The capacity of the average open-hearth furnace is about 60 tons of metal.

Supposing the furnace to be at a moderate heat, ready for the charge, the tapping hole, which leads from the lowest part of the basin through the far side of the furnace, is stopped by ramming into it a quantity of magnesite from the outside.

The charge consists of pig iron, limestone, usually iron oxide, and steel scrap if any of this is available. A quantity of limestone, determined by experience, is first thrown in through the charging door. The pig iron is then brought molten in a large ladle from the mixer or directly from the blast furnace, and is transferred from the ladle to the furnace hearth by means of a portable refractory-lined trough. Sometimes solid pigs may be thrown in at the charging door, but this is not the best practice, as there is considerable saving of fuel and handling by using molten pig. A quantity of steel scrap is next thrown in if available, but scrap must not be used unless its composition is known to be suitable to the grade of steel to be made. Steel scrap of the proper composition is highly desirable, as its impurities have been greatly reduced in its manufacture. A small quantity of iron ore, low in sulphur and phosphorus, is added to the charge.