Carnegie made his fortune in the steel industry, controlling the most extensive integrated iron and steel operations ever owned by an individual in the United States. One of his two great innovations was in the cheap and efficient mass production of steel by adopting and adapting the Bessemer process, which allowed the high carbon content of pig iron to be burnt away in a controlled and rapid way during steel production. Steel prices dropped as a result, and Bessemer steel was rapidly adopted for rails; however, it was not suitable for buildings and bridges.