Grazia et al. Herit Sci (2020) 8:1 https://doi.org/10.1186/s40494-019-0345-z RESEARCH ARTICLE Open Access Shades of blue: non-invasive spectroscopic investigations of Maya blue pigments. From laboratory mock-ups to Mesoamerican codices Chiara Grazia1,2, David Buti3* , Anna Amat2, Francesca Rosi2, Aldo Romani1,2, Davide Domenici4, Antonio Sgamellotti1,2 and Costanza Miliani2* Abstract Maya blue is a hybrid pigment where an organic component, indigo, is incorporated in a porous clay. Despite its widespread use in the Mesoamerican artistic production and numerous studies devoted to understand the type of interactions between indigo and the host framework, its technology has not been completely unravelled yet. In this study portable non-invasive UV–vis refection spectroscopy is proposed as a robust method for in situ investigation of Maya blue pigments. Laboratory mock-ups of powder Maya blue have been prepared employing diferent synthesis procedures (varying the nature of: clays, indigo–clay ratios, heating temperatures and time). The goodness of the pre- pared Maya blue samples—namely the occurrence of indigo–clay interactions—have been probed by micro-Raman spectroscopy and related UV–vis spectral markers have been identifed. DFT calculations as well have been performed to deeply explain UV–vis profles. The set of spectral markers have been fnally exploited to interpret spectra recorded on Mesoamerican pictorial codices, through a multi-technique approach based on exploring the UV–Vis properties of the blue paint supported by the FT-IR vibrational study of the inorganic clays. The characterization of blue colours on pre-Hispanic and colonial Mesoamerican codices contribute to a better understanding of the compositional variabil- ity of these painting materials and to point out the existence of diferent technological traditions of colour prepara- tion in ancient Mesoamerica. Keywords: Maya blue, Organic–inorganic hybrid pigment, Indigo–clay interactions, Host framework, UV–vis refection spectroscopy, DFT calculations, Mesoamerican codices Introduction time as well as the destructions brought by the Spanish Mesoamerican codices represent the cultural legacy of conquest and the subsequent campaigns of extirpation of pre-Hispanic indigenous people who recorded histori- idolatry. cal, religious, calendrical, and astronomical information In the last years, non-invasive in situ spectroscopic using colourful images and logophonetic writing. Only a techniques [1] allowed to study ancient Mesoamerican small part [namely 13 pre-Hispanic codices mostly dated codices (not only pre-Hispanic but also colonial) in view to the Late Postclassic Period (AD 1250–1521)] of the of their capability to yield information about the materi- Mesoamerican library holdings survived the ravages of als composition and overall preserving the integrity of these fragile and valuable artefacts [2–14] for which any sampling and transportation are absolutely unthinkable. *Correspondence: [email protected]; [email protected] 2 CNR-ISTM (Istituto CNR di Scienze e Tecnologie Molecolari), Via Elce di Te fnal aim of this study is to explore beyond the lim- Sotto 8, 06123 Perugia, Italy its of today’s knowledge of Mesoamerican codices paint 3 CATS, Statens Museum for Kunst, Sølvgade 48-50, 1307 Copenhagen K, technology and to provide material science and art-his- Denmark Full list of author information is available at the end of the article torical insight by interpreting data recorded through © The Author(s) 2020. This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creat iveco mmons .org/licen ses/by/4.0/. The Creative Commons Public Domain Dedication waiver (http://creat iveco mmons .org/publi cdoma in/ zero/1.0/) applies to the data made available in this article, unless otherwise stated in a credit line to the data. Grazia et al. Herit Sci (2020) 8:1 Page 2 of 20 a non-invasive multi-technique approach on Maya indigo with edge silanol units of the clay, preventing the blue paints from 13 codices (eight pre-colonial and fve entrance of the colourant molecules in the channels. Chi- colonial). ari et al. [27] proposed that indigo can ft into the chan- In order to optimize the spectral interpretation and nels of palygorskite, or the external grooves [28] forming thus maximizing the information of this non-invasive hydrogen bonds with structural water of the clay (only study, in situ data were necessarily compared with results through its CO groups, with the NH groups remaining from reference samples as well as interpreted taking into free). Another model involves interactions with Al3+ or account the