Federica Pozzi

Sample treatment considerations in the analysis of organic colorants by surface-enhanced Raman scattering.

The introduction of surface-enhanced Raman spectroscopy (SERS) in the field of cultural heritage has significantly improved the analysis of the organic dyes and their complexes that have been used as textile dyes and pigments in paintings and other polychrome works of art since antiquity. Over the last five years, a number of different procedures have been developed by various research groups. In this Article, we evaluate the effect of pretreating samples by exposing them to hydrofluoric acid (HF) vapor prior to SERS analysis, a step designed to hydrolyze the dye-metal complexes and increase analyte adsorption on the nanosized metallic support, thus enhancing the SERS signal. Materials studied include pure colorants, commercial lake pigments, and fibers from dyed textiles, as well as actual aged samples, such as microscopic fragments of lakes on paper and ancient pigments and glazes from several works of art, covering a wide range of time, from the second century B.C. to the early 20th century. In each case, SERS spectra obtained with or without HF hydrolysis were critically evaluated. The pretreatment with HF vapor resulted in faster analysis and increased sensitivity in most cases, with the exception of dyed silk fibers, where silk protein hydrolyzates were found to interfere with SERS analysis. As a final point, a two-step procedure including SERS on untreated and treated samples is proposed as a standard approach: by analyzing a sample first without hydrolysis, and then, following removal of the colloid, upon HF treatment, the best and most reliable results for a great number of dyes and substrates are assured.

Statistical methods and library search approaches for fast and reliable identification of dyes using surface-enhanced Raman spectroscopy (SERS)†

The identification of organic colorants is of high importance in the cultural heritage field, where they are found as paint components and textile dyes, and in forensic science, because of their use in inks and paints, food colorants and textile dyes. Surface-enhanced Raman spectroscopy (SERS) has emerged as a promising technique for the detection of these materials, yet concerns over the sensitivity of SERS spectra of dyes to chemical and instrumental variables (such as pH, choice of SERS substrates and/or aggregants, and excitation wavelength) have prevented its widespread use in analytical applications. Over the last few years, the development of several microanalytical approaches has considerably increased the chances of success in the identification of minute amounts of dyes by SERS. However, the need for searchable databases is still largely to be fulfilled. In this work, we have assembled the core of a comprehensive library which contains 100 Raman and SERS reference spectra of natural and synthetic organic colorants. Experiments to classify 20 query SERS spectra of dyes from a variety of museum objects were conducted using principal component analysis (PCA) and the correlation coefficient (CC) algorithm. The effect of spectral transformations such as baseline correction, selecting a standard frequency range, normalization, smoothing, as well as carrying out the search on the second derivative of the spectra, was systematically evaluated. With this study we demonstrate that SERS spectra of organic colorants can be reliably matched against a well constructed spectral library regardless of the instrumentation and the colloids used, and of the pH conditions at which the measurements were carried out.

Winsor & Newton original handbooks: a surface-enhanced Raman scattering (SERS) and Raman spectral database of dyes from modern watercolor pigments

BackgroundWinsor & Newton Ltd. has been one of the main fine art products providers since its establishment in 1832, being responsible for the manufacture of a wide assortment of materials ranging from oils and pigments to brushes and papers. All the items produced over the years have been indexed in a comprehensive historical archive. Original Winsor & Newton handbooks are a powerful resource which can offer insight into the world of artists’ materials, and knowledge of artists’ choices through the identification of substances employed to obtain particular colors. Scientific analyses of various kinds have been carried out on Winsor & Newton art materials over the years; however, a detailed study of the organic dyes contained in the watercolors manufactured by the company has never been performed thus far to our knowledge.ResultsIn the present study, we examined a number of color washes on drawing paper from two historical Winsor & Newton catalogues dating to the 19th and 20th century. An appropriate database was thus built, including surface-enhanced Raman scattering (SERS) and Raman spectra of organic colorants from a wide variety of shades. While the selection of colors offered by the company in the 19th century mostly included lakes prepared from plant and insect dyes, i.e. madder and cochineal, some tints based on synthetic dyes were also found in the 20th century handbook.ConclusionsThe present article sheds new light on the chemical composition of a number of original Winsor & Newton color washes in terms of organic colorants contained in each shade. A special attention was dedicated to the analysis of those colors for which the formulation was ambiguous or not specified by the manufacturers, such as dragons’ blood and most of the alizarin-based pigments. In addition, we were able to correct erroneous indications provided by Winsor & Newton on the composition of some tints, as in the case of violet carmine, and study how the formulation of certain pigments has been modified over the centuries.

