For over three centuries during the Song Dynasty (960–1279) China experienced sustained growth in per capita income and population, structural change in the economy, and increased pace of technological innovation. Movable print, improved seeds for rice and other commercial crops, gunpowder, water-powered mechanical clocks, the use of coal as a source of fuel for a variety of industries, improved techniques for iron and steel production, pound locks and many other technological innovations transformed the economy. In north China, the main fuel source for ceramic kilns and iron furnaces shifted from wood to coal.