Chronological Methods: Obsidian Hydration Dating

Obsidian Flake
Copyright: Melanie Magdalena

Volcanic glass is known as obsidian. Humans in the past often used obsidian as a raw material when making chipped stone tools like arrowheads and spear points. Obsidian is formed by the rapid cooling of silica rich lava. Its precise chemical composition often varies from one outcrop to another, but it always contains seventy percent silica by weight.

Geologists Irving Friedman and Robert Smith, investigated obsidian's potential as a time marker. They introduced the obsidian hydration dating method to the archaeological community in 1960. It may be used in two ways: as a relative dating method to determine if one artifact is older or younger than another, or as an absolute dating method where a calendar date (AD/BC) is produced. The decision to use it as a relative or absolute dating method depends upon whether the environmental conditions of the archaeological site are known.

How does obsidian hydration dating work? Obsidian contains about 0.2 percent water. When a piece of obsidian is chipped and exposed to atmospheric water, it attracts the water to the surface and begins to diffuse into the glass. The result is the formation of a water rich hydration rind that increases in depth with time. The hydration process continues until the fresh obsidian surface contains about 3.5 percent water, this is the saturation point. The thickness of the hydration rind can be identified in petrographic thin sections cut normal to the surface and observed under a microscope. Friedman and Smith reasoned that the degree of hydration observed on an obsidian artifact could tell archaeologists how long it had been since that surface was created by a flintknapper.

There are some limitations to dating using obsidian hydration. Hydration forms at different rates on different obsidians. The rate of hydration can differ throughout the world. Soil temperature and relative soil humidity need to be well estimated. It is best to use a sample with known age and similar composition to the sample being dated.

Another limitation is that an artifact, for example, a tool that was used to skin a deer is used and then discarded. Several hundred years later, a second individual finds the tool and sharpens it and uses the refurbished tool for a completely different use - like to shave bark of a tree. When the archaeologists finds it in a site and sends it to be dated, instead of dating the surface that was used to shave bark, the surface that was used to skin deer is dated, giving an erroneous date for the site it was in.

References:

"Obsidian Hydration Dating." Accessed on 25 January 2012.

About the Author

I'm Jose Pierre and I like learning about all aspects of culture, both ancient and modern. I enjoy learning how they communicated, expressed themselves, and their technology.