Caitlin O’Grady hopes to crack a mystery that has puzzled archaeologists and potters for more than 100 years.
It surrounds small pieces of broken Hopi pottery, some of which are now in O’Grady’s lab in the Materials Science and Engineering (MSE) department at The University of Arizona.
O’Grady, an MSE Ph.D. student, recently sat at her lab bench and turned one of these potsherds over in her hands. “These ceramics are beautiful and incredibly well made,” she said. “The artists who made them were amazingly skilled and able to very precisely manipulate the materials and technology. It’s remarkable. I wish I had their skills.”
The Hopi artists created what archaeologists call Jeddito ware between about 1200 and 1650 A.D., O’Grady explained. The potsherds that she’s studying are a subset of Jeddito called Sikyatki Polychrome. They’re named for a site that early archaeologists excavated on Northern Arizona’s Hopi reservation, where a large number of complete and broken pots were found.
No One Knows How They Were Made
The mystery is how they were made. No one knows for sure, and no one has been able to consistently produce ceramics with this even, tan-yellow buff surface since the last ones were fired around the time that Spanish settlers arrived in the Southwest.
O’Grady’s task is to unravel the Sikyatki technology that has been lost in time.
It’s an incredibly complex assignment.
Where does the yellow color come from, for instance? Is it a chemical source? Is it due to sulfur, iron or vanadium in the clay fabric? Or does it have something to do with what was going on in the atmosphere when the pots were being heated in a coal fire?
O’Grady also is studying the paints used to decorate Sikyatki pots and sleuthing out paint recipes. She knows, for instance, that most of them were mineral paints, made from iron oxide, manganese dioxide, or various combinations of the two.
She also knows the pots were fired outdoors in temporary kilns that were fueled by coal. In fact, Sikyatki Polychrome is one of a very few prehistoric ceramics that were coal-fired to especially high temperatures.
Most ancient pottery was fired over wood or dung fires. But wood is scarce near the Hopi mesas and there was little dung from domestic animals until the Spanish brought sheep and horses to the area. However, there are vast coal seams and clay beds near the mesas. The clay has a high quartz content and is ideal for making fine pottery.
O’Grady’s task now is to determine how these resources were manipulated to make what her advisor, Professor Pamela Vandiver, calls “the porcelain of the Southwest.”
O'Grady is calling on several high-tech scientific tools to find the answers. In addition to the traditional techniques of radiography and petrography, she's using a scanning electron microscope (SEM) with energy dispersive x-ray spectrometry (EDS), x-ray diffraction (XRD), transmission electron microscopy (TEM), and even an electron beam microprobe in UA's Lunar and Planetary Laboratory. She also is using the Proton-Induced X-ray Emission (PIXE) unit at Arizona State University's ion beam facility, as well as other instruments.
Could Coal Firing be the Secret?
Some experts believe the high temperatures produced by coal firing are the secret to producing Sikyatki Polychrome because some of the clay turned to glass when it was fired.
O’Grady has been refiring small bits of the potsherds to a range of likely temperatures and then creating fresh fractures in the refired samples. Next she uses electron microscopy to determine what percentage of the ceramic paste has a glassy structure. When the amount of glass exceeds the original, she knows that she's exceeded the original firing temperature, allowing her to determine the temperature at which each shard was fired.
“The glassy phase corresponds to the temperature and length of time the pots were fired at that temperature,” she said. “We’re finding a very glassy microstructure that indicates these pots were really high fired, most between 1,100 and 1,200 degrees Celsius (2,000 to 2,200 degrees Fahrenheit).
The problem now is to determine the exact chemical composition of the pottery samples, how long those chemicals were fired, in what temperature range, and how these components interacted during the firing process.
Field Testing Comes Next
“I’m hoping that we can start eliminating possibilities and narrow down what might be responsible for the unique color and composition of these ceramics,” O’Grady said. “Then we need to go through a replication firing because you can theorize forever, but at some point you have to prove your theory in the field.”
O’Grady will need permission from Hopi tribal officials to conduct field tests because she needs to collect clay and coal samples from the reservation. Over the years, several Hopi potters have been intrigued by Sikyatki Polychrome, and their interest may work in O’Grady’s favor.
“Hopefully, I’ll be able to come up with a good theory of how this pottery was made, go through the process of testing the theory in the field and actually answer the questions surrounding Sikyatki Polychrome that have intrigued both archaeologists and potters for so long,” O’Grady said.
O'Grady's research is part of UA's Heritage Conservation Science Program. Students in this program learn to stabilize, preserve and better understand ancient artifacts and how they were created and used.
The curriculum, which combines engineering, anthropology, architectural history and art history, is particularly important today because many of the material links to our past are disintegrating, while the ancient technologies that created them are disappearing.
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