Surface-enhanced Raman scattering (SERS) is a powerful technique for the detection of natural dyes found in archeological and historical textiles, in paintings, and in other works of art. Natural organic products historically used as textile dyes or lake pigments are often fluorescent under normal dispersive Raman measurement conditions. To add to the fluorescence problem, the amount of dye actually present on works of art is minimal, requiring extremely sensitive analytical techniques. The enhancement of the Raman signal and the quenching of the background fluorescence resulting from the adsorption of dye molecules on metal nanoparticles in SERS concur to solve the problems encountered when studying dyes by Raman spectroscopy. Reproducible spectra of several reference dyes were obtained, and extraction and adsorption protocols to optimize the analysis of samples from actual work of art were developed. SERS supports evaluated include citrate-reduced and hydroxylamine-reduced Ag colloids, as well as Tollens mirrors and silver nanoisland films. Dyes for which SERS spectra were observed include: alizarin, purpurin, laccaic acid, carminic acid, kermesic acid, shikonin, juglone, lawsone, brazilin and brazilein, haematoxylin and haematein, fisetin, quercitrin, quercetin, rutin, and morin. The incompatibility with SERS of techniques traditionally used to extract dyes from artwork samples was demonstrated, and a nonextractive hydrolysis technique specially suited to prepare SERS microscopic samples was developed. Finally, SERS was successfully used to identify alizarin in a 1mm by 50 µm (diameter) single fiber sample form a sixteenth-century tapestry. Copyright © 2006 John Wiley & Sons, Ltd.

Application of surface-enhanced Raman scattering techniques to the ultrasensitive identification of natural dyes in works of art

The use of ultraviolet and visible diffuse reflectance spectrophotometry as a preliminary technique in the investigation of illuminated manuscripts is discussed. Because ancient manuscripts are amongst the most fragile and precious artworks, characterisation of the materials used in their decoration should be performed using non-invasive analytical methods. Ultraviolet and visible reflectance spectrophotometry with optical fibres (FORS) allows non-invasive identification of several colourants used by ancient illuminators, causing no damage or mechanical stress to the artworks subjected to analysis. Identification is usually based on the comparison of analytical data with a spectral database built from painted areas on parchment, created by preparing paints according to ancient recipes as described in medieval technical treatises. Such database and the spectral features of the colourants analysed are discussed, along with the benefits of extending the spectral range of analysis into the shortwave infrared (to 2500 nm). FORS can be best appreciated as a rapid preliminary tool that offers an overview on the main colourants employed and guides the selection of painted areas of manuscripts on which more selective techniques, such as X-ray fluorescence or Raman spectroscopy, can be employed for a more complete and accurate identification.

Characterisation of colourants on illuminated manuscripts by portable fibre optic UV-visible-NIR reflectance spectrophotometry

Dyes are among the most significant components in works of art and archaeological findings. In the scientific examination of historical artefacts, the identification of natural dyestuffs is a challenging task, due to the complexity of their chemical composition and the possible presence of mixtures of chromophores and degradation products. For this reason, in the last few decades, new analytical procedures and techniques have been developed and improved for the characterization of organic dyes and their identification in microsamples. This review looks at the chemical composition of natural organic dyeing materials used in the field of the cultural heritage and focuses on several analytical methods based on spectrometric and chromatographic techniques that have contributed to the study of organic dyes in works of art and archaeological findings.

Analytical Methods for the Characterization of Organic Dyes in Artworks and in Historical Textiles

Scientific studies of works of art are usually limited by severe sampling restrictions. The identification of organic colorants, a class of compounds relevant for attribution and provenance studies, is further complicated by the low concentrations at which these compounds are used and by the interference of the protein-, gum-, or oil-binding media present in pigment and glaze samples. Surface-enhanced resonance Raman scattering (SERRS) was successfully used to identify natural organic colorants in archaeological objects, polychrome sculptures, and paintings from samples smaller than 25 μm in diameter. The key factors in achieving the necessary sensitivity were a highly active stabilized silver colloid, obtained by the reproducible microwave-supported reduction of silver sulfate with glucose and sodium citrate, and a non-extractive hydrolysis sample treatment procedure that maximizes dye adsorption on the colloid. Among the examples presented are the earliest so far found occurrence of madder lake (in a 4,000 years old Egyptian object dating to the Middle Kingdom period), and the earliest known occurrence in Europe of the South Asian dyestuff lac (in the Morgan Madonna, a 12th century polychrome sculpture from Auvergne, France).

