Identification of the pre-Columbian pigment Maya blue on works of art by non-invasive UV-Vis and Raman spectroscopic techniques

UV-Vis Reflectance Spectroscopy and Raman Microspectroscopy provide a rapid way to unequivocally identify the pre-Columbian pigment Maya blue. Spectra of modern synthetic materials are compared with data from a contextualized archaeological sample and from an object in the collection of the Los Angeles County Museum of Art. UV-visible spectroscopy and Raman Microspectroscopy, together with complementary techniques such as Fourier Transform Infrared Microspectroscopy (FTIR) reveal significant differences between spectra of indigo as pure crystalline solid or as complexed by palygorskite. These techniques are thus extremely specific, being able to identify Maya blue as a prepared pigment, rather than just detecting its ingredients, indigo and palygorskite. Fiber optics UV-Vis Reflectance Spectroscopy (FORS) and Raman Microspectroscopy (RM) present the additional advantage of being completely non-invasive, and therefore extremely amenable to the study of works of art. Lightweight portable FORS devices make it possible to perform measurements in situ on wall paintings and other immovable objects. The spectral differences between pure indigo and the indigo-palygorskite complex can be interpreted in terms of different hydrogen bonding configurations for the indigo molecule