An early method used for the manufacture of steel involving the heating together of scrap, pig iron, and hot metal in a refractory-lined shallow open furnace heated by the burning of producer gas in air. This causes excess carbon and other impurities to be burnt out. The method was developed to overcome the very high temperatures that are required. Developed in the nineteenth century, it was gradually replaced during the twentieth century with basic oxygen and electric arc furnaces due to its comparatively slow operation.