beginning of the twenty-first century, there were over 100 tree ring programs (Dean 1997, 33). The technique entails the use of tree growth rings for dating and studying climatic variations in the past. Although Douglass had been researching tree growth rings since the beginning of the twentieth century, it was in the 1920s that he applied the technique to archaeology by constructing a chronology for Pueblo ruins of the U.S. Southwest. A tree adds a ring (xylem growth) each year, so by counting the number of rings one can obtain an estimate of age. The thickness of the ring is dependent on climatic variations: wet years produce thick rings; dry years, thin rings. Ring patterns from different species of trees are compared and cross-dated with known age samples to form a baseline with which to compare older samples, thus building up a sequence of tree ring growth back into the past.

The Radiocarbon Revolution

One of the most important archaeometric techniques is radiocarbon dating, one of a number of techniques that use radioactive decay. It was pioneered by willard libby, then of the University of Chicago, in the late 1940s and is based on the premise that all living organisms have an uptake of radiocarbon (carbon–14) that ceases when they die. As carbon–14 is unstable, it decays at a half-life of what Libby thought was 5,568 years. By measuring the amount of radioactive carbon of organic matter in an archaeological sample (charcoal, bone, or shell) of known age and comparing it to the radiocarbon in living matter, Libby was able to estimate the time lapse since the organism died.

Radiocarbon dating has undergone many changes since its advent, in particular with regard to refinement in the calculation of the half-life, the development of a calibration curve to give calendar years, and in the techniques for measuring the amount of carbon–14 in a sample. Accelerator mass spectrometry (AMS), for instance, is a relatively new method that determines the amount of carbon–14 in a sample by measuring the atoms directly. Although more expensive than the conventional C-14 dating, this method can utilize much smaller samples.

The importance of radiocarbon dating for both chemistry and archaeology cannot be overestimated. Libby was awarded the Nobel Prize for Chemistry in 1960, and laboratories were set up at the University of Pennsylvania in 1951 and in various centers in Europe soon after the one at Chicago. For the first time, chronologies could be established for areas where conventional dating techniques were difficult to apply. The human time depth of areas such as Australia, Southeast Asia, and the Pacific was recognized for the first time, and a “revolution” occurred in dating Europe’s prehistory, which had been previously based on cross-dating to objects of known age. For a full discussion on the radiocarbon revolution on European and U.S. archaeology see Renfrew (1976) and G. Marlowe (1999), respectively.

Luminescence Dating

Although first suggested in the mid-1950s and there was an application to archaeological pottery in the 1960s, it has only been since 1970 that thermoluminescence dating (TL) has been applied routinely in archaeology with levels of precision of plus or minus10 percent. Major developments occurred after dedicated laboratories were set up, such as that by Michael Aitken at Oxford University during the 1960s, and a book on the technique and its application to archaeology was published in 1979 (Fleming 1979).

The advantage of TL is that it dates the artifact itself. TL has been used to date archaeological materials that have mineral grains, such as quartz and feldspar, and have been exposed to heating: pottery, flint, burned stone, earth ovens, etc. TL is based on the assumption that energy from long-term exposure to ionizing radiation is built up or trapped over time in the lattice of crystals. When heated above 500 degrees centigrade, this stored energy is released in the form of emitted light called thermoluminescence, and the TL clock reset back to zero. The measurement of TL emitted can indicate the time elapsed since the last firing.

Major advances have occurred since the development of the technique, including the use of optical dating (OD). Rather than dating minerals