Glass Hydration Essay

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The explicit presentation of the mathematics involved in the Arrhenius equation (Michels, Tsong, and Smith 1983) rendered the parameters involved in glass hydration understandable. However, the actual implementation of the physical processes described in the mathematical equations into a model replicating the natural environment is a complicated problem hard to solve (Stevenson 1998). THE DEVELOPMENT OF MODELS FOR THE UNDERSTANDING OF OBSIDIAN HYDRATION RATES In order to understand the obsidian hydration dating method, a model that can demonstrate the process of glass hydration under a certain range of environmental conditions is needed. To implement this, the nature of the concerned materials i.e. glass and water should be adequately known so that the interaction between them can be predicted on the basis of the properties of each substance (Stevenson et al. 1998). There is no consensus among archaeologists and geochemists on how best to model hydration rates. They have long debated on how hydration-rind thickness could tell us about time. The confusion seems to be based on two points. First, after hydration begins, water is no longer reacting with the pure glass. Rather it reacts with an already hydrated layer. The classic diffusion model breaks down at this point. Second, the different variables that determine the hydration rate are mutually interacting. Several researchers have observed a shift from classic diffusion between hydration-rind thickness and time to a linear relation, based on changes in pH. In turn, pH, in a closed system, is influenced by how long the hydration has occurred (Tremaine 1989). As alkali ions leach into solution and are replaced by hydrogen or hydronium ions, the pH of the solution will increase (Bra... ... middle of paper ... ...dity variation, largely because RH is not a variable expressed in the Arrhenius equation. It is likely that the Copan researchers, like the experimental parameters of Michels and colleagues (1983) might consider the RH of tropical soils to be always 100 percent. For contexts deeper than 50 cm, this may be a reasonable assumption. However, it should be borne in mind that the majority of the variability in hydration rates occurs between RH values of 90 and 100 percent, and a 1 percent change in RH translates to approximately a 3 percent change in hydration rate (Tremaine 1989). So if blades from Copan were recovered from contexts with 90 percent RH, their OHD dates would be in error by approximately 30 percent, excluding all other sources of error. Braswell (1992) was hence right to have questioned the validity of the hydration dates obtained from the Copan region.

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