Commercial magnesium oxide powders have been examined before and after electrofusion and subsequent stages of the manufacturing process to evaluate the effect of these stages on the impurity content. Particular emphasis has been paid to the identification of magnetic phases as some of these have been found to have deleterious effects on the insulating properties of the powder which is widely used, as "electrical grade magnesia" as the refractory dielectric filler in heating elements X-ray diffractometry was used for structural phase analysis supplemented by energy dispersive analysis by X-rays to aid interpretation of the more complex X-ray diffraction results. Iron, magnesioferrite, manganese oxide, aluminium silicate, Iron II oxide (and many other impurities at less than 2% abundance) were detected. Reference standards were also obtained from powdered doped MgO single crystals of known composition. Measurements of magnetic susceptibility, made with a Gouy magnetometer, showed that during the fusion stage of the commercial process, the magnetic impurities diffused towards the centre of the melt, accumulating in a distinct band bounded on either side by less magnetic material, K ~ + 450 J T(^-2) m(^-3) for the former, K ~ + 50 J T(^-3) m(^-3) for the latter compared with -20 J T (^-2) m (^-3) for pure MgO). Control experiments were made on heat treated Fe/MgO, obtained from ground single crystals, to establish details of the formation and properties of the magnesio-ferrite phase precipitated in the MgO matrix. These studies also established that in the commercial powders the annealing type heat treatment increased the susceptibility of the resultant material whilst the quenchlng-type heat treatment and especially the magnetic separation stages reduced it
