The presence of certain minor elements also is vital to the successful production of each type of iron. For example, nucleating agents, called inoculants, are used in the production of gray iron to control the graphite type and size. Trace amounts of bismuth and tellurium are used in the production of malleable iron, and the presence of a few hundredths of a percent magnesium (Mg) causes the formation of the spheroidal graphite in ductile iron.