In 1864, Carnegie was one of the early investors in the Columbia Oil Company in Venango County, Pennsylvania. In one year, the farm yielded over $1,000,000 in cash dividends, and petroleum from oil wells on the property sold profitably. The demand for iron products, such as armor for gunboats, cannons, and shells, as well as a hundred other industrial products, made Pittsburgh a center of wartime production. Carnegie worked with others in establishing a steel rolling mill, and steel production and control of industry became the source of his fortune. Carnegie had some investments in the iron industry before the war.