Photoacoustic, EPR and electrical conductivity investigations of three synthetic mineral pigments: hematite, goethite and magnetite N. Guskos, G.J. Papadopoulos, V. Likodimos, S. Patapis, D. Yarmis, A. Przepiera, P. K. rzepiera, J. Majszczyk, J. Typek, M. Wabia, K. Aidinis, Z. Drazek Materials Research Bulletin37 (6), 1051 (2002)