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Archeology for Interpreters > 5. How Do Archeologists Figure Out How Old Things Are?

Absolute dating

Obsidian hydration

Obsidian is the volcanic glass that was sometimes used as raw material for the manufacture of stone tools. Obsidian is found in the western United States, Alaska, Central America, and elsewhere. When an archeologist has identified the source of the obsidian from which an artifact is made, he or she may be able to date the artifact using the obsidian hydration technique. This technique of dating obsidian artifacts measures the microscopic amount of water absorbed on freshly broken surfaces. The principle behind obsidian hydration dating is simple—the longer the artifact surface has been exposed, the thicker the hydration band will be. Obsidian hydration can indicate an artifact's age if the datable surfaces tested are only those exposed by flintknapping. Obsidian hydration is not effective on surfaces that are uneven due to gradual weathering caused by natural forces.

Fun fact

The volcanic glass of Obsidian Cliff in Yellowstone National Park is a National Historic Landmark, and was an important source of raw material for the manufacture of stone tools by Native Americans. To find out more about obsidian and its importance to people who once occupied national park land, search for “obsidian” on the National Park Service home page.

Try it yourself

Obsidian Hydration Dating
See a movie about obsidian hydration dating.

Thermoluminescence

Thermoluminescence dating is used for rocks, minerals, ceramics and burned features. It is based on the fact that almost all natural minerals are thermoluminescent—they emit light when heated. Energy absorbed from ionizing radiation frees electrons to move through the crystal lattice and some are trapped at imperfections. In the lab, samples are heated releasing the trapped electrons and producing light. The light is measured to determine a date. Thermoluminescent dating is used to date archeological deposits, detect ceramic fakes in art collections, and even date burned flint artifacts.

Fun fact

At Hopewell Culture National Historical Park archeologists collected ceramic samples for TL (thermoluminescence) dating. The abundance of decorated ceramics from the park will be helpful in fixing the time of occupation. Ceramic samples will be subjected to mineralogical analysis to determine whether or not they were made of local clays.

For your information

Luminescence Dating
Learn more about luminescence dating.

TSM/MJB