Hydration of obsidian

FIGURE 88 Dating methods. Shortly after the discovery of radioactivity, at the beginning of the twentieth century, it was found that the decay of radioactive elements could be used to keep track of time. Many of the dating techniques developed since then are, therefore, based on radioactive decay phenomena, but others, such as the hydration of obsidian, amino acid racemization, and dendrochronology, are based on other physical, chemical, or biological phenomena. 

The rate of hydration of obsidian, which is diffusion limited, forms the basis for Obsidian Hydration Dating [f]. A date refers to "the time a fresh surface of obsidian was created, either naturally or by man.. ..Laboratory and field studies have confirmed that the time indicated by a hydrated layer is proportional to the thickness squared of the layer. The hydration rate is independent of the relative humidity of the environment, but the chemical composition of the obsidians affect the rate by orders of magnitude. Si02 increases the rate, whereas CaO, MgO, and H20 decrease it. A 6 - 8 °C temperature increase causes doubling of the rate." The method is quite inexpensive, and it is applicable to ages between a few hundred and several million years. 

Freshly exposed surfaces of obsidian, such as those created when obsidian breaks or is flaked, react with environmental moisture (i.e., water), and the product of the reaction forms a thin layer of water-rich obsidian on the obsidian bulk. The surface is said to become hydrated while the underlaying bulk remains unaltered, as it is affected by neither the water nor other weathering processes (see Textbox 25). Microscopic studies have shown that the thickness of the hydrated layer depends on the relative amount of the water... 

The natural composition of obsidian includes very little water, generally less than 0.1%. When new obsidian surfaces are created, either by the natural breakdown of obsidian bulks or by human activity, the exposure of a new surface to humidity in the air or to water brings about a process known as hydration the surface adsorbs (takes up) water and becomes... 

Once initiated, and provided the surface continues to be exposed to the environment, the process of hydration continues at a slow, but measurable rate. The adsorption of the water is accompanied by changes in the physical properties of the obsidian. The refractive index of the obsidian, for example, is altered as it becomes hydrated. If the obsidian was subjected to alternative wet and dry periods, successive hydrated layers are formed on the surface. The differences in refractive index between the bulk and the hydrated layer (or layers) creates an interface between the bulk and the hydrated layer, and between the layers, that stands out sharply when observing a cross-cut section of obsidian under a microscope (see Fig. 23). Thus the thickness of the hydrated layer, or layers, can be measured. 

In simple terms, the meaning of the equation is that by measuring the thickness of a hydration layer on the surface of a piece of obsidian or of an obsidian tool, it is possible to calculate when the surface was first created and became exposed to the environment (Stevenson et al. 2000 Friedman and Smith 1960). 

Ericson, J. E., J. D. McKenzie, and R. Berger (1976), Physics and chemistry of the hydration process in obsidian, in Taylor, R. E. (ed.), Advances in the Study of Obsidian Glass Studies, Noyes, Park Ridge, pp. 46-62. 

Stevenson, C. M., M. Goitesnian, and M. Macko (2000), Redefining the working assumptions of obsidian hydration dating, /. California and Great Basin Anthropology 22, 223-226. 

Erickson, J.E. 1981. Exchange and production systems in Californian prehistory The results of hydration dating and chemical characterization of obsidian sources. British Archaeological Reports International Series, 110, 1-240. 

Stevenson, C.M., Abdel-Rehim, L. and Novak, S.W. (2004). High precision measurement of obsidian hydration layers on artifacts from the Hopewell site using secondary ion mass spectrometry. American Antiquity 69 555-567. 

Stevenson, C. M., Abdelrehim, I. M., and Novak, S. W. (2001). Infra-red photoacoustic and secondary ion mass spectrometry measurements of obsidian hydration rims. Journal of Archaeological Science 28 109-115. 

Of potentially greater significance is surface hydration which occurs concurrently with alkali diffusion at relatively low temperature. The average activation energy of water diffusion in obsidian can be estimated at 75kJ between 95° and 245°C (25). A nuclear resonance hydration profile of obsidian at 25°C has yielded a diffusion coefficient of 5xlO-20 cm2-s 1... 

Studies of obsidian artifacts and sources can be used to examine resource procurement patterns, to identify long-distance exchange networks, to study manufacturing processes, and to establish site chronologies. In addition, obsidian artifacts may be used to extract dating information through hydration dating. 

The sources of obsidian exploited by prehistoric peoples were almost entirely restricted to more recent volcanic eruptions due to the hydration process. Few obsidian sources are greater than 10 million years old, and many are less than 100,000 years of age. In addition to the Andes Mountains of South America, obsidian is found in the Mediterranean, Turkey, Africa, central Europe, central Mexico, western United States, Alaska, Japan, and the islands of the South Pacific. 

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