On the Raman spectrum of maya blue

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. Copyright (C) 2006 John Wiley & Sons, Ltd.