Obsidian obeys the property of mineral hydration and absorbs water, when exposed to air at a well-defined rate.
Over time, water slowly diffuses into the artifact forming a narrow "band", "rim", or "rind" that can be seen and measured with many different techniques such as a high-power microscope with 40–80 power magnification, depth profiling with SIMS (secondary ion mass spectrometry), and IR-PAS (infra red photoacoustic spectroscopy).
[3][4] Obsidian hydration dating was introduced in 1960 by Irving Friedman and Robert Smith of the U.S. Geological Survey.
The use of Secondary ion mass spectrometry (SIMS) in the measurement of obsidian hydration dating was introduced by two independent research teams in 2002.
It has also been applied in South America, the Middle East, the Pacific Islands, including New Zealand and Mediterranean Basin.
This sample is ground down to about 30 micrometers thick and mounted on a petrographic slide (this is called a thin section).
In case of measuring the hydration rim using the depth profiling ability of the secondary ion mass spectrometry technique, the sample is mounted on a holder without any preparation or cutting.
Although relatively infrequent the use of SIMS on obsidian surface investigations has produced great progress in OHD dating.
SIMS in general refers to four instrumental categories according to their operation; static, dynamic, quadrupole, and time-of-flight, TOF.
In essence it is a technique with a large resolution on a plethora of chemical elements and molecular structures in an essentially non destructive manner.
An approach to OHD with a completely new rationale suggests that refinement of the technique is possible in a manner which improves both its accuracy and precision and potentially expands the utility by generating reliable chronological data.
In Rhodes, Greece, under the direction and invention of Ioannis Liritzis,[11] the dating approach is based on modeling the S-like hydrogen profile by SIMS, following Fick's diffusion law, and an understanding of the surface saturation layer (see Figure).
In fact, the saturation layer on the surface forms up to a certain depth depending on factors that include the kinetics of the diffusion mechanism for the water molecules, the specific chemical structure of obsidian, as well as the external conditions affecting diffusion (temperature, relative humidity, and pressure).
[12] Together these factors result in the formation of an approximately constant, boundary concentration value, in the external surface layer.
Using the end product of diffusion, a phenomenological model has been developed, based on certain initial and boundary conditions and appropriate physicochemical mechanisms, that express the H2O concentration versus depth profile as a diffusion/time equation.
This latest advance, the novel secondary ion mass spectrometry–surface saturation (SIMS-SS), thus, involves modelling the hydrogen concentration profile of the surface versus depth, whereas the age determination is reached via equations describing the diffusion process, while topographical effects have been confirmed and monitored through atomic force microscopy.
[13][14][15][16] Several factors complicate simple correlation of obsidian hydration band thickness with absolute age.
[9] The reliability of the method based on Friedman's empirical age equation (x²=kt, where x is the thickness of the hydration rim, k is the diffusion coefficient, and t is the time) is questioned from several grounds regarding temperature dependence, square root of time and determination of diffusion rate per sample and per site, as part of some successful attempts on the procedure and applications.
3) which relates the inverse gradient of the fit polynomial to well dated samples: where Ds = (1/(dC/dx))10−11 assuming a constant flux and taken as unity.