The purpose of this operation is to remove, so far as can be done by the process, the silicon, manganese, carbon, phosphorus and sulphur in the charge. The removal of sulphur is difficult and uncertain, phosphorus is removed only in the basic process, and the remaining ingredients are usually reduced below the quantities desired in the finished steel and are re-introduced at the end of the process.

The doors are tightly closed after charging and the heat is regulated to melt the whole charge gradually, requiring from 2 to 4 hours. The furnace temperature begins at once to rise, and the limestone (CaCo3) begins to decompose, forming CaO and Co2. The increase of heat soon causes the silicon, manganese and carbon to begin to oxidize. The oxygen for this purpose is supplied mainly from the iron ore in the charge, to a less extent from the Co2 of the lime, and to a small extent from the air entering through the regenerators. As the charge becomes more and more fluid, the iron and scrap become mixed, distributing their impurities evenly throughout their combined mass. The lime (CaO) and iron oxide float to the surface of the molten iron and become fused, mixing with the slag which has begun to form from the oxidized silicon and manganese and from the earthy matter of the charge. The slag spreads out evenly over the bath of iron, protecting it from the oxidizing action of the flame.

Unlike puddling, no stirring or rabbling is done, though the bottom of the basin is raked over by a long iron bar inserted through the small openings in the charging doors to loosen any part of the charge which may have stuck to the hearth.

From the time the metal is thoroughly melted, samples are occasionally dipped from the bath by means of a small ladle with a long handle. These samples are cast in a small iron mould kept my the melter, and when cold, are taken from the mould and broken. The melter judges instantly by inspection of the fracture the amount of carbon and phosphorus contained, and regulates the process accordingly. Another practice, more reliable, is to take the sample, after it has been cast and cooled, to the laboratory nearby and determine the quantity of these elements by exact chemical methods, requiring 15 or 20 minutes.

As the silicon and manganese decrease in the metal, the oxidation of carbon increases, causing the charge to 'boil" as in wrought-iron making, due to the formation and escape of CO.

The melter watches the progress of the operation through peep holes in the furnace doors, protecting his eyes with dark-colored glasses. His experience enables him to regulate the furnace temperature to suit requirements. He must so reduce the temperature that carbon will be burned out last. If the carbon is burning too fast, it is necessary to "pig up" the charge by adding solid pig to increase the carbon and chill the bath. If phosphorus (the last element to be attacked) is going too fast, as compared with the carbon, as shown by the sampling, the consumption of carbon may be hastened by "oreing down," that is by adding iron to supply oxygen to consume the carbon. It is essential that an excess of iron oxide should not be added, particularly toward the end of the process, as an undue amount of iron oxide cannot be carried by the slag, and, at the end of the process, distributes itself throughout the steel, greatly impairing its quality.

Toward the last of the process when the heat is still intense, and the bath is comparatively quiet from the cessation of other chemical action, the phosphorus is removed by becoming oxidized and at once combining with lime to form a stable compound. This compound, phosphate of lime, enters the slag. The burning out of the carbon continues at varying rates throughout the entire operation, and the last of it is not burned out until after the removal of the phosphorus.

When the carbon has been burned out, the purified metal has a higher melting point than before, and would "come to nature." or collect in plastic masses as in wrought-iron making were it not for the high heat of the furnace to keep it thoroughly fluid.

In making high-carbon steel it is the practice to stop the process when the carbon has burned out to just below the per cent desired in the steel, and the small quantity needed is introduced by re-carburizing as in the Bessemer process.

The elimination of sulphur is very irregular. It is safest to use iron for the charge which has a sulphur content below that allowable in the steel, but this is not always practicable. Some sulphur will unite with lime and enter the slag, if very fluid, a condition assisted by throwing into the furnace a quantity of fluor-spar. Manganese ore added to the charge causes the formation of manganese sulphide, which also enters the slag.

The melter judges by the appearances in the furnace and particularly by the sampling, when the heat is finished.