Monobromoindigo: The Singular Chromatic Biomarker for the Identification of the Malacological Provenance of Archaeological Purple Pigments from Hexaplex Trunculus Species

Abstract

The determination of the malacological provenance of archaeological purple pigments used in textile dyeings and in wall and stone paintings is possible due to the chromatic fingerprinting of the pigment into its individual components via High Performance Liquid Chromatography. The common component of all molluskan purple pigments from all oceans and seas is the doubly-brominated indigo dye, dibromoindigo (DBI). However, the singularity of the Hexaplex trunculus sea snails that sets them apart from all other purple-producing whelks is that they contain colorants unique to this species. Besides DBI, H. trunculus pigments also contain monobromoindigo (MBI) as well as indigo (IND), where the latter two are negligible in other species. While indigo can also be extracted from certain plant sources, the chromatic biomarker that is unique to H. trunculus pigments is the existence of a significant quantity of MBI. All archaeological pigments analyzed to date contain a sizable amount of MBI dye, and hence all purple pigments were produced in antiquity from the H. trunculus species. Further, two varieties of this species exist: one produces DBI-rich reddish-purple pigments, and the other yields IND-rich bluish-purple pigments. Other reddish Muricidae pigments were generally only used as additions to reddish-purple H. trunculus pigments.