In 1904, the Sacred Cenote of Chichén Itzá was dredged by archaeologist Edward H. Thompson. Thompson recovered one hundred human skeletons and hundreds of objects sacrificed by the Maya between about AD 500 and the Spanish conquest. Many of the objects recovered by Thompson are currently curated in Chicago's Field Museum, where they recently provided Wheaton College archaeologist Dean Arnold and coauthors with new information about the tenacious turquoise pigment known as Maya Blue. The report appears in the March 2008 issue of Antiquity.
Sacred Cenote (Well of the Sacrifices), Chichén Itzá, Mexico
Photo Credit: Oscar Anton
Maya Blue, like other ancient manufactured pigments (such as Egyptian Blue and Chinese Purple) is quite stable, maintaining its bright turquoise color after hundreds of years and despite exposure to harsh climatic conditions. The sacred cenote at Chichén Itzá is a natural well into which humans and jade, wood, rubber and leather objects were thrown as sacrifices. Before these objects were thrown into the cenote, according to historical accounts, the Maya heated some of the objects and painted many of them blue.
Studies of Maya Blue in the 1960s revealed that the pigment was made of a combination of palygorskite and a tiny bit of indigo (about .5 to 1%). Palygorskite is a white clay called sak lu'um in the Maya language, and it was used for medicinal purposes and pottery temper. Indigo (the source of the blue color) is a plant (Indigofera spp); it was called ch'ooh in the Maya language and was also used for medicinal purposes. Researchers soon discovered that simply combining the two elements, however, did not produce a stable pigment. Only if the combination was exposed to sustained low-heat temperatures, about 150 degrees centigrade, would classic Maya Blue be obtained.
Mural at Bonampak, Chiapas, Mexico with Maya Blue background
Photo Credit: Nick Leonard
Some of the artifacts recovered from Thompson's 1904 dredgings are curated today in Chicago's Field Museum. Among the artifacts is a decorated tripod Mayapan bowl made between about AD 1300-1460. Dean Arnold was perusing the Thompson collection at the Field Museum when he noted a label "Blue on copal in bowl." Energy dispersive x-ray analysis of the material in the bowl revealed a combination of copal, palygorskite and indigo, suggesting to Arnold that the bowl had been used in an attempt to produce Maya Blue.
Copal (called pom) is a form of hardened resin from the sap of the copal tree (Protium copal), and it is basically a young form of amber. Copal melts at a temperature of about 150 degrees centigrade; it burns slowly and consistently with a pungent smoke. It had a widely documented role as incense in ceremonies throughout the cultures of Middle America.
Maya tripod pottery bowl containing copal recovered from the Sacred Cenote from Chichén Itzá, Yucatán. Accession No. 189262
Photo Credit: John Weinstein (c) the Field Museum
The presence of partially burned copal, indigo and palygorskite within this bowl in the Sacred Cenote, combined with historical documents from Spanish chroniclers such as Bishop Landa, indicates that the production of Maya Blue was part of the ritual of religious sacrifice. Objects to be sacrificed would have been heated, painted or dusted blue and then tossed into the murky green water with a ceremonial sizzle.
Although he didn't understand it at the time, Edward Thompson identified evidence of the association of Maya Blue with sacrificial practice, when he dredged the Sacred Cenote in 1904. In his field notes he indicated that a 14 foot thick layer of blue silt was found at the bottom of the cenote. Arnold and colleagues believe this represents the accumulated remains of Maya Blue washed from hundreds of sacrificial objects.
Arnold, Dean E., Jason R. Branden, Patrick Ryan Williams, Gary M. Feinman and J.P. Brown. 2008. The first direct evidence for the production of Maya Blue: Rediscovery of a technology. Antiquity 82:151-164.
Berke, Heinz 2007 The invention of blue and purple pigments in ancient times. Chemical Society Reviews 36:15–30.
Chiari, G., et al. 2008 Pre-columbian nanotechnology: reconciling the mysteries of the Maya Blue pigment. Applied Physics A 90(1):3-7.
By the way, there's a very interesting blog on the physical mechanics of this mixture at the Backreaction site: Cookies, Palygorskite, and Maya Blue.