As pig iron comes from the blast furnace it still possesses certain impurities, such as carbon and silicon. The mission of the Bessemer converter is to get rid of these. It is a huge egg-shaped steel container, lined with fire-brick, and mounted on trunnions so that it may be tilted and its liquid contents poured out. By means of a blower a tremendous supply of oxygen is driven through the molten metal, taking off with it all the carbon as it goes. After the blower has been turned off, a scientifically measured quantity of carbon is added to give the contents the characteristics of carbon steel.