The Mayan blues: Unravelling a mystery

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  • Published: Apr 15, 2013
  • Author: David Bradley
  • Channels: Chemometrics & Informatics
thumbnail image: The Mayan blues: Unravelling a mystery

Ancient colour

Golden Mayan warrior with Maya Blue, Azul Maya, background. Copyright-free image from Wikimedia c/o Constantino Reyes

Maya Blue is an ancient pigment that was used for centuries in Mesoamerica. Unfortunately, the recipe for recreating this azure substance was lost with the cultures that created it. Scientists know from analyses that it was made from a plant dye, indigo, and a type of clay known as palygorskite. But, having a list of ingredients is not a recipe, one also needs to know how to combine and cook them up to make the final product. Now, a team of chemists from the University of Valencia and the Polytechnic University of Valencia in Spain has used a new kinetic analysis of cyclic voltammetry, infrared and ultraviolet spectroscopic data to confirm their 2006 hypothesis for how ancient artists got the blues.

We have reported on various analyses of Maya blue in the spectroscopyNOW ezines over the years but while the analytic data seems to comes thick and fast, it is not until now that a Spanish recipe may well have hit upon just how the pigment was prepared.

Sky blue ceremonial

Maya blue was used to adorn palace walls, sculptures, codices and pieces of pottery with the colour of the sky. In addition to palaces and buildings of the Maya nobility, this pigment was associated with religious ceremonies rituals and may even have been used during human sacrifices given that containers holding traces of the pigment have found at the bottom of some natural and man-made wells on the Yucatán.

This same pigment was used elsewhere by other Mesoamerican cultures and while visually it is its startling blueness that impresses aesthetically, to the chemist, it is the fact that it is highly resistant to chemical and biological deterioration that makes it more intriguing still. Indeed, samples tested today seem unchanged despite their having been first prepared several centuries ago. Understanding how such a resilient and highly coloured material was prepared by those ancient materials scientists could help inform archaeologists about the history of those cultures and perhaps give scientists new clues as to how to prepare modern composites with similarly persistent properties. There have, of course, been many hypotheses put forward regarding the recipe for Maya blue, but science is yet to reach a consensus, the Spanish work may help to rectify that situation.

Two-stage pigment

The dominant theory proposes that there is a single type of Maya Blue that was also prepared in a unique way and that a specific type of bond holds the two components - the organic indigo from the plant Indigofera suffruticosa and the inorganic crystalline but porous palygorskite clay.

The Spanish research seems to contradict this 'monoist' version of the Maya blue story. "We detected a second pigment in the samples, dehydroindigo, which must have formed through oxidation of the indigo when it underwent exposure to the heat that is required to prepare Maya Blue," explains Antonio Doménech-Carbó of the University of Valencia. "Indigo is blue and dehydroindigo is yellow," he adds. The presence of both coloured pigments in varying proportions would determine how much of a greenish hue a particular sample of Maya Blue would have. "It is possible that the Maya knew how to obtain the desired hue by varying the preparation temperature, for example heating the mixture for more or less time or adding more of less wood to the fire," he adds. "The central idea is that, upon anchorage to the clay, a significant fraction of indigo converts into dehydroindigo, a yellow dye resulting from the oxidation of indigo. Then, the Maya could modulate the hue of the pigment from ultramarine blue to turquoise green by varying the proportion of dehydroindigo. This can be accomplished, simply, by varying the temperature (between 100 and 180 Celsius) at which the indigo plus palygorskite mixture is treated," Doménech-Carbó told us. "Our 2013 article provides a kinetic study on the physico-chemical mechanism leading to the formation of the Maya Blue from a mixture of the dye and the clay," he told us.

Another of the unsolved questions is how the dye molecules are distributed in the channels of palygorskite crystal network. Some research has hinted at the indigo bonding to the exterior of the clay structure but other work suggests that the indigo molecules penetrate the crystal channels. This latter theory is supported by the most recent work by the Valencia team published in the journal "Microporous and Mesoporous Materials."

The results reveal that two stages occur when both components are heated to temperatures between 120 and 180 Celsius. In the first and fastest of the two stages water evaporates from the palygorskite and the indigo bonds to the clay, although some of it oxidises to form dehydroindigo. In the second stage it seems that the dye disperses through the channels in the clay. Exactly what binds the organic to the inorganic component and gives Maya Blue's its chemical resistance remains a mystery, however.

"The 'core' idea, the coexistence of indigo plus dehydroindigo, was published by us in 2006 and supported by archaeometric data published in 2007, " Doménech-Carbó told SpectroscopyNOW. "Really, our [new] results are a re-affirmation of the previous ideas based on a novel kinetic study). "Our previous analysis of genuine Maya Blue samples from 32 archaeological sites in Yucatan and Campeche suggested that there was no unique recipe of preparation of Maya Blue," he adds. "On the contrary, we believe that the recipe experienced local variations and also varied during the prolonged duration of the Maya civilization."

Related Links

Microporous Mesoporous Mater, 2013, 166, 123-130: "Application of solid-state electrochemistry techniques to polyfunctional organic-inorganic hybrid materials: The Maya Blue problem"

J Phys Chem B, 2006, 110, 6027-6039: "Dehydroindigo: a New Piece into the Maya Blue Puzzle from the Voltammetry of Microparticles Approach"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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