How a stone measures time
Obsidian Hydration Dating (OHD) is a method that determines the age of an obsidian artifact by measuring the microscopic layer of water it has absorbed since its creation. This volcanic glass, formed by the rapid cooling of silica-rich lava, contains very little water when formed, typically around 0.2% by weight. Geologists Irving Friedman and Robert Smith first introduced the technique to archaeology in 1960.
The process begins when a new surface of obsidian is exposed to the atmosphere, for example, when a stone tool is made. This fresh surface immediately begins to absorb water from its surroundings. Water molecules slowly diffuse into the glass, creating a distinct hydration "rim" or "rind." This layer of absorbed water increases in thickness over time. The process continues until the surface is saturated, containing about 3.5% water. The thickness of this rim, often just a few micrometers even after thousands of years, is directly related to how long the surface has been exposed.
To measure the hydration layer, a technician cuts a very thin slice from the artifact, grinds it down to a thickness of about 30-50 micrometers, and mounts it on a microscope slide. Under a high-power microscope, often with polarized light, the hydration rim becomes visible because it has a different density and refractive index than the unhydrated interior glass. The layer's thickness is then precisely measured, with an accuracy of 0.1 to 0.2 micrometers.
Reading the clock's variables
While the principle of OHD is straightforward, converting a rim thickness into an absolute calendar date is complex. The rate at which obsidian absorbs water is not constant. The most significant factor influencing the hydration rate is temperature. Higher ambient temperatures accelerate the diffusion process, causing the hydration layer to grow faster. An artifact buried deep in cool soil will have a different hydration rate than one found on the sun-baked surface.
Other factors also affect the rate, including the obsidian's specific chemical composition and the intrinsic water content of the glass. Humidity and even soil chemistry can play a role. Because of this variability, a single, universal hydration rate does not exist. Instead, archaeologists must develop source-specific and site-specific calibration curves.
To create a reliable timeline, researchers must correlate obsidian hydration measurements with other dating methods, like radiocarbon dating of associated organic materials. By dating charcoal found in the same stratigraphic layer as obsidian tools, scientists can establish a local hydration rate for that specific type of obsidian under those particular environmental conditions. For long-term studies, scientists may even bury thermal cells at an archaeological site to record soil temperature and humidity over time, helping to refine the "effective hydration temperature" used in age calculations. More advanced analytical techniques, such as Secondary Ion Mass Spectrometry (SIMS), can measure the hydrogen profile in the glass directly, offering a non-destructive way to achieve higher precision.