Application of spectroscopic and imaging techniques for the study of historical natural dyes

The Michelangelo Guggenheim’s textile collection has been studied using a multi analytical approach. Specifically, False color imaging, Fiber Optics reflectance spectroscopy (VIS range) and Surface Enhanced Raman Scattering (SERS) spectroscopy were used to identify the dyestuff in the perspective of checking their compatibility with both declared historical period and the origin. Most of them are dated to the XVI century, while the stylistic analysis together with the technical characteristics traced them back to an Italian manufacture. The cross checking of spectroscopic data allowed to confirm most of the results acquired in a previous non-invasive investigation. However, new data obtained in micro-invasive mode resulted fundamental to understand the complexity characterizing some of the sampled threads. In many cases colors were obtained by mixing several dyes: all the presented methodologies have well known limitations but they often compensate each other allowing for the identification of different components co-existing in the same sample. In particular, in FORS, absorption bands of natural dyes are often broad, tend to overlap or to generate new structures; in parallel, SERS if often blind to some dyes, especially when mixtures involve components with very different cross sections. Finally, in FCI colours deriving from the contemporary presence of several compounds can interfere creating new shades difficult to interpret. Raman spectra of the Guggenheim collection mainly showed cochineal in red-based artefacts and indigo in blue ones. Additionally, other interesting dyes such as orcein or annatto were identified in many samples. At the same way, VISRS spectral profiles were affected by indigotin on green textile fragments, on which SERS gave information about the yellow dye. However, yellow dyes are particularly challenging and despite the application of a multi-technique approach, in some of the fragments it was not possible to recognize any specific compound. In some cases, equivalent results were obtained for the three tested techniques, while in others more complex situations emerged, showing the utilization of mixture instead of single dyes as previously assumed based on pure non-invasive results. This highlights the importance of utilizing a set of complementary analytical techniques when approaching historical textiles. FCI can also be a very interesting technique to obtain preliminary non-invasive data, but the study highlighted the need for the creation of a complete database of historical dyes employed with different dyeing procedures, substrates, mordants, and as mixtures with different proportions.