Surface-enhanced resonance Raman spectroscopy reveals the earliest colour chemistry.
Detecting organic pigments in ancient artefacts, such as textiles, paintings and sculptures, is harder than for inorganic pigments. The use of pigments such as madder, lac and cochineal reveals information on not only the provenance of the artwork, but on the trade routes at the time, because they each had specific geographical origins. Their detection, however, is hampered by their low concentration (often in media that interfere with detection) and the small sample size available for detection — typically <200 μm from a painting.
Now, Marco Leona of the Metropolitan Museum in New York has used a surface-enhanced resonance Raman scattering technique1 involving colloidal silver to study some of the museum's collection. The highly active colloid was prepared using a microwave-assisted reaction, and with glucose and sodium citrate as reducing and protective agents, respectively. The samples were pre-treated with hydrogen fluoride vapour to partially attack the binding medium and release the dye. When the colloid was added, dye molecules adsorbed onto the silver particles and their characteristic Raman spectrum was recorded.
Red pigments from a 4,000-year-old Egyptian quiver were found to contain madder, predating the earliest previously known use by seven centuries. This new technique represents the earliest evidence of the chemical knowledge required to extract a dye from a plant or animal source.
Leona, M. Microanalysis of organic pigments and glazes in polychrome works of art by surface-enhanced resonance Raman scattering. Proc. Natl Acad. Sci. USA 10.1073/pnas.0906995106 (2009).