Raman spectroscopy of synthetic organic pigments used in 20th century works of art

Raman microscopy allows a non‐destructive characterisation of inorganic and organic painting materials such as pigments and organic dyestuffs. The objectives of this study are the more recent organic pigments typically present in paintings and other art works from the 20th century. More than 20 organic synthetic pigments from different chemical classes could be identified by Raman spectroscopy using different excitation wavelengths (457.9, 476.5, 487.9, 514.5, 632.8, and 1064 nm). To evaluate the performance for real paint samples, varying paint mixtures of the Hansa Yellow pigment PY 3 and the binding medium Mowilith, a polyvinyl acetate (PVAC) compound, were characterised; PY 3 was determined at a 1 wt% level in the binder. In addition, commercial tube paints containing the quinacridone violet PV 19 were studied. The pigment was clearly identified in all of these more complex oil and acrylic paints. Finally, alizarin (PR 83) and a green copper phthalocyanine pigment (PG 7) could unambiguously be identified by Raman microscopy in the painting ‘Woman with mandolin in yellow and red’ of Max Beckmann dating 1950. The discovery of a red naphthol AS pigment by Raman spectroscopy in a sample from the ‘Three field workers’ by Georg Baselitz (1964/1965) demonstrated that in some cases complementary chromatographic methods are needed for a comprehensive identification of the organic pigments. Copyright © 2008 John Wiley & Sons, Ltd.     

The use of Raman microscopy and laser desorption ionization mass spectrometry in the examination of synthetic organic pigments in modern works of art

Synthetic organic pigments are widely used in modern and contemporary works of art. They have been examined by a variety of techniques including spectroscopic methods such as Fourier transform infrared spectroscopy, Raman, and X‐ray powder diffraction as well as chromatographic or mass spectrometric techniques such as pyrolysis‐gas chromatography/mass spectrometry and laser desorption ionization mass spectrometry (LDI–MS). Often, a combination of techniques has been used to examine these pigments. This paper describes use of Raman spectroscopy to create a database of colorants including two pigments not previously reported, PO1 and PO2. Then, using Raman spectroscopy in combination with LDI–MS, samples from modern works of art by artists including Mark Rothko, Barnett Newman, and José de Rivera were examined in order to identify the pigments present. One finding was that Rothko used a variety of red pigments over the course of his career including PR11 which has not been previously reported in artwork, and PO2 found with its positional isomer PR1. Knowledge of the colorants serves to inform conservators about display and treatment decisions. Published 2014. This article is a U.S. Government work and is in the public domain in the USA.     

In situ Raman microscopy applied to large Central Asian paintings

Procedures and versatile Raman instruments are described for the non‐destructive in situ analysis of pigments in large paintings. A commercial Raman microscope is mounted on a gantry for scanning paintings with dimensions exceeding 1 m². Design principles and the physical implementation of the set‐up are outlined. Advantages/disadvantages and performance of the gantry‐based instrument are compared with those of a mobile Raman probe, attached to the same Raman microscope. The two set‐ups are applied to Central Asian thangka paintings. The utility of the gantry‐mounted Raman microscope is demonstrated on a 19th century Buddhist painting from Buriatia, South Siberia. Surprisingly, three arsenic‐based pigments, i.e. orpiment, realgar, and pararealgar, are found all in the same painting. Pararealgar is used for painting the numerous yellow areas. Realgar is admixed with red lead for adjusting its orange tint. Finally, orpiment is blended with Prussian blue for producing green. Traditional malachite is used in addition as a non‐adulterated green pigment. The mobile Raman probe was employed for examining a Tibetan painting of the 18th century from Derge monastery in the Kham area of Sichuan. The highly unique painting could be dated well and its origin accurately located. In fact, the painter's workshop, where the thangka has been executed, is shown in great detail on the painting itself. The painter's palette of this thangka matches the canonical set of pigments used in Tibet for more than 10 centuries. Copyright © 2009 John Wiley & Sons, Ltd.     

An example of the complementarity of laser‐induced breakdown spectroscopy and Raman microscopy for wall painting pigments analysis

Laser‐induced breakdown spectroscopy (LIBS) and Raman microscopy were used for the identification of pigments in wall painting. Raman spectroscopy, which provides the molecular ‘fingerprint’ of the compound, is nowadays widely used by the archaeometry community, especially for pigment analysis. LIBS, which provides the elementary composition of samples, is a rapid noncontact method, enabling layer‐by‐layer analysis through a precise laser ablation of the sample. This work deals with the behavior of pigments after a LIBS analysis, by trying to identify the compounds before and after the laser shot. Six commercial pigments prepared with the fresco technique were investigated: ultramarine blue, red lead, charcoal, a yellow and a red ochre, and a green earth. Raman spectra, acquired on the sample surface and in the crater induced by LIBS analysis, were compared. The results show that these pigments are well recognized after a LIBS measurement. The analysis of green earth illustrates that the combination of these two techniques gives complete information from a sample. Copyright © 2007 John Wiley & Sons, Ltd.     

