The smelting process always leaves in iron a small amount of this element. Up to 2% it increases tenacity and hardness, but beyond that amount it causes brittleness. It tends to eliminate sulphur and neutralize silicon, hence its effect within this range would act to render iron less hard by decreasing the hardening effects of these elements. Much manganese increases the soluble power of cast iron for carbon.

In foundry iron, manganese is added, if not already present, up to about 1.5% for its effect in making a hard, close-grained iron and in eliminating sulphur absorbed during re-melting for the purpose of casting.

In steel making, manganese is always added up to about .05% for improving the working qualities of the steel, particularly when hot.

The extensive use of manganese as an ingredient in steel making has made common two alloys of manganese and iron. One, known as spiegel-eisen, contains from 1.5 to about 20% of manganese, and presents a brilliant fracture. The other, known as ferro-man-ganese, contains from 20 up to 86%, has a light gray fracture and is so brittle that it may be readily pulverized in a mortar. Both of these products are obtained from the oxide of manganese, a mineral.