Combining SERS and microspectrofluorimetry with historically accurate reconstructions for the characterization of lac dye paints in medieval manuscript illuminations

Abstract

In the present study, a number of dark red microsamples from nine illuminated manuscripts of three important medieval Portuguese monasteries (St. Mamede of Lorvão, Holy Cross of Coimbra and St. Mary of Alcobaça) were analyzed using a multi-analytical approach to identify the dyes, fillers, binders and other additional paint components. Historically accurate reconstructions of lac dye paints were prepared according to recipes from medieval treatises and characterized as reference materials. Its purpose was to create better means to study this complex dye, by using the materials and techniques as close as possible to the medieval ones and ultimately build a solid database to support the interpretation of the results obtained from the spectroscopic techniques. Surface-enhanced Raman spectroscopy (SERS) and microspectrofluorimetry were used here for the first time as complementary techniques to characterize lac dye paints, whereas Raman microscopy and micro-Fourier transform infrared spectroscopy were employed to detect binders and fillers. While SERS was able to offer a conclusive molecular fingerprint of lac dye, microspectrofluorimetry provided useful information on the global formulation of the red paints.

Organic dyes in illuminated manuscripts: a unique cultural and historic record

“…The preparation of the medieval binders is described elsewhere [13,15]. In the electronic supplementary material, infrared spectra for glair and gum arabic binders, single or admixed, are also shown for comparison with the original paint colours.…”

“…Our research has demonstrated that dyes are more specific of a certain cultural milieux than inorganic pigments, and we also know that they are prone to fading-for all these reasons, they have been at the centre of our research. Recently, we have shown that microspectrofluorimetry and surface-enhanced Raman spectroscopy (SERS) may be combined to unambiguously characterize lac dye red in medieval illuminations [13]. While SERS provided unequivocal identification of laccaic acid A (the main component of lac dye), microspectrofluorimetry described the chromophore in its environment in situ.…”

“…Additionally, we foresee that a dye difficult to assess by Raman microscopy will probably be a medium/strong emitter and, therefore, will provide a good signal-to-noise (S/N) ratio for microspectrofluorimetry. In this work, we assess whether, by combining these two techniques, we transform a weakness into a strength [11,13,14], addressing the characterization of dyes in different manuscript typologies that were produced between the thirteenth century and the sixteenth century.…”

Microspectrofluorimetry and chemometrics for the identification of medieval lake pigments

“…The importance of sensitivity is clear when the following facts are considered: some of the dyes used in the past to create bright colours may have faded or may have been applied as very thin coats over, or mixed with, an inorganic pigment or extender, and therefore they may be present in very low concentrations. The possibility of in situ analysis of ancient colourants is a considerable advantage, particularly when considering that the techniques currently employed for dye analysis (HPLC-DAD-MS, microFTIR and SERS) require micro-sampling [15][16][17]. Microspectrofluorimetry also presents some drawbacks, namely the absence of a molecular fingerprint as disclosed in infrared spectra.…”

“…Microspectrofluorimetry also presents some drawbacks, namely the absence of a molecular fingerprint as disclosed in infrared spectra. This limitation may be overcome by combining surface-enhanced Raman spectroscopy (SERS) and fiber-optics reflectance spectroscopy in the visible (FORS) and by using a consistent database build up with historically accurate reproductions of references for colourants, binders and colour paints, which are the result of research into written sources of medieval techniques [13,14,17]. They are part of reproducing the process described in the source material as well as molecular identification and comparison with the original colours.…”