In situ chemical analysis of modern organic tattooing inks and pigments by micro‐Raman spectroscopy

The chemical composition of tattooing pigments has varied greatly over time according to available technologies and materials. Beginning with naturally derived plant and animal extracts, to coloured inorganic oxides and salts, through to the modern industrial organic pigments favoured in today's tattooing studios. The demand for tattooing is steadily growing as it gains cultural popularity and acceptance in today's society, but ironically, increasing numbers of individuals are seeking laser removal of their tattoos for a variety of reasons. Organic pigments are favoured for tattooing because of their high tinting strength, light fastness, enzymatic resistance, dispersion and relatively inexpensive production costs. Adverse reactions have been reported for some organic inks, as well as potential complications, during laser removal procedures stemming from the unintentional creation of toxic by‐products. Currently, regulatory bodies such as the US Food and Drug Administration have not approved any coloured inks to be injected into the skin, and tattoo ink manufacturers often do not disclose the ingredients in their products to maintain proprietary knowledge of their creations. A methodology was established using micro‐Raman spectroscopy on an animal model to correctly identify the constituents of a selection of modern, organic tattoo inks in situ or post procedure, within the skin. This may serve as a preliminary tool prior to engaging in Q‐switched laser removals to assess the risks of producing potentially hazardous compounds. Likewise, the pigments responsible for causing adverse reactions in some patients may be quickly identified to hasten any corresponding treatment. Copyright © 2008 John Wiley & Sons, Ltd.     

Three fabricated pigments (Han purple,indigo and emerald green) in ancient Chinese artifacts studied by Raman microscopy,energy‐dispersive X‐ray spectrometry and polarized light microscopy

In this study, we analyzed three fabricated pigments from ancient artifacts in China. The purple pigment was obtained from a painted pottery figurine unearthed from the Chu Tombs group of the Western Han dynasty in Xuzhou, Jiangsu Province. The dark blue dye was from silk textiles in the Palace Museum. The green pigment was from decorative paintings on ancient architectures in the Palace Museum. These pigments were analyzed with Raman microscopy (RM), energy‐dispersive X‐ray (EDX) analytic spectroscopy and polarized light microscopy (PLM). By comparing their Raman spectra with standard samples, the primary ingredients of the purple pigment and the dark blue dye were found to be Han purple and indigo, respectively. However, the green pigment could not be identified by RM because of strong fluorescence. It was then confirmed to be emerald green by using EDX analytic microscopy and PLM. We also describe the traditional manufacturing methods of these pigments and their applications on artifacts in Chinese history. Copyright © 2007 John Wiley & Sons, Ltd.     

Integration of correlative Raman microscopy in a dualbeam FIB SEM

We present an integrated confocal Raman microscope in a focused ion beam scanning electron microscope (FIB SEM). The integrated system enables correlative Raman and electron microscopic analysis combined with focused ion beam sample modification on the same sample location. This provides new opportunities, for example the combination of nanometer resolution with Raman advances the analysis of sub‐diffraction‐sized particles. Further direct Raman analysis of FIB engineered samples enables in situ investigation of sample changes. The Raman microscope is an add‐on module to the electron microscope. The optical objective is brought into the sample chamber, and the laser source, and spectrometer are placed in a module attached onto and outside the chamber. We demonstrate the integrated Raman FIB SEM function with several experiments. First, correlative Raman and electron microscopy is used for the investigation of (sub‐)micrometer‐sized crystals. Different crystals are identified with Raman, and in combination with SEM the spectral information is combined with structurally visible polymorphs and particle sizes. Analysis of sample changes made with the ion beam is performed on (1) structures milled in a silicon substrate and (2) after milling with the FIB on an organic polymer. Experiments demonstrate the new capabilities of an integrated correlative Raman–FIB–SEM. Copyright © 2016 John Wiley & Sons, Ltd.     

