Late 19th century accounts of Indian yellow: The analysis of samples from the Royal Botanic Gardens, Kew

Indian yellow has always been a mysterious pigment. In 1883, an eye-witness account of its production described it as a urinary sediment from cows fed a mango leaves rich diet; this same account describes a set of manufacturing materials and samples sent to the Royal Botanic Gardens, Kew (England). Most of these original materials were recovered in 2016 from the Economic Botany Collection at Kew, and analyzed alongside known reference materials, to characterize the pigment materials, and to search for evidence of an animal origin. Component characterization and identification was performed using py (TMAH)-GC-MS, LC-MS, FTIR, Raman Spectroscopy, SEM-EDS, UVA-induced visible fluorescence, and polarized light microscopy. All the materials were shown to contain variable ratios of reported components of Indian yellow (euxanthic acid, euxanthone, and a sulfo-derivative of euxanthone), and some presented hippuric acid, a ruminant metabolite found in urine. This is the first substantial link between animal urine and the pigment Indian yellow. When possible, the interior and exterior of the pigment-balls were characterized, and a higher amount of euxanthone was consistently detected on the exposed exterior of the ball. It is proposed, that euxanthone may be a chemical marker for the degradation of Indian yellow. A laboratory prepared euxanthone was used for comparison; this paper presents the characterization data for euxanthone, much of which is the first time it has been reported in the literature.