In 1929, 73 million tons of iron ore was transported on the Lakes, and an almost equal amount of coal, limestone, and other products were also moved.[14] Two defining characteristics of bulk carriers were already emerging: the double bottom, which was adopted in 1890,[5] and the triangular structure of the ballast tanks, which was introduced in 1905.[8] After World War II, an international bulk trade began to develop among industrialized nations, particularly between the European countries, the United States and Japan.[11] Due to the economics of this trade, ocean bulk carriers became larger and more specialized.[12][15] In this period, Great Lakes freighters increased in size, to maximize economies of scale, and self-unloaders became more common to cut turnaround time.