Example: Can EMPA Identify Coronado's Musket Balls?
Above: A painting by American artist Frederic Remington (1861-1909), best known for his paintings of the American West, titled "Coronado Sets Out to the North." Source: Texas Council for the Humanities.
Francisco Vázquez de Coronado had balls. Musket balls, that is. And like all 16th-century Spanish conquistadors, Coronado used his musket balls to "explore" Central and North America, leaving behind many of these lead bullets along the expedition's trail and for us to find in the archaeological record. The precise path taken by his expedition, consisting of over 300 Spaniards, 1300 Native Americans, and an unknown number of African slaves, is not known. Reconstructing Coronado's route is an interest of historians and historical archaeologists. Some of the researchers hope that musket balls from known Coronado camp sites can be utilized to identify suspected ones. In 2005, I was contacted by a group of such researchers interested in this, and I agreed to help.
Therefore, in 2005 and 2007, I did analyses of musket balls from known and suspected Coronado camp sites. The researchers were very enthusiastic and hoped that the analyses would enable them to positively identify suspected Coronado camp sites at the real thing. I, though, was cautious and had my doubts that we could establish any firm identification. At best, it might help support or debunk the sites when the rest of their evidence was considered. At worst, it could be totally worthless. It was worth a shot, I figured (no put intended).
Those researchers have offered their interpretations elsewhere: there is a website (http://chichilticale.com/), a new article ("The Chichilticale Camp of Francisco Vázquez de Coronado: The Search for the Red House," New Mexico Historical Review, vol 82, no 4, fall 2007), and upcoming publications in the same journal.
I offer my insights here, not in the context of those outlets above, for a few reasons. First, this work has particular significance because, as I wrote about here on my Geo/Arch/Sci Blog, reporters recently learned that the FBI had concluded in 2004 that trace-element fingerprinting of modern lead bullets does not work, but the government has neglected to reevaluate criminal cases in which the debunked technique was the key to a conviction. Second, my interests in this project have different foci that the other researchers. Finally, I want to show how analyses can be iterative processes and how I made a mistake, realized a problem, and corrected it.
First let's review a little history about Francisco Vázquez de Coronado...
Below: The suspected route of Coronado's expedition, 1540-1542. Source: National Park Service.
Coronado and his expedition, composed of more than 1300 individuals, set out from Mexico in 1540 to discover the Seven Cities of Gold, which, of course, they planned to pillage. They headed north, with Coronado and his soldiers ahead of the main expeditionary group, and entered present-day New Mexico. Coronado headed east into Arizona, probably along the San Pedro River, and then their path becomes hazy. At some point, the expedition stopped at a location they called Chichilticale ("red house" in Nahuatl) and likely met the Apache. Many people have speculated (e.g., Sauer 1932, Haury 1984, Duffen and Hartmann 1997) about the location of Chichilticale. Finding the site has apparently become some sort of Holy Grail for Coronado enthusiasts.
Identifying a suspected site as Chichilticale was the goal of the researchers who contacted me. They had found a possible location for Chichilticale as well as possible Coronado camps up- and down-trail from there. The site has landscape features that fit chronicles of the expedition, and there are a few artifacts from the period (e.g., part of a Spanish crossbow, a horseshoe nail). There are also ruins of red clay bricks -- remember Chichilticale means "red house." Unfortunately, there is also evidence of a later presence: a coin dated 1774. (As you might have guessed based on only the artifacts listed here, the surveyors used a metal detector.)
Also among the artifacts were musket balls. These, the researchers hoped, could aid the identification. They had musket balls from known Coronado campsites and hoped that there may be a recognizable signature. There was reason to believe this would be true: the FBI had been using "comparative lead-bullet analysis" since the Kennedy assassination, and it was not yet known such tests had been debunked.
