A study on the influence of the type of magnesium oxide on the structure of the ceramic coating and consequently on the magnetic properties of a 3% Si steel was carried out. Decarburized samples coming from industrial equipment were coated with magnesia slurries and submitted to final annealing at 1200degreesC for 15 hours under H-2 atmosphere. We used magnesia synthesized from magnesium chloride and dolomite. The magnesia samples were characterized using granulometry measurements, scanning electron microscopy (SEM), nitrogen adsorption-desorption analyses, helium pycnometry, thermal analyses (TG and DTA), reactivity tests and calcination mass loss. The magnesia sintered from brine has the highest density, surface area and porosity. The rheological behavior of magnesia slurries, the magnetic properties of the steel and the ceramic film structure were correlated with the type of magnesia used. The slurry made with MgO from dolomite has the highest viscosity and the slurry made with oxide from brine the smallest one. Brine magnesia films are darker, thicker and rougher and sea water magnesia films are light gray, thinner and smoother.