[1] Univ Parma, Dept Phys & Earth Sci, Parco Area Sci 7-A, I-43124 Parma, Italy
[2] CNR, ICVBC, Via Cozzi 53, I-20125 Milan, Italy
[3] STFC Rutherford Appleton Lab, Cent Laser Facil, Res Complex Harwell, Oxford OX11 0QX, England
[4] Metropolitan Museum Art, Dept Sci Res, 1000 Fifth Ave, New York, NY 10028 USA
[5] Univ Ghent, Dept Archaeol, St Pietersnieuwstr 35, B-9000 Ghent, Belgium