By Jason H. Shapiro
If I could save time in a bottleJim Croce
The first thing I’d like to do
Is to save every day
‘Til eternity passes away
Just to spend them with you.
The plasma laboratory at the Center for New Mexico Archaeology in Santa Fe is filled with the background hum of vacuum pumps. The gentle noise comes from an island of equipment: stainless steel piping and valves, glass chambers, wires and electrodes, gas cylinders, a radio frequency tuner, and some metal boxes with gauges, knobs, and switches. If you are there at the right time, the glass chambers glow with a faint white or lavender light, and cups of liquid nitrogen boil over in frosty cascades. There is something innovative unfolding in that laboratory, and Marvin Rowe is making it happen.
Archaeologists generally are not in the business of physics and chemistry; they are more interested in uncovering patterns of past human behavior. Nevertheless, the field of archaeology has come to rely upon some extraordinarily sophisticated and complex laboratory science, including a new and developing scientific procedure that enables archaeologists to fulfill their core mission: to discover, understand, and present the story of us.
If the public takes any interest in archaeology, it is usually in a romanticized, media-inspired picture of sunburned field workers toiling away with trowels and screens trying to uncover “treasures.” Fieldwork is indispensable to archaeology, but more and more our knowledge about the past is uncovered in laboratories such as the one Rowe, a research associate with the Museum of New Mexico, created and operates.
Rowe may not use a trowel, but his carefully choreographed ritual of opening and closing valves, monitoring readouts, and resetting timers is part of an evolving dating technique. It may provide answers to the kinds of nettlesome questions about which archaeologists heretofore have only offered informed speculation, such as, “When was this example of rock art created?” or “Can we accurately date a unique and fragile object such as the Shroud of Turin without the destructive sampling that even sophisticated modern dating analysis requires?”
Despite the intimidating moniker—low energy plasma radiocarbon sampling—this new procedure is, at its heart, a variation of the radiocarbon dating that has been the mainstay of archaeology for decades. It has the potential to be an indispensable tool for archaeologists looking to explain the past.
Dating and Archaeology
The essential business of archaeology is interpreting patterns of earlier human behavior. Our ability to understand that behavior depends upon our capacity to accurately place it in the context of time. When something occurred is as critical as what, where, and why it occurred. Archaeologists separate the past into chronological sequences called periods, phases, or components based upon the evidence of human activity at the time. Those sequences are used to conceptually organize past behavior in ways that bring to light what people were doing and how these behaviors changed over time. In order to decode and make sense of the sequences, archaeologists must uncover time.
There are many ways to mark the passage of time, from the tiny fractions of seconds measured by physicists to the billions of years measured by geologists and astronomers. In the broadest sense, archaeologists are concerned with cultural history that occurred yesterday to some point between two and three million years ago (when stone tools, the oldest forms of preserved human cultural behavior, were first produced). Essentially, they uncover all available information in order to answer the question, “How old is it?”
Historically, archaeologists have approached this question using two broad categories of techniques: relative and absolute dating. Relative dating involves ordering objects or events from earlier to later without a precise idea as to the amount of time that passed between those objects or events. Relative age is determined primarily through the concept of superposition: the idea that the archaeological record is comprised of layers or strata, with older layers located underneath newer layers. In its simplest terms, relative dating is based upon the sequential layering of cultural deposits.
The problem with relative dating is that it merely provides a progression from earliest to latest, without being able to specifically tie the sequences to a definite chronological date; but it was really all that archaeologists had until 1949. That’s when everything changed.
The second category of dating techniques is called absolute or chronometric dating. All of the various techniques within this category are based upon the observation of continuously occurring natural events that change at constant measurable rates. These techniques provide a specific date; they allow objects to be dated to thousands, tens of thousands, or even millions of years.
