4 CONCLUSIONS

The electronic structure of indigo is different for molecules of the pure dye and molecules located within the structure of palygorskite, as in the pre-Columbian pigment Maya blue. Different microenvironments surrounding the indigo molecule cause modifications in the electronic structure of the indigo molecule, giving rise to different spectral characteristics. Simple spectroscopic measurements can easily differentiate between indigo and Maya blue. UV-Vis FORS can successfully identify Maya blue since the electronic spectrum of the pre-Columbian pigment displays a less broad and more prominent band in the 540–730 nm range, while the spectrum of pure indigo is always characterized by a very broad and asymmetric band in the 420–730 nm range. Micro Raman and FTIR spectroscopy distinguish Maya blue from pure indigo, based on modifications of the vibrational spectrum related to changes in the charge distribution of the indigo molecule, as a result of its fixation on palygorskite during the synthesis of the Maya blue pigment.

Moreover, techniques such as FORS and micro Raman offer the important advantage of in situ, noninvasive identification of the pigments, thus resulting in a simple and safe methodology for the investigation of different objects of cultural significance.

ACKNOWLEDGEMENTS

The idea for this project came from an inquiry from Dulce Maria Grimaldi, Coordinacion Nacional de Conservacion del Patrimonio Cultural, Mexico, DF, Mexico, who kindly provided Marco Leona with the Maya blue sample from the Templo Mayor. The authors would also like to thank Jun Zhao, Chromex Inc., for recording the SERDS spectrum of modern Maya blue. Virginia Fields, curator of pre-Columbian art, LACMA, allowed Marco Leona to analyze the blue coating on the Jaina figurine from LACMA. Giacomo Chiari was extremely helpful to the authors and kindly provided advance copies of his publications on Maya blue. Analytical work at LACMA was made possible by a generous grant from the Andrew W. Mellon Foundation.