3D-Fluorescence Supported by Optical Reflectance and Raman Spectroscopy for the Analysis of Natural Dyes in Ancient Textiles: Potential and Limitations.

Optical fluorescence spectroscopy has emerged as a powerful, highly sensitive, and non-invasive tool for identifying natural dyes in historical textiles, especially when chromatographic techniques are unavailable or sampling is not permitted. In this study, 3D-fluorescence was at the core of a multi-analytical approach involving optical reflectance and Raman spectroscopy. This approach was adopted to investigate three ancient carpets from the Oudayas Museum in Rabat, Morocco. The aim was to identify the original dyes to reproduce naturally dyed wool for restoration purposes. Madder (Rubia tinctorum L.) was recognized as the source of the reds, Cochineal (Dactylopius coccus) insect extract as the source of the pinks, weld (Reseda luteola), or Daphne gnidium, as the source of the yellows, and indigo (from Indigofera tinctoria or Isatis) as the source of the blues. The colour palettes were enriched through plant combinations and iron treatment. Additionally, XRF measurements revealed the presence of mordanting elements. Based on these results, red, yellow, green, and blue-dyed wools were successfully reproduced and characterized employing 3D-fluorescence and optical reflectance. Overall, 3D-fluorescence, particularly when combined with other complementary non-invasive techniques, is a powerful tool for real-time non-destructive analysis of natural dyes in ancient textiles. It enables the rapid acquisition of spatially resolved spectral data, revealing patterns that suggest either uniform or varied dye sources. However, while this technique is effective in revealing dye mixtures and in distinguishing between animal-derived and plant-derived red dyes, its main limitation is that it cannot identify the precise botanical origins of plant-based dyes.