Obsidian Hydration Studies

Obsidian hydration rim (between arrows)

Introduction to Obsidian Hydration Studies

INTRODUCTION | PREPARATION GUIDELINES | PREPARATION METHODS | REFERENCES

INTRODUCTION

The obsidian hydration dating method was introduced to the archaeological community in 1960 by Irving Friedman and Robert Smith of the U. S. Geological Survey (Friedman and Smith 1960). The potential of the method in archaeological chronologic studies was quickly recognized and research concerning the effect of different variables on the rate of hydration has continued to the present day by Friedman and others.

When a new surface of obsidian is exposed to the atmosphere, such as during the manufacture of glass tools, water begins to slowly diffuse from the surface into the interior of the specimen. When this hydrated layer or rind reaches a thickness of about 0.5 microns, it becomes recognizable as a birefringent rim when observed as a thin section under a microscope. Hydration rims formed on artifacts can vary in width from less than one micron for items from the early historic period to nearly 30 microns for early sites in Africa (Michels et al. 1983a; Origer 1989).

Formation of the hydration rim is affected not only by time but also by several other variables. The most important of these are chemical composition and temperature, although water vapor pressure and soil alkalinity may also play a role in some contexts. The effects of these variables have often been summarized and will not be discussed further here (Michels and Tsong 1980; Friedman and Obradovich 1981; Freter 1993; Hull 2001; Stevenson et al., 1993, 1998, 2000; Friedman et al. 1994, 1997; Morganstein et al. 1999, Ridings 1996; see Skinner and Tremaine 1993 for additional references).

Once a hydration layer has been measured, it can be used to determine the relative ages of items or, in some circumstances, can be converted into an estimated absolute age. In order to transform the hydration rim value to a calendar age, the rate of the diffusion of water into the glass must be determined or estimated. The hydration rate is typically established empirically through the calibration of measured samples recovered in association with materials whose cultural age is known or whose age can be radiometrically determined, usually through radiocarbon dating methods (Meighan 1976). The hydration rate can also be determined experimentally, an approach that has shown increasing promise in recent years (Friedman and Trembour 1983; Michels et al. 1983a, 1983b; Tremaine 1988, 1993).

PREPARATION GUIDELINES

Normally, a single small hydration cut is made for each sample at a location selected by our hydration analyst. If you have any special requests regarding the location of measurements on any artifact or would like measurements made at more than one location on the artifact, please attach a separate sheet with a photocopy or tracing of the sample(s) and well-marked hydration measurement locations. It is best not to mark the artifact directly with any type of paint (such as correction fluid) since this may make the hydration rim more difficult to measure (and will also interfere with any accompanying trace element studies). There are no size restrictions for samples submitted only for obsidian hydration analysis, although very small samples may be completely destroyed during slide preparation.

PREPARATION METHODS

An appropriate section of each artifact is selected for hydration slide preparation. The location of the section is determined by the morphology and the perceived potential of the location to yield information on the manufacture, use, and discard of the artifact. Two parallel cuts are made into the edge of the artifact using a lapidary saw equipped with 4-inch diameter diamond-impregnated .004" thick blades. These cuts produce a cross-section of the artifact approximately one millimeter thick which is removed from the artifact and mounted on a petrographic microscope slide with Lakeside thermoplastic cement. The mounted specimen slide is ground in a slurry of 600 grade optical-quality corundum abrasive on a plate glass lap. This initial grinding of the specimen reduces its thickness by approximately one half and removes any nicks from the edge of the specimen produced during cutting. The specimen is then inverted and ground to a final thickness of 30-50 microns, removing nicks from the other side of the specimen. The result is a thin cross-section of the surfaces of the artifact.

The prepared slide is measured using an Olympus BHT petrographic microscope fitted with a filar screw micrometer eyepiece. A Panasonic color CCTV camera is mounted on the filar eyepiece and the image is directed to a Panasonic color video monitor. The filar eyepiece is used to measure the thickness of the hydration band projected onto the high resolution monitor. The specimen is first scanned under crossed polarizers and a first-order red (gypsum) plate in order to identify the birefringent hydration layer; the hydration layer is scanned under a magnification of 500X, 750X, or 1250X. The magnification is generally selected with regard to the opacity of the obsidian and the width of the hydration layer. When a clearly defined hydration layer is identified, the section is centered in the field of view to minimize parallax effects. Four rim measurements are typically recorded for each artifact or examined surface. Narrow rinds (under approximately two microns) are usually examined under a higher magnification. Hydration rinds smaller than one micron often cannot be resolved by optical microscopy.

Hydration thicknesses are reported to the nearest 0.1 micron and represent the mean value for all readings. Standard deviation values for each measured surface indicate the variability for hydration thickness measurements recorded for each specimen. It is important to note that these values reflect only the reading uncertainty of the rim values and do not take into account the resolution limitations of the microscope or other sources of uncertainty that enter into the formation of hydration rims (Meighan 1981, 1983; Skinner 1995:5.13-5.19; Anovitz et al. 1999). Any attempts to convert rind measurements to absolute dates should be approached with great care and considerable skepticism, particularly when rates are borrowed from existing literature sources. When considered through long periods, the variables affecting the development of hydration rims are complex, and there is no assurance that artifacts recovered from similar provenances or locales have shared thermal and cultural histories.

REFERENCES

Anovitz, Lawrence M., J. Michael Elam, Lee R. Riciputi, and David R. Cole. 1999. The Failure of Obsidian Hydration Dating: Sources, Implications, and New Directions. Journal of Archaeological Science 26:735-752.

