(From Latin indicum, “Indian”). The active ingredient of deep-blue indigo dye and pigment is found in the leaves of the indiagofera tinctoria plant, which thrives in tropical climates. In addition to “true indigo” from indiagofera tinctoria, similar blue pigments can be extracted from other plants, including Japanese indigo (Polygonum tinctoria), Chinese indigo (Persicaria tinctoria), and Guatemalan indigo (Indigofera suffruticosa). Indigo was exported from India to Europe, where it was used in painting and as a textile dye from medieval times, but few instances of indigo used in painting are known after the early 18th century, when it was largely replaced by Prussian blue. Since 1870 indigo pigment has been manufactured synthetically. Web resources here and here. Video here.

Wang Youdun. Fan with Poem in Clerical Script. Qing Dynasty. First third of the 18th c. Harvard Art Museums, Arthur M. Sackler Museum, Cambridge, MA.