The use of electron spin resonance spectroscopy for determining the provenance of classical marbles

The possibility of identifying the provenance of classical marbles and solving related questions, such as the joining fragments problem, via electron spin resonance spectroscopy has been reexamined. The method is based on characterization of the Mn²⁺ impurity ubiquitously present in marbles. Six different, although correlated, spectroscopic variables, such as intensity, linewidth, metal hyperfine splitting and others have been measured, together with some petrographic properties, for over 500 samples belonging to 14 different Italian, Greek and Turkish quarrying sites. The work, still in progress, is aimed to establish a marble database including all the historically relevant sites within the Mediterranean basin. The experimental data matrix has been analyzed with the aid of multivariate statistical techniques, primarily linear and quadratic discriminant analysis, and the ability of the method to classify correctly unknown samples has been estimated through standard techniques (resubstitution, jackknife), but also employing control “unknown” samples. The essential result is that, although the all-variables approach may describe the data set very accurately, the predictive power is due to four variables only, which are a suitable combination of spectroscopic and petrographic information. In this way 82.4% of the control samples could be assigned correctly, whereas inclusion of additional variables in the classification rule may result in substantially poorer performance. The conclusion is that ESR spectroscopy, although not providing a complete and general solution for the marble provenance problems, is probably, at the moment, the most developed methodology for identifying marbles. Its results, coupled with artistic historical information, deal correctly with a number of relevant archaeometric problems.