Abstract
On the problem of dating method by means of obsidian artifact, an important contribution from petrology to archaeology has been presented by FRIEDMAN and SMITH (1960). Obsidian is a homogeneous natural glass which contains a small amount of water, usually less than 0.3%. While the obsidian is exposed to the atmosphere or buried in the soil, its surface will be hydrated. Other conditions being equal, the time required to form the hydration layer would be proportional to the squares of thickness of the layer. The hydration rate may be controlled mainly by two factors, temperature of circumstances and petrographic character of obsidian. In order to obtain a reliable result by this method, a working curve based upon radiocarbon date is required at each climatic region, and obsidian samples must be similar in their petrographic features.
Under the special caution above mentioned, a nearly linear working curve (Fig. 1) was prepared based upon 6 sites of nonceramic to after-Jomon cultures excavated from Hokkaido, Japan. A significant increase of the hydration rate from the late Pleistocene to the Holocene is found. It would be attributed to the warming of climate in the Holocene. Even though other factors are the same, the working curve at any region would not bear a simple linear relation due to the climate change in the Quaternary.
