Geologists and archaeologists originated a technique to determine directly the date of manufacture of some stone artifacts, enabling them better to detemine the age of the cultures that produced those artifacts.
From the beginnings of archaeology, archaeologists have struggled to date the objects that they unearth. This has not been easy, since some artifacts—particularly stone tools—have been made for hundreds of thousands or even millions of years and can bear considerable similarity to one another. Early archaeologists thought they had solved the problem when they reasoned that earlier tools would be simpler than later tools; to their minds, sophistication was a simple index of recency. Unfortunately, this notion was wrong. In North America, for example, one of the earliest stone spear point types also is one of the most technologically sophisticated and most aesthetically pleasing.
For decades, archaeologists were left with little choice but to date their sites by one of two methods: stratigraphy (based on the layering of earlier levels below later ones) and seriation (the placing of artifact forms in sequences where each form is only slightly different from its predecessor). Each of these methods, however, had significant drawbacks, and the dating of sites and artifacts remained contentious.
The development of radiocarbon dating at the midpoint of the twentieth century solved many archaeologists’ problems and allowed them to begin turning more of their attention to some of the vexing issues of human behavior in the ancient past. There remained one difficulty, however. Radiocarbon dating is an isotopic method that relies on material that once was alive; it calculates how long since the death of the animal or plant from which the material came. Many archaeological sites contain only artifacts that cannot be dated by this technique.
Even in the cases where charcoal or other radiocarbon-datable materials are present, it is not always clear that the charcoal is from the same date as the tools. Ground movement with frost, burrowing by rodents or even insects, and a host of other mechanisms can place a chunk of organic material next to an archaeological tool, though the two may come from eras millennia apart. In the late 1950’s, Irving Friedman and Robert L. Smith, geologists working with the U.S. Geological Survey,
Obsidian is a naturally occurring volcanic glass, usually black, but sometimes gray or even greenish. Obsidian occurs around the world, and wherever it has been available, people have used it to make flaked stone tools. Its earliest use probably was about one million years ago, since it has been found in Acheulean hand axes from the Kariandusi site in Kenya, where it dates to 900,000
Friedman and Smith based their technique on the recognition that the surfaces of obsidian were chemically distinct from the interior portions of a piece. Specifically, the outer surfaces were “hydrated”—they had molecules of water between the silica molecules that composed the obsidian. The hydrated rind, only a few microns thick and invisible to the naked eye, was denser than the interior surfaces and was visible under magnification by virtue of its higher refractive index and tiny cracks that developed at the line of contact between the hydrated and unhydrated portions. This was known from previous studies, but their prior work showed that the changes in the outer rind took place after its volcanic formation.
Friedman and Smith also demonstrated that the changes were the result of a more or less straightforward diffusion process, where the water molecules mechanically seeped into the obsidian. Since diffusion is a well-known process that follows predictable and quantifiable rules, it seemed likely that the hydration process would take place at a more or less consistent rate, making obsidian hydration a good basis for a dating technique.
Primarily through the assistance of Betty J. Meggers and Clifford Evans, archaeologists at the Smithsonian Institution, Friedman and Smith obtained hundreds of archaeological specimens of obsidian that came from sites and levels that had already been dated by the radiocarbon dating technique. They reasoned that the hydration rind began forming the moment a tool was made by the removal of flakes. Assuming the radiocarbon dates to be accurate (as most were), they reasoned that the thickness of this rind would be greater for the older tools. Further, they reasoned that they could derive an equation to calculate the age of a specimen on the basis of the thickness of its hydration layer.
To test this hypothesis, Friedman and Smith cut thin sections of obsidian from each of the hundreds of tools provided by the archaeologists. Each of these was examined under a microscope, where the thickness of the rind was measured. On the basis of their results, they decided that their hypothesis was fundamentally correct: Obsidian hydration layers become thicker with time at a regular rate and can be used for dating.
A problem, however, became evident. Since diffusion processes proceed faster at higher temperatures, a tool from the tropics would appear falsely older than a tool of the same age from the Arctic. Friedman and Smith established several hydration rates to use in the calculation of dates on specimens from regions of different climates. Subsequent researchers have found it necessary to calculate local hydration rates for relatively small areas. Friedman and Smith published their findings in the spring of 1960.
A further complication was noted by Friedman and Smith, though it was left for others to recognize its full significance: Obsidians of different chemical compositions have somewhat different hydration rates, even if held under identical temperatures. In 1961, Donovan L. Clark, then a graduate student working on his doctoral dissertation, recognized that relatively subtle chemical differences in obsidian could result in significantly different hydration rates. This meant that a few hydration rates for different temperatures no longer would suffice; each locality of obsidian potentially has a different composition from all others, so each type of obsidian needs its own family of hydration rates, one for each temperature. This made the situation much more complicated; therefore, for the next three decades, archaeologists calculated hydration rates for scores of different obsidian sources. Friedman advocated developing a general equation that would take composition of the obsidian into account in the calculation of a date, but no one has yet figured out how to do so.
A final difficulty with obsidian hydration is a mathematical one. The calculation of a hydration rate is a statistical process in which one attempts to summarize a series of findings with a single equation or curve. Unfortunately, the real world is an untidy and unpredictable place, and a series of tiny errors and chance variations make the data from which the curve or equation is constructed considerably less regular than one might wish. As a result, more than one equation can summarize the data; and, while most versions of the equation will give similar results, the results can be quite different for specific cases. Various researchers champion different forms of the equation.
