Indigo is a vat dye that is one of the oldest classes of dyes known.   Until the late 19th century indigo was obtained from plants, the most significant being Indigofera tinctoria.  Experience, patience, and a sensitivity for subtle color changes are necessary for successful dyeing.  Obtain the darkest blues through a properly reduced vat and repeated dyeing to build color depth.  Indigo requires a reducing agent, Thiox, and an alkali Lye.  When the fabric is removed from the yellow-green dye vat and begins to oxidize, the subtle changes from yellow-green through blue are captivating.