Indigo is probably the most widely used dyestuff of all time - indeed denim jean material is still coloured using indigo dye as it is extremely wash fast. It was used many centuries before the Christian era in the Far East. Marco Polo saw indigo being prepared in China during the 13th century; at that time European dyers were obtaining blue colours from woad (Isatis tinctoria) which contains the same indigo molecule. Traditional methods of indigo dyeing can be observed today in Africa, Mexico, India and Japan; indigo vats can be found in almost any country in the developing world, as dyers have adapted age-old techniques to their local situations.
Indigo dye is used extensively for batik and shibori.
Indigo is obtained from plants which contain the indican molecule. These belong to different plant families; the most common are of the genus Indigofera, but they also belong to others including the buckwheat family, the Lonchocarpus cyanescens found in Africa, or the Indonesian Marsdenia, and of course Woad. Although the indican molecule is contained in all these plants, producing the traditional blues requires more than merely steeping fibres with the plants in a pot full of water.
Synthetic indigo, prepared in the laboratory, contains the identical molecular structure to the natural indigo, but it has a much higher percentage of indigo per weight than the natural form. It is necessary to use only 1/4 to 1/3 as much synthetic indigo as natural. The powdered form is easiest, as the lump indigo must be ground or pounded to reduce it for use.
Although the indigo powder is blue, the indigo molecule does not produce its blue colour until it is oxidised. The indigo blue powder must be dissolved in an alkali bath with the combined oxygen removed. This is done by adding spectralite (thiourea dioxide) to the indigo vat during its preparation. Indigo is only soluble in an alkaline solution made by dissolving sodium carbonate (as Soda Ash or Washing Soda) or caustic soda in water. The resultant solution is a yellow-green in colour.
The alkalinity of the solution is controlled by the amount of Soda Ash dissolved. This can be tested with the Universal Indicator Paper. The pH needs to be between 9-11. At pH 11 it is easiest to reduce the indigo, but pH 9 is gentler on silk and wool. For your first indigo bath it helps to work at the higher range but not above pH11 and wash the dyed fabric in vinegar as the final rinse.
The Dyeing Process
The indigo dyeing process involves making the soluble, yellow-green indigo in the bath turn blue while still attached to the fibre. This is accomplished by immersing the wetted (but not dripping) fibre, yarn or fabric into the indigo bath and leaving it under the surface initially for a period of 5-10 minutes. It is then removed very carefully and slowly to prevent dripping, splashing or the introduction of oxygen into the bath. It turns from yellow-green to blue as it takes in oxygen from the air.
Indigo builds colour, so the more immersion-oxidation cycles, the deeper the colour. This must be done with care to prevent introduction of oxygen to the dye bath. Subsequent immersion times must be kept to a minimum to ensure that the newly attached molecules of indigo are not stripped off by the solution.
Indigo dyed materials are also very wash-fast, since the indigo blue is not water soluble, except in alkali.
Before dyeing, it is important to check whether the item is 'Prepared for Dyeing' (PFD) or requires scouring to remove any grease, oil or starch. Run a few droplets of cold water onto the fabric. If they soak in quickly, no scour is necessary. To remove starches, size and oils, add 5mls of Synthrapol (a non-ionic detergent) along with 2-3 litres of water for each 100gms of material. Stir gently over a 15 min period, and then rinse thoroughly in warm water. It is possible to use household detergent, but the alkaline residue may affect the final colour or wash fastness.