Obsidian hydration dating has failed to have the major impact on a variety of fields that radiocarbon dating has produced. The lingering technical problems noted make the application of obsidian hydration dating to a new area and new obsidian types a time-consuming task, since a considerable investment in time must be made to calculate the applicable hydration rates. Consequently, the use of obsidian hydration dating tends to be spotty, with intensive use in such areas as California, Mexico, and Japan.
Where it has been used, however, it has presented archaeologists with several important benefits. First, it is the only major dating technique available widely to archaeologists that permits the dating of the tool itself. Dates obtained by other methods always are subject to the criticism that the material that actually is dated may not truly be contemporary with the artifacts or other materials for which the date is desired. Second, the technique is inexpensive. Archaeological excavations operate under the same fiscal restraints as almost all other scientific research, and a technique that allows the researcher to stretch his or her budget a bit further is very welcome.
For the cost of one radiocarbon date, an archaeologist can have about fifteen obsidian hydration dates. Since any single date (obtained by any technique) can be in error, a sizable array of dates is desirable, and this technique can provide them affordably. Further, the equipment necessary for the procedure is moderately simple, and the archaeologists often are able to save money by doing the obsidian hydration dating themselves, a luxury not afforded by radiocarbon dating and its complex apparatus.
While the impact of obsidian hydration has been strongest in archaeology, it has seen applications elsewhere. Geologists have used it to date obsidian flows and domes, which in turn have been used to date other geological events. There are limits, however, to the use of obsidian hydration dating in geology. Hydration rinds that exceed a certain thickness tend to chip off, making a specimen appear younger than it really is. The practical limit of the technique appears to be around one million years, a limit that encompasses nearly all of the archaeological record but only the most recent portion of the geological record.
Finally, art historians have used the technique to test the age of disputed artworks of obsidian, particularly prehistoric Mexican and Central American sculptures. Usually a thin wedge can be cut from the base of a piece without disfiguring it.
When Friedman and Smith introduced obsidian hydration dating, their article was paired with an archaeological assessment of the technique by Meggers and Evans. The archaeologists concluded that the technique had considerable promise if the problems surrounding the calculation of the hydration rate (or rates) could be solved. After years of attempts, the problems foreseen by Meggers and Evans have been only partly solved, and new ones unanticipated by them have also developed. Still, obsidian hydration dating has yielded thousands of accurate, low-cost dates to archaeology.
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citation-type="booksimple" xlink:type="simple">Ericson, Jonathan E. “Egalitarian Exchange Systems in California.” In Exchange Systems in Prehistory, edited by Timothy K. Earle and Jonathan E. Ericson. New York: Academic Press, 1977. A good example of how obsidian hydration can be used in concert with other archaeological techniques to help reconstruct past behavior from archaeological remains. The author is a major proponent of obsidian hydration dating in archaeology. -
citation-type="booksimple" xlink:type="simple">Evans, Clifford, and Betty Meggers. “A New Dating Method Using Obsidian, Part 2: An Archaeological Evaluation of the Method.” American Antiquity 25 (April, 1960): 523-537. The companion article to the original treatment by Friedman and Smith, this article presents the first archaeological application and assesses the value of the technique. Many of the difficulties noted subsequently have been overcome. Examples used are from Latin America. -
citation-type="booksimple" xlink:type="simple">Friedman, Irving, and Robert L. Smith. “A New Dating Method Using Obsidian, Part 1: The Development of the Technique.” American Antiquity 25 (April, 1960): 476-522. This is the original article in which the technique of obsidian hydration dating was presented. While subsequent research has pointed out some areas of complication, the article remains substantially current. -
citation-type="booksimple" xlink:type="simple">Friedman, Irving, and Fred W. Tremblour. “Obsidian: The Dating Stone.” American Scientist 66 (January/February, 1978): 44-51. While there are no summaries of obsidian hydration dating written for the layperson, this is the closest approximation. It is a highly readable, brief, nonjargonistic treatment that summarizes the technique, its assumptions, and some of its successes. Photographs illustrate obsidian artifacts and their obsidian hydration rinds, and charts graphically depict the hydration rates and results of analysis. -
citation-type="booksimple" xlink:type="simple">Michels, Joseph W., and Ignatius S. T. Tsong. “Obsidian Hydration Dating: A Coming of Age.” In Advances in Archaeological Methods and Theory. Vol. 3, edited by Michael B. Schiffer. New York: Academic Press, 1980. An excellent summary of obsidian hydration dating, its history, its problems, and its future. The author presents a balanced discussion of relevant controversies and differing viewpoints. Technical. -
citation-type="booksimple" xlink:type="simple">Nash, Stephen E., ed. It’s About Time: A History of Archaeological Dating in North America. Salt Lake City: University of Utah Press, 2000. Compilation of essays detailing various developments in archaeological dating technologies. Includes “Obsidian Hydration Dating, Past and Present,” by Charlotte Beck and George T. Jones. Bibliographic references and index. -
citation-type="booksimple" xlink:type="simple">Romer, John. Great Excavations: John Romer’s History of Archaeology. London: Cassell, 2000. Popular history of archaeological methodologies and developments, produced as a tie-in with a television series. Bibliographic references and index. -
citation-type="booksimple" xlink:type="simple">Tite, M. S. Methods of Physical Examination in Archaeology. New York: Seminar Press, 1972. A basic handbook of scientific analytic methods in archaeology. The book is densely written and difficult, but it is a classic in its field. One portion of a chapter is devoted to obsidian hydration dating, but other dating techniques also are discussed.
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