Maya blue is a marvelous pigment with extraordinary properties. It was invented by the Maya around VII-VIII century and used by many Mesoamerican peoples in prehispanic times. It is made by encapsulating natural indigo into an inorganic clay matrix of palygorskite. The palygorskite-indigo mixture becomes acid-resistant when a moderate thermal treatment is applied. The chemical reasons of the unusual stability of the pigment and the exact mechanism of interaction between the indigo and the clay are not well understood. We present a Raman study of different preparations of Maya blue and other mixtures of indigo with other inorganic materials. We found that the unheated mixture of indigo with palygorskite presents the same Raman spectrum as Maya blue, indicating that the differences with respect to the indigo spectrum are not due to the interaction produced during the thermal treatment, which makes the mixture acid-resistant. Moreover, indigo mixed with other clays, like sepiolite or montmorillonite, presents a Raman spectrum very similar to that of Maya blue. Some chemical mechanisms that could explain these spectra, and the suitability of Raman spectroscopy for identifying Maya blue are discussed.

Direct identification of organic colourants in heterogeneous matrices of cultural heritage objects with very little or no sampling remains a challenging analytical task. A Raman procedure was investigated for the direct identification of archaeological organic-coloured pigments found in works of art, without solvent extraction. Conventional micro-Raman spectroscopy (MRS) using different excitations and surface-enhanced Raman scattering (SERS) were tested on microscopic samples of paints and cosmetics containing dyes from different sources (animal or vegetal). A tiny lump of purple pigment discovered during excavations in the ancient Minoan city of Akrotiri, on the SantoriniIsland in Greece (1650 B.C.) and Greco-Roman pink cosmetics were studied. In some cases the results were compared with a range of lake pigments made in the laboratory following historical recipes.