First evidence of purple pigment production and dyeing in southern Arabia (Sumhuram, Sultanate of Oman) revealed by mass spectrometric and chromatographic techniques

Archaeological excavations carried out in the ancient settlement of Sumhuram (3rd century BC-5th century AD) in the area of Khor Rori (Dhofar Governorate, southern Oman), brought to light pottery showing a pink-violet substance. In order to reveal the nature of this pink-violet colour, the substance was chemically examined by laser desorption-ionization mass spectrometry (LDI-MS) and high performance liquid chromatography-diode array detection (HPLC-DAD). The analytical investigations provided a detailed molecular composition of the organic fraction of the pink-violet material, highlighting the presence of 6,6'-dibromoindigo, 6-monobromoindigo, 6,6'-dibromoindirubin, 6- and 6'-monobromoindirubin, indigo and indirubin. The results revealed that shellfish purple was the source. In addition, in some of the pottery fragments, alizarin and purpurin, which are the molecular markers of madder type dyestuffs, were also ascertained by LDI-MS. The analytical results enabled us to draw hypotheses not only on the possible function of such vessels as dye baths, but also that, since Sumhuram was one of the most important harbours in southern Arabia during the pre-Islamic time, it was also possibly a centre for the production and trade of purple pigments.