Ai-gusa, Indigo Plants
There are a lot of indigo plants’ varieties, and these have been used as materials of blue colored dye in all over the world since ancient times.
In Japan, some kinds of indigo plants such as Ryukyu indigo, originally coming from Assam area in India passing through Un-Nan area in China, or Tade (water pepper) Ai, coming from South-East Asia also via China are used.
For extracting the blue pigments and preserving well, these indigo plants are fermented and processed to Sukumo or Chin-den ai (precipitated indigo), and then, they are used to make dye liquid.