The hydration-rind dating method is based on the ability of a fresh surface of obsidian to adsorb water from the air or soil, and the ability of this adsorbed water to then slowly diffuse into the body of the obsidian at a rate which is dependent mainly on temperature and somewhat on the composition of the glass, but which is independent of humidity. The thickness of hydration rinds on the surface of obsidian (volcanic glass of rhyolitic composition) increases with age and has been used for dating obsidian artifacts that were manu factured 200 to 250,000 years ago. Hydration rinds have also been used to measure the age of rhyolite volcanic flows. -- AATA