Pig iron of a certain range of composition must be selected for this process. It must not contain more phosphorus nor sulphur than is allowable in the steel, as these elements are not burned out in the acid Bessemer process. Silicon is desirable to increase the heat in the charge when it burns in the converter, and its is also the chief slag producer, although too much silicon would unduly prolong the blow while burning it out and this prolonged oxidation would consume much iron.

The converter charge is considerably increased in temperature by the oxidation from the blast and it may become at times so hot that the chemical reactions are upset, and much iron is consumed. To reduce the temperature, either a quantity of steel scrap of correct composition is thrown into the converter during the blow, or steam is forced through with the air, absorbing heat as it is decomposed in the charge.

Bessemer steel is not regarded as the equal of open-hearth or crucible steels in purity, as the removal of impurities is under better control in the two last-named processes. The Bessemer product is used for railroad rails, structural steel for buildings, steel castings, forgings, and other purposes where the strength of the steel is not put to its supreme test.