Combined analytical techniques for the investigation of dyes in the Coptic collection of the Egyptian Museum of Turin

Many aspects shall be considered when approaching a scientific survey of archaeological collections by means of instrumental analytical techniques. As a matter of fact, the integrity of the objects is a priority, therefore non-invasive or micro-invasive analytical techniques are of choice in the analysis. Here, the analytical strategy for the analysis of dyes in the textiles of the Coptic collection of the Egyptian Museum of Turin is described. The collection includes about 250 items. Many of them are decorative parts detached from tunics or fragments cut out from larger textiles, although the collection also encompasses whole tunics and headdresses. The term “Coptic textiles” indicates woven objects produced in Egypt from 30 to 1300 AD, i.e. from Roman to Medieval period. Therefore “Coptic” mainly refers to specific stylistic features and culture and does not identify an historical period. Coptic textiles were obtained exploiting the skilled weaving techniques of the era of the Pharaohs, but introducing the use of wool, which was preferred for the colorful Coptic decorations as it is easier to be dyed, whereas flax was employed for the structure in plain weave fabric. As far as the present collection is concerned, a minor part of it derives from Schiaparelli's excavation in Ashmunein (Hermopolis), while the provenance of most of the textiles, which were donated to the Turin Museum by the Cairo Egyptian Museum at the beginning of the 1900, is unknown. The collection [1] encompasses textiles dating from the 3rd to the 11th century AD, some of which are excellently preserved, whereas others show the effects of bad restorations carried out at the beginning of the past century. Decorations were obtained by monochrome (mainly red-brown) or polychrome wool threads, whether woven, attached or, rarely, embroidered. The decoration motifs are from the Pharaonic to Christian traditions, with geometric and plant themes on medallions and stripes with interlaced plants, series of animal and human figures. Dyes in Coptic textiles have been investigated mainly by liquid chromatography (HPLC) coupled with photometric, spectrophotometric and/or mass spectrometric detectors. HPLC separates the chemical species that contribute to the final color and enables the identification of the natural source of the dyes. Nevertheless, HPLC analysis is time- and solvent-consuming, and needs few milligrams of fibers to be detached from the archaeological finds. Therefore the collection was subjected to a non-invasive screening by Vis-reflectance spectroscopy [2] and fluorimetry [3] employing portable instruments equipped with fiber optics. Robust information could be obtained by performing a large number of analyses on the same object and by considering together the information obtained from the two analytical techniques. The non-invasive survey yielded a first overview of the dyestuffs employed. In particular, the presence of indigotin was evidenced in blue, green and purple samples. As far as red dyestuffs are concerned, they were detected in red, purple and orange decorations. In particular, madder-type dyestuffs were evidenced and a possible partition into two sub-groups was suggested. The non-invasive survey also indicated which, among the many textiles of the collection, were dyed by employing scale insects, although the identification of the dyestuff down to the species level cannot be achieved by a non-invasive approach. The non-invasive screening of the collections allowed us to select a limited number of representative samples that were subjected to HPLC-DAD-MS analysis that permitted to extend the information on the employed dyestuffs throughout the whole collection.