Archaeologists use X-rays to distinguish iron from different periods of America’s colonial past
Florida Museum of Natural History
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Using X-ray spectrometry, archaeologists have found a way to distinguish iron from different time periods in America's colonial past, which may result in long-anticipated discoveries.
view moreCredit: Florida Museum of Natural History
Key points
- Iron artifacts from early Spanish expeditions in North America often look too similar to tell apart, making it difficult to establish the exact routes that were taken.
- In a new study, archaeologists analyzed iron artifacts spanning more than 400 years of American colonial history using X-ray fluorescence spectroscopy. Their results show that differences in the purity of iron and the trace elements it contains can be reliably used as a diagnostic feature to identify iron artifacts from different time periods.
- This method may be sensitive enough to distinguish iron artifacts from Spanish expeditions separated by only a few decades, but the study authors say more data needs to be collected to be sure.
On a dark night in late May 1543, a group of men snuck through the Native American town of Guachoya and stopped at the gated wall, where a body had recently been buried. Working quietly, they disinterred the body and carried it to a nearby river, where they wrapped it in shawls filled with sand and dropped it in the water. Thus ended the brief and brutal history of Hernando de Soto, a Spanish soldier who helped conquer Nicaragua, overthrow the Inca empire in Peru and famously led an extensive expedition and military campaign from present-day Florida up through South Carolina and west to Arkansas.
His men had decided to bury his body at first, but given that he’d convinced the Indigenous inhabitants that he was a god before he died of an unknown illness — a decidedly ungodlike thing to do — they later committed his body to a tributary of the Mississippi River, hoping no one would find him.
De Soto’s expedition represented the longest sustained 16th-century incursion of Europeans into North America, but it was preceded and followed by several others, 15 in all. That’s a problem for archaeologists. The Spanish left behind detailed records of their exploits in the Americas, but because they only had a vague sense of where they were at any given time, the exact routes they took remains unclear.
Archaeologists have sidestepped this issue by looking for things the Spanish left behind, especially iron, which they brought with them in great quantities. But the various expeditions, which often overlapped, makes things complicated.
“A wrought-iron nail from the 1500s looks like a wrought iron nail from the 1600s,” said Charles Cobb, the Lockwood chair in historical archaeology at the Florida Museum of Natural History.
Nails account for more than half of all metal artifacts found in North America. This, of itself, is no small problem, said Lindsay Bloch, a courtesy faculty member at the Florida Museum and principal investigator at Tempered Archaeological Services. “Archaeologists find lots and lots of rusty nails and other rusty iron objects. We often can’t even tell what they are, so they get weighed, counted and put back in their bag. And usually, no one ever looks at them again,” she said.
The Spanish had more than just nails. They used iron to make axe blades, horseshoes, breastplates, helmets, spokes, spears, knives, guns and more. They even brought along blacksmiths and farriers on their expeditions to repair and repurpose things on the go. But these objects, like nails, are typically indistinguishable through time. From the moment Christopher Columbus laid anchor in the Bahamas through the conquest of Florida, there were too few changes in the style of metalworking for there to be readily observable diagnostic differences between iron objects made by the Spanish.
That may be about to change. Both Cobb and Bloch are coauthors of a new study in which they demonstrate that microscopic differences in iron from this time period can be spotted using X-ray fluorescence spectrometry. They made this discovery by analyzing objects of unknown affinity, which they now think may have come from the de Soto expedition.
Custer’s last stand elevated the status of metal detectors, resulting in big discoveries
The new methodology follows on the heels of a quiet revolution that’s been taking place in southeast archaeology, namely the recent adoption of metal detectors in large-scale survey work.
This kind of change might seem like a no-brainer from the outside. If an absolute novice were told to find ancient metal artifacts, a metal detector is probably the first thing they’d reach for. But these devices are in many ways antithetical to long-established — and successful — methods of archaeological excavation.
Before metal detectors existed, archaeologists relied solely on their own experience and intuition to find things. They’d set up a transect in a likely spot and dig test holes at regular intervals or scour a predetermined area for objects that had been exposed to the elements. Whenever they found something, they’d clear away an excavation plot and slowly, meticulously work their way down through the sediment horizons while noting the exact location of each object. These methods are rigorously thorough and still in use. They’ve contributed the majority of what we know about ancient cultures that wasn’t put down in writing.
When the first portable metal detectors were invented in the 1930s, archaeologists didn’t have much of a need for them. But metal detectors did catch on with another group: Hobbyists and treasure hunters began finding metal objects all over the place, which they often kept or sold for profit. This didn’t sit well with archaeologists, who were of the opinion shared by Indiana Jones that such things belong in a museum.
“Metal detectors have a bad reputation in archaeology because they are often the go-to for people who loot historic sites,” Bloch said.
So, for several decades, most archaeologists wouldn’t have been caught dead with a metal detector, until 1983, when a wildfire in Montana cleared away the dense vegetation that covered the site of Lt. Col. George Armstrong Custer’s last stand at the Battle of Little Bighorn. The conflict between the U.S. Army and an alliance of Indigenous tribes had taken place over a large area, which made the standard archaeological approach of shovel testing impractical. An enterprising research team, not wanting to waste the narrow window of opportunity, decided to give metal detection a try.
Their gumption paid off. Archaeologists recovered such a great amount and variety of munitions that they were able to retrace the harried steps of U.S. troops, who were routed during the two-day battle.
Other archaeologists took note. “That made it a bit more legitimate for people, but it still took a long time for it to catch on, and it still hasn’t caught on completely,” Cobb said.
