Fe2O3 or another Fe oxide is converted with 10-20% by weight of powdered Al and the hard, brittle product pulverized. If the materials are present in stoichiometric amounts a fused mixture of Al2O3 and metallic Fe is obtained. If Al is in excess, a homogeneous mixture of Al, Al2O3 and alloys is formed; with less Al a mixture of Al2O3, Fe, and Fe oxide (magnetite) is obtained. Activating substances such as K compounds, Mn, and Cu can be added. In similar way as the oxides of Fe, the oxides of Cr, Co, Th, Ni, and Cu can be treated; instead of Al pulverized amorphous Si is suitable. Example: 40 parts Fe2O3 and 6 parts, or about 13%, of Al powder are mixed in an earthenware vessel, and the mass is ignited with an acetylene burner. The vigorous exothermic reaction is quickly ended, and a hard, porous, strongly magnetic mass, with the appearance of lava, is obtained.