Investigation of pH-Dependent Wash Fastness of Indigo Carmine in Replica and Historical Textile Fibers

Indigo carmine, a semi-synthetic blue dye encountered in historical textiles, poses conservation challenges due to its poor wash fastness and tendency to transfer during aqueous treatments. Textile fibers dyed with indigo carmine often exhibit color migration during conservation wet cleaning, complicating preservation efforts. Due to the nature of indigo carmine interactions with proteinaceous textile fibers, this study assessed the pH-dependence of the wash fastness of indigo carmine combined with absorbance spectroscopy as a simple identification technique, mostly to differentiate it from non-soluble and more stable blue dyes. First, the pH-dependent wash fastness of wool samples dyed with indigo carmine was assessed and compared to samples dyed with natural indigo and madder as a negative control. Wash fastness tests conducted using water adjusted to pH 4, 5, 7, and 8.5 revealed strong pH dependence for only indigo carmine, with minimal dye migration at acidic pH, some dye migration at neutral pH, and the highest degree of dye migration occurring at pH 8.5. pH-dependent microextraction tests were then conducted for few-fiber samples using sub-milliliter wash volumes, successfully detecting indigo carmine in both replica and historical samples. HPLC was used to confirm the presence or absence of indigo carmine in selected historical samples. We note that identification of indigo carmine could be complicated by the fiber’s age, sample size, color, presence of a water-insoluble finish, or degradation. Overall, these findings contribute to the understanding of indigo carmine’s behavior and provide guidance for historical dye identification and decision-making in textile conservation treatments.