Identification of dyes is of great importance for the protection of textiles artefacts, as well as for further understanding the manufacturing process of ancient dyes. Moreover, the evaluation of degradation status can help conservators to reconstruct the original appearance of historical textile objects and predict the stability of their colors.
Natural organic dyes on textile are usually detected through molecular analytical methods.
In particular, liquid chromatography (LC) and gas chromatography (GC) have been widely used in the dye analysis with high accuracy. However, they are invasive methods, as they are based on the analysis of organic extracts.
Recently, non-invasive methods based on spectroscopy technology, such as reflectance spectroscopy and fluorescence spectroscopy, received great attention. Reflectance spectroscopy is used for the characterization of paint films and pigments. In addition, Kubelka-Munk’s theoretical reflectivity of diffuse medium provides chances to quantitatively analyze mixtures of pigments by reflectance spectroscopy. When the scattering coefficient is independent of the wavelength, the typical absorption spectrum measured by the reflection can be used as a substitute indicator for the actual absorption spectra.
Fiber optic reflectance spectroscopy (FORS) takes the advantages of being non-invasive, portable, and not being limited by the volume of the tested object, and is proper for on-site inspection . In the last years a certain understandings of the reflectance spectra of dyes on textiles has been acquired, although some problems still exist in the practice: the inflection points are not obviously observed, leading to infeasible interpretation of the dye features; the absorption of natural dyes is in the range of UV and Vis, while textile fibers absorb both the NIR and UV (the latter interfering in the evaluation of dyes); the absorption peak assignment to the corresponding chemical dye components are not identified clearly; a spectral library with multiple fiber materials has not been established.
In the paper Non-invasive study of natural dyes in textiles of the Qing Dynasty using fiber optic reflectance spectroscopy by Li Ding et al. published on the Journal of Cultural Heritage, Volume 47, 2021, Pages 69-78 doi.org/10.1016/j.culher.2020.10.013, the authors made a step forward.
In this work, 4 kinds of textile fibers (cotton, polyester cotton blend, silk and wool) with 11 kinds of natural dyes were selected to prepare a set of modern dyeing references. The reflectance spectra of references were collected via the UV–vis–NIR FORS. This reference collection showed that the absorption spectrum of Kubelka–Munk transform is suitable for dye analysis and provides the basis for peak assignment in the UV–Vis range; in the NIR range, it helps to discriminate fiber types.
Afterwards, the technique was applied to a collection fabric from Tibet Museum, which was named ‘Lohan Rubbing’ made during the the Qing dynasty. The rubbing is from a stele depicting No. 4 of 16 arhats (Lohans, Buddhist saints), whose original painting is attributed to Guanxiu (832-912 AD). It was mounted with dyed silk, showing elaborate designs with elements from the dragon robe pattern. The textiles show a wide range of colors in various shades, including yellow, red, blue, purple, green and black.
Fiber optics reflectance spectroscopy proved to be useful for the identification and differentiation of the natural dyes and their mixtures on textiles. The absorption characteristics of dye molecules on fiber medium and in the solution show similarity by comparison, and most of dyes show the absorption peaks with discernable features. The set of references prepared and characterized in this work allowed the identification of sappanwood, madder, indigo and amur-cork tree, and their mixtures, on the historical textiles. In addition, by comparing the dye components reflecting from the spectra, possible dye processes were discussed as well.
Therefore, FORS can provide a useful tool for the study of valuable textile pieces in a rapid way. It yields information about the fibers and dyes without the need for sampling, shedding light on the historical dye process and reconstructing the original appearance of the historical artefact.