Ineke Joosten

The colorant behaviour of cochineal and kermes insect dyes in 141 experimentally-dyed and 28 artificially-aged samples of silk and wool was investigated using ultra-high performance liquid chromatography coupled to photodiode array detector (UHPLC-PDA), liquid chromatography electrospray ionisation mass spectrometry (LC-ESI-MS) and image scanning electron microscopy – energy dispersive X-ray spectroscopy (SEM-EDX). Partial-least squares discriminant analysis (PLS-DA) was then used to model the acquired UHPLC-PDA data and assess the possibility of discriminating cochineal insect species, as well as their correspondent dyed and aged reference fibres. The resulting models helped to characterize a set of 117 red samples from 95 historical textiles, in which UHPLC-PDA analyses have reported the presence of cochineal and kermes insect dyes.

Analytical investigation of the experimentally-dyed and artificially-aged fibres has demonstrated that the ratio of compounds in the insects dye composition can change, depending on the dyeing conditions applied and the type of fibres used. Similarities were observed when comparing the UHPLC-MS and SEM-EDX results from the dyed and aged references with the historical samples. This was verified with PLS-DA models of the chromatographic data, facilitating the classification of the cochineal species present in the historical samples. The majority of these samples were identified to contain American cochineal, which is in agreement with historical and dye identification literature that describe the impact of this dyestuff into European and Asian dyeing practices, after the Iberian Expansion in the 16th century.

The analytical results emphasize the importance of using statistical data interpretation for the discrimination of cochineal dyes, besides qualitative and quantitative evaluation of chromatograms. Hence, the combination of UHPLC-PDA with a statistical classification method, such as PLS-DA, has been demonstrated to be an advisable approach in future investigations to assess closely related species of natural dyes in historical textile samples. This is particularly important when aiming to achieve more accurate interpretations about the history of works of art, or the application of natural dyes in old textile production.

In 2014, a large collection of textile fragments was found in a 17th-century shipwreck (named BZN17) that had sunk in the Wadden Sea, near the island of Texel, The Netherlands. Captured in a time capsule for centuries, this collection represents a unique example of 17th century fashion, comprising over 150 textile fragments, including costumes, parts of costumes and interior textiles. Although buried for centuries, they are in remarkably good condition, which might be related to the archaeological environment and the high quality of the fabrics. Indeed, these are almost entirely made of silk (except one woollen carpet), and embroidered or woven with metal thread. The most striking characteristic of this collection is the well-preserved deep red colour displayed by many of the textiles.
To understand the origin and function of these textiles, as well as their state of conservation for future preservation strategies, it was important to characterize their materials. Hence, a group of 12 objects was selected for evaluation with ultra-high performance liquid chromatography with diode array detector (UHPLC-PDA) and a scanning electron microscope coupled to an energy dispersive x-ray spectrometer (SEM-EDX).
With UHPLC-PDA, coccid insect dyes were identified as the main colorants on the textiles. However, the ratio of their dye compounds seems to have been affected by the maritime archaeological conditions: dcII and flavokermesic acid were not reported in any of the textiles; and an unusually high kermesic acid equivalent was detected, in relation to the major compounds kermesic and carminic acids. Even though American cochineal and kermes could be possibly attributed in some cases, these results substantially hinder the precise identification of the insect sources. Besides these, madder was often found mixed with the insect dyes or in textile parts that are not directly visible on the costumes.
The evaluation of the fibres with SEM-EDX indicated that some fibres show evidence of microorganism attack, and that the majority has preserved their flexibility; although those from fabrics of lower quality (e.g. lining) are more friable. On the metal threads, silver sulphide crystals were observed, and these correspond to the degradation product of silver. Also, gold was detected on few threads, which indicates that they were probably gilded. Due to the corrosion of silver, most gold particles could have been lost in the maritime environment.
While the provenance of these textiles, and the ship that carried them, is still under investigation, the analytical results obtained undoubtedly prove that a very rich finding has been unearthed. Nevertheless, the archaeological conditions certainly had a preponderant influence on their original appearance and this deserves future research.

The preservation of woollen textiles in the prehistoric salt mines of Hallstatt is a lucky chance for the study of prehistoric textile dyeing. The Hallstatt textiles collection of the Department of Prehistory at the Natural History Museum Vienna shows the creativity of the prehistoric people. Already in the Bronze Age, the main materials for wool dyeing had been discovered: Woad served as indigo plant for dyeing blue, and rhizomes of Rubiaceae for dyeing red. Plants provided dyes for yellow and tannins for black. Out of the 12 analysed samples from 11 Bronze Age textile fragments, 9 are dyed and 3 are probably dyed. In textile fragments of the Hallstatt Period a greater number of yellow dyes were detected pointing to the use of plants which up to now had not been known for the Bronze Age. Green was dyed through combining woad blue with yellow. The palette of reds seems to be expanded through the use of dye insects and lichens. Frequently, woad blue was combined with other dyes and tannins to achieve dark blues. They went perfectly together with clasps, belts and necklaces made of shiny bronze. Out of the 67 analysed samples from 49 Iron Age textile fragments, 42 were dyed, 8 were probably dyed and 17 probably not dyed.

Textiles from the Bronze Age and Iron Age have been preserved for more than 3000 years in the salt mine of Hallstatt, Austria. Copper originating from prehistoric mining tools made of bronze has probably altered the colour of many of the textiles. Three woven bands from the Iron Age were chosen for reproductions in order to show how they might originally have looked, and to acquire knowledge of prehistoric dyeing technology. Dyeing techniques documented in historical, ethnographic, and experimental archaeological literature were analysed. Fibre, dye and element analyses of the prehistoric bands formed the basis for the experimental development of dyeing methods using woad (Isatis tinctoria L.), weld (Reseda luteola L.) and scentless chamomile (Tripleurospermum inodorum (L.) Sch. Bip.). The hand spun yarns were woven with rep band and tablet weaving techniques. Each band was successfully reconstructed in two possible colour variants. The light fastness of the dyed woollen yarns ranges between level 3 and 6 and matches everyday requirements today. Element and dye analyses and a post-mordanting experiment with copper acetate explain today's colours of the woven bands. A detailed picture of conceivable dyeing techniques in the Hallstatt Culture is provided, concerning the handling of textile material during dyeing, woad processing and dyeing procedures, mordanting techniques, and the tools and resources required. Dyeing with natural dyes is an ancient cultural technology that is simple in terms of equipment and resources, but sophisticated in terms of the knowledge required. It fully reflects the comprehensive knowledge prehistoric people had of the chemical properties of natural substances, the effect of temperature on (bio)chemical processes, and the ability to control and manage these processes. In central Europe, the beginning of this knowledge dates back to Bronze Age, the 2nd millennium BC, as proven by the textile finds in Hallstatt.