Freter, AnnCorinne. 1993. Obsidian-Hydration Dating: Its Past, Present, and Future Application in Mesoamerica. Ancient Mesoamerica 4(2):285-303.

Friedman, Irving and John Obradovich. 1981. Obsidian Hydration Dating of Volcanic Events. Quaternary Research 16:37-47.

Friedman, Irving and Robert L. Smith. 1960. A New Dating Method Using Obsidian: Part I, The Development of the Method. American Antiquity 25:476-522.

Friedman, Irving and F. W. Trembour. 1983. Obsidian Hydration Dating Update. American Antiquity 48(3):544-547.

Friedman, Irving, F. W. Trembour, and Richard E. Hughes. 1997. Obsidian Hydration Dating. In Chronometric Dating in Archaeology, edited by R. E. Taylor and M. J. Aitken, pp. 297-321. Plenum Press, New York, New York.

Friedman, Irving, Fred W. Trembour, Franklin L. Smith, and George I. Smith. 1994. Is Obsidian Hydration Dating Affected by Relative Humidity? Quaternary Research 41(2):185-190.

Hull, Kathleen L. 2001. Reasserting the Utility of Obsidian Hydration Dating: A Temperature-Dependent Empirical Approach to Practical Temporal Resolution with Archaeological Obsidians. Journal of Archaeological Science 28:1025-1048.

Meighan, Clement W. 1976. Empirical Determination of Obsidian Hydration Rates from Archaeological Evidence. In Advances in Obsidian Glass Studies, edited by R. E. Taylor, pp. 106-119. Noyes Press, Park Ridge, New Jersey.

Meighan, Clement W. 1981. Progress and Prospects in Obsidian Hydration Dating. In Obsidian Dates III, edited by Clement W. Meighan and Glenn S. Russell, pp. 1-9. University of California Institute of Archaeology Monograph No. 6, Los Angeles, California.

Meighan, Clement W. 1983. Obsidian Dating in California. American Antiquity 48:600-609.

Michels, Joseph W. and Ignatius S. T. Tsong. 1980. Obsidian Hydration Dating: A Coming of Age. In Advances in Archaeological Method and Theory, Volume 3, edited by M. B. Schiffer, pp. 405-444. Academic Press, New York, New York.

Michels, Joseph W., Ignatius S. T. Tsong, and Charles M. Nelson. 1983a. Obsidian Dating and East African Archeology. Science 219:361-366.

Michels, Joseph W., Ignatius S. T. Tsong, and G. A. Smith. 1983b. Experimentally Derived Hydration Rates in Obsidian Dating. Archaeometry 25:107-117.

Morganstein, Maury E., Carolyn L. Wicket, and Aaron Barkatt. 1999. Considerations of Hydration-rind Dating of Glass Artifacts: Alteration Morphologies and Experimental Evidence of Hydrogeochemical Soil-zone Pore Water Control. Journal of Archaeological Science 26:1193-1210.

Origer, Thomas M. 1989. Hydration Analysis of Obsidian Flakes Produced by Ishi During the Historic Period. In Current Directions in California Obsidian Studies, edited by Richard E. Hughes, pp. 69-77. Contributions of the University of California Archaeological Research Facility No. 48, University of California, Berkeley, California.

Ridings, Rosanna. 1996. Where In the World Does Obsidian Hydration Dating Work? American Antiquity 61:136-148.

Skinner, Craig E. 1995b. Obsidian Hydration Studies. In Archaeological Investigations, PGT-PG&E Pipeline Expansion Project, Idaho, Washington, Oregon, and California, Volume V: Technical Studies, by Robert U. Bryson, Craig E. Skinner, and Richard M. Pettigrew, pp. 5.1 5.51. Report prepared for Pacific Gas Transmission Company, Portland, Oregon, by INFOTEC Research, Inc., Fresno, California, and Far Western Anthropological Research Group, Davis, California.

Skinner, Craig E. and Kimberly J. Tremaine. 1993. Obsidian: An Interdisciplinary Bibliography. International Association for Obsidian Studies Occasional Paper No. 1, San Jose, California.

Stevenson, Christopher M., Mike Gottesman, and Michael Macko. 2000. Redefining the Working Assumptions of Obsidian Hydration Dating. Journal of California and Great Basin Anthropology 22:223-236.

Stevenson, Christopher M., Elizabeth Knaus, James J. Mazer, and John K. Bates. 1993. Homogeneity of Water Content in Obsidian from the Coso Volcanic Field: Implications for Obsidian Hydration Dating. Geoarchaeology 8:371-384.

Stevenson, Christopher M., James J. Mazer, and Barry E. Scheetz. 1998. Laboratory Obsidian Hydration Rates. In Archaeological Obsidian Studies: Method and Theory, edited by M. Steven Shackley, pp. 181-204. Advances in Archaeological and Museum Science Series. Plenum Publishing Co., New York, New York.

Tremaine, Kimberly J. 1989. Obsidian as a Time Keeper: An Investigation in Absolute and Relative Dating. Master's Thesis, Sonoma State University, Rohnert Park, California.

Tremaine, Kimberly J. 1993. Temporal Ordering of Artifact Obsidians: Relative Dating Enhanced Through the Use of Accelerated Hydration Experiments, in There Grows a Green Tree, edited by Greg White, Pat Mikkelsen, William R. Hildebrandt, and Mark E. Basgall, pp. 265-275. University of California Center for Archaeological Research at Davis Publication No. 11, Davis, California.

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Last Updated: 06/06/2006
Northwest Research Obsidian Studies Laboratory