Raman spectroscopy of minerals and mineral pigments in archaeometry

Minerals, as raw structural materials or pigments, play a fundamental role in archaeometry, for the understanding of nature, structure and status of an artefact or object of interest for cultural heritage. A detailed knowledge of the mineral phases is crucial to solve archaeological problems: Raman spectroscopy is a powerful investigation technique and has been applied extensively in the last 30 years on mineral identification and on pigment degradation. Here we report an updated review, covering the last decade, of the applications of Raman techniques to issues in which raw minerals, including mineral pigments, are involved. Particular attention is devoted to cases where the Raman analysis of minerals is deeper than a simple identification of the phases present in an archaeological or artistic object. Copyright © 2016 John Wiley & Sons, Ltd.     

Yellow pigments in painting: characterisation and UV laser‐induced modifications

Artistic yellow pigments, commonly employed from antiquity, were investigated by morphological, spectrophotometric and compositional analyses. Namely, scanning electron microscopy–energy dispersive X‐ray (SEM/EDX) characterisations and reflectance, attenuated total reflection–Fourier transform infrared spectroscopy (ATR‐FTIR) and Raman spectroscopy were carried out on egg yolk tempera models to discriminate the pigments. The models were irradiated with excimer KrF (248 nm) laser at different working conditions, and the effects induced on colour were related to chemical and physical modifications through the same analyses, as a function of laser parameters. It came out that the effects on the pictorial layers are always related to the modifications of the binding medium and, in addition, the laser radiation induces dehydration and transformation of iron oxides in natural earths. Copyright © 2009 John Wiley & Sons, Ltd.     

Raman identification of green and blue pigments in Etruscan polychromes on architectural terracotta panels

Micro‐Raman measurements were performed on two Etruscan polychromes on architectural terracotta panels now on display at the Villa Giulia Etruscan Museum in Rome. These painted panels, dated from 530 to 520 B .C ., are of particular interest because of the unusual presence of green and blue layers. Etruscans in the Archaic Age indeed mainly used white, red, and black colours for painted terracotta panels. Raman spectra allowed the analytical identification of green (malachite) and blue (Egyptian blue) pigments employed by Etruscans for this kind of artistic production. This finding provides evidence for a larger use of malachite and Egyptian blue, previously well documented only in Etruscan wall paintings. The use of different pigments to obtain different colour tones has been also observed. Egyptian blue is indeed mixed with malachite to obtain different green tones, and a black pigment seems to have been applied over the Egyptian blue layer to obtain a dark blue tone. Copyright © 2006 John Wiley & Sons, Ltd.     

Pigment study by Raman microscopy of 23 paintings by the Portuguese artist Henrique Pousão (1859–1884)

Twenty‐three paintings by Henrique Pousão—a 19th century Portuguese painter—belonging to the collection of Museu Nacional Soares dos Reis, Porto, Portugal, were analysed by Raman microscopy. The fine focus of a 100× objective allowed the visualisation and individual identification of small grains. As a result, thirty‐seven compounds, namely, anatase, barium white, basic lead sulfate, brochantite, cadmium red, cadmium yellow, calcium carbonate, carbon‐based black, celadonite, chrome green, chrome orange, chrome yellow, cobalt blue, cochineal lake, copper sulfide, emerald green, iron(III) oxyhydroxide, iron(III) oxide, kaolinite, lead antimonate yellow, lead carbonate, lead white, lead sulfate, madder lake, malachite, Prussian blue, quartz, realgar/pararealgar, red lead, rutile, Scheele's green, strontium yellow, ultramarine blue, vermilion, viridian, zinc white and zinc yellow, were identified. Not all these compounds are pigments; some are extenders, others trace components and others probably products of reactions between pigments. Special attention was given to the Raman characterisation of celadonite, chrome orange, basic lead sulfate and lead antimonate yellow. Complementary techniques were used to confirm the identities of certain pigments and to characterise reference samples. Pousão, whose work has not previously been studied spectroscopically, was found to have used a remarkably wide range of pigments over his painting periods, without showing significant preference for any particular set of pigments. Copyright © 2007 John Wiley & Sons, Ltd.     

Origin of spectral interferences in femtosecond stimulated Raman microscopy

In femtosecond stimulated Raman microscopy (FSRM), a spectrally narrow (Raman pump) pulse and a broad (Raman probe) laser pulse are employed to generate the Raman spectra of microscopic objects. The resulting spectra exhibit, in addition to the Raman bands, spectral modulations of comparable amplitude. Here a model is devised that attributes these modulations to a four‐wave mixing (FWM) process. Two light fields of the probe pulse and one field of the pump pulse serve as input fields. The resulting FWM field experiences a heterodyne amplification by the probe field. Simulations based on this model reproduce the appearance of the spectral modulations. Furthermore, the amplitude of the modulations exhibits dependences on the energies of pump and probe pulses as well as on the nonlinear refractive index n₂, which are in line with the model. Copyright © 2011 John Wiley & Sons, Ltd.     

