We investigated bricks with ages from modern to late 19th century. The experimental and theoretical analysis of hysteresis loops and magnetization (ARM, IRM) acquisition curves show presence of two magnetic phases with high and low coercivity, respectively, which might be related to nanoparticles of oxidized and/or cation-substituted magnetite and epsilon iron (III) oxide. The fired clay (bricks) thus can be viewed as a model system in which epsilon iron (III) oxide forms readily.