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

Mayan civilization stands out among the pre-Columbian cultures for its writing (similar to hieroglyphs), sophisticated art, and advances in mathematics and astronomy. It developed in Mesoamerica around 4,000 years ago in what today is the Yucatan peninsula (Mexico), Guatemala, Belize, and part of Honduras and El Salvador. Several archaeological sites, such as Bonampak, preserve beautiful examples of Mayan paintings. The blue shades of these murals contain the hybrid pigment called Maya blue, which is characteristic of Mayan art but other civilizations such as the Aztecs used it as well. 

The Maya blue is a precursor of the modern nanocomposites, which are materials that contain at least one component with dimensions in the range of nanometers. It is a mixture of indigo with a clay, for example, palygorskite or sepiolite. Scientists classify Maya blue as a hybrid pigment because the two components interact and create a new compound.

Chiara Garzia and colleagues’ research aimed to understand the technology and use of Maya blue in different codices. Their approach was multi-analytical and was focused primarily on non-invasive techniques. They studied Maya blue samples prepared in the laboratory using different proportions of the dye and the clay and followed different procedures.

The authors used UV–Vis (200–1100 nm) and infrared (4000–400 cm^-1) spectroscopies, both in reflection mode, for the analysis. The reflection mode allows performing the measurements in a non-invasive way; it detects the signal reflected from the surfaces of the object after illuminating them with specific radiations. Another advantage of the reflection mode is that it can be performed in situ and does not require moving the object or sampling; unfortunately, the data interpretation is tricky since some distortions in the spectra are produced because of the surface texture.  

They compared the analytical results from the laboratory samples prepared using the information obtained from 13 codices: eight from the pre-Columbian period (there are only 13 pre-Columbian codices around the world) and five from the early colonial period. 

The results from the samples prepared in the lab allowed the authors to differentiate between a simple mixture of indigo and the clay and a real Maya blue. When palygorskite is the clay host, the genuine Maya blue pigment is formed after heating the materials. The bands at 294–292 nm characterize this Maya blue in the UV–Vis spectra. They also identified other spectral modifications that indicate a change in the electronic structure of indigo produced by the interaction with palygorskite. When sepiolite was the host, the Maya blue formed even without heating only by grinding the two components. The bands at 646–640 nm characterize this pigment in the UV–Vis spectra (Figure 1a). 

 

Figure 1. a) UV–Vis reflectance spectra of indigo-palygroskite mixtures with (24 h at 130 °C, 160 °C, and 180 °C) and without (black line) applied heat. The dash-dotted black line is the reflectance spectra of solid indigo dispersed in BaSO₄ for comparison. The spectra displayed in a, c, and d show results for different concentrations (1, 0.3, and 5 wt%) of indigo in the mixture.

The FTIR spectra provided information on the inorganic host used for the production of the Maya blue. The spectra showed the characteristic peaks of silicates (978 cm^-1 and 1023 cm^-1), common for both the clays, and specific peaks from sepiolite (422 cm^-1) and palygorskite (516 cm^-1). These signals are present in an inverted shape because of the distortions produced by the reflection mode. 

By comparing the results from the samples prepared in the lab, they identified two sources for the blue hues in the codices (Figure 2): the indigo-based pigments and pigments based on extracts from the plant Commelina coelestis (presented in detail in another paper from Garzia et al.).  The authors classified the indigo-based colorants into four types:

• the genuine Maya blue hybrid pigment, which was present in the majority of the codices. Type 1 is made with palygorskite and Type 3 with sepiolite.
• two variations: Type 2 is obtained using palygorskite and heating it at high temperatures or using a low concentration of indigo, and Type 4 is produced by mixing the indigo dye and the clays without creating the hybrid pigment.

Figure 2. Left: Detail of the Codex Vaticanus B, the sepiolite-based Maya blue was identified in the light blue areas and the simple mixture of indigo and clays in the dark blue areas. Right: UV–Vis reflectance and FTIR spectra collected on the Codex Vaticanus A and B. a) and b) UV–Vis reflectance spectra, c) and d) FTIR spectra in reflection mode.

This research is a great starting point for the study of Maya blue and offers information regarding its preparation technique. The results allowed the identification of different types of blue pigments prepared with indigo, only two of them are true Maya blue, which were used to obtain different shades of blue in the codices. Scientists can use the methodology proposed to analyze other codices and to study other kinds of artifacts, such as mural paintings in a non-invasive and portable way.  

All figures reproduced/adapted with permission from: “Chiara Garzia, et al., Shades of blue: non-invasive spectroscopic investigations of Maya blue pigments. From laboratory mock-ups to Measoamerican codices in Heritage Science, 2020, Spring Nature. Copyright 2020 Spring Nature”