wide, vividly debated and sometime also con- Mg2+ ions substituting Si in the crystal lattice with or troversial literature on Maya blue. without coordinated H 2O [29, 30]. Te heating treat- Maya blue is the well-known hybrid pigment that can ment, inducing the removal of zeolitic water, is always be considered as a precursor of modern nanocomposite reported as necessary for the indigo molecules to fll the materials, in which an organic guest, indigo, is incorpo- channels [28, 29]. rated in an inorganic porous host framework, providing a On the basis of the wide literature, UV–Vis and Raman stable and long-standing pigment. can provide valuable information on the efect of Maya In ancient Mesoamerica indigo (xiuhquilitl in Náhuatl, blue synthesis conditions on its colour [31–35]. the language of the Aztecs; añil in modern Spanish) was In the present paper a detailed study of specifcally extracted from the plant Indigofera sufructicosa [15, 16] conceived reference samples was carried out in order with a process described in two early colonial Mexican to defne UV–vis spectral markers for the non-invasive sources (the Florentine Codex, also known as Historia characterization of Maya blue on real artworks. Te UV– General de las Cosas de Nueva España, written by Fray Vis study on reference samples was supported by color- Bernardino de Sahagún [17], and Historia Natural de imetry and DFT (density functional theory) calculations. Nueva España by Francisco Hernandez [18]). Te his- Micro-Raman spectroscopy was performed on the refer- torical sources didn’t mention the process of producing ence materials to control the quality of the synthesized Maya blue; nevertheless, recent studies proposed that blue samples (i.e. the formation of indigo–clay interac- the Náhuatl term texotli could refer to a blue hybrid tions upon heating) on the basis of literature data [33, organic–inorganic colour [19]. After considerable eforts, 36]. Te non-invasive multi-technique approach carried the mystery concerning composition and synthetic out in situ on the ancient codices was based on UV–Vis manufacture of Maya blue was solved in the 1960s. Te supported by FT-IR (Fourier-transform infrared) spec- remarkable stability of Maya blue had led to the conclu- troscopy in refection mode and colorimetry. sion that it was an entirely inorganic pigment, identifed by Gettens as palygorskite, a white clay [20]. Te pres- Materials and methods ence of an organic dye, responsible for the blue colour, in According to the literature [22, 37, 38], the Maya blue pig- addition to the clay was frst hypothesized by Shepard in ment can be produced (i) by adding the dye from a solu- 1962 [21]. Few years later Van Olphen fnally succeeded tion (containing indoxylacetate or indigo by vat-dying) to in producing a stable blue pigment, resembling Maya the clays (palygorskite or sepiolite) or (ii) by mixing the blue, by mixing and heating indigo with either palygor- powdered components and heating the mixture. It is gen- skite or sepiolite, both being fbrous clays with a tunnel erally reported that 1 wt% is the “optimum” amount of structure [22]. Palygorskite is a magnesium aluminium indigo used to prepare genuine Maya blue pigment [32]. phyllosilicate with ideal composition Si8O20(Al2Mg2) However, in our study we have also synthesized labora- (OH)2(OH2)4∙4H2O. Zeolite-like channels running along tory mock-ups including a wider range of indigo loadings the fbres are flled by weakly bound, non-structural (specifcally 0.3 and 5 wt%) to evaluate the sensitivity and (zeolitic) water, while magnesium and aluminium cati- defning the spectral markers and colorimetric proper- ons complete their coordination with tightly bound water ties for Maya blue on models with the aim of interpreting molecules (structural water) [23]. In sepiolite, a complex non-invasive in situ results. magnesium silicate (Si12O30Mg8(OH)4(OH2)4(H2O)8), the In this work reference samples of palygorskite-based structure defnes channels of 10.6 × 3.7 Å dimensions, and sepiolite-based Maya blue were prepared by ball wider with respect to palygorskite (6.4 × 3.7 Å) [24]. milling (10 min at a frequency of 15 Hz) weighted mix- Several spectroscopic and computational studies were tures of synthetic indigo from Sigma-Aldrich and two devoted to unveil the main location of indigo in the types of clay from Kremer (i.e. sepiolite cat. nr. 58945, clay framework and to identify the functional groups palygorskite cat. nr. 58903). Particle size (determined involved in the interaction which to date are still contro- by laser difraction and expressed as average volume versial [25].