Shades of blue: non-invasive spectroscopic investigations of Maya blue pigments. From laboratory mock-ups to Mesoamerican codices

Chiara Grazia,Anna Amat,David Buti,Aldo Romani,Antonio Sgamellotti,Costanza Miliani,Francesca Rosi,Davide Domenici

doi:10.1186/s40494-019-0345-z

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 reflection 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 different synthesis procedures (varying the nature of: clays, indigo–clay ratios, heating temperatures and time). The goodness of the prepared 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 identified. DFT calculations as well have been performed to deeply explain UV–vis profiles. The set of spectral markers have been finally 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 variability of these painting materials and to point out the existence of different technological traditions of colour preparation in ancient Mesoamerica.

Highlights

Mesoamerican codices represent the cultural legacy of pre-Hispanic indigenous people who recorded historical, religious, calendrical, and astronomical information using colourful images and logophonetic writing
Preliminary characterization of Maya blue reference samples Pre-resonance micro-Raman analysis of palygorskitebased and sepiolite-based Maya blue replicas were aimed at investigating dye–clay interactions as function of differences in weight ratio, temperature and heating time
UV–vis spectral markers and effect of synthesis parameters on the optical properties of palygorskite‐based and sepiolite‐based hybrids UV–Vis reflection properties of palygorskite‐based hybrids Indigo, or indigotin (2,2′-bis(2,3-dihydro-3-oxoindolyliden), is an organic dye which owes its blue colour to the presence of a doubly cross-conjugated system of two electron donor groups and two electron acceptors groups

Summary

Introduction

Mesoamerican codices represent the cultural legacy of pre-Hispanic indigenous people who recorded historical, religious, calendrical, and astronomical information using colourful images and logophonetic writing. Non-invasive in situ spectroscopic techniques [1] allowed to study ancient Mesoamerican codices ( pre-Hispanic and colonial) in view of their capability to yield information about the materials composition and overall preserving the integrity of these fragile and valuable artefacts [2,3,4,5,6,7,8,9,10,11,12,13,14] for which any sampling and transportation are absolutely unthinkable. Few years later Van Olphen succeeded in producing a stable blue pigment, resembling Maya blue, by mixing and heating indigo with either palygorskite or sepiolite, both being fibrous clays with a tunnel structure [22]. A complex magnesium silicate (Si12O30Mg8(OH)4(OH2)4(H2O)8), the structure defines channels of 10.6 × 3.7 Å dimensions, wider with respect to palygorskite (6.4 × 3.7 Å) [24]

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