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Keywords:

  • orcein;
  • litmus;
  • SSRS;
  • UV-visible fluorescence;
  • lichen dye

Ultraviolet and visible (UV-vis) fluorescence and subtracted shifted Raman spectroscopy have exploited to identify orchil in a fragment from the ninth century Bible de Théodulphe and a parchment fragment sampled from a 16th century map of Auvergne.

Orchil is a purple lichen dye widely used since ancient times in place of the more expensive Tyrian purple and therefore known as the ‘poor person's purple’. Unlike the latter, orchil has a low lightfastness, and in ancient works, it is often faded. Orchil is a complex mixture of different coloured compounds, and they all share a common structure resulting from phenoxazone with a number of different substituents. In this work, UV-vis fluorescence combined with resonance Raman spectroscopy allowed for the identification of orchil in a fragment from the ninth century Bible de Théodulphe. Raman spectroscopy has been applied also for studying a parchment fragment sampled from a 16th century map of Auvergne. In both cases, subtracted shifted Raman spectroscopy (SSRS) has been exploited for removing the strong fluorescence background. Overall results have been confirmed by liquid chromatography with quadrupole time-of-flight mass spectrometry (LC/ QTOF-MS).

Both the electronic and vibrational spectral features of ancient orchil showed some interesting differences with respect to those of the standard sample. Taking into account the poor lightfastness of the purple dye, the same investigation has been carried out on photochemically aged orchil reproducing the spectral modification observed on the ancient fragments. Copyright © 2013 John Wiley & Sons, Ltd.