Radiocarbon dating (C-14) is probably the most widespread and well known of the absolute techniques used in archaeology. In 1949, the chemist William Libby came up with the process as he was studying cosmic radiation and the manner in which radioactive carbon atoms are produced and decay. Libby understood that all living things incorporate fresh carbon atoms into their tissues until death, whereupon the process stops. The older, atomically unstable isotope C-14 in the tissues degrades into the more stable C-12. The proportion of C-14 relative to C-12 changes at a constant measurable rate over time. The measurable rate for C-14 is described as its atomic half-life of 5,730 years, give or take forty years.
Archaeologists are often dealing with extremely small amounts of material, particularly with older sites—since, for example, after 23,000 years, less than 7% of the original C-14 in any sample is still present. C-14 dating is generally applied to objects between 100 and 40,000 years old, although various enhancement techniques have pushed the application to samples up to 80,000 years old.
One of the problems associated with traditional C-14 dating is that it is destructive. In order to date an object, a small portion must be removed and burned to release CO2. The CO2 is converted to graphite (carbon) that allows one to measure the C-14 to C-12 ratio, and determine the last point in time when new C-14 molecules were incorporated into the living tissue that became the artifact, i.e., when the organism died. If all that one has is a tiny sample or a single, unique artifact, standard radiocarbon dating may not always be feasible.
Low Energy Plasma Radiocarbon Sampling
In the late 1980s, Marvin Rowe was a professor of chemistry at Texas A&M University when he was asked by an archaeologist about the possibility of dating rock art pictographs. Pictographs are notoriously difficult to date; while styles and pigment sources changed over time, establishing a precise date for any particular example had always been more of an exercise in “informed guesstimation” than hard science. That question got Rowe thinking about how one might extract dateable carbon samples from rock art pigments that don’t contain very much organic carbon.
Shortly thereafter, in an example of pure serendipity, Rowe read an article on an airplane that described how a Swiss scientist was using hydrogen plasmas in the controlled restoration of heavily oxidized metal objects. No one had thought to use the plasma-based process for the direct dating of artifacts, but the Swiss study piqued Rowe’s interest. Plasma is often described as an electrically charged gas, but it is really a fourth state of matter (the other three being liquids, solids, and gases).
Plasma is found in fluorescent lighting, your flat-screen plasma TV, and lightning strikes, so it is something that we “know about,” even if we do not think much about it. Using some donated and reconfigured equipment, Rowe created a connected system of chemical treatments, radio frequency tuners, and electronic analysis capable of dating rock art pigments. The system had some of the same complications associated with existing dating techniques such as required sample size, external contamination, and calibration, but it showed genuine promise—promise that Rowe has persistently refined and improved upon for the past thirty-plus years.
How and Why Plasma Dating Works
The most appealing quality of Rowe’s method is that it is nondestructive. This is a major consideration, as many artifacts are simply too fragile or unique to have pieces removed for standard types of radiocarbon dating. Non-destruction is the aspect about which museum curators and restoration specialists are most excited, because they want to refrain from damaging even the tiniest portions of their prized artifacts. In addition, this process can potentially be used to uncover fake artifacts whose actual age may be inconsistent with its alleged provenance.
Rowe’s methodology solves the artifact damage problem by placing the entire artifact in a sealed vacuum chamber and gradually igniting an oxygen plasma that slowly oxidizes the surface of the artifact to produce CO2 for C-14 analysis. This electrically excited plasma produces the “glow in the bottle” that is the signature of this method. In some ways this process is akin to a slow-motion, very controlled, and extremely limited burn, especially as the rate of burn can be controlled by the level at which the operator energizes the plasma.
The critical part of the process is maintaining control of the rate of oxidation across the entire surface of the artifact so that its integrity is not impacted. In addition, great care (and a variety of pretreatment chemical rinses) are used in order to ameliorate the effects of potential environmental contaminations, such as humic acids in soils.
