography combined with mass spectroscopy revealed the presence of organic binding media and enabled to distinguish between fresco and secco paintings.     

The potential of field spectroscopy for the assessment of sediment properties in river floodplains

Investigations have shown that visible-near-infrared (VNIR) spectroscopy can accurately determine soil properties under laboratory conditions. In situ assessment of soil properties is of great benefit for several applications, as spectra can be acquired fast and almost continuously. The present study used partial least squares (PLS) regression to establish a relationship between soil reflectance spectra measured under field conditions and the organic matter and clay content of the soil. Spectra were acquired with a fieldspectrometer in a recently reconstructed floodplain along the river Rhine in The Netherlands. Several spectral pre-processing methods were employed to improve the performance and robustness of the models. Results indicate that, under varying surface conditions, field spectroscopy in combination with multivariate calibration does result in a qualitative relation for organic matter (R²=0.45) and clay content (R²=0.43) while under laboratory conditions more accurate results are obtained (R²=0.69 and 0.92, respectively). Soil moisture and vegetation cover had a negative influence on the prediction capabilities for both soil properties. Although the performance of the spectra measured in situ is not as accurate as physical analysis, the accuracy obtained is useful for rapid soil characterisation and remote sensing applications.     

Synthesis and characterization of environmentally benign calcium-doped Pr2Mo2O9 pigments: Applications in coloring of plastics

A new class of environmentally benign rare earth pigments of general formula Pr_2−xCa_xMo₂O_9−δ (x ranges from 0 to 1.0) displaying colors ranging from green to yellow were synthesized by traditional solid-state route, as alternatives to lead, cadmium and chromium colorants. The products were characterized by X-ray powder diffraction, UV-vis spectroscopy and CIE-L*a*b* 1976 color scales. The coloring mechanism is based on the strong absorptions of the pigments in the blue and red regions due to electronic transitions between 4f²→4f¹5d¹ states of Pr³⁺. The designed pigments consist of non-toxic elements and further found to be thermally and chemically stable. The yellow-green pigments were found to be interesting alternatives to existing toxic pigments for coloration of plastics.