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. An appropriate section of each artifact is selected for hydration slide preparation.
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. Formation of the hydration rim is affected not only by time but also by several other variables. 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.
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. 1999, Ridings 1996; see Skinner and Tremaine 1993 for additional references).