LARRY COPENHAVER

Tucson Citizen

It's a treasure of the Hopi Nation and a centuries-old mystery.

A University of Arizona doctoral student is using polarized-light microscopes and other 21st-century technology to determine which primitive method was used to create the unique yellow-ware pottery between 1350 and 1630

The pottery is recognized for its fine, pale ivory paste, said Caitlin O'Grady, 29, during an interview in her lab at the UA materials science and engineering department.

"These ceramics are beautiful and incredibly well-made," she said while sorting sherds collected about 25 years ago.

O'Grady, who has a bachelor's degree in art history and a master's in conservation and art history, estimated the pottery was fired at temperatures close to 2,000 degrees. While no evidence of a kiln system has been found, there is plenty of evidence the high temperatures were generated by burning coal, harvested from ample open deposits in the area of northeastern Arizona, she said.

"They made elegant-looking vessels because they had the ability to control the firing even though they didn't have a kiln to control the temperatures," O'Grady said, pointing out the pottery's porcelain characteristics. "You can see the glass elements through the microscope."

But just what was the technology? wonders O'Grady, who has worked on the project for nearly 18 months. Is the yellow from chemicals or from sulfur, iron or vanadium in the clay? Could it have been something in the air back in those days?

She expects it will take another year of research to unravel enough of the mystery to publish her findings.

Others have tried to find the secrets, she said. Modern Hopi potters have revived several ancient traditions, including Sikyatki polychrome.

"But reproduction has been inconsistent, and further analysis is needed to replicate the manufacturing technology," O'Grady said.

One clue might be in the chemical reaction of extraordinarily hot environments produced by burning coal with the clay or adding to the clay some finely ground pieces of pottery made with lower-temperature firings, usually by burning wood or dung as fuel, she said. Those fuels produce temperatures of only 1,300 to 1,500 degrees.

Sikyatki, a subset of Jeddito pottery, is significant for several reasons, said Mike Jacobs of the Arizona State Museum, where several intact samples of the pottery are on display.

"Because it was fired with coal, it's more durable, so it lasts longer," he said. "It's a kind of pottery that is so distinctive in texture and composition and decoration that it's readily recognizable. Sikyatki polychrome also is a good marker for trade."

The pottery is protected by several federal laws, including the 1906 Antiquities Act, he said. It also is tightly controlled by the Hopi Nation because artifacts, including sikyatki polychrome, are extremely important historically and culturally.

O'Grady will need permission from Hopi tribal members when it comes time to do field-testing, including replication of her theories.

The research is part of UA's Heritage Conservation Science Program, through which students learn to stabilize, preserve and better understand ancient artifacts.