Obsidian is found in the western United States, Alaska, Central America, and elsewhere.When an archeologist has identified the source of the obsidian from which an artifact is made, he or she may be able to date the artifact using the obsidian hydration technique.
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. In Chronometric Dating in Archaeology, edited by R.
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.
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).
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. 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. This technique of dating obsidian artifacts measures the microscopic amount of water absorbed on freshly broken surfaces.The principle behind obsidian hydration dating is simple—the longer the artifact surface has been exposed, the thicker the hydration band will be.At Hopewell Culture National Historical Park archeologists collected ceramic samples for TL (thermoluminescence) dating.The abundance of decorated ceramics from the park will be helpful in fixing the time of occupation.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. 1999, Ridings 1996; see Skinner and Tremaine 1993 for additional references).