Exploring plant dyes: why is "blue taken from indigo"?

Traditional Chinese blue dye plants include Polygonum tinctorium, Indigofera tinctoria, Isatis tinctoria, Strobilanthes cusia, and Brassica juncea. Polygonum tinctorium was cultivated in China as early as the Xia dynasty, and by the early Han dynasty, China had already established a specialized division to collect and weigh indigo plants before handing them over to dyers. In the Tang dynasty, the government established a "dyeing institute." Japanese Polygonum tinctorium was introduced from China during the Tang dynasty and was once called "Tang blue," later "Wu blue," and now "Awa blue." Blue dyeing has evolved from direct dyeing and boiling dyeing to fermentation and reduction dyeing. After soaking, the fabric turns yellow-green and gradually turns blue when exposed to oxygen. The invention of indigo manufacturing technology allowed for repeated dyeing to achieve deeper blue, which is the origin of "blue taken from indigo." When not dyeing, the dye vat must be stirred daily, which is called "nurturing the vat." In the eyes of dyers, "the beauty of indigo dyeing" is alive. As Fumi Shimura said, each indigo vat contains the life of blue. Indigo dyeing is usually combined with different techniques, such as tie-dyeing, resist dyeing, clamp dyeing, wax dyeing, and dip dyeing.