Disturbing interference patterns in femtosecond stimulated Raman microscopy

Femtosecond stimulated Raman microscopy (FSRM) is an upcoming technique in nonlinear microscopy which facilitates rapid chemical mapping. It employs femtosecond white‐light pulses as probe pulses and intense picosecond pulses as pump pulses. Stimulated Raman scattering (SRS) occurs at the focus of a scanning microscope. Chemical constituents in the sample are identified via their Raman signatures. In this article, disturbing interference patterns in FSRM are reported. They are caused by a broadening of the pump pulse due to nonlinear interactions in the focal region of the microscope and reduce the signal‐to‐noise ratio. The properties of these modulations are explored, and the methods to suppress them are presented. Copyright © 2009 John Wiley & Sons, Ltd.     

Identification of Raman spectra through a case‐based reasoning system: application to artistic pigments

This paper presents a methodology conceived as a support system to identify unknown materials by means of the automatic recognition of their Raman spectra. Initially, the design and implementation of the system were framed in an artistic context where the Raman spectra analyzed belong to artistic pigments. The analysis of the pigmentation used in an artwork constitutes one of the most important contributions in its global study. This paper proposes a methodology to systematically identify Raman spectra, following the way analysts usually work in their laboratory but avoiding their assessment and subjectivity. It is a three‐phase methodology that automates the spectral comparison, which is based on one of the most powerful paradigms inmachine learning: the case‐based reasoning (CBR) systems. A CBR system is able to solve a problem by using specific knowledge of previous experiences (well‐known spectral library of patterns) and finding the most similar past cases (patterns), reusing and adapting them to the new problem situation (unknown spectrum). The system results in a global signal processing methodology that includes different phases such as reducing the Raman spectral expression by means of the principal component analysis, the definition of similarity measures to objectively quantify the spectral similarity and providing a final value obtained by a fuzzy logic system that will help the analyst to take a decision. The major benefit of a Raman spectral identification system lies in offering a decision‐support tool to those who are not experts or under difficult situations with respect to Raman spectroscopy. Copyright © 2011 John Wiley & Sons, Ltd.     

Lancaster delftware: a Raman spectroscopy,electron microscopy and Mössbauer spectroscopy compositional study

In 2008, excavations were conducted by the Northern Ceramic Society at the site of the former Lancaster delftware potworks, which operated between 1754 and about 1790. The recovered sherds have been non‐destructively examined by Raman and electron microscopies and the iron phases in the biscuit by Mössbauer spectroscopy. These methods have provided a new understanding of the mineralogy of the delftware produced at Lancaster using clay imported from Carrickfergus in Ireland and blended with the local ferruginous‐aluminous clays. This has implications for the attribution of delftware produced at Liverpool, Bristol, Scotland and Ireland. The Carrickfergus clay has been found to be highly dolomitic resulting in the body of the delftware forming diopside and the magnesian olivine forsterite, when fired. Brookite had not converted to rutile, nor had tridymite, nor cristobalite formed; the K‐feldspars did not undergo further chemical reactions, and the observation of metakaolin would suggest that the biscuit firing temperature was in the order of 800–900 °C. Chalcedony containing moganite was established as the silica source. A cobalt‐containing lead‐tin glaze was applied to the biscuit body and, after decorating, was fired. The mineralogy of the pigments used to decorate the objects indicate the yellow to be the PbSbSn triple oxide, the green to be a copper silicate mixed with lead‐tin yellow, the purple and browns to be manganese silicates and the blue to contain cobalt spinels and cobalt pyroxenes. Diopside crystals together with recrystallised tin agglomerates have been observed floating within the glaze. Copyright © 2015 John Wiley & Sons, Ltd.     

Historical organic dyes: a surface‐enhanced Raman scattering (SERS) spectral database on Ag Lee–Meisel colloids aggregated by NaClO4

In the present study, several natural organic dyes used in antiquity, especially in textile dyeing, were analysed by surface‐enhanced Raman scattering (SERS) spectroscopy, in order to build a wide database that could integrate the data previously published in the literature. In particular, we reported for the first time the SERS spectra of 11 dyes: dragon's blood, sandalwood, annatto, safflower yellow and red, old fustic, gamboge, catechu, kamala, aloe and sap green. Silver colloids (Ag colloids) prepared according to the Lee–Meisel procedure, i.e. by reduction of a silver nitrate (AgNO₃) aqueous solution with trisodium citrate dihydrate, were used as substrate. As its efficiency had been tested in a previous work, sodium perchlorate (NaClO₄) 1.8 M was again employed as aggregating agent, giving the best results when added to the silver nanoparticles after the analyte. Copyright © 2011 John Wiley & Sons, Ltd.     

