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 arenow 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 andturned one of these potsherds over in her hands. “These ceramics arebeautiful and incredibly well made,” she said. “The artists who madethem were amazingly skilled and able to very precisely manipulate thematerials and technology. It’s remarkable. I wish I had their skills.”
The Hopi artists created what archaeologists call Jeddito warebetween about 1200 and 1650 A.D., O’Grady explained. The potsherds thatshe’s studying are a subset of Jeddito called Sikyatki Polychrome.They’re named for a site that early archaeologists excavated onNorthern Arizona’s Hopi reservation, where a large number of completeand 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 onehas been able to consistently produce ceramics with this even,tan-yellow buff surface since the last ones were fired around the timethat 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 achemical source? Is it due to sulfur, iron or vanadium in the clayfabric? Or does it have something to do with what was going on in theatmosphere when the pots were being heated in a coal fire?
O’Grady also is studying the paints used to decorate Sikyatki potsand sleuthing out paint recipes. She knows, for instance, that most ofthem were mineral paints, made from iron oxide, manganese dioxide, orvarious combinations of the two.
She also knows the pots were fired outdoors in temporary kilns thatwere fueled by coal. In fact, Sikyatki Polychrome is one of a very fewprehistoric ceramics that were coal-fired to especially hightemperatures.
Most ancient pottery was fired over wood or dung fires. But wood isscarce near the Hopi mesas and there was little dung from domesticanimals until the Spanish brought sheep and horses to the area.However, there are vast coal seams and clay beds near the mesas. Theclay has a high quartz content and is ideal for making fine pottery.
O’Grady’s task now is to determine how these resources weremanipulated 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 theanswers. In addition to the traditional techniques of radiography andpetrography, she's using a scanning electron microscope (SEM) withenergy dispersive x-ray spectrometry (EDS), x-ray diffraction (XRD),transmission electron microscopy (TEM), and even an electron beammicroprobe in UA's Lunar and Planetary Laboratory. She also is usingthe Proton-Induced X-ray Emission (PIXE) unit at Arizona StateUniversity's ion beam facility, as well as other instruments.
Could Coal Firing be the Secret?
Some experts believe the high temperatures produced by coal firingare the secret to producing Sikyatki Polychrome because some of theclay turned to glass when it was fired.
O’Grady has been refiring small bits of the potsherds to a range oflikely temperatures and then creating fresh fractures in the refiredsamples. Next she uses electron microscopy to determine what percentageof the ceramic paste has a glassy structure. When the amount of glassexceeds the original, she knows that she's exceeded the original firingtemperature, allowing her to determine the temperature at which eachshard was fired.
“The glassy phase corresponds to the temperature and length of timethe pots were fired at that temperature,” she said. “We’re finding avery glassy microstructure that indicates these pots were really highfired, most between 1,100 and 1,200 degrees Celsius (2,000 to 2,200degrees Fahrenheit).
The problem now is to determine the exact chemical composition ofthe pottery samples, how long those chemicals were fired, in whattemperature range, and how these components interacted during thefiring process.
Field Testing Comes Next
“I’m hoping that we can start eliminating possibilities and narrowdown what might be responsible for the unique color and composition ofthese ceramics,” O’Grady said. “Then we need to go through areplication firing because you can theorize forever, but at some pointyou have to prove your theory in the field.”
O’Grady will need permission from Hopi tribal officials to conductfield tests because she needs to collect clay and coal samples from thereservation. Over the years, several Hopi potters have been intriguedby 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 thispottery was made, go through the process of testing the theory in thefield and actually answer the questions surrounding Sikyatki Polychromethat have intrigued both archaeologists and potters for so long,”O’Grady said.
O'Grady's research is part of UA's Heritage Conservation ScienceProgram. Students in this program learn to stabilize, preserve andbetter understand ancient artifacts and how they were created and used.
The curriculum, which combines engineering, anthropology,architectural history and art history, is particularly important todaybecause many of the material links to our past are disintegrating,while the ancient technologies that created them are disappearing.
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