The effect of changes in relative humidity on the hydration rate of Pachuca obsidian

The effect of relative humidity on the hydration rate of obsidian and other glasses has been debated since the early work of [I. Friedman, R. Smith, Am. Antiquity 25 (1960) 476]. While more recent work has been in general agreement that a relative humidity dependence does exist, hydration profiles as a function of relative humidity have not been obtained. In this paper we present the results of a study in which samples of Pachuca obsidian were hydrated for approximately 5 days at 150 °C at relative humidities ranging from 21% to 100%, and the resultant profiles were measured by secondary ion mass spectrometry (SIMS). The results suggest that the hydration rate is, indeed, a function of relative humidity, but for the relative humidity levels commonly observed in most soils the effects on hydration dating are expected to be relatively small. In addition, analysis of the surface values as sorption isotherms and comparisons with nitrogen sorption isotherms suggests that water is relatively strongly bound to the obsidian surface. By assuming a situation in which the ‘surface’ refers to active centers within the glass we have shown that an adsorption model provides a useful approach to modeling the diffusive process.