Purple-Red Dyeing of Silk by Leaves of Indigo Plant

Abstract

Purple-red dyeing is successfully carried out from leaves of Ryukyu-ai (Stobilanthes Cusia O. Kuntze). Two methods were established for the formation of the source dye indirubin of the color. Indirubin was chemically synthesized by the treatment of the dry and smashed leaves in an acidic solution of isatin and hydrosulfite as reducing reagent and finally addition of calcium hydroxide. In another method, indirubin was prepared by keeping the leaves of Ryukyu-ai at -20 deg in a freezer for 2 weeks and extraction in the boiling water. The reactions in the frozen leaves are discussed. A piece of silk cloth was dipped in the solutions and vat-dyed into purple-red color with the oxidation process.