DOI: 10.1007/s00339-007-4287-z Appl. Phys. A 90, 3–7 (2008) Materials Science & Processing Applied Physics A g. chiari 1, ✉ r. giustetto 2 j. druzik 1 e. doehne 1 g. ricchiardi 2 Pre-columbian nanotechnology: reconciling the mysteries of the maya blue pigment ∗ 1 Getty Conservation Institute, 1200 Getty Center Drive, Suite 700, Los Angeles, CA 90049, USA 2 Università di Torino, Via Valperga Caluso 35, 10125 Torino, Italy 3 Instituto Nacional de Antropolog´ ıa e Historia in M´ exico, Mexico Received: 6 October 2006/Accepted: 11 December 2006 Published online: 31 October 2007 • © J. Paul Getty Trust 2007 ABSTRACT The ancient Maya combined skills in organic chem- istry and mineralogy to create an important technology – the first permanent organic pigment. The unique color and stability of Maya Blue can be explained by a new model where indigo dye fills the grooves present at the surface of palygorskite clay, forming a hydrogen bonded organic/inorganic complex. Ex- isting theory assumes the dye is dispersed inside the channels of an opaque mineral. Based on data from thermal analysis, synchrotron and neutron diffraction, ESEM and chemical mod- elling calculations, our new concept of Maya Blue structure resolves this contradiction and suggests some novel possibilities for more durable, environmentally benign pigments. PACS 61.66-f; 62.23.St; 61.66.Fn; 61.66.Hq 1 Introduction The composition of Maya Blue has confounded re- searchers for decades. How could the bright turquoise hue and extreme stability of this hybrid organic/inorganic pigment be explained? How did the Maya combine skills in organic chem- istry and mineralogy to create an important technology – the first permanent organic pigment – by binding indigo dye to a clay mineral substrate? Maya blue was first produced by the Mayas about the 8th century AD and extended well into the Spanish colonial era before the technology was lost [1]. Due to its attractive turquoise colour and light fastness, Maya blue was widely used in mural paintings, ceramics, codices [2]. Maya blue is extremely stable: it can resist the attack of con- centrated nitric acid, alkali and organic solvents without los- ing its colour [3]. In the famous wall paintings at Bonampak and Cacaxtla, Maya blue has displayed remarkable durability over hundreds of years of exposure under humid conditions (Fig. 1). Over the past 50 years, analytical advances have pro- vided the pieces to the puzzle of Maya Blue. X-ray powder ✉ Fax: +1-310-440-7711, E-mail: gchiari@getty.edu ∗ This study is dedicated to our mentor and friend Constantino Reyes- Valerio, who, with his expertise, dedication and enthusiasm was able to instill in all of us the desire to study what was one of his life loves: Maya Blue. None of our studies could have been possible without the tremendous contribution of Constantino. We thank him deeply. diffraction (XRPD) demonstrated the presence of the clay mineral palygorskite. Later, indigo was identified [4–6] and confirmed with infrared spectroscopy [5, 7, 8, 10]. Maya blue is a complex formed between palygorskite (or sepiolite) clay and indigo dye obtained from the common plant indigofera suffruticosa. However, a simple mixture of palygorskite and indigo is not resistant to chemical attack. Eventually the recipe was rediscovered: the dye/clay mixture requires heat- ing to 100 ◦ C to produce Maya Blue [2, 6, 9]. Although the pieces of the puzzle – indigo and paly- gorskite clay – had been identified, researchers have strug- gled since to explain how they fit together to create a ma- terial with such remarkable properties. Photoluminescence spectroscopy 1 showed how the experimental spectra of Maya Blue could be obtained by superposing those of the clay and indigo. Molecular modelling calculations on Maya blue were carried out [11–13], demonstrating that no impediment ex- isted for indigo to be inserted in interior channels, which run the whole length of the clay fibres. Transmission electron mi- croscopy (TEM) analysis suggested that nanoscale Fe, Ti and Mn impurities discovered in Maya Blue samples may influ- ence its appearance [14]. Chiari et al. [11] refined the crystal structure of palygorskite and Maya blue using synchrotron ra- diation XRPD and found that the indigo molecules located in the channels were highly disordered. Neutron diffraction on Maya blue prepared with deuterated indigo [15, 16] posi- tioned without ambiguity indigo inside the clay channels. The six-fold disorder of the indigo molecule postulated in the syn- chrotron diffraction study was confirmed by the occupancy factor of the deuterium atoms [17]. 2 Experimental results The crystal structure of palygorskite containing in- digo as refined by Chiari et al. [11] shows that an indigo molecule occupies three unit cells along the channel (Fig. 2). In order for the indigo molecules to fill the channels, it is first necessary for the zeolitic water to be removed. Once a dye molecule enters the channel, it forms two hydrogen bonds with the clay framework, which have to be simultaneously broken for the molecule to move one step further in. Only 1 D. Ajò, M. Favaro, C. Reyes-Valerio, G. Chiari, R. Giustetto, presented at the ARCHEOMETRY 2000, 32nd Int. Symp. on Archeometry, Mex- ico City, 2000 (unpublished).