https://www.pnas.org/content/pnas/106/35/14757.full.pdf

Microanalysis of organic pigments and glazes in polychrome works of art by surface-enhanced resonance Raman scattering

Raman spectroscopic studies of carbonaceous materials are, until now, mainly devoted to geological and industrial materials. On the other hand, it is known from artistic literature that many varieties of carbon-based black pigments were produced and used in different places and times, and according to the artist’s preferences. The ability of Raman spectroscopy to analyse particles down to 1 μm and its non-destructiveness make it an ideal tool for pigments investigation. Anyway, the discrimination among different types of carbon-based black pigments is affected by various aspects, one of which is the lack of reference spectra as well as of specific nomenclature. In this paper, reference materials have been studied by means of Raman spectroscopy to provide reference spectra. All the pigments showed two broad bands of carbon, but sometimes specific excitation conditions were required to record a good quality Raman spectrum. The obtained Raman signatures are discussed, on the basis of the specificities of the pigment (natural or artificial origin; structural implications related to the raw materials used or to production processes; etc.). Therefore, on the basis of the Raman spectra of painting materials, further knowledge can be obtained on the type of carbon-based black pigments in works of art. Copyright © 2015 John Wiley &S ons, Ltd.

/pdf/raman-spectroscopy-for-the-investigation-of-carbon-based-ilm9yz5r67.pdf

Raman spectroscopy for the investigation of carbon‐based black pigments

Ultraviolet-visible reflectance spectroscopy is a rapid and accurate technique for the investigation of painted surfaces. The use of a high efficiency beveled-tip fiber optic probe and a pulsed xenon source gives improved detection efficiency and minimizes the interference from ambient light, eliminating the need to shield the analyzed area and allowing non-contact, non-invasive analysis. Thin washes of organic colorants, often impossible to sample, are ideally suited to this kind of analysis. This study describes the in situ identification of indigo, Prussian blue and a red insect dye (cochineal carmine or lac dye) in two paintings by Katsushika Hokusai.

https://www.jstor.org/stable/info/1506806

Fiber optics reflectance spectroscopy: a unique tool for the investigation of Japanese paintings

A primary classification of carbons is into crystalline and non-crystalline forms. The only crystalline carbon encountered as a pigment is graphite; a secondary classification for non-crystalline forms is into flame carbons (formed in the gas phase), cokes (formed in a liquid or plastic phase), chars (formed entirely in the solid phase) and natural forms such as coal. The main features, molecular structures and physical and chemical properties of these carbon types, so far as they are relevant to pigment studies, are reviewed in outline. Some better-known pigments based on carbon are summarized, with an indication of which form or forms of carbon each contains. X-ray diffraction patterns for graphites, and for examples of the non-crystalline types, are described and discussed. Particle morphology of various carbons and pigments, as elucidated from scanning electron micrographs, is described in some detail, and notes are added on the use of optical microscopy in studying these materials.

The characterization of pigments based on carbon

(1984) Natural adhesives in East Asian paintings Studies in Conservation: Vol 29, Preprints of the Contributions to the Paris Congress, 2-8 September 1984 Adhesives and Consolidants, pp 117-120

Natural adhesives in East Asian paintings

Japanese paintings of the twelfth to the sixteenth centuries in the collection of the Freer Gallery of Art has been determined. In 12 out of 13 Chinese paintings, and in all three Korean paintings, basic lead carbonate (lead white) was found. Of the 29 Japanese paintings, six had basic lead carbonate, 20 had basic lead chloride (which occurred as two different compounds), and two carried both of these species. One Chinese painting had lead sulphate and one Japanese painting lead phosphate. Historical evidence suggests that two forms of 'lead white' were known and distinguished during the eighth century in Japan; the possible connection between this and the present findings on Japanese paintings is discussed.

John Winter

Papers

Fiber optics reflectance spectroscopy: a unique tool for the investigation of Japanese paintings

The characterization of pigments based on carbon

Natural adhesives in East Asian paintings

‘Lead white’ in Japanese paintings

Studies using scientific methods : pigments in later Japanese paintings