Characterisation of the pigment “Jaune Peruvien” using in-situ SERS reveals murexide-Zn(II): From guano/snake faeces to pigment palette

• Historical “Jaune Peruvien” samples confirmed to contain Murexide. • Murexide-Zn(II) found in a yellow pigment using in-situ SERS and XRF. • This is the first evidence that Murexide extracted from snake may have been used. • Results suggest potential application of in-situ SERS and XRF for other pigments. Accurately identifying natural, organic dyes and pigments is crucial for art conservation, preservation, and historical interpretation. While past studies showcase the sensitivity and selectivity of surface-enhanced Raman scattering (SERS) for red lake pigments using various treatments, investigations of yellow pigments and paints remain limited. This study addresses this gap by employing in-situ SERS spectroscopy, coupled with non-invasive XRF analysis, to characterise a yellow organic pigment from two historical French Lefranc & Cie catalogues. The analysis successfully identifies both samples as murexide, a pigment containing zinc as the mordant metal. This finding provides the first scientific evidence supporting Lefranc & Cie's claims of using snake excrement for pigment production. Because 19th-century literature suggests efficient extraction methods for murexide from snake excrement, definitively proving the source material in these samples remains a challenge. Beyond expanding our understanding of historical artistic practices, this discovery highlights the potential of in-situ SERS and XRF for unlocking the secrets of historical pigments. Further research is needed to refine and broaden the application of these techniques for various pigments while simultaneously considering the ethical implications of micro-invasive approaches.