The main dye component of Indigofera Tinctoria (true indigo) is indigotin. There are other indigotin bearing plants and I will only mention the most common ones here. Persicaria tinctoria, Polygonum Tinctorium, Indigofera Suffruticosa, European Woad (Isatis Tinctoria) and Chinese Woad (Isatis Indigotica). Persicaria tinctoria and Polygonum Tinctorium are most often called Japanese Indigo and may be the most widely propagated indigo-bearing plants because they grow well in most environments and are easy to propagate from cuttings and seed.
European and Chinese Woad also grow very well in most environments, but they produce less indigotin. Chinese Woad produces a little more indigotin than European Woad and I have found it easier to extract than European Woad, Isatis Tinctoria.
Indigofera Tinctoria and Indigofera Suffruticosa grow well in warmer and tropical environments, and contain a good deal more indigotin than the other indigo bearing plants I mentioned. It is easy to understand that the historic European woad growing economy fell to the amazing amount of blue indigo than could be obtained from plants like Indigofera Tinctoria and Indigofera Suffruticosa brought in from India. The woad plant was no match at all.
Indigo can provide a whole range of blue shade from light blue to dark, almost black, and has excellent lightfastness and washfastness. Indigo is one of the most lightfast natural dyes, by whom others are measured.
Dyeing fibers blue with indigo is a completely different process than dyeing with other natural dyestuffs. There are many various methods that can be learned and studied to affix the indigotin to fiber. Dyeing with indigo is an amazing art.
Indigo powder, or indigo itself is not water soluble and cannot be attached to a fiber without it being changed from the oxidized, insoluble powder or form that it is in, to a reduced state that you see visually as a greenish yellow pigment, which is when it is soluble. In this soluble state the dyes can affix to fibers before being again oxidized by the oxygen in the air when they are brought out of the dye vat.
Indigo can be reduced by making a natural vat using fructose syrup or some type of sugar and hydrated lime for a 1-2-3 type of vat, or by using a chemical reducing agent such as sodium hydrosulphite or thiourea.
Indigo bearing plant leaves can be used fresh and kneaded with salt and your fiber to extract a light blue or greenish blue. This is an easy, no skill way to dye with indigo, but the results will probably not be as lightfast. Often you will hear of ice water and blender methods. I ‘think’ that using salt and kneading with your hands gave as good results.
Indigo can be easily overdyed with yellows or pinks to give a whole palette of green and purple shades. While indigo does not need a mordanted fiber, because it is a whole different process……if you want to overdye with a natural dye stuff that requires a mordant to make a chemical bond with the fiber, then you will need to mordant the fiber.
Indigo stayed the ‘queen of the dyes’ until the development of synthetic indigo at the end of the 19th century.