602 ANNUAL REPORT SMITHSONIAN INSTITUTION, 196 2 



The potentialities of these various refined methods of physical anal- 

 ysis in the examination of antiquities are very great and they are 

 capable of yielding information of great interest to archeologists. 

 There is little doubt that they will in the future be used on an increas- 

 ing scale. The only drawback to their extended use is the high cost 

 of the necessary equipment. 



ARCHEOLOGICAL DATING 



As a final illustration of the versatile role played by chemistry and 

 physics in the service of archeology, there can surely be no better 

 example than that of the modern methods which have been developed 

 for the dating of certain classes of antiquities. The first of these 

 is the radiocarbon method due to the pioneer work of Libby, which 

 began in 1946. The principle of this method depends upon the fact 

 that the carbon in all living organisms contains a constant proportion 

 of the radioactive isotope (carbon-14) derived from the atmosphere, 

 and such organisms are thus slightly radioactive. When the organ- 

 ism dies, replenishment of the radioactive isotope by the uptake of 

 atmospheric carbon dioxide ceases, and the existing radioactivity 

 decays at a known rate, determined by the "half-life" of carbon-14 

 which has been estimated at about 5,700 years. This means that the 

 radioactivity of the sample dwindles to half its value in 5,700 years to 

 a quarter in 11,400 years, and so on. Thus, by measuring the actual 

 radioactivity of an object containing carbon — such as wood, charcoal, 

 bone — it is possible to estimate the time that has elapsed since the 

 death of the organism. Although the amount of radioactivity that 

 has to be measured is extremely small — "modern" carbon having a 

 specific activity of only about 14 disintegrations per minute per 

 gi-am — it is possible to obtain reasonably accurate measurements by 

 the use of refined techniques. The reliability of the method has been 

 established by applying the method to samples of known age, and 

 fmding that the results agree. This method of dating is of paramount 

 importance to the archeologist, who is concerned with the chronology 

 of the prehistoric past, because he now has for the first time reli- 

 able absolute dates upon which he can base his system of chro- 

 nology. There are now many radiocarbon laboratories all over the 

 world, including three in Great Britain, and their lists of dates are 

 published annually in a special Radiocarbon Supplement of the 

 American Journal of Science. 



The radiocarbon method of dating is, of course, limited to material 

 which contains carbon, and therefore it is of interest that two other 

 methods of dating have recently been suggested, which can be applied 

 to a different class of archeological material. The first of these is 

 known as magnetic dating, and can be applied to baked clay of arche- 

 ological origin (e.g., the walls of a pottery kiln, or bricks or associated 



