Evidence of indigo dyeing in Japan appears to go as far back as the 10th century when indigo was expensive. By the 18th century, all types of people wore indigo products and it was thought of as a good deterrent for bugs and snakes!
Japanese indigo comes from the Polygonum plant. The leaves are gathered and left to ferment for three months. Every 3-4 days within that time, water is added to the leaves, mixed in and then covered with a straw mat. At the end of the three months, the leaves become a soil consistency and now contain the indigo colourant, but this substance cannot be used directly as it's not soluble in water. The soil substance is sent to dyers and they then place 7kg of indigo in a vessel with 1kg of bran and ash from wood and place it under ground. The dyers apparently check the alkalinity every few days, using their tongue! After several days the indigo is reduced and this is indicated by bubbles on the surface of the vat. The bubbles are formed by reoxidization of the reduced indigo. The dye is then finally ready to use!