Surface-enhanced Raman spectroscopy: a direct method to identify colorants in various artist media.

Surface-enhanced Raman spectroscopy (SERS) has been developed as a direct, extractionless, nonhydrolysis tool to detect lake pigments and colorants of various classes used in a variety of artist materials. Presented first is the SERS analysis of the natural colorant turmeric (Curcuma longa L.), main component curcumin, as present in dry lake pigment grains, dyed textile yarns, and reference paint layers containing the lake pigment bound in animal glue painted on glass. This experiment demonstrated that it is possible to detect the chromophore in various matrixes of increasing complexity, allowing its unambiguous identification in a wide range of artists' materials, even at very low concentration and in the presence of binders such as glue. In addition, removal of the colorant from the complex with the inorganic substrate or mordanted yarn was not necessary for identification. This proof-of-concept study was then extended to include analysis of several pastel sticks from a historical pastel box and two samples from a pastel artwork, both attributed to American painter Mary Cassatt (1844-1926). This study represents the first extractionless, nonhydrolysis direct SERS study of multiple artist materials, including identification of natural and synthetic colorants and organic pigments contained in historic artists' pastels spanning a broad range of chemical classes: polyphenols, rhodamines, azo pigments, and anthraquinones. Successful identification is demonstrated on samples as small as a single grain of pigment.

Ad-hoc surface-enhanced Raman spectroscopy methodologies for the detection of artist dyestuffs: thin layer chromatography-surface enhanced Raman spectroscopy and in situ on the fiber analysis.

Tailored ad-hoc methods must be developed for successful identification of minute amounts of natural dyes on works of art using Surface-Enhanced Raman Spectroscopy (SERS). This article details two of these successful approaches using silver film over nanosphere (AgFON) substrates and silica gel coupled with citrate-reduced Ag colloids. The latter substrate functions as the test system for the coupling of thin-layer chromatography and SERS (TLC-SERS), which has been used in the current research to separate and characterize a mixture of several artists' dyes. The poor limit of detection of TLC is overcome by coupling with SERS, and dyes which co-elute to nearly the same spot can be distinguished from each other. In addition, in situ extractionless non-hydrolysis SERS was used to analyze dyed reference fibers, as well as historical textile fibers. Colorants such as alizarin, purpurin, carminic acid, lac dye, crocin, and Cape jasmine were thus successfully identified.