Recent Advances on the Analysis of Polychrome Works of Art: SERS of Synthetic Colorants and Their Mixtures With Natural Dyes

The development and application of proper sample pretreatments is often a key step toward the successful analysis of dyes used as artists' materials by surface-enhanced Raman spectroscopy (SERS). Complexation of the organic colorants with metal ions to dye fabrics and produce lake pigments, as well as undesired interactions with other matrix components such as substrate, binding media, fillers, and extenders, are just some of the issues that typically complicate dye identification in minute samples from invaluable artworks and museum objects. These concerns may be addressed by using, prior to SERS analysis, ad-hoc sample pretreatments that, in addition to increasing the technique's sensitivity, favorably affect its selectivity toward certain molecules or molecular classes. The present work describes a newly developed sample pretreatment based on the use of nitric acid that has proven crucial for the successful detection of aniline and xanthene dyes–the first synthetic organic colorants to be used in printing and painting, among other art forms–in microscopic samples from works of art such as a 19th-century silk fabric, paper cut-outs by Henri Matisse, Vincent Van Gogh's Irises, and Japanese woodblock prints. This treatment promotes the hydrolysis of the dye-metal bond in mordant dyes or lake pigments, resulting in a more efficient adsorption of the dye molecules on the SERS-active substrate and, hence, enabling the acquisition of high-quality spectra. In the case of synthetic colorants, this method shows advantages over hydrolysis with hydrofluoric acid–a procedure previously established for the analysis of red lakes prepared from natural dyes. The nitric acid treatment presented here may be integrated into a multi-step methodology that, by exploiting differences in solubility of various dyes and lake pigments, has enabled for the first time to successfully characterize intentional mixtures of natural and synthetic colorants of the xanthene and anthraquinone molecular classes, i.e., eosin Y and carmine, in a selection of Japanese prints of the Meiji era. The present study paves the way for the systematic identification of synthetic dyes in objects of artistic and archeological interest, even when they are present in mixtures with natural organic colorants

Time-resolved photoluminescence spectroscopy and imaging: New approaches to the analysis of cultural heritage and its degradation

Applications of time-resolved photoluminescence spectroscopy (TRPL) and fluorescence lifetime imaging (FLIM) to the analysis of cultural heritage are presented. Examples range from historic wall paintings and stone sculptures to 20th century iconic design objects. A detailed description of the instrumentation developed and employed for analysis in the laboratory or in situ is given. Both instruments rely on a pulsed laser source coupled to a gated detection system, but differ in the type of information they provide. Applications of FLIM to the analysis of model samples and for the in-situ monitoring of works of art range from the analysis of organic materials and pigments in wall paintings, the detection of trace organic substances on stone sculptures, to the mapping of luminescence in late 19th century paintings. TRPL and FLIM are employed as sensors for the detection of the degradation of design objects made in plastic. Applications and avenues for future research are suggested

A timeline for the introduction of synthetic dyestuffs in Japan during the late Edo and Meiji periods

Abstract A widespread belief among scholars and connoisseurs of the Japanese color woodblock print (nishiki-e) holds that synthetic dyes were imported from the West in the 1860s, and soon came to be used for all nishiki-e colorants during the Meiji period. These “cheap imported aniline dyes” are widely described as “gaudy”, “garish”, and “strident”, and thought to stand in sharp contrast to more muted “natural” colorants that preceded them. This study calls this narrative into question through an analysis of the colorants of nishiki-e from 1860 until 1900, using surface-enhanced Raman spectroscopy coupled with micro-Raman, XRF and fiber optic reflectance spectroscopies. The results show that the introduction of synthetic dyes was gradual and selective, and that most of the customary colorants of the late Edo period continued in use. The results revealed a series of key turning points after 1860: (1) In 1864, the purple dye rosaniline became the first synthetic dye to be used in nishiki-e, at first in combination with Prussian blue for a more bluish color. From 1875, it was usually mixed or replaced with methyl violet for a stronger purple. (2) In early 1869, a dramatic and until recently unrecognized transition took place, from the longstanding use of safflower as the dominant red, to its total replacement by imported cochineal carmine. Carmine remained the primary red for the next two decades, often combined with vermillion. (3) In 1877, eosine appeared as the first synthetic red dye in nishiki-e, used alone for pink, and in mixture with carmine for red. (4) Finally, from 1889, a succession of red naphthol dyes of more striking color appeared. Just about this time, however, a tendency to more restrained use of color and more painterly effects began to emerge in nishiki-e, and with the exception of a burst of dynamic color in prints depicting the Sino–Japanese War (1894–95), the uses of strong colors in Meiji prints receded. A final key finding is that colorants were often combined, either through mixture in a bowl or on the printing block, or by two-step overprinting

MOESM1 of A timeline for the introduction of synthetic dyestuffs in Japan during the late Edo and Meiji periods

Additional file 1: Table S1. Table indicating: date of production, artist and title of the Meiji prints collection analyzed in this study. The prints in italic are part of the Metropolitan Museum of Art collection. Table S2. Table indicating: XRF, Raman and SERS analysis of the pigment materials used in the Meiji prints collection considered in this study

Multianalytical Study of Historical Luminescent Lithopone for the Detection of Impurities and Trace Metal Ions

