The development and application of analytical methods for the identification of dyes on historical textiles

Abstract

• The analytical investigation of several historically important natural yellow and red dye sources is presented, extending previous work on the chemical characterisation of the acid hydrolysed extracts from dyed yarns. The PDA HPLC studies of weld (Reseda luteola L.) dyed yarn extracts found that neither the substrate nor additional steps in the dyeing process significantly altered the relative ratio of the main flavonoid components. Furthermore, their relative photo-degradation rates on the dyed yarns were similar, presenting few problems in the identification of the biological source. In contrast, the relative ratio of the main flavonoid components in dyer’s greenweed (Genista tinctoria L.) dyed yarn extracts was highly dependent on the dyeing process. The PDA HPLC and HPLC ESI MSn investigation of the acid hydrolysed extracts of yarn dyed with the Serratula species, sawwort (Serratula tinctoria L.) and Serratula coronata L., identified their characteristic dyeing components to be luteolin, apigenin, quercetin, kaempferol and 3-O-methylquercetin. Similar studies performed on young fustic (Cotinus coggygria Scop.) dyed yarn extracts identified the main colouring components to be fisetin and sulfuretin. Due to the relatively high photo-degradation rates of many of the characteristic components in both sawwort and young fustic, the identification of historical yarns dyed with these sources was problematic. The dye components characterisic for Mexican cochineal (Dactylopius coccus Costa) were all found to be structurally related anthraquinones, with similar photo-degradation rates on the dyed yarns. Analysis of the principal dye components from brazilwood (Caesalpinia sappan L.) and logwood (Haematoxylon campechianum L.) using negative ion ESI MSn enabled the identification of new, structure dependent, mass spectrometric breakdown pathways. The mechanisms were supported by deuterium labelling experiments and the results were shown to be useful for the structural determination of previously unidentified components. An elimination product observed in the PDA HPLC analysis of hematein dyed yarn extracts was structurally characterised using NMR techniques. A similar component, derived from brazilein, was also observed in brazilwood dyed yarn extracts. A novel approach towards mordant identification using inductively coupled plasma techniques was investigated. ICP MS analysis was successful in identifying the metallic ions present in the acid hydrolysed extracts from both reference and historical yarns. However, interpretation of the results from historical samples was challenging due to the observation of large amounts of both aluminium and iron in the sample extracts. Within the Monitoring of Damage to Historic Tapestries (MODHT) project, yellow and green yarns from a selection of 15th – 17th century tapestries were investigated. Analysis of the yarn extracts identified weld and dyer’s greenweed as the most prevalent yellow dye sources, with young fustic frequency identified in the metal thread core extracts.