AMONG the scientific tools devised to help to satisfy man’s curiosity about his past, none is better known than the radiocarbon clock. This method of dating organic material in ancient artifacts is based on measurement of the radioactive carbon that is formed by cosmic rays in the atmosphere and taken up by plant life. It is most useful for dating things made of wood, charcoal and plant or animal fibers. Its workable range goes back more than 10,000 years.

An accuracy of within fifty to one hundred years in the date is now expected. It is true that divergences larger than this have been found between the “radiocarbon age,” as calculated from the radioactivity, and the real age of known samples, but this may be taken into account with a calibration curve measured in several laboratories.

This curve is based chiefly on wood taken from long-lived trees that have been dated by counting their annual rings. For example, a piece of wood 7,000 years old according to the ring count may give a radiocarbon age of only 6,000 years. So the 1,000 years is applied as a correction to be added to the radiocarbon age of any sample from that era.