Dean Arnold pieces together mystery of Maya blue

A chance rainstorm in the Mexican state of Yucatan in 1965 set Chicago anthropologist Dean Arnold on a lifelong, somewhat accidental quest to illuminate the mysteries of a sky-blue pigment known as Maya blue.

Arnold, then a graduate student at the University of Illinois, had been in Ticul learning about modern Maya firing techniques from local potters. As the rainy season set in, late in May, Arnold became intrigued with one potter's careful attentions to a pile of hard, light, chalky white rocks. Every time it rained, the man would run outside to cover up the pile. Once, though, he forgot to cover the rocks, and to Arnold's astonishment, they turned to white mud.

The material was palygorskite, a clay-like substance with unusual properties. Chemists would soon show that, mixed with a small amount of indigo, a natural dye, over slow heat, palygorskite transforms into Maya blue, "a rich blue pigment that is very stable over time. It doesn't fade," Arnold said.

Maya blue can still be seen among the ruins at Chichen Itza in Yucatan, on murals that are easily 800 years old. Only a few man-made pigments emerged from any ancient civilizations; most were natural dyes or minerals like lapis lazuli.

The ancient Maya used their brilliant colorant — the blue of the clear sky, of the nearby Caribbean Sea — to paint pots, sculptures, murals, jewelry, clothing, altars and, chillingly, the human beings they sacrificed to garner favor with the rain god, Chaak.

In a dozen trips that began in 1968 (two with Bruce Bohor of the United States Geological Survey), Arnold, now an adjunct curator of anthropology at the Field Museum of Natural History in Chicago, and emeritus professor at Wheaton College, Wheaton, Illinois, located two ancient sources of palygorskite in Yucatan — at Sacalum and Ticul.

In a paper published in the Journal of Archaeological Science in March, Arnold, with the help of several colleagues, and deploying tests that included laser ablation inductively coupled plasma mass spectrometry, x-ray fluorescence and x-ray diffraction, showed that the ancient Maya had mined the sites for the palygorskite they used in Maya blue. Not only that, but Arnold also showed that potters still mine one of those sites for materials, and that they understand the distinctive properties of palygorskite.

Arnold's most celebrated discovery came several years ago, when he happened upon a three-legged pot at the Field Museum, recovered in 1904 from Chichen Itza's Sacred Cenote, a water-filled sinkhole the Maya revered as a portal to Chaak's home. The pot contained a hunk of resin called copal, which the Maya burned as incense and considered food for the gods.

Scanning electron microscopy and x-ray analysis confirmed that white and blue traces on the copal were palygorskite and indigo. All three materials were used in Maya healing arts; the copal could have provided the slow heat required to create Maya blue.