Obsidian hydration dating on the south coast of peru

When an unworked nodule of obsidian is initially fractured, there is typically less than 1% water present.Over time, water slowly diffuses into the artifact forming a narrow "band," "rim," or "rind" that can be seen and measured under 40-80 power magnification.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.

Several factors complicate simple correlation of obsidian hydration band thickness with absolute age.Obsidian is too brittle to be used as a pounding (e.g., axe, adze) or grinding tool.Obsidian obeys the property of mineral hydration, and absorbs water when exposed to air.Experimental and empirical data indicate that this hydration band grows roughly as the square root of time (measured in years).Thus, the hydration rind initially grows quickly, then slows with age.

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