The process is so sensitive that, in order to get an accurate date, the sample must be scrupulously cleaned of as many impurities as possible. The payoff is that the nondestructive nature of the process allows fragile textiles, artworks, wooden objects, seeds, or human and animal remains to be dated without destroying even a small portion of the artifact. At the present time the techniques are not automated but are hands-on, labor-intensive actions that rely on trained specialists to initiate and monitor the entire process.
Rowe has applied his dating process to a variety of organic materials such as rock paintings, wood, charcoal, animal skin, bone, and paint pigments on a pottery sherd. In one experiment, Rowe successfully applied the technique to a single turkey feather that was not only successfully dated, but whose overall appearance and structural integrity were not altered. In other words, Rowe demonstrated how even the most delicate objects can potentially be dated without fear for their destruction. As Jeffrey Cox, who works with Rowe, has described the nondestructive quality of plasma dating: “It’s like using a loofah scrub on your arm. The loofah scrubs off some outer cells, but the arm remains virtually unaffected and unchanged.”
Given the rarity and fragility of many artifacts being dated, that element alone makes the technique revolutionary. Indeed, the Archaeological Institute of America was sufficiently impressed that in 2010 its popular Archaeology magazine named low energy plasma radiocarbon sampling as one of its top ten discoveries.
In addition to its nondestructive aspect, low energy plasma carbon sampling can be applied to incredibly small sample sizes such as the organic paint on a single pottery sherd, or even a few linen threads from a painted canvas. One experiment involved dating several small threads on the edge of an alleged signed and dated Pablo Picasso painting. Five dates were obtained from the threads that revealed the painting was younger than the date painted on the corner, and therefore fraudulent.
Plasma extraction radiocarbon dating has been applied to samples as small as 25-30 millionths of a gram, and the Center for New Mexico Archaeology Plasma Lab typically works with samples with a maximum weight of 100 millionths of a gram of carbon. Field archaeologists have been trained to secure much larger sample sizes because they are aware that in traditional radiocarbon dating, unlike the plasma approach, much of the sample material is destroyed during standardized decontamination pretreatments that rely on a series of relatively harsh acidic and alkaline chemical washes. By eliminating the need for these destructive washes, the plasma-based technique can be applied directly to extremely small samples without the destruction that occurs during standard radiocarbon dating.
Despite its successes and potential, low energy nondestructive plasma carbon dating is still being refined—not so much in the accuracy of the dates, which has always been good, but with a greater specificity and clarity of the internal operating procedures. Whether one views it as still experimental or demonstrably effective depends somewhat on what one is trying to date.
Although the aforementioned turkey feather is considered a useful and replicable application of the process, consider the problem of trying to date a sample of powdered soot from a burned structure. This is the kind of “experimental stretching” that the archaeologists at the Office of Archaeological Studies are trying to achieve. It is not the soot per se that concerns the archaeologists; rather it is the burning event that produced the soot that is of interest, because dating that soot provides an answer to the question, “When did someone burn this log or structure?”
After that date is known, one can move on to considering who did the burning and why they did it. It is the sensitivity of the plasma dating process that enables a researcher to date that specific event.
The detailed and demanding nature of the process is underscored by the fact that Rowe’s laboratory is one of only four similar laboratories in the entire world. The complexity and demanding nature of the procedures, the need for specifically trained technicians, some resistance from within the C-14 dating establishment, and the fact that one major institution has tried and failed to make the process work with sufficient ease and effect have so far limited its widespread use.
Despite—or maybe even because of—all these complexities, Rowe and his team have made it work. Their ultimate goal of having a dependable, accurate, and nondestructive dating technique that is a useful tool for a variety of researchers with highly specialized dating challenges appears to be only a question of … time.
Jason S. Shapiro J.D., Ph.D., is a retired archaeologist living in Santa Fe. In addition to several prior contributions to El Palacio, Dr. Shapiro is the author of Before Santa Fe: The Archaeology of the City Different (Museum of New Mexico Press, 2008), the first comprehensive synthesis of the archaeology of the Santa Fe region.