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).
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 video micrometer unit and a digital imaging video camera. 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.
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