impurities found in the pig iron, such as silicon, manganese, and carbon in the form of oxides. These oxides either escape as gas or form a solid slag. The refractory lining of the converter also plays a role in the conversion — clay linings may be used when there is little phosphorus in the raw material, and Bessemer himself used ganister sandstone – this is known as the acid Bessemer process. When the phosphorus content is high, dolomite, or sometimes magnesite, linings are required in the basic Bessemer limestone process, see below. In order to produce steel with desired properties, additives such as spiegeleisen (a ferromanganese alloy), can be added to the molten steel once the impurities have been removed. Managing the process When the required steel had been formed, it was poured into ladles and then transferred into moulds while the lighter slag was left behind. The conversion process, called the "blow", was completed in approximately 20 minutes. During this period, the progress of the oxidation of the impurities was judged by the appearance of the flame issuing from the mouth of the converter. The modern use of photoelectric methods of recording the characteristics of the flame greatly aided the blower in controlling final product quality. After the blow, the liquid metal was recarburized to the desired point and other alloying materials were added, depending on the desired product. A Bessemer converter could treat a "heat" (batch of hot metal) of 5 to 30 tons at a time. They were usually operated in pairs, one being blown while another was being filled or tapped. Predecessor processes By the early 19th century the puddling process was widespread. Until technological advances made it possible to work at higher heats, slag impurities could not be removed entirely, but the reverberatory furnace made it possible to heat iron without placing it directly in the fire, offering some degree of protection from the impurity of the fuel source. Thus, with the advent of this technology, coal began to replace charcoal fuel. The Bessemer process allowed steel to be produced without fuel, using the impurities of the iron to create the necessary heat. This drastically reduced the costs of steel production, but raw materials with the required characteristics could be difficult to find. High-quality steel was made by the reverse process of adding carbon to carbon-free wrought iron, usually imported from Sweden. The manufacturing process,