Archaeo-chemical analysis of Royal Purple on a Darius I stone jar

Abstract.High-performance liquid chromatography (HPLC) coupled with photodiode array (PDA) detection was used for the microchemical analysis of a purple residue on the surface of a 2500-year old stone jar. This 30 × 37 cm pear-shaped marble vessel contains carved inscriptions praising the Persian king Darius I and is unique due to the use of quadrilingual writings on a vessel of that king. The major colorants identified in the purple pigment are 6,6′-dibromoindigo, 6-monobromoindigo, and 6,6′-dibromoindirubin, with negligible contributions by indigo and 6-bromoisatin. This analysis establishes that a marine mollusk was the source of the purple pigment, which is the famous Royal Purple or Tyrian Purple of the ancients. A comparison with the relative dye compositions of various Muricidae species (Hexaplex trunculus, Bolinus brandaris, and Stramonita haemastoma), and with their newly formulated Di-Mono Index values, suggests that the biological provenance of this ancient pigment was probably an indigo-deficient Hexaplex trunculus sea snail. The entire exterior of the vessel – including its base – was originally painted purple by using a fresco-type technique. This is only the second chromatographic finding of a molluskan purple colorant in use as an ancient paint pigment and not as a textile dye, and the only example yet discovered where it is the sole paint pigment on such a large royal art object.