The production of indigo dye
The production of indigo dye from the green indigo leaves is a long and demanding process. Indigo does not appear in plants as a directly usable pigment or dye, it is an extract obtained by fermentation. The chemical precursor to indigo in plants is indican, a colourless glycoside that is concentrated primarily in the vacuoles of the leaves. When leaves are broken, hydrolysis splits indican into two parts: glucose and indoxyl. The exposure of indoxyl to atmospheric oxygen under alkaline, anaerobic conditions produces the blue pigment indigotin, essential for the production of indigo dye.
The ideal production system for indigo dye would be a closed circulatory system, moving from soil to dye to textiles and back to soil. Our process consists of four basic steps: indigo growing, indigo harvesting, indigo pigment extraction, and indigo dyeing.
At BioSain in the Waldviertel, a northern region of Austria, we grow four different indigo plants that have historically been grown in different parts of the world. These are Persicaria tinctoria from Asia, Indigofera arrecta from West Africa, Indigofera suffructicosa from Central America and Isatis tinctoria from Europe. While woad (Isatis tinctoria) can be planted directly in the field, the other indigo species are frost sensitive and their seedlings must be grown in the greenhouse and not transplanted outdoors until the last threat of frost.