Obsidian, or volcanic glass, is formed by the rapid cooling of silica-rich lava. Although its precise chemical composition varies from one outcrop to another, it always contains >70% silica by weight. Humans often used obsidian as a raw material when making chipped stone tools.
In 1948, two geologists, Irving Friedman and Robert Smith, began looking into 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 (eg. soil temperature and soil relative humidity) of the archaeological site are known.
How does Obsidian Hydration Dating work?
Obsidian hydration dating is based on the fact that a fresh surface is created on a piece of obsidian in the tool manufacturing, or flintknapping, process. Obsidian contains about 0.2 percent water. When a piece of obsidian is fractured, atmospheric water is attracted to the surface and begins to diffuse into the glass. This results in 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. A distinct diffusion front can be recognized by an abrupt change in refractive index at the inner edge of the hydration rind. These fronts or rinds of hydration are more dense than the unhydrated inside, and the unhydrated zone has different optical properties. 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.
Hydration begins after any event which exposes a fresh surface (e.g. cracking of a lava flow on cooling, manufacture of an obsidian artifact, or glacial abrasion of an obsidian pebble). Providing one can identify which process created the exposed surface or crack in the rock, it is possible to date when that process took place. Hydration rind thickness is a (non-linear) function of time. The hydration rate is primarily a function of temperature, though chemical composition of the sample is also an important factor. For this reason, it is necessary to calibrate the samples within a limited geographical area against a sample of known age and similar chemical composition.
Hydration forms at different rates on different obsidians. Under the same conditions of temperature and humidity some glasses will hydrate rapidly while others are very slow. What controls the process? There is a very strong relationship between the rate of hydration and the quantity of intrinsic water found within the glass. This is the water trapped in the obsidian at the time the lava hardens into a natural glass. The presence of intrinsic water opens up the glass structure and allows the atmospheric water to diffuse inward from the surface to form the hydration rim. The more intrinsic water present within an obsidian artifact, the faster it will hydrate and the faster the hydration rim will form
How is an Obsidian Sample Processed?
Three steps are required to determine a calendar date from an obsidian artifact. These are the determination of: 1) the hydration rate, 2) the thickness of the hydration rim, and 3) the soil temperature and soil relative humidity at the archaeological site.
A hydration rate is determined for every artifact through a measurement of the amount of intrinsic water that is present. This is done by either a direct infrared spectroscopic measurement of the volcanic glass, or by a determination of the volcanic glass density made by submersion in a heavy liquid. Once the quantity of water is known a hydration rate is estimated.
A small sample is cut out perpendicular to the edge of the obsidian artifact using a diamond-impregnated saw. A lapidary machine is used to grind down the obsidian sample until it is very thin. It is glued to a clear microscope slide with Canada balsam. The obsidian sample is ground a second time until it less than 50 microns in thickness. A microscope is then used to optically measure the hydration rind on the petrographic thin section. The hydration layer is measured at 800x using a Watson image-splitting instrument. This is the most precise optical instrument that can be used. It has an error factor of about 0.1 microns.
In order to adjust the experimental hydration rate to the conditions at the archaeological site, the soil temperature and soil relative humidity need to be well estimated. On short term projects, ambient conditions can be estimated from weather records. For studies that take longer than a year, thermal cells can be buried at the archaeological site. The small capsules are placed at multiple depths that typically span a depth range of 5 cm to 100 cm below ground surface. About 8 cells are required to establish a temperature and relative humidity curve for the site. With this background work done, the environmental conditions can be determined for any context at a site.
The Limitations of Obsidian Hydration Dating
Using this technique, any sample of obsidian can be dated. There are several limitations, however.
The Diffusion Laboratory
The Northwest Research Obsidian Studies Laboratory
International Association for Obsidian Studies
Freter, AnnCorinne. 1993. Obsidian-Hydration Dating: Its Past, Present, and
Future Application in Mesoamerica. Ancient Mesoamerica 4(2):285-303.
Friedman, Irving and F. W. Trembour. 1983. Obsidian Hydration Dating Update. American Antiquity 48(3):544-547.
Friedman, Irving Fred W. Trembour,Franklin L. Smith, and George I. Smith. 1994. Is Obsidian Hydration Dating Affected by Relative Humidity? Quaternary Research 41(2):185-190.
Michels, Joseph W. and Ignatius S. T. Tsong. 1980. Obsidian Hydration Dating: A Coming of Age. In Advances in Archaeological Method and Theory, Volume 3, edited by M. B. Schiffer, pp. 405-444. Academic Press, New York, New York.
Michels, Joseph W., Ignatius S. T. Tsong, and Charles M. Nelson. 1983. Obsidian Dating and East African Archeology. Science 219:361-366.
Origer, Thomas M. 1989. Hydration Analysis of Obsidian Flakes Produced by Ishi During the Historic Period. In Current Directions in California Obsidian Studies, edited by Richard E. Hughes, pp. 69-77. Contributions of the University of California Archaeological Research Facility No. 48, University of California, Berkeley, California.