In-Situ Identification of Palygorskite in Maya Blue Samples Using Synchrotron X-ray Powder Diffraction
Lori Polette, Norma Ugarte and Russell Chianelli
Department of Chemistry, The University of Texas at El Paso, El Paso, Texas 79912
Presentation at the Workshop on Synchrotron Radiation in Art and Archaeology, SSRL, 18 October 2000
In spite of the abundance of colors found in the murals and other objects encountered in ancient Mayan sites, a peculiar hue of blue has captured the interest of researchers. The blue color, resembling that of the Caribbean Sea, is now known as "Maya Blue." However, the pigment is found not only on wall paintings and pottery from Mayan sites, but in many other locations in Central America as well. Thus, there are open questions as to whether there was ancient technology transfer by the Mayans to other cultures or if the other cultures discovered this technology on their own.
Reconstruction of the method that pre-Hispanic artists used to create exceptional paints has been a challenge. The pigment is not a copper mineral and has no relation to natural ultramarine, ground Lapis Lazuli or Lazurite as originally thought. The Maya Blue is composed of a natural clay (palygorskite) and a natural organic pigment (indigo), and has unprecedented stability when exposed to acid, alkalis, solvents, and resists biodegradation. While the exact technique that the Maya employed to synthesize such a sophisticated paint remains a conundrum, it can be reproduced in a laboratory today but is has been unknown as to why the clay-indigo complex imparts the material it's stability, although here we present substantial evidence in this report that reveals Maya blue stability.
Traditional x-ray diffraction though very successful is ultimately limited by weak conventional x-ray sources. Recently, powerful x-ray sources from synchrotron sources have revolutionized the field of x-ray diffraction. Because of the intense and monochromatic x-rays provided by synchrotron sources, high-resolution and unprecedented precision in the determination of crystal structures and the separation of mixed phase materials with very similar structures can be obtained.
Synchrotron data of Maya blue is helping to further validate the assertion that indigo is inserted into the channels of the palygorskite clay structure, which gives the paint its stability. Additionally, for the first time, palygorskite has been identified from original whole mural fragments whereas other studies were performed using material removed from the murals and ground into powders and data were taken using a conventional x-ray source. This is particularly important because conventional x-ray studies did not provide the resolution necessary to see specific impurities in the palygorskite phase.
Synchrotron data analysis will eventually allow us to elucidate the origin of the starting material used by the Mayans and further understand their technology transfer. The quality of the data is such that we will be able to completely extract the palygorskite pattern and refine the lattice parameters, which describe the atomic structure. Our preliminary data already suggest that the structure is significantly different from the reported structure. Obtaining a refined structure of the palygorskite clay found in the mural fragments from various Maya regions will enable us to determine the sources of the materials the Maya used to create the color because palygoskite from different regions can contain different impurities and can even vary in structure.
One sample of palygorskite (called sak-lum in Mayan) originating from the Yucatan peninsula that we have also analyzed (diffraction data not shown) only indicates the presence of dolomite as an impurity. This data alone already suggests that the clay used by the Maya in the Totonaca region, which clearly showed the presence of calcite rather than dolomite, is not the same clay used by the Maya in the Yucatan. Perhaps this suggests that while the clay was not traded between these regions, the secret of making Maya blue was.
A mural fragment of Maya blue from Bonampak yields yet another Maya secret. In Bonampak, the artists painted using a multiple coating technique. The synchrotron diffraction data indicates the presence of both calcite and dolomite. This is due to the fact that a calcite (gesso) layer was applied prior to the Maya blue paint.
We are continuing to analyze mural fragments from various Maya regions. A complete analysis will reveal some of the ancient Maya secrets and their technology transfer of materials and techniques.
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