Introduction

Developed in 1960, Obsidian Hydration Analysis (OHA) is an inexpensive technique archaeologists and geoarchaeologists use to find the age of a site they have excavated. This method is most often used as a means of relative dating , but an absolute date may also be estimated in some circumstances.

What is obsidian?

Obsidian is an igneous rock formed when volcanic magma cools quickly. It is a natural glass and is usually black in color, though it has been found in a variety of hues. It can readily be found in recent lava flows. This rock has little to no crystalline structure, which makes its breaking pattern easy to anticipate. Obsidian has been a favorite material of flint knappers since the beginning of stone tool production, and is found at archaeological sites around the world.

Obsidian was a common rock used in stone tool making. Obsidian is used mainly because of its availability in prehistoric tool making and its unique quality of rehydrating itself after a fracture.

The Development of Obsidian Hydration Analysis

Irving Friedman and R.L. Smith discovered the factors determining the hydration rate of obsidian in 1960. It was suggested that the technique could be applied as a direct dating method for obsidian artifacts once the factors which affect hydration rates could be controlled for. The proposed factors were the composition of the obsidian, temperature, and relative humidity. It was also determined by Friedman and Smith that erosion and burning could reduce the hydration layer thickness.

How does obsidian become "hydrated?"

Rainbow-colored obsidian

When obsidian is newly exposed to the atmosphere, its surface begins to absorb water from the air, which gradually seeps into the interior of the stone. Several factors can affect the obsidian's water absorption, including soil type, climate, time and geochemistry.

When viewed under a microscope, the layer permeated by moisture (known as a "rind") becomes visible as a rim when the rind reaches a width of 0.5 microns (a micron being one millionth of a meter). The greater the rind thickness, the greater the age of the exposed obsidian.

The OHA lab procedure

When an archaeologist would like a piece of obsidian dated, s/he sends it to one of the several OH labs around the country. The lab technicians cut a sample off the artifact, usually perpendicular to the edge of a flake and about 4.5 mm deep. This allows for better light transmission during the analysis. The wedges are then cemented to microscope slides and polished to remove any small nicks and scratches left in the obsidian from the cutting process. The sample is then prepared on a microscope slide and the hydration band is measured using polarized light, white light, or both according to the flake's translucency. Several measurements on each rind are taken, and the samples are often re-evaluated after a week for accuracy. Mathematic calculations are made of the sample's translucency at the various powers of magnification of the microscope. These measurements are then sent back to the archaeologist so he/she can make an assessment of the age of the artifact in question.

The actual measurement of hydration involves using light transmission to determine the amount of hydration, and therefore the relative age of the sample. The prepared slides are observed by means of a microscope to determine the amount of light transmission. White light and/or polarized light may be used in the analysis of the sample to measure hydration. These calculations are used to determine the hydration of the sample, which indicates the age of the sample.

Obsidian hydration is a very advantageous method when identifying the relative age of an artifact. OH can also be beneficial in finding out the seismic history of an area.

References

Hamilton, Jeff. Obsidian Hydration Analysis Service. Pacific Internet, 1996.

Obsidian Hydration Analysis Service. http://www.ohas.com/ International Association for Obsidian Studies, 1997.

Stevenson, Christopher M., Wal Ambrose, Peter J. Sheppard, and Douglas G. Sutton. Advances in the Hydration Dating of New Zealand Obsidian. The Obsidian Hydration Laboratory, 1998.

Simon & Schuster's Guide to Rocks and Minerals. Annibale Mottana, Rodolfo Crespi and Giuseppe Liborio. Published by Simon and Schuster Inc. 1978.

What is Obsidian Hydration Analysis?. Website: http://www.pacific.net/~ohas/wisoha.html

Methodological Notes on the Use of Obsidian Hydration Data. Scott Byram. 1995.

Website: http://www.peak.org/obsidian/byram1.html

By: Linda Robinson