We have explored the performance of an integrated multianalytical approach to the analysis of a series of microsamples of historical lithopone (a coprecipitate of ZnS + BaSO₄) produced at the beginning of the 20th century, based on the combination of spectrally- and lifetime-resolved photoluminescence (PL) microscopy imaging and electron paramagnetic resonance (EPR) spectroscopy. Multispectral imaging of the PL emission from microsamples revealed the presence of different luminescence centers emitting in the visible spectrum, which we have hypothesized as trace Cu and Mn impurities unintentionally introduced into the ZnS crystal lattice during synthesis, which act as deep traps for electrons. Time-resolved PL imaging analyses highlighted the microsecond decay-kinetic behavior of the emission, confirming the trap state nature of the luminescence centers. EPR confirmed the presence of Cu and Mn, further providing information on the microenvironment of defects in the ZnS crystalline lattice related to specific paramagnetic ions. The multianalytical approach provides important insights into the historical synthesis of lithophone and will be useful for the rapid screening and mapping of impurities in complex semiconductor pigments and other artists’ materials

A novel classification method for multispectral imaging combined with portable Raman spectroscopy for the analysis of a painting by vincent van gogh

In this work, a novel combination of portable micro-Raman spectroscopy and semi-automatic methods of data treatment are proposed for the classification and mapping of visible multispectral imaging data for the analysis of a painting on paper by Vincent Van Gogh. Analysis of multispectral imaging data with the sequential maximum-angle convex cone (SMACC) and spectral angle mapper (SAM) algorithms differentiated the surface into areas on the basis of the presence of pigment mixtures. Complementary analytical information was obtained through portable Raman spectroscopy was performed on a few selected points of the painting, allowing for the determination of Van Gogh's palette and the mapping of pigment mixtures on the painting's surface; the number of mixtures employed is varied and at least two different blues are present. The results obtained were integrated with the information from prior ultraviolet (UV)-induced luminescence analysis performed on the same painting to better understand the materials used by the artist. The mathematical treatment of multispectral data using the proposed methods could be extended to the analysis of other painted surfaces

Photoluminescence properties of zinc white: an insight into its emission mechanisms through the study of historical artist materials

While the photophysical properties of ZnO nanostructures have been widely explored, less research has focused on the bulk material present in artist pigments. This study is based on the analysis of historical pastels, representative of artist materials available at the turn of the twentieth century, and of the pure powder pigment as the control sample. The study of the intensity of the photoluminescence emission as a function of the fluence and of the nanosecond and microsecond emission decay kinetic properties allows the elucidation of the emission mechanisms in control ZnO and historical samples containing ZnO. Data suggest that in historical samples the near-band-edge free-exciton photoluminescence emission, typically occurring in the pure semiconductor, is influenced by the interaction of the pigment with surrounding organic binding material. Conversely, crystal defects, typically expected in historical samples following the imperfect synthesis process available at the beginning of the twentieth century, introduce minor modifications to the photoluminescence emission. The study further suggests that zinc carboxylates, detected in all historical samples and known to introduce characteristic groups on the surface of ZnO, could be responsible for changes in emission mechanisms. Research demonstrates how photoluminescence decay kinetics and the study of the dependence of the emission intensity on the fluence are powerful methods for elucidating the nature of the mechanism processes in luminescent semiconductor pigments

An integrated approach based on micro-mapping analytical techniques for the detection of impurities in historical Zn-based white pigments

In this work we propose an integrated approach, based on synchrotron analysis with micrometric spatial resolution and sub-ppm sensitivity and m-Raman mapping, for investigating impurities and heterogeneous inclusions in historical samples of Zn-based white pigments. Analysis was performed at the LUCIA beamline at the SOLEIL synchrotron radiation facility for the simultaneous detection of the elemental distribution in suitably prepared pigment samples using micro-X-ray fluorescence (m-XRF) mapping and for the investigation of oxidation states and coordination of metals using micro-X-ray near edge absorption (m-XANES) spectroscopy. The identification of specific molecular signatures and the detection of their spatial distribution throughout samples by m-Raman measurements supported and complemented X-ray analysis, allowing the identification of Cr- and Fe-based inclusions in historical samples. In ZnO pigments, common impurities are due to the production process and include Fe and, depending on samples, Cd, Cl and Pb. In one of the Zn-containing pigments, identified as Lithopone, m-XRF mapping revealed the presence of Co, both as highly concentrated micrometric inclusions and as impurities throughout the pigment

Van Gogh's Irises and Roses : the contribution of chemical analyses and imaging to the assessment of color changes in the red lake pigments

Abstract Vincent van Gogh’s still lifes Irises and Roses were investigated to shed light onto the degree to which the paintings had changed, both individually and in relation to each other since they were painted, particularly in regard to the fading of the red lakes. Non-invasive techniques, including macroscopic X-ray fluorescence mapping, reflectance imaging spectroscopy, and X-radiography, were combined with microanalytical techniques in a select number of samples. The in-depth microchemical analysis was necessary to overcome the complications that arise when evaluating by non-invasive methods alone the compositions of passages with complex layering and mixing of paints. The results obtained by these two approaches were complemented by color measurements performed on paint cross-sections and on protected edges, and with historical information provided by the artist’s own descriptions, early reviews and reproductions, and the data was used to carry out digital color simulations that provided, to a certain extent, a visualization of how the paintings may have originally appeared. Graphical abstract Irises, 1890, Vincent van Gogh. The Metropolitan Museum of Art #58.187. Zn (upper right), Pb (bottom left), and Br (bottom right) distribution maps acquired by XRF imaging