"How old is it?" is frequently the first question raised following the discovery of any type of artifact at a prehistoric archaeological site. Prior to the introduction of radiocarbon dating, archaeologists frequently classified artifacts - particularly stone tools, spearheads and pottery - based on their style, shape and design. Those artifacts of a similar shape, design and other characteristics would be classified as belonging to a certain age. Of course, this method of classification and comparison failed to furnish an answer as to the specific age in terms of years of the artifact.

The archaeological revolution that served to alter this classification approach began in 1949 when nuclear physicist Willard Libby (University of Illinois) first developed radiocarbon dating. Carbon, the sixth most abundant element in the universe, has been known since ancient times. Carbon has three naturally occurring isotopes, that is, atoms of the same atomic number but different atomic weights. These isotopes are designated C-12, 13 and 14 respectively. There are also differences in terms of the frequency with which these isotopes occur. Carbon consists of 99 percent of C-12, less than 1 percent of C-13 and about one part per trillion (0.0000000001) of C-14. Unlike the other two carbon isotopes, C-14 is unstable and therefore radioactive. Hence, the term "radiocarbon" is applied when referring to the C-14 isotope.

Radiocarbon dating is based on the fact that all living organisms (plants and animals) maintain a constant amount of radiocarbon (C-14) that disintegrates at a known rate following the death of the organism. The rate of decay is determined by the half life of radiocarbon which is 5,730 years. This simply means that one half of the radioactive carbon decays every 5,730 years after the death of the organism. Through the use of sophisticated measuring devices, by calculating how much radiocarbon remains in an organic specimen, it is possible to determine, within a range, an accurate age for the specimen. If one-fourth of the radiocarbon is still present, an object would be 11,460 years old; if one-eighth of the radiocarbon is present it would be 17,190 years old; and so forth.

After an object is dated, the results are reported in the following example format: 5000 ± 100 years B.P. The initials B.P. stand for "before present" with the "present" being measured from A.D. 1950, the year that radiocarbon dating was established. The date of 5000 ± 100 means that there is a 68 percent probability that the date will range between 4900 to 5100 B.P. if the same sample is rerun. To increase the probability to a higher percentage of confidence, the ± would be increased. For example, to increase the level of confidence to 95 percent, the specimen may be reported as 5000 ± 200 years B.P., or 4800 - 5200 B.P.

Some organic materials are better to use for radiocarbon dating than others: charcoal, plant remains, wood, bone, and shell, in descending order of confidence. The reason that charred plant remains are the most desirable for purposes of radiocarbon dating is that charcoal is chemically inert and because plants absorb radiocarbon directly from the atmosphere whereas animals derive radiocarbon from eating plants.

For his discovery of radiocarbon dating, Libby was awarded the Nobel Prize for chemistry in 1960. In a subsequent article, the discussion will address certain of the limitations associated with radiocarbon dating, and some of the other methods being used to date artifacts.