When cosmic rays hit the earth's atmosphere striking nitrogen atoms, radioactive carbon, called carbon 14 (C 14) is created. Carbon 14 is unstable, and eventually, through the process of radioactive decay, becomes nitrogen again. Plants, by breathing in carbon dioxide, and animals, through the food they eat, absorb carbon 14, maintaining a relatively constant ratio of carbon 14 to normal carbon within their tissues. Once the plant or animal dies, the intake of carbon 14 ends, and what remains breaks down into nitrogen at a constant rate. Because the loss of carbon 14 is measurable, scientists can estimate how old once-living things are by the amount of carbon 14 they contain. For example, in about 5,730 years, the amount of carbon 14 left in organic remains drops to half of its original level, then, in about 11,460 years, decreases to one-fourth its original level.

Measuring carbon 14 in bone, wood, charcoal, or other remains requires complicated chemical processes to purify the specimen and then transform it into a gas or liquid, which is then measured for the amount of carbon present relative to more stable carbon. Through this process, scientists can estimate the age of a specimen up to about 50,000 years. Some advanced laboratories can extend the dating to 100,000 years.

Errors can occur in the test, however, to alter the date, sometimes by hundreds of years, identifying the specimen as older or younger than it actually is. Consequently, archeologists prefer having a number of objects from a site carbon dated for comparison. They also examine where the carbon-dated materials were found to see if artifacts known to originate during a particular era were nearby Studying the soil layers above and beneath a carbon 14 tested specimen to determine which period of artifacts preceded and followed it, is also useful.

Other methods are also helpful to determine dates. Radioactive potassium is examined, for example, in volcanic material because it eventually reverts to argon. This test works best when researchers are looking for dates millions of years ago because half of the radioactive p6tassium will break down in about 1.3 billion years.

To learn when a ceramic piece was made, thermoluminescence dating is useful. Clay is heated above 38 degrees centigrade, then the light emitted and radioactive material present are measured to learn when the clay was first fired.

Fission track dating is the study of submicroscopic damage trails left by the decay of uranium atoms½ in volcanic materials, glasses, and other substances to learn how old they are.

Most of these techniques are relatively new and continue to be refined. They are also costly. Dating specirnens, however, doesn't always require an advanced degree in chemistry or great expense. Thomas Jefferson, sometimes called the Father of American Archeology, was among the first to suggest another dating method still used today-studying tree rings. Called dendrochronology-which is the comparison of changes in annual tree rings caused by climate-the technique has proven especially helpful in some regions such as the arid American Southwest where wood has helped date old Indian structures.

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