On the positive side, there are ways that the lead could have some sort of chemical signature. Antimony is added sometimes to lead in order to strengthen it, and different musket ball makers could've thought different amounts of antimony were necessary, if any at all. Lead ores commonly contain copper, silver, and/or zinc, so variation in the concentrations of these elements might prove useful. The three principal ores of lead are galena (PbS), anglesite (PbSO4), and cerussite (PbCO3). Therefore, the presence of sulfur would suggest either galena or anglesite were smelted to produce the lead. More important, these minerals can have different impurities. For instance, bismuth, arsenic, cadmium, antimony, zinc, thallium, and selenium can occur as trace elements in galena, and their relative amounts can indicate something about the conditions of their formation.
One problem, though, is that I'd classify musket balls as "expedient tools." No one spends a great amount of time making a really nice musket ball. They are made by pouring molten lead into a mold, and after they're used once, musket balls are often lost and not reused. Musket balls don't even have to fit perfectly into the barrel -- they were usually wrapped in linen or paper in order to achieve a tight fit. More importantly, their composition really isn't that critical. A musket ball that is 99% lead is no more or less dangerous than one that is 97% lead. That's the reason modern bullet manufacturers use recycled lead from, for instance, old car batteries -- minor amounts of calcium or strontium don't, in practice, affect a bullet's performance. This is especially true for muskets, which, in Coronado's period, weren't accurate beyond 40 or 50 meters. We shouldn't expect that the musket-ball maker controlled their compositions closely. Each batch of musket balls could be extremely different.
Another possible complication would include the shear number and variety of musket balls carried by the Coronado expedition. As I mentioned above, there were over 300 Spaniards on the expedition. I'm guessing that there could easily have been over dozens, even hundreds, of muskets and pistols carried with them. Their guns may have had different calibers -- for instance, the British "Brown Bess" shot 3/4-inch ammunition. The expedition members might have had to haul their own ammunition, might have had a shared reserve, or even might have been responsible for making their own ammunition. Even a common stash may have included musket balls from multiple makers and/or batches. Would they have brought all their ammunition with them? Or would they have expended the musket balls brought with them and had to make more ammunition along the way? There could've easily been great variation in musket balls on the Coronado expedition. Furthermore, iron was also utilized by the Spanish to make musket balls during the sixteenth century (either "often" or "sometimes" depending on one's source). Were the iron musket balls considered "better" or the lead ones? Would Coronado have had the "better" ammunition? These are the types of questions we must ask before we're able to formulate meaningful conclusions.
Despite the concerns, I figured that there was no harm in trying, so I agreed to do the analysis. In 2005, I was sent fragments of five musket balls from three different sites. Later, I received eleven more samples from four sites. All samples were mounted and polished according to our usual sample preparation procedures -- these samples were embedded in epoxy plugs, which were polished using a series of silicon carbon discs and diamond suspensions on fabric pads. They were then coated with a layer of carbon to ensure conductivity.
The samples were first observed using secondary-electron imaging...
... and backscattered-electron imaging (the field of view for all images is 1 x1 mm).
These electron images reveal a few things. You can see that there are apparently particles in the lead matrix. The particles appear dark in the images because their average atomic number must be lower than that of lead -- darker areas are lighter (have a lower atomic number) than brighter areas. You may also notice in the SE images that the particles have a better polish than the surrounding lead, which looks mottled. This is because lead metal is ductile, malleable, and soft. It is notoriously difficult to polish. Pure lead metal is never used as a standard for microprobe analyses for this reason -- we have to use lead compounds, like galena, instead.
I then used our energy-dispersive spectrometer (EDS) and the five wavelength-dispersive spectrometers (WDS) to determine which elements were present in the musket ball samples. I detected small amounts of carbon (from the applied coats), oxygen, antimony, silicon, arsenic, tin, aluminum, copper, and silver. The maps below illustrate the element spatial distributions that I observed (the field of view is 1 x 1 mm):
Of the elements here, three are homogeneously distributed, four are heterogeneously distributed. We believed the musket ball matrix would be mostly lead. We also see two other homogeneously distributed elements: arsenic and antimony. Arsenic was a metalloid typically alloyed with lead, a practice dating as far back as the Bronze Age -- its presence shouldn't be a surprise. Like antimony, arsenic hardens lead. Antimony has been used in shot since the 1800s, but did the Spanish use it in musket balls in the 16th century? I don't know.
