Searching for blue: Experiments with woad fermentation vats and an explanation of the colours through dye analysis

The starting point for this research was the requirement to produce replicas of Iron Age textiles from Hallstatt in Austria using traditional methods. Three traditional processing and dyeing methods using woad (Isatis tinctoria L.) were successfully recreated in an iterative experimental process: dyeing with fresh leaves, with green and couched woad and with woad pigment. During these experiments, several colours other than the typical blue also emerged. The light fastness of all colours was fairly good. Dye analysis using high-performance liquid chromatography with photo diode array detection (HPLC-PDA) showed that the most predominant component in the blue samples was indigotin. The colours mint, purple, beige and green were achieved when indirubin and flavonoids appeared in higher concentrations. The composition of the woad-related components detected on dyed samples enabled us to retrace the dyeing methods used. Antraquinones originating from madder (Rubia tinctorum L.) used in the madder-bran vat were also detected, but in different ratios to that of madder mordant dyeings. Further research is required to prove whether the components detected in reference samples can be used to identify woad dyeing or the use of madder-bran vats in historic/archaeological textiles. The ultraviolet-visible absorption spectra obtained by HPLC-PDA were used to calculate RGB values, which provide a better understanding of the colour observed.