Spectroscopy in Art

Our favourite works of art shouldn’t always be taken at face value. Take, for example, Winslow Homer’s famous watercolour, For to be a farmer’s boy (right). The painting had long perplexed gallery-goers who couldn’t understand why the boy depicted appears to be standing in a sunset and yet the sky is pale. It took Raman spectroscopy to reveal the truth – the painting had originally shown a vivid sunset (see Chemistry World, May 2007, p12). In fact, Raman spectroscopy is allowing art conservators to unearth all sorts of secrets hidden in artworks and national treasures. Art conservators have long been armed with analytical techniques to explore the chemical make-up of paints and dyes but in the past five years a method called surfaceenhanced Raman spectroscopy (SERS) has started to attract their attention. Experts predict that in the next five years it will become an established part of the art world’s analytical toolbox. Conservators routinely use chromatography, spectroscopy, and microscopy; each technique has obvious advantages and disadvantages. For example, high-performance liquid chromatography (HPLC) provides detailed information about the pigments and other components in dyes and paints but needs relatively large quantities of sample and laborious pre-treatment methods. With fragile and unique works of art, investigators don’t have the luxury of taking large samples. Ultraviolet–visible absorption and fluorescence spectroscopy are alternatives, but they lack the necessary specificity to pinpoint chemical signatures.