For one site (one that is a Coronado battlefield site, I think), some, but not all, samples had a few percent antimony present -- remember that antimony could've been added deliberately for strength or could have originated from the galena. This suggests that there are at least two types of musket balls at the site.
There are tiny silver-, copper-, and aluminum-based inclusions in the lead matrix. Both silver and copper probably originated in the ore: as mentioned above, the elements commonly occur with lead. The aluminum-rich inclusions probably occur as alumina (aluminum oxide) or aluminosilicates, and either form may well have occurred naturally surrounding the lead ore. Most obvious, though, are the larger silicon-rich particles.
The silicon-rich particles varied in size and abundance in the various samples -- for instance:
These silicon-rich particles appeared dark in BSE images, and it does not make sense that these would be metallic silicon particles. Investigation with the EDS system showed only a silicon X-ray peak, but I'd expected this, though, because the EDS system cannot detect elements lighter than sodium, so I wouldn't see an oxygen peak. The most likely interpretation was that these are quartz grains, possibly added deliberately.
I sent one of the Coronado researchers my quantitative analyses and X-ray maps, and I discussed my findings and observations with him. He was intrigued by these quartz grains and decided to consult a few individuals who make musket balls -- I'm not sure if these were Civil War reenactors, people interested in living history, or what. When he called back, he said that musket-ball makers will often throw a handful of sand into the molten lead. The individuals apparently didn't know why this was done, just that it was (according to someone). I've been unable to confirm that anyone did this, much less sixteenth-century Spanish conquistadors.
If throwing a handful of sand into the molten lead is indeed a common practice, how do we control for this? Would handfuls of sand ever have the same composition? Would different people have different ideas about the size of a "handful" of sand? Would musket balls from even the same batch have different amounts of sand? It seemed that quartz grains would have to be excluded from the analysis. The electron beam could be placed on spots free from quartz, and the lead itself could be to focus of study. The quartz grains could still be observed, and their sizes and abundance could be recorded and kept in mind as a possible indicator.
But there's a problem -- those aren't all quartz grains. Quartz is silicon and oxygen, but look at the element maps I collected after I started to realize that there was a problem:
There is no oxygen associated with those largest silicon-rich particles. If one examines a combination map (red = aluminum, green = oxygen, blue = silicon), the white arrows point to some of the small particles which contain both silicon and oxygen -- they are quartz. The blue particles, though, contain no oxygen -- they cannot be quartz. The particles can't really be silicon metal, at least not in these particular samples.
What are these silicon-rich particles then? I did a WDS scan for light elements because the EDS spectrum still only showed a silicon characteristic X-ray peak and no other elements. I soon learned my answer. These particles were contamination, and here is the source of the contamination: the silicon-carbide polishing discs:
Remember that we described lead as very soft, ductile, and malleable? Well, during polishing, some of the silicon-carbide particles were pressed into the soft lead, and the lead deformed around these particles, incorporating them within its matrix. This doesn't happen with rocks, glass, or other typical samples. It wasn't until I thought about the situation for a time that I released I should go back and reexamine these particles. Fortunately, I realized my error, discovered the true identify of the particles, and figured out why they were present.
During this reexamination, I was able to confirm there were a few micron-scale silica, alumina, and aluminosilicate inclusions, but these were so small that they wouldn't have been deliberately added. Such grains are probably the remainders of minerals that surrounded the lead ore. I think we can rule out handfuls of sand being tossed into the molten lead because these particles are very fine and silt-sized or even smaller. It is possible that, like the silicon-carbide particles from the polishing discs, these particles are due to contamination during preparation: some of the polishing supplies were also used with rocks rich in silicon, aluminum, and calcium.
