One of the most common artifacts at many archaeological sites across the world are tools, ornaments and weapons made from a volcanic glass called obsidian. This versatile material was highly prized by ancient and prehistoric peoples due to its fracturing properties, its lusterous appearance and its sharp cutting edge. When properly fractured, raw obsidian blocks can be "flaked" into a variety of tool forms. An additional property of obsidian that makes it valuable to archaeologists is the fact that the fresh surfaces of a newly flaked obsidian tool interact with water in the atmosphere and ground, producing a "hydration rim", the thickness of which is dependent on time. In 1960, a USGS geologist, Irving Friedman, developed a method of dating obsidian artifacts by measuring the width of the thin hydrated rims (around .002 to .01 mm thick) with an optical microscope. Dr. Friedman named his technique "obsidian hydration dating" or OHD. Unfortunately, despite the promise of the method to produce inexpensive and accurate dates, it has failed far more often than it has succeeded.
In 1998, NSF awarded a grant to a team of researchers at the University of Tennessee and Oak Ridge National Laboratory who had done a preliminary reassessment of OHD using secondary ion mass spectrometry (SIMS) to precisely measure the concentration of hydrogen in the hydrated rim as a function of depth from the surface. This technique provides not only precise measurement of the width of the interaction rim, but data on the glass-water chemistry as well. During the three years of NSF support, the team carried out further research on both obsidian that was experimentally hydrated under controlled laboratory conditions and independently dated archaeological specimens (mainly from Mesoamerica). As part of the laboratory work, the team developed a new type of reaction vessel system for performing in-laboratory hydration of fresh samples, and is currently under patent review.
Thus far, the team's has made a number of crucial observations and discoveries. First, they demonstrated that optical measurement is inherently imprecise and cannot be used for accurate dating. Second, they showed that the rate equations used in traditional obsidian dating are incorrect, as the rate of hydration is not constant with time, but rather varies with age. Third, they noted that the glass is absorbing only hydrogen and not the whole water molecule as previously thought, indicating that the mechanics of the hydrogenation process are far more complex than previously assumed, requiring significant re-evaluation of glass-water interaction theory. Fourth, the team was able to demonstrate that using the shape and depth of the hydrogen concentration profile, a series of artifacts from the Chalco, Mexico site could be precisely and accurately dated. However, this method requires at least one independently dated artifact from any particular site for calibration. Fifth, SIMS analysis suggests that the hydrogen isotope ratios of the rim may preserve information on the average temperature of hydration, which is a crucial factor controlling hydration rates. If confirmed, this would provide a means of constraining hydration temperature, and would allow obsidian dating to be a completely independent method. Finally, the results of this study suggest that the long-term durability of nuclear waste glass forms should be re-evaluated, as previous breakdown models may be based on flawed assumptions.
The research team has been awarded an additional three years support to continue this exciting and important research. Objectives of the project for the next three years include: 1) to understand the interaction chemistry and structure of the hydrogenation zone; 2) refine the hydration model to make it a fully independent and robust dating method; 3) determine if temperature data can be extracted from obsidian artifacts; and 4) test the method at various archaeological sites. To accomplish these goals, the team will expand their research to Old World obsidian (Melian obsidian from Greece) and have already established ties with a team of Greek archaeologists (University of the Aegean) to obtain the necessary samples. This makes this project truly international in scope benefiting not only U.S. scientists in multiple disciplines (e.g. archaeology, geology, chemistry, physics, materials science), but scientists throughout the world.
