Dyes as dating markers: the case of the “Coptic” textiles collection of the Museo Egizio di Torino

The occurrence of specific dyes in 1st millennium AD textiles found in Egypt has been tentatively linked to their age in previous investigations [1]. This aspect was considered here by revealing the dyes employed in the collection, which encompasses fragments from tunics, household textiles and dressing accessories. Each find was dated on stylistic bases, and the presence of finds distributed from the Roman to the Islamic period was assessed . About 200 textiles (carrying monochrome or polychrome decorations obtained with dyed wool) were considered. They were all analysed in situ by non-invasive spectroscopic techniques (UV-Vis diffuse reflectance spectroscopy and fluorimetry, both employing fibre optics). Then, 33 samples were taken from selected textiles, treated for extracting the dyes and the solutions were analysed by high performance liquid chromatography coupled with photodiode array detection and mass spectrometry. As previously highlighted, the most significant information for dating purposes derives from red and purple hues. A preliminary picture was obtained here by the non-invasive spectroscopic investigation, which allows us to cluster textiles dyed with red dyes from plants (such as madder) and with dyes from scale insects. Moreover, purple textiles obtained by combining a red dye and a blue one, and those dyed by true shellfish purple were recognized. Micro invasive analysis by HPLC-DAD-MS was instead necessary to elucidate the family of the insect dyes. i.e.: cochineal, kermes or lac dye. The overall results would indicate the need of revisiting the picture on the age of the textiles that emerges from the stylistic attribution by taking into consideration also the information that derives from the detection of the dyes. In general, a systematic investigation of the age of the textiles by means of physical methods would also be helpful in defining a more robust model for linking the age with the dyeing materials.