An innovative surface‐enhanced Raman spectroscopy (SERS) method for the identification of six historical red lakes and dyestuffs

Abstract

Surface-enhanced Raman spectroscopy (SERS) was used in this work to obtain highly detailed spectra of artists' red lake pigments and colorants. In the past, Raman spectroscopy has been successfully employed to identify many pigments and modern synthetic dyes. Unfortunately, red lake pigments and dyes commonly employed in artistic production from antiquity to the mid-nineteenth century are often extremely fluorescent, making identification with Raman spectroscopy difficult or impossible. This work presents an innovative SERS technique that quenches fluorescence, significantly enhances the weak Raman scattering effect, and requires very little sample material and minimal sample handling. A silver island film (AgIF), approximately 6-8 nm thick, is deposited on the substrate by electron beam (e-beam) deposition. The SERS-active surface is then analyzed with a confocal dispersive Raman microscope, at an excitation wavelength of 632.8 nm. Reference materials including the synthetic dyestuffs alizarin, purpurin, and eosin, high-purity carminic acid, and historic red lake pigments such as madder lake, cochineal, brazilwood, lac lake, and kermes were studied. The proposed method has great potential for the unambiguous identification of red dyes applied in different media on a variety of substrates, as demonstrated by the highly detailed Raman spectra presented here.

Non-invasive study of natural dyes on historical textiles from the collection of Michelangelo Guggenheim

“…8-right well fit profiles reported by literature [11,74,77], showing reflectance maxima centered at approx. 463 nm, absorptions at approx.…”

“…The use of these colorants was historically assessed for various purposes (such as painting, inks, textile dyeing, etc.) and it is known that kermes, an insect native to southern Europe, was widely employed until the late XVI century, when cochineal, able to provide a ten times more efficient dye in terms of coloring power, started to be imported from Mexico and to completely replace it [72][73][74].…”

“…Acquired images were qualitatively compared with spectra from literature [5,74]: as a general result, images could not allow the discrimination of colorants since a low absorption was generally observed for red, blue and yellow areas (Fig. 12a-d), compatible with the spectral behavior of anthraquinones, indigotin and flavonoids.…”

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

“…Normal Raman spectra acquired in the red lake pigment layers in this cross-section present bands at ca. 642, 712, 1174, 1280, 1345, 1501 and 1620 cm −1 , that are consistent with the presence of an eosin lake [25], and a strong feature at 978 cm −1 due to PbSO 4 [26]. In Fig.…”

Raman Spectroscopy of cultural heritage Materials: Overview of Applications and New Frontiers in Instrumentation, Sampling Modalities, and Data Processing

“…Fabricated by drop-casting and self-assembly of microspheres (usually silica or polystyrene) on a glass slide, subsequently covered by a thin layer (approximately 200 nm) of vapor deposited Ag or Au, these substrates generally give high-quality spectra with higher enhancement factors than colloids [150], and can be tuned by appropriately choosing the size of the microspheres used so as to give rise to LSPRs at different wavelengths [278,303,304]. The major drawback of these systems is that the analyte needs to be brought into solution, and often they suffer from contamination of carbonaceous and other spurious materials that get preferentially adsorbed on the metallic coating of the microspheres.…”