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

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.

Colors and dyes of archaeological textiles from Tarapacá in the Atacama Desert (South Central Andes)

“…In the case of the green samples (Samples 12 and 18), FORS results suggest the presence of indigo (Fig. 4c), from the apparent absorption maxima at 648 nm and the inflection point at 717 nm, in agreement with the spectra of indigo references and the results of FORS analysis of Mexican codices [107] and polychrome ceilings [108].…”

Colors and dyes of archaeological textiles from Tarapacá in the Atacama Desert (South Central Andes)

“…In the case of the green samples (Samples 12 and 18), FORS results suggest the presence of indigo (Fig. 5c), from the apparent absorption maxima at 648 nm and the in ection point at 717 nm, in agreement with the spectra of indigo references and the results of FORS analysis of Mexican codices [103], and polychrome ceilings [104].…”

Colour stability of wood coatings pigmented with natural indigo from Isatis tinctoria after accelerated weathering

“…A peak shift could also be observed for the ultramarine‐containing varnished sample, even although the peak only shifted slightly. The spectral changes of the paint samples after 40 days of exposure showed practically a full loss of features related to blue pigments, and the spectra finally had a similar shape to the sample that contained only titanium dioxide as the pigment 35,36 . The final shape in the varnished samples containing synthetic indigo and ultramarine had features of the blue pigment in the visible wavelength region.…”