Obsidian hydration layer

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. 

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... 

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. 

FIGURE 23 Hydration layer in obsidian. When obsidian is broken into two or more pieces, new surfaces are created. As a new surface is exposed to the environment, water (from atmospheric humidity, rain, or the ground) penetrates the surface gradually, the water diffuses into the bulk and forms hydrated obsidian, that is, obsidian containing water. With time, the thickness of the hydration layer, as such a layer is known, gradually increases the rate of increase is affected by such factors as the vapor pressure of the water in the atmosphere, the environmental temperature, and the composition of the surrounding environment as well as of the obsidian. If the hydration layer reaches a thickness of 0.5 microns or more, it becomes discernible under a microscope, the thickness can be measured, and the age of the surface calculated. The microphotograph shows an hydration layer on obsidian. 

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). 

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. 

Obsidian is a volcanic glass frequently used as tool by prehistoric men. The thickness of the hydration layer that has developed on the surface of the artifact since it was made can be used to date it. Measurement of the hydration layer is carried out by a variety of instrumental techniques. Among them are nuclear resonance reactions, LM, PAS, XPS, XANES, sputter-induced optical spectrometry (SIPS), and SIMS [70]. 

Liritzis I, Ganetsos Th (2006) Obsidian hydration dating from SIMS H-l- profiling based on saturated surface (SS) layer using new software. Appl Surf Sci 252 7144-7147. 

There has been a renewed interest in the obsidian dating technique which was developed in the early 1960s. In this dating methodology age determination is related to the thickness of hydration layer in the obsidian artifact. The surface of a freshly carved obsidian artifact slowly absorbs moisture and builds up a hydration rim over a period of time. The rate with which the hydration rim is acquired depends greatly on the composition of the obsidian as well as on environmental factors (28,31,59,60, 73-76). 

XAFS characterization of the hydration layer of obsidian (Lytle et al. 2002)... 

Lytle FW, Pingitore NE Jr (2002) Iron valence in the hydration layer of obsidian characterization by X-ray absorption spectroscopy. Microchemical J 71 185-191... 

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