This and the Bessemer process convert pig iron into steel by first burning the impurities from the molten iron, but the equipment used in the open-hearth process differs considerably from that used in the Bessemer process.

The open-hearth process was developed along the lines of the puddling process for making wrought iron, and was made successful only after an improvement in the furnace used and after a modification of some of the steps in the puddling process. The two obstacles which had to be overcome in perfecting the open-hearth process were (1) supplying suitable fuel and burning it in such a way that higher temperature could be maintained than in the puddling furnace, and (2) building a furnace capable of resisting this high heat and serving the practical needs of the process. It was found that in burning out all the carbon and other impurities a much higher heat was necessary in order to maintain the purified iron in a molten state and not let it "come to nature" in a pasty condition, as in the puddling process.

To supply higher heat, the manufacture of producer gas was developed, and the principle of the regenerative stove, as used with the blast furnace, was applied to the steel furnace. A better furnace, to withstand the heat,' was obtained principally by a careful selection of pure refractory materials and by a skillful mixing and burning of these into high-grade refractory bricks.

The open-hearth process includes both the Siemens process, which ures iron ore to assist in decarbonizing the pig iron, and the Martin process which melts scrap steel with pig to dilute the impurities in the latter. The combination of these two is the Siemens-Martin process, but their individual distinctions are now lost sight of as all their combinations are included in the one designation of open hearth.

The composition of available iron ores from different deposits of the earth renders necessary the use of both the acid and the basic methods in open-hearth steel making, and the control now possible of the various steps of this process, particularly in the removal of impurities, renders the product thoroughly reliable as a mild steel for high grade uses.