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Maya paint secrets revealed

Researchers have used Accelrys' Cerius2 software to unlock the chemical structure of a remarkable ancient pigment, Maya Blue.

The Maya, probably the best known classical civilisation, rose to prominence in the Yucután region of Central America around 250 AD. The area, better known today as southern Mexico, Guatemala, northern Belize and western Honduras, still shows the physical signs of this once great civilisation with pyramids soaring above the steaming jungles. But, this wonderful architecture aside, the Maya have had a much greater influence on the modern day world through their intellectual achievements. Their advanced mathematics tracked the planets, predicted eclipses and produced an accurate calendar, and Maya scribes developed a written hieroglyphic language. They were also known for their artwork, with elaborately decorated murals, statues, and ceramics. The colours used were possibly the most spectacular of the time period and were derived from natural materials, e.g. iron, manganese oxides, and charcoal. The raw ingredients were mixed with specific binding agents depending on application and thus required a working knowledge of chemistry.

One of the most striking colours used was Maya Blue, a turquoise that resembles the surface of the Caribbean sea. Despite their age and the harsh weathering conditions, ceramics coloured by Maya Blue have not faded over the centuries. What is even more remarkable is that the colour is resistant to extremes of pH, chemical solvents, and biodegradation. The pigment is a mixture of both organic and inorganic ingredients - a natural clay, palygorskite, and a plant derivative, indigo. What is particularly surprising is that no known organic pigments show such stability. As a result, Maya Blue's chemical complexity and remarkable physical properties have attracted much scientific attention for over 50 years. Even though it is known how to manufacture the paint in the laboratory, the secrets to its stability remain a mystery, until now.

In a joint collaboration, Lori Polette, Norma Ugarte, and Russell Chianelli at the University of Texas, and José Yacamán at the Instituto Nacional de Investigaciones Nucleares, Mexico City, used Accelrys' Cerius2 molecular modelling and simulation software to first create a virtual model of the crystal structure of palygorskite. The clay has a fibrous, channel-containing structure with water molecules held loosely inside the channels. These water molecules are removed by heating and, once heated, the channels absorb molecules. In the case of Maya Blue, indigo molecules are absorbed, resulting in the pigment's stability. The scientists simulated this absorption process using Cerius2 and, once conditions for absorption had been optimised, found that the indigo molecules were held in the channels by a strong chemical bond.






Simulations of the structures of Maya Blue using Accelrys' Cerius2 software


Polette says "Maya blue is a particularly significant material because it has unprecedented stability but does not contain heavy metals. This is important to the paint and pigment industry, which annually consumes large amounts of strategically important and environmentally unfriendly metals. Replacement of these metalliferous pigments with those such as Maya Blue would greatly improve the current environmental situation and result in considerable savings in strategic metals."

Polette emphasises the importance of the Accelrys' software in the research "We synthetically produced Maya Blue in the laboratory and tried to thoroughly understand the complex interaction of palygorskite and indigo from the macroscopic to the atomic scale. Using programs such as Cerius2 is an example of applying the most advanced techniques in modern science." Polette continues "Nearly all of the results we obtained from instrumental techniques can be modeled on Cerius2 to compare theoretical and experimental hypotheses and conclusions. For example, we used the sorption and HRTEM programs to model the superlattice created by indigo intercalating into the palygorskite clay. We also used simulated X-ray diffraction patterns to model the various mineral phases present in the raw materials."


These results provide the first evidence of indigo in the palygorskite channels and, most importantly, the chemical bonds binding the molecules for centuries. "We are currently developing additional paints based on the ancient Maya technology and are working with the paint and coatings industry to develop these paints on a large-scale" Polette continues.

References

For the full paper, see: Scientific American Discovering Archaeology, August 2000, p. 46 (http://www.discoveringarchaeology.com).

For background information on the Maya, see: http://www.civilization.ca/membrs/civiliz/maya/mmc01eng.html.




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