The Ming-dynasty scientist Song Yingxing wrote the following description in Tian Gong Kai Wu (1637), his encyclopedic description of the technologies of his time: "To make indigo dye... they steep the leaves and stems in water for seven days, to naturally produce a liquor.
宋應星《天工開物》記載：「凡造澱，葉與莖...... 水浸七日，其汁自來。