Application of forensic photography for the detection and mapping of EGYPTIAN BLUE (cas 10279-60-4) and madder lake in Hellenistic polychrome terracottas based on their photophysical properties

Add time:07/31/2019    Source:sciencedirect.com

Two of the oldest synthetically produced and most widespread pigments employed in the decoration of ancient polychrome artifacts are EGYPTIAN BLUE (cas 10279-60-4), a vitreous blue compound, and madder lake, a hybrid organic-inorganic composite. Both of these pigments are known to be photoluminescent after electronic excitation by photons in the visible and re-emission of photons in the near infrared (NIR) and visible (Vis) respectively. The emissions of Egyptian blue are typically due to d-d electronic bands and for madder lake generally due to π-π* type transitions. In this research, the photophysical properties of these two pigments were examined by means of forensic photography. The usefulness of this type of imaging is based on the properties of Egyptian blue and madder lake in exhibiting characteristic near infrared and visible emissions when excited by red and green light respectively providing reliable fingerprint markers for their identification and mapping. To probe and capture the characteristic excitation and emission photon energies of Egyptian blue and madder lake an alternate light source (ALS) and a modified digital single-lens reflex (DSLR) camera with suitable optical filters were employed in the examination and analysis of two Canosa polychrome terracotta vases from the J. Paul Getty Museum collection dated to the third century BCE. Reference materials and supplementary analysis by fiber optic reflectance spectroscopy (FORS) and X-ray fluorescence (XRF) spectroscopy were used for verification.

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