Work Package 5 - Substances in and on textiles explores the burial process by looking at the chemicals that remain on the textile fragments and are ingrained in the gypsum itself. Detecting the presence and location of any organic residues and identifying the nature of the organic residues will tell us a lot about how the Romans cared for those they interred and the societal status of interred individuals.
How to detect low levels or organic residues
A multitude of chemical analytical techniques will be used to look for evidence of oils and resins that may have been used to anoint a body, and for dyes used in any textiles use to adorn and clothe the body prior to burial.
The in-situ search for surface features on the gypsum
Fibre Optic Reflectance Spectroscopy (FORS) is a non-destructive technique that will be taken to the gypsum casing so that the analysis can be carried out within the Museum or Museum store. Light covering the ultraviolet and visible to near infrared wavelengths is focused onto the gypsum surface and the back-scattered light is measured.
This technique allows us to generate a chemical profile for both inorganic and organic molecules on the surface of the gypsum. Through this, we can see variations in the gypsum mineral itself, identify the fabric used in some of the textiles and have detected lipid/wax which is thought to be candle wax, from when Victorians building York’s railway station first uncovered the burial!
The search for oils and resins ingrained in the gypsum
Resin analysis will be performed using Gas Chromatography-Mass Spectrometry (GC-MS). Gas chromatography separates individual chemical compounds based on how volatile and how hydrophilic (water-loving) or hydrophobic (water-repelling) the molecules are. Mass spectrometry then determines what each chemical compound is by removing an electron from it molecules, causing them to fragment into distinct pieces.
The result is a chemical signature, or fragmentation pattern, that acts like a fingerprint which can be searched against a large database to determine what each compound is. The next step is to link the identified compound to known biomarkers, specific to a particular plant or tree, and hence determine the source of the resin or oil.
The search for dyes on textiles trapped in the gypsum
Despite the passage of time, some of textile remains could still contain remnants of dyes originally used to colour fabrics, even if the colours are no longer visible by eye. There are cases of archaeological textiles where visible shades of brown are determined to have been once blue, green or yellow when analysed at the molecular level.
Textile fragments will be tested for evidence of dyes using Liquid Chromatography-Tandem Mass Spectrometry (LC-MS/MS). Liquid chromatography separates out the compounds based on how hydrophilic (water-loving) or hydrophobic (water-repelling) the molecules are. MS/MS takes each molecule one at a time, breaks it into pieces then measures each piece with such accuracy and precision that the molecular formula can be derived. Comparing the pieces generated with those measured from reference materials will enable us to confirm the identity of any dyes and hypothesise where they have come from.
Textile dyes were commonly derived from a variety of sources such as plants, insects, and marine snails. The rarity of these raw materials, coupled with the complex processes required to extract their colours, meant that certain dyes were considered more precious and indicative of luxury.
Significance of the origins of resins, oils, and dyes
Dyes used on textiles during the Roman period could originate from local sources or be imported from far-flung regions. The discovery of exotic dyes in fabric fragments highlights extensive trade networks throughout the Roman empire, as well as signalling the wealth and social standing of those who possessed such items.
Similarly, the oils and resins detected in archaeological samples may derive from plants or trees whose geographic distribution suggests they were acquired through long-distance trade. The presence of these non-local materials, whether dye or resin, acts as a marker of affluence, demonstrating connections with distant lands and the ability to access luxury commodities.