Non-destructive and non-invasive methodology for the in situ identification of Mexican yellow lake pigments

Abstract

According to historical sources, yellow colorants have been used since ancient times; despite this, their identification in textiles, Mesoamerican codices and other colonial documents is still a difficult task to perform. Most of the analytical techniques traditionally employed in the identification of yellow dyes, like HPLC-mass spectrometry, require object sampling, an undesirable and often forbidden procedure when dealing with cultural heritage objects. While some information can be obtained by non-invasive and non-destructive multi-technique analyses with portable equipment (allowing in situ studies); only few works have dealt with the in situ analysis of yellow dyes using such techniques, most of them with unsuccessful results. This is due to a combination of factors, mainly the low concentration of the dye on the object (caused by their high coloring power), their fast degradation, and the similarity of the molecule structures responsible for the yellow hues. In this work we present a non-destructive and non-invasive multitechnique methodology for the in situ analysis of Mexican yellow lake pigments with portable equipment: Raman spectroscopy, colorimetry, and fiber optic reflectance spectroscopy (FORS), as well as UV, false color infrared (FCIR) and false color ultraviolet imaging. The colorant specimens studied include: zacatlaxcalli (Cuscuta tinctoria), xochipalli (Cosmos sulphureus), annatto (Bixa orellana), old fustic (Maclura tinctoria), marigold (Tagetes erecta), sweet-scented marigold (Tagetes lucida) and weld (reseda luteola), the latter of European origin. The results presented in this paper shed some light on the non-destructive identification of yellow lake pigments - elaborated with the Mexican traditional specimens mentioned above - with the aim to allow their in situ identification on original artworks.