text: Several factors complicate simple correlation of obsidian hydration band thickness with absolute age. Temperature is known to speed up the hydration process.  Thus, artifacts exposed to higher temperatures, for example by being at lower elevation, seem to hydrate faster. As well, obsidian chemistry, including the intrinsic water content, seems to affect the rate of hydration.  Once an archeologist can control for the geochemical signature of the obsidian (e.g., the "source") and temperature (usually approximated using an "effective hydration temperature" or EHT coefficient), he or she may be able to date the artifact using the obsidian hydration technique. Water vapor pressure may also affect the rate of obsidian hydration.
  The reliability of the method based on Friedmans 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. The SIMS-SS age calculation procedure is separated into two major steps. The first step concerns the calculation of a 3rd order fitting polynomial of the SIMS profile (eq. 1). The second stage regards the determination of the saturation layer, i.e. its depth and concentration. The whole computing processing is embedded in stand-alone software created in Matlab (version 7.0.1) software package with a graphical user interface and executable under Windows XP. Thus, the SIMS-SS age equation in years before present is given in eq. 2:
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  Eq. 1 Fitting polynomial of the SIMS profile
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  Eq. 2     The SIMS-SS age equation in years before present
  Where, Ci is the intrinsic concentration of water, Cs is the saturation concentration, dC/dx is the diffusion coefficient for depth x=0, k is derived from a family of Crank's theoretical diffusion curves, and  is an effective diffusion coefficient (eq. 3) which relates the inverse gradient of the fit polynomial to well dated samples:
  :D = aD + b/ (10D) = 8.051eD+0.999/(1022D), 	Eq. 3'
  where Ds = (1/(dC/dx))10  assuming a constant flux and taken as unity. The eq. (2) and assumption of unity is a matter of further investigation.
  Several commercial companies and university laboratories provide obsidian hydration services.