FT-Raman Analyses of Blue Dyes Stuff Common in Swedish Folk Art from Hälsingland during 18th and 19th Century

Abstract

The overall aim of this study is to gain a better understanding of the historic use of woad in Sweden in the 18th and 19th centuries. A key question is, whether it is possible to distinguish between indigo from woad and exotic indigo using Raman spectroscopy? Reference samples prepared according to historic recipes using woad, exotic indigo, logwood and brazilwood - were used to dye different textile fibres - cotton, wool and linen. In addition, various reference compounds - indigotin, indirubin, kaempferol, luteolin, 3-hydroxyflavone and tryptanthrin - as well as the ‘flower’ of the woad dye bath were analysed to identify specific fingerprint regions for each compound. Blue threads from four historic textiles - a skirt and three rag balls - from the region of Halsingland were then analysed in comparison to the references. A specific peak at 993 cm-1 was identified in the spectra taken from the flower of the woad vat, which we suggest possibly arises from 3-hydroxyflavone. DSA-ToF-MS analysis suggests flavoneglycosides as possible candidates. However, cellulosic substrates show a broad vibration at 999 cm-1 overlapping the flavone signature, thus rendering it non-diagnostic, but it might be possible to use this peak in Raman analyses supported by mass spectrometry to distinguish between indigo from woad and exotic indigo if the substrate is wool or if it is a pigment. Raman analyses of the historic samples identify the colourants as indigo and possibly woad and logwod mixed with brazilwood.