A process of producing magnesium-treated iron such as spheroidal graphite iron (SGI), compacted graphite cast iron (CGI) containing inclusions that deform plastically during machining, said process comprising the steps of: a) producing base iron; b) desulfurizating the base iron produced in step a) with a magnesium free reagent, if it is sulfur concentration exceeds 0.02 % (wt.); c) controlling the oxygen potential and temperature of the base iron to facilitate silicon control of oxygen, if the amount of oxygen exceeds 10 ppm; d) adding aluminum, calcium and/or calcium-containing oxides to the base iron in amounts designed to form dicalcium aluminate deoxidation product or low melting point calcium aluminum silicate deoxidation product; e) treating the base iron with magnesium containing inoculant to attain desired condition for desired nodularity; and f) continuing the process of producing magnesium-treated iron in a per se known manner.