1. Department of Physics and Mathematics, Surface Analysis Laboratory; University of Rome “Roma Tre”; Via della Vasca Navale 84 00146 Rome Italy
2. Department of Industrial Engineering; University of Rome “Tor Vergata”; Via del Politecnico 1 00133 Rome Italy
3. Istituto Superiore per la Conservazione ed il Restauro; Via di San Michele 23 00153 Rome Italy