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. 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. 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. 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. Three steps are required to determine a calendar date from an obsidian artifact.