Methodological evolutions of Raman spectroscopy in art and archaeology

During the last decades, Raman spectroscopy has grown from research laboratories to a well-established approach that is increasingly often used in archaeometry and conservation science. When looking at these research fields, some novel trends can be detected and therefore we would like to review the recent literature on the technical aspects and new evolutions of Raman spectroscopy applied to art analysis. This article reviews Raman instrumentation, with a special focus on the use of mobile and portable instruments, recent developments in the field of surface-enhanced Raman spectroscopy (SERS), and the introduction of spatially offset Raman spectroscopy (SORS) in the field of art and archaeology.

Multi-technique characterization of dyes in ancient Kaitag textiles from Caucasus

Kaitag textiles, named after the Kaitag district of Southwest Daghestan, Russia, where it is being manufactured, are a unique embroidered textile art form. They were used by families on special occasions such as the birth, marriage or death of one of their members and were thus passed down from generation to generation as family heirlooms. Today, only a few hundred of these precious antique specimens can still be found, and surviving examples are mostly from the seventeenth and eighteenth centuries. In this article, an extensive work for the scientific analysis of Kaitag textiles is presented as the logical continuance and updating of the investigations performed by thin layer chromatography almost two decades ago. A multi-technique approach involving the combined use of micro-invasive and nondestructive techniques suitable for in situ analyses was used, aiming to identify the colourants of Kaitag textiles and the inks employed for the underlying drawing. Analyses were performed by high-performance liquid chromatography, surface-enhanced Raman spectroscopy, scanning electron microscopy combined with energy dispersive X-ray analysis, as well as visible reflectance spectroscopy and X-ray fluorescence. In addition, infrared reflectography and ultraviolet fluorescence were employed to visualise underlying drawings and possible restorations. Corrosion phenomena observed in brown- and black-dyed areas were also investigated.

Identification of Natural Dyes on Laboratory-Dyed Wool and Ancient Wool, Silk, and Cotton Fibers Using Attenuated Total Reflection (ATR) Fourier Transform Infrared (FT-IR) Spectroscopy and Fourier Transform Raman Spectroscopy

Attenuated total reflection (ATR) infrared and Fourier transform (FT) Raman spectra were obtained from wool threads dyed in the laboratory with natural dyes used in antiquity, following a procedure similar to ancient methods for dyeing wool. The ATR spectra were primarily dominated by the signals of the wool, making it difficult to identify the dye on the fibers only by visual inspection of the infrared spectrum. However, the Raman spectra showed more significant characteristics attributable to the dyes as previously studied in the literature on modern synthetic dyes. A library-search method was thus applied to the second derivatives of both the ATR and Raman spectra to verify the possibility of identifying the dye. Two libraries were constructed, one consisting of the ATR spectra of undyed wool (raw, washed, and mordanted) and the transmission spectra of pure dyes and the other consisting of the Raman spectra of undyed wool and of pure dyes. Correlation and first-derivative correlation search algorithms were used. The results presented here suggest that the two types of spectroscopy are complementary in this kind of work, allowing the almost complete identification of historic dyes on wool. In fact, through the combined use of the two searches, most dyes were identified with a good index of similarity and within the first five hits. Only for annatto was identification totally impossible using either technique. Subsequently the same method was applied to wool, silk, and cotton threads taken from ancient Caucasian and Chinese textiles.

Surface-Enhanced Raman Spectroscopy: Using Nanoparticles to Detect Trace Amounts of Colorants in Works of Art

In recent years, powerful physical processes occurring in the vicinity of nanoscale metal surfaces have been exploited in the art world for the detection of trace amounts of colorants with surface-enhanced Raman spectroscopy (SERS). With this technique, naturally occurring and man-made organic molecules used as dyes and pigments in objects from antiquity to the present day are being detected with high molecular specificity and unprecedented sensitivity. This chapter reviews the broad spectrum of SERS analytical methodologies and instrumental improvements that have been developed over the years in the field of cultural heritage science, and discusses significant case studies within different types of works of art and archaeological artifacts.

László Moholy-Nagy's Painting Materials: From Substance to Light

ABSTRACT This article presents results from an extensive scientific examination of the painting materials used by László Moholy-Nagy. The artist employed modern materials, such as metals and plastics, alongside more traditional artists’ oil paint and canvas, creatively manipulating these diverse media to generate a unique visual vocabulary. This study highlights the intimate link between the material properties and the expressive content of Moholy-Nagy’s painted works.