Maya blue paint: an ancient nanostructured material

I must be going through a blue period. After posting on the Blue Hour and just the day before yesterday on International Klein Blue, yesterday's New York Times brings us The Grim Story of Maya Blue by Kenneth Chang (subscription required). Nathan tells me I have to post about it. On the Web, Maya Blue is color #73C2FB.

I will not write about Chaak, the rain god, and of human sacrifice. You already read that in yesterday's newspaper. In fact, we happened to visit Chichén Itzá in Yucatán back in 1994 and nearby we even visited the Cenote of Sacrifice shown below at right. The guide explained us those sacrifices, noting they were only performed as a last resort during draughts.

Until 1993, Maya Blue paint was a mistery, because it survived the harshest conditions in the jungle, and even modern acids, without fading. No need for Ingeborg Tastl's Fading Tool when you use the original Maya Blue pigment!

The mistery was solved by Constantino Reyes-Valerio (1922 - 2006), who is also the creator of the Maya Blue Web site. His research revealed that the indigo dye from the leaves of añil was encased in a superlattice of palygorskite, a magnesium aluminium phyllosilicate. He contributed the beautiful image below at left.

You can read about the science behind this paint in the Science magazine of 12 July 1996 in the article Maya Blue Paint: An Ancient Nanostructured Material by M. José-Yacamán, Luis Rendón, J. Arenas, and Mari Carmen Serra Puche (subscription required).

They describe that the añil dye was obtained from the sprigs of the xiuquilit (indigophera sp.) plant. However, the superlattice of palygorskyte mixed with indigo molecules does not explain by itself the color tone of Maya blue.

They found that the low concentration impurities of Fe, Mn, and Cr must also be present. Indeed, metallic and oxide particles of nanometer dimensions can strongly influence the optical properties of a material, and small impurity concentrations are sufficient to produce particles.

Nanoparticles can have both linear and nonlinear optical properties. Linear absorbance of small particles can create a resonance feature related to surface-plasmon excitation. The main effect of the particles is classical in nature, although quantum effects are not excluded. Particles much smaller than the wavelength of visible light have an extinction spectrum dominated by the imaginary part of the refractive index. The blue color comes from an absorption curve peaked in the visible part of the spectrum. The oxide metals can account at least in part for the color. In addition, many of the particles are nonspherical, and planar defects may play an important role in the optical properties. The color of Maya blue could be at least partially associated with the presence of nanoparticles.

The authors further report that it is likely that impurities were added to the indigo during the preparation of the añil (heating clay in añil at 100ºC). Oxidation or encapsulation of particles in the substrate explains the acid-resistant character of the paint. The indigo molecules encapsulated in the palygorskite pores probably also contribute to the final color. The combination of an intercalated clay forming a superlattice and the metallic and oxide nanoparticles supported on an amorphus substrate makes the ancient Maya blue look like modern nanostructured materials.