Determination by Raman scattering of the nature of pigments in cultured freshwater pearls from the mollusk Hyriopsis cumingi

The nature of pigments in naturally colored pearls is still under discussion. For this study, Raman scattering measurements were obtained for 30 untreated freshwater cultured pearls from the mollusk Hyriopsis cumingi covering their typical range of colors. The originality of this work is that seven different excitation wavelengths (1064 nm, 676.44 nm, 647.14 nm, 514.53 nm, 487.98 nm, 457.94 nm, 363.80 nm) are used for the same samples at the highest possible resolution. All colored pearls show the two major Raman features of polyenic compounds assigned to double carbon–carbon (CC) – at about 1500 cm⁻¹ – and single carbon–carbon (C C) – at about 1130 cm⁻¹ – bond stretching mode, regardless of their specific hue. These peaks are not detected in the corresponding white pearls, and therefore seem directly related to the major cause of body color. Additionally, the exact position of CC stretching vibration shows that these compounds are not members of the carotenoid family. Moreover, some changes are observed in intensities, shape and positions of the two main characteristic polyenic peaks from one sample to the next. Similar changes are observed also using several excitation wavelengths for the same point of the same pearl. The exact position of C C stretching vibration of polyenic molecules depends strongly on the number of double bonds (N) contained in their polyenic chain. Hence, using a constrained decomposition of this band for different excitation wavelengths, up to nine different pigments may be detected in the same pearl. Their general chemical formula is R‐( CHCH )_N‐R′ with N = 6–14. All our colored samples contained at least four pigments (N = 8–11). Different colors are explained by different mixtures, not by a simple change of pigment. The chemical nature of the chain ends is still unknown, because it cannot be detected with Raman scattering. However, it is possible that these polyenes are complexed with carbonate molecules of the nacre. Similar coloration mechanisms are found in products from other living organisms (e.g. parrots feathers). Moreover, it seems that a similar series of pigments is found in other pearls also, as well as in some marine animals living in similar environments (e.g. corals). Copyright © 2006 John Wiley & Sons, Ltd.     

Confocal Raman microscopy: how to correct depth profiles considering diffraction and refraction effects

A new approach to obtain corrected depth profiles by confocal Raman microscopy, which considers diffraction and refraction effects is presented. The problem of diffraction effects encountered intrinsically in the confocal configuration can be described using a linear Fredholm integral equation of the first kind, which correlates apparent and true Raman intensities with the depth resolution curve of the instrument. Refractive index differences between air and the polymer sample, which cause further errors in the obtained depth profile due to strong aberration effects have been considered. This has been carried out using an empirical variation of the depth resolution function, which is able to simulate the broadening of the depth of focus with depth and also the discrepancy between nominal and measured depth scales. It is shown that considerable differences between apparent and corrected depth profiles exist at the surface and that these depend on the gradient of the profile and the depth resolution of the Raman microscope. Copyright © 2007 John Wiley & Sons, Ltd.     

Molecular orientation analysis of organic thin films by z‐polarization Raman microscope

Polarization‐dependent Raman microscopy is a powerful technique to perform both structural and chemical analyses with submicron spatial resolution. In conventional Raman microscopy, the polarization measurements are limited only in the direction parallel to the sample plane. In this work, we overcome the limit of conventional measurements by controlling the incident polarization by a spatially modulated waveplate. In this method, the polarization perpendicular to the sample surface (z‐polarization) can be detected together with the parallel polarization (xy‐polarization). Because of this unique polarization control, our Raman microscope has the ability to image the molecular orientation, together with the molecular analysis. Here, we have investigated thin films of pentacene molecules that are widely studied as an organic semiconductor material. The orientations of pentacene molecules are imaged with a spatial resolution of 300 nm. Our results clearly indicate that the lamellar grains show the lower tilt angles compared to the neighboring islands, which has not been proved in conventional methods. The substrate effects and the thickness dependence of the film are also studied. These results provide knowledge about the relationship between the devise performance and the film structures, which is indispensable for future device exploitations. Copyright © 2012 John Wiley & Sons, Ltd.