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Obsidian hydration dating (OHD), an artifact-dating technique that measures the thickness of the rind that forms on the surface of obsidian (a volcanic glass), can be used to estimate when prehistoric humans used the sharp glass to shape stone tools. The use of OHD requires careful calibration based on a sample's climatic context and temperature fluctuations, variations in water vapor pressure, and differences in glass chemistry are known to affect the rate of rind growth.