Clearly, then, silicon (and carbon) must be excluded from the analyses. I won't detail the exact results here as the Coronado researchers are planning to publish the data and make them available on their website. Suffice it to say variations were found mainly in silver, arsenic, and antimony. Arsenic and antimony would have been deliberately added on purpose to increase strength. Silver likely would have come from the ore. There also were variations in elements like copper and sulfur. The sample size, though, was very small -- a mere sixteen samples total from six different sites -- especially considering the likely complications discussed above.
What could be said at the end? Well, I agreed with the Coronado researchers that the musket balls from the place suspected to be Chichilticale are not inconsistent with musket balls from known Coronado sites. Yes, that is not saying too much: they're not inconsistent. If I was feeling particularly generous, I might even say that musket balls unearthed at the site are consistent with musket balls from known Coronado sites. (Update -- 04/25/08: Read my response to a reader's comments and criticisms about this paragraph.)
The problem with this project is that I do not know what fraction of musket balls are consistent with those found at Coronado campsites. Are 10% of musket balls consistent with them? Or are 90% consistent with them? I wasn't provided with even one musket ball from a site not associated with Coronado's expedition. I had no samples from Portuguese explorers, Hernan Cortes, De Soto, Lewis and Clark, fur trappers, or anyone else. Without the proper stratigraphic information or dating, even the musket balls found (using metal detectors?) at known Coronado sites shouldn't be immediately assumed to be contemporaneous with Coronado. This boils down to some musket balls unearthed at one place are not terribly different from musket balls found at a few other places. Would these same musket balls be consistent with George Washington's ammunition? Perhaps.
Between a really small sample size, no tests of musket balls known not to have been used by Coronado, and the unknowns about the musket-ball supplies used by the Coronado expedition, this line of investigation shouldn't be provided much weight. Much more work would have to be done on these artifacts. Even then, the findings could possibly be no better than those of the FBI: fingerprinting bullets doesn't really work. The FBI concluded that any particular bullet could have millions of compositional "twins" and that all the bullets sold together in one box didn't necessarily (or even usually) have the same trace elements (read more here).
Lead isotopes are so popular for sourcing in archaeology that there is a tiny number of other studies on the trace elements in historical musket balls. Among the best projects are two unpublished Master's theses. One is Sarah Workman's Lead Musket and Pistol Ball Analysis from the O'Connell Mission Site (1999, Florida State University, Tallahassee, Florida), and the other work is Jamie R. Lockman's Elemental Analysis of Nineteenth Century Lead Artifacts from Lewis and Clark and Hudson's Bay Sites of the Pacific Northwest (2006, University of Montana). It appears that, beyond these theses and the works they cite, little else has been done.
This was an interesting project, but I also had mixed feelings about studying these musket balls. Why? Well...
Above: What conquistadors did with musket balls. Credit: National Geographic.
The individual above is the first forensically known gunshot victim in the New World. The skull of this Inca man was perforated by a Spanish musket ball in 1536 during an insurrection against the conquistadors in Peru. The skeleton pictured above is only one of 72 found at the site, all apparently killed during the uprising by conquistador Francisco Pizarro and his soldiers. It is important to remember the brutal uses of these musket balls.
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Update -- 04/25/08: I recently received comments and criticisms from a reader about this page, and I've posted my responses in a new post titled Example: EMPA and Coronado's Musket Balls - Part 2 -- please that post too.
12/6/07
Electron Microprobe Analysis in Archaeology
Electron microprobe analysis (EMPA), also known as electron probe microanalysis (EPMA), is an analytical technique that combines scanning electron microscopy (SEM) and compositional analysis using x-ray spectrometry. The ability to determine structure and chemistry of samples makes EMPA very versatile. This is a dominant analytical technique in geology, but it is not as commonly used in archaeology despite similar materials in studied both fields. Here I will post about topics in EMPA, artifacts I have analyzed, archaeological studies that use EMPA, etc. If there is a topic you'd like to see posted here, please let me know.
Ellery Frahm
Doctoral Candidate, Archaeology
Research Fellow, Geology & Geophysics
University of Minnesota - Twin Cities
