Bright Light: Microspectrofluorimetry for the Characterization of Lake Pigments and Dyes in Works of Art

Abstract

Color is an important component in the perception of beauty and in an artist's original intent when creating a work. Better conservation of our cultural heritage requires detailed knowledge of artwork materials and the complex evolution they have endured over time. Organic dyes have been used from ancient times, and their characterization is a challenge that has been successfully addressed over the past few years by the development of advanced techniques, such as microspectrofluorimetry. In this Account, we describe the application of microspectrofluorimetry to the study of medieval illuminations, paint cross sections, millenary textiles, and wall paintings. In our research into color in medieval Portuguese illuminations, we chose to emphasize the importance of the experimental design and the use of microspectrofluorimetry in the context of other analytical techniques, such as microFTIR, microRaman, and micro-X-ray fluorescence (microXRF). Within this framework, we were able to unveil the full complexity of a medieval colorant and to address issues not yet explored, such as the influence of Arab, Jewish, and Christian cultures on the production and underlying technology of Portuguese illuminations. The analysis of individual pigment particles or aggregates (by excitation with an 8 mum diameter spot) in paint cross sections from works by Vincent van Gogh and Lucien Pissarro highlights the technique's advantage of high spatial resolution. Its high spectral resolution proved to be useful not only for better characterizing the dyes used to color Andean textiles but also for detecting mixtures of relevant chromophores; the emission signals for the reds in Paracas and Nasca textiles were shown to be due to the presence of purpurin and pseudopurpurin. Finally, the complexity of the study of yellow dyes and the importance of accurate historical reproductions is addressed in a study of Asian organic colorants on historic Chinese wall paintings. Microspectrofluorimetry offers high sensitivity, selectivity, fast data acquisition, good spatial resolution, and the possibility of in-depth profiling. It has proved to be an invaluable analytical tool in identifying dyes and lake pigments in works of art. As Saint-Exupéry's protagonist said in Le Petit Prince, "L'essentiel est invisible pour les yeux," or "What is essential is invisible to the eye"--but it may be unveiled with kind love, a prepared mind, and a little help from microspectrofluorimetry.

Approaches for Detecting Madder Lake in Multi-Layered Coating Systems of Historical Bowed String Instruments

“…For these reasons, a different analytical approach is often required. The topic has been considered for paintings [12][13][14][15][16] and textiles, and novel approaches for detecting the dyes have been reported in a recent work [17]. As for lakes in finishing layers applied on wood, only a recent paper discusses a multi-technique characterization of three reference madder lakes containing different metal cations [18], which were brushed on wooden panels in order to obtain three-layered systems.…”

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

“…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

“…In previous publications, we proved that confocal microfluorescence is a powerful tool for in situ analysis of colourants based on natural dyes [13,14,25]. Natural dyes may be described as weak to medium emitters.…”

“…Microspectrofluorimetry offers high sensitivity and selectivity combined with good spatial resolution and the possibility of in-depth profiling. It can also be used in situ without any contact with the sample or work of art to be analyzed, for movable objects that can be transported in the laboratory [13,14]. 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.…”

“…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.…”