Lead Glaze Tells Archaeologist a Tale
by J. McNulty, UCSC
reprinted with permission
They come from different worlds and at times don’t seem to speak the same language, but archaeologist Judith Habicht-Mauche and geochemist Russell Flegal have built a bridge between their two disciplines. In the process, this unlikely duo has opened the gates of time and shed new light on the activities of Native Americans in the 1400s.
Habicht-Mauche, an associate professor of anthropology, is an expert on Southwestern pottery. Flegal, a professor of earth sciences, has spent most of his career in toxicology, analyzing levels of trace metals in the environment. Their shared interest is lead, an element that began appearing in the glazes of some pottery in the northern Rio Grande Valley of New Mexico in the 14th century and a source today of considerable environmental contamination.
Unlike clay, which is ubiquitous, lead ores are found in discrete areas. In glazes, lead provides viscosity and helps the glaze melt at lower temperatures an advantage when firing pots in open pits that reach a maximum of 900 degrees centigrade, as Native Americans did.
For Habicht-Mauche, whose cabinets at UCSC are filled with dusty bags of broken pottery, the shards decorated with leaded glazes hold clues about the patterns of trade and interaction that made up everyday life for the ancestors of the eastern Pueblos. For years, she wondered where the lead on those fragments came from and how the potters acquired it.
Lucky for her, researchers like Flegal can track the origins of even the smallest traces of lead. Lead has four stable isotopes, and researchers can match lead samples to their sources by measuring the ratios of the isotopes. Flegal is one of the pioneers of stable lead isotope analysis, and it turns out that the ratios of samples from different areas vary quite strikingly, enabling researchers to pinpoint lead sources with confidence.
In August 1996, Habicht-Mauche and Stephen Glenn, formerly an assistant in the UCSC anthropology labs and currently a doctoral candidate in geochemistry at Duke University, spent two weeks in New Mexico gathering lead samples from about 20 known prehistoric and historic mining sites scattered across an area that covers approximately one-fourth of the state. Flegal pitched in over a long weekend, but it was back at UCSC where his talents really came into play.
With about 100 lead samples in hand, Habicht-Mauche was eager to identify the 'signature' ratios of each sample and then do the same for about 60 ceramic pieces gathered from four different sites.
For Flegal, who is used to working with lead samples that weigh one-billionth of a gram, handling golf ball sized hunks of lead sulfide ore was a real change. Using a lead isotope analysis technique called ICP-MS (inductively coupled plasma mass spectrometry), Glenn determined the "fingerprint" of each sample, with the assistance of Genine Scelfo and Rob Franks, research associates with the Insititute of Marine Science who developed the methodology for highly precise analyses with ICP-MS.
The results have been "very spiffy, very clean," said Habicht-Mauche: "Not only do each of the four mining districts produce lead ore with a distinct 'signature,' but all of the lead in the pottery glazes shares the same 'fingerprint,' meaning it came from the same place." Indeed, Habicht-Mauche and Flegal are confident that it all came from deposits on the south side of the Cerrillos Hills area. It is a striking finding because it suggests that people traveled considerable distances to acquire lead from Cerrillos despite lead mines that in some cases were adjacent to their own villages.
"My own hunch is that trade had as much to do with developing social and political ties between communities as it did with just moving stuff from place to place."
"People who were producing pottery in different places were all getting their lead from the same place," said Habicht-Mauche. "The pattern of acquisition doesn’t fit with logistical proximity people were not doing the easiest, most rational thing, which is to go to the place that’s nearest to them."
"This shows that there were regional exchange networks at least in this area and during this time period," she said. "And we know they weren’t trading because others had something they didn’t have. My own hunch is that trade had as much to do with developing social and political ties between communities as it did with just moving stuff from place to place. Trade creates social ties between people, and you’re less likely to go to war with people when you have economic and social relations with them."
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