CONSERVATION OF ANTIQUITIES — WERNER 603 



pottery that has remained in situ). This method depends upon two 

 essential facts. The first is that the secular variations in the direction 

 of the earth's magnetic field — as defined by the magnetic declination 

 and the angle of dip — have been recorded in different centers over the 

 past 400 years, and the second is that this information has also been 

 preserved in baked clay by the phenomenon of thermo-remanent 

 magnetism. Hence, by measuring the magnetic direction in clay that 

 can be presumed to have remained in situ since it was baked (as would 

 be the case with a pottery kiln) and comparing the measurements with 

 the known secular variations, an estimate of the date of baking can 

 be obtained. Details of the method and its archeological applications 

 are given in a recent book by Aitken [19] . 



The other possible method of dating, which is known as thermo- 

 Imninescent datmg, has been suggested by Professor Kennedy of the 

 University of California. This depends upon measuring the radiation 

 damage produced in clay minerals owing to the presence in them of 

 small amounts of radioactive impurities such as uranium and thorium. 

 This radiation damage consists of defects in the crystal lattice, which 

 act as electron traps. If the clay sample is heated to about 400° C, 

 the excess energy of the electrons is released as visible light, which 

 can be measured by a photo-multiplier. The amount of light is pro- 

 portional to the amount of damage, wdiich is itself proportional to the 

 length of time during which the radiation damage has been accumulat- 

 ing. For pottery this would be the time elapsed since it was originally 

 fired, and herein lies the value of this dating method in archeology. 

 Potsherds are among the commonest of archeological material, and, 

 unlike the magnetic dating method, the thermoluminescent dating 

 method would not require the finding of the sample in situ. If further 

 research should establish this as a reliable method of dating, it will 

 undoubtedly be of enormous value for prehistoric archeology. 



To the archeologist, dating is of vital importance, and the subtle 

 methods which the scientist is now evolving — and may continue to 

 evolve — will be of great value in throwing light on many problems 

 of chronology. 



CONCLUSION 



The foreffoins: account of the kind of work that is carried out in a 

 museum laboratory has of necessity been selective, since space has only 

 permitted the discussion of a limited range of problems. No reference 

 has been made, for example, to the use of scientific methods in problems 

 of authenticity. However, the selected topics will serve to demonstrate 

 that the work involved is very varied, both as regards the type of prob- 

 lem itself — whether it be of conservation or of scientific examina- 

 tion — and the new methods of investigation used by chemists and 

 physicists to solve these problems. In this fascinating field of activity, 



