Obsidian hydration dating

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Obsidian hydration dating is a geochemical method of determining age in either absolute or relative terms of an artifact made of obsidian.

Obsidian is a volcanic glass that was sometimes used as raw material for the manufacture of stone tools such as projectile points, knives, or other cutting tools through the process of flintknapping. Obsidian is too brittle to be used as a pounding (e.g., axe, adze) or grinding tool. Obsidian obeys the property of mineral hydration, and absorbs water when exposed to air. When an unworked nodule of obsidian is initially fractured, there is typically less than 1% water present. Over time, water slowly diffuses into the artifact forming a narrow "band," "rim," or "rind" that can be seen and measured under 40-80 power magnification. Experimental and empirical data indicate that this hydration band grows roughly as the square root of time (measured in years). Thus, the hydration rind initially grows quickly, then slows with age.

To measure the hydration band, a small sliver of material is typically cut from an artifact. This sample is ground down to about 30 micrometres thick and mounted on a petrographic slide. The hydration rind is measured under a high-power microscope outfitted with some method for measuring distance, typically in tenths of micrometres. measures the microscopic amount of water absorbed on freshly broken surfaces. The principle behind obsidian hydration dating is simple–the longer the artifact surface has been exposed, the thicker the hydration band will be.

In order to use obsidian hydration for absolute dating, the conditions that the sample has been exposed to and its origin must be understood or compared to samples of a known age (e.g. as a result of radiocarbon dating of associated materials.

Several factors complicate simple correlation of obsidian hydration band thickness with absolute age. Temperature is known to speed up the hydration process. Thus, artifacts exposed to higher temperatures, for example by being at lower elevation, seem to hydrate faster. As well, obsidian chemistry, including the intrinsic water content, seems to affect the rate of hydration. Once an archeologist can control for the geochemical signature of the obsidian (e.g., the "source") and temperature (usually approximated using an "Effective Hydration Temperature" or EHT coefficient), he or she may be able to date the artifact using the obsidian hydration technique. Water vapor pressure may also affect the rate of obsidian hydration.

Several commercial companies provide obsidian hydration services. Costs typically range from $12 to $20 per artifact.

[edit] History

Obsidian hydration dating was introduced in 1960 by Irving Friedman and Robert Smith of the U.S. Geological Survey. Their initial work focused on obsidians from archaeological sites in western North America.

Today the technique is applied extensively by archaeologists to date prehistoric sites and sites from protohistory in California and the Great Basin of North America. It has also been applied in South America, the Middle East, and the Pacific Islands, including New Zealand.