The blowing of air through the molten pig iron introduces oxygen into the melt which results in oxidation, removing 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.