An archaeology of natural dyes and mordants: bridging gaps in Iron Age textile chaînes opératoires

Throughout the history of textile production, plant-derived adjective dyes were applied to alter the color of fiber yarns and fabrics. A chaîne opératoire approach to conceptualizing textile workflows, including dyeing and mordanting processes, enables recognition of durable traces of production practices that may be preserved at archaeological sites. An Iron-Age fireplace and an adjacent pit feature at Castro de Sâo Martinho in central Portugal provide evidence for the production of a chemical mordant used to fix natural dyes from wildflower plants. At São Martinho, magnetite-rich dolerite was the precursor raw material for iron acetate, a mordant known to have been used in Bronze and Iron Age Europe. Sequential iron speciation and spectroscopic identification in controlled laboratory trials demonstrate the chemical pathway for the formation of ferrous iron salt from the dolerite. Actualistic experiments with Cistus seed capsules and flower calyces, a species and tissues that match macrobotanical remains from the archaeological pit feature, yielded a colorfast, deep olive green color on linen. The archaeological association of spindle whorls, a fireplace, dye plants, and crushed dolerite debris is coherently explained by the hypothesis that Castro de São Martinho evidences small-scale or household dye and mordant fiber practices, even though, to date, no fiber artifacts have been recovered from the site. Experimental archaeology is substantiated as an integral component of the archaeological method and theory necessary to fully interpret the social relevance of textiles in the past.