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46 Abstract Hydration of obsidian is a diffusion-reaction process , the rate of which is dependent on temperature. Chronological analyses based on obsidian hydration must therefore account for the effects of the temperature history to which the artifact has been subjected. For chronological analyses, the temperature history is summarized by the concept of effective hydration temperature (EHT). Prior studies, cited herein, have developed a mathematical model of the hydration process that relates EHT to obsidian characteristics and temperature parameters at an archaeological site, and the model has been validated with measured temperature data. Drawing on these results, this chapter summarizes a modelbased method for calculating EHT of an artifact and for applying a correction to the measured hydration rim value prior to computing age. The correction process accounts for the effects of burial depth, local temperature conditions, and paleoclimatic change. 1. Introduction Computing the age of an obsidian artifact based on measurement of hydration rim thickness requires correcting for its temperature history. This chapter describes an analytical technique for determining the temperature correction factor, based on recent advances in the theory of hydration (Anovitz et al. 1999; Doremus 2002; Rogers 2007a, 2008a). The method was developed for use with hydration rims measured by optical microscopy, and for rate equations computed from obsidian-radiocarbon associations, but could be employed with other measurement techniques. Temperature at an archaeological site varies both diurnally and annually, which causes corresponding variations in hydration rate. The effect of this variability is summarized by the effective hydration temperature , or EHT: a constant temperature yielding the same hydration outcome as the actual time-varying temperature over the same period of time. EHT is always higher than the mean temperature; further, a higher EHT corresponds to a greater hydration rate and thus a larger rim thickness for a given age. Historically, the primary method used to estimate EHT has been the Lee equation (Lee 1969:430, eq. 12). It was originally developed for estimating mean temperature for forestry monitoring and uses data from a Pallmann cell, a device that measures an effective reaction temperature based on a sucrose chemical reaction. Lee developed his equation to convert the reaction temperature of the Pallmann cell to arithmetic mean temperature, the reverse of the way it is typically applied in archaeology. The chief limitation of Lee’s equation, fully discussed by Lee, is that it is based on a single period of temperature variation, while there are actually two, annual and diurnal; Lee attempted to account for annual variation by summing the two variations, but admitted it was not really successful, and suggested limiting the use of the cells to a single season. For obsidian analysis it has a second limitation, in that the activation energy of the sucrose reaction is not the same as the activation energy for obsidian hydration. Chapter 4 Temperature Correction for Obsidian Hydration Dating Alexander K. Rogers Temperature Correction for Obsidian Hydration Dating | 47 The method described here avoids both these problems. The discussion below presents a summary of hydration theory and a method of computing EHT derived from a consideration of time-dependent hydration rates. A method of computing temperature parameters at a site based on regional temperature scaling is described, with an example. Correction factors for rock shelters and caves are suggested, as are a correction for paleoclimatic change and a systematic process for chronological analysis. 2. Hydration theory Whenafreshlyexposedsurfaceofobsidianisexposed to water, either liquid or vapor, water molecules diffuse into the glass at a predictable rate (Doremus 2002; Stevenson et al. 1998). As the water diffuses into the glass it causes a change in refractive index in the hydrated layer; if a small cross-sectional sample is cut from the obsidian, mounted on a microscope slide, and polished to transparency, the interface between hydrated and unhydrated volumes (the “hydration front”) can be observed under a polarizing microscope (Anovitz et al. 1999; Scheetz and Stevenson 1988). The hydrated volume is referred to as the hydration rim, and its thickness in most archaeological cases is of the order of microns. If the rate of hydration is known or can be inferred, the time since the surface was exposed can be estimated. The rate of hydration is dependent on the obsidian chemistry (Friedman and Smith 1960; Friedman and Trembour 1983), the intrinsic water content of the glass (Mazer et al. 1992; Stevenson et al. 1993; Stevenson et al. 2000), and temperature and relative humidity (Ebert et al. 1991; Friedman and Long 1976; Friedman et al. 1994; Hull 2001...

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