A violet colour excavated from the archaeological site of Pompei has been investigated by micro-Raman spectroscopy in order to ascertain its organic-lake nature. Fluorescence was partly avoided by excitation in the near infrared region with a diode laser emitting at 785 nm. Sample degradation was prevented by keeping a low laser power. Due to the similarity of purple dyes, an indigo and a pure 6,6'-dibromoindigotin specimens were analysed for spectral comparison. Reference spectra of some red natural organic colours were also measured, namely madder, lac, brazilwood, kermes and cochineal. By comparing the pigment spectrum with reference data, the presence of an indigotin-derivative chromophore was excluded, while a remarkable similarity was observed with madder lake, suggesting the presence of an anthraquinone-like dying component.
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