For his part, Cobb has no qualms with metal detectors and has made consistent use of them for the last several years. His first big breakthrough came in 2015, when he participated in an archaeological survey in Mississippi. The survey was primarily undertaken to locate ancestral Chickasaw sites. On a whim, they decided to bring out a few metal detectors. Native Americans often traded iron objects they obtained from the Spanish, so it was reasonable to expect there might be a few items lying around that might be indicative of a former habitation. Instead, they discovered what was likely the site of a major battle between de Soto’s army and the Chickasaw that archaeologists had spent decades searching for.
Other breakthroughs soon followed. Before about 15 years ago, less than 100 European objects had been found at North American Indigenous sites outside of Florida. That number has since swiftly increased, leaving archaeologists looking for better ways to determine who these objects belonged to.
One option would be to analyze impurities in the iron. The process of refining iron ore by smelting and forging it leaves a substance that is very nearly 100% iron, but not quite. The more forging a metal is subjected to, the purer its content will be, but trace elements like copper, vanadium and manganese stubbornly resist removal. The proportions of those trace elements are specific to the geographic location where they were deposited. These differences can theoretically be used to determine where a hunk of iron had originally been mined from.
Metals lend themselves well to X-ray analysis because of their high density, and fortunately for Cobb, an expert on the subject happened to work just down the hall from his office.
“She won’t brag on herself, but Lindsay literally wrote the manual on how archaeologists should use X-ray fluorescence spectrometry when she was a grad student,” Cobb said.
The purity of iron artifacts and the trace elements they contain differ through time
Since their study was intended as a proof of concept, the authors decided to cast a wide net by looking at iron material from multiple places and time periods associated with Spanish colonialism. Among the sites included were the first Spanish colony in the Americas, established by Columbus in 1492; several Spanish missions; the 16th century capital of La Florida (located in present-day South Carolina); the de Soto/Chickasaw battle site Cobb had helped excavate; a British fort; and three 19th century plantations.
They also included iron from a site in Alabama known as the Marengo complex. The area comprises excavations from several villages and is believed to be somewhere near the Indigenous town of Mabila, where de Soto’s men engaged in an even more devastating battle than the one they’d have with the Chickasaw a few months later.
Although objects from the Chickasaw battle unambiguously came from the de Soto expedition, the provenance of those from the Marengo complex is less certain. De Soto certainly traveled through the area and left behind a significant store of supplies after the loss of life incurred during the fighting, which reduced the expedition’s ability to haul heavy equipment through the wilderness.
But two decades later, another Spanish expedition came through the area led by Tristán de Luna, who established a nearby settlement that has also yet to be discovered. Grueling starvation and conflict forced de Luna to abandon the settlement, leaving behind supplies that would have been virtually indistinguishable from those of his predecessor. Thus, archaeologists working at the Marengo complex can’t be sure which of the two expeditions artifacts in the area came from.
Though de Soto and de Luna visited some of the same places and were met with similar fates (though De Luna was merely deposed by his men rather than dying outright), their expeditions differ in one important regard. De Soto lived in Spain when he began assembling ships, men and supplies for the trip, and he sourced all of his iron equipment from Europe. De Luna got his supplies from New Spain in South America. He also strongly relied on his men to bring their own assorted iron tools along.
In an early test of the X-ray fluorescence spectrometry method in archaeology, a 2013 study of iron artifacts from Pensacola (known to have come from De Luna) and others from Tallahassee (thought to have come from de Soto) yielded tantalizing but inconclusive results indicating there were elemental differences between the two.
Hoping to find something more robust, Bloch scanned the assemblage of artifacts they’d gathered with a handheld X-ray spectrometer and held her breath.
As a proof of concept, the results were a success. The types of impurities in iron varied markedly through time. The authors say the differences were so consistent that, going forward, they can be reliably used as a diagnostic feature. Small amounts of manganese, for example, were found in some artifacts from the 16th century, but this element was almost entirely absent in iron from later periods. Bismuth was more likely to show up in 18th and 19th century artifacts, and several impurities — including titanium, ruthenium and zirconium — were associated with iron from the late 16th and the 17th centuries.
The overall quality of iron also differed through the ages. Iron artifacts from the mid-16th century had the fewest impurities, and of these, horseshoes had the highest iron content. There was a significant dip in iron quality associated with the 16th and 17th centuries, corresponding with the greater variety of impurities in artifacts from this time. The quality of iron from later periods improved, but it never reached the level of purity found in objects from the early expeditions.
Their results also suggested that the iron recovered from the Marengo complex had likely come from de Soto, but the authors say it’s still too soon to tell. To be certain, they’ll need to take measurements from additional objects that they can pinpoint to specific expeditions that can be used as a standard. And X-rays, while proven to be informative in archaeological contexts, are the quick and dirty way to collect data. To really get down to the fine-grained differences, Cobb said, they will need use a method called isotopic analysis, which gives more precise (and expensive) results. The authors are currently in the process of applying for a grant that would allow them to do just that.
The authors published their study in the International Journal of Historical Archaeology.
Additional authors of the study are Nicolas Delsol of the Université Laval; Gifford Waters of the Florida Museum of Natural History; Edmond Boudreaux III of Mississippi State University; Chester DePratter, James Legg and Steven Smith of the University of South Carolina; Ashley Dumas of the University of West Alabama; V. James Knight Jr. of the University of Alabama; and Brad Lieb of the Chickasaw Nation.
Journal
International Journal of Historical Archaeology
Article Title
Spanish Signatures? XRF Analysis of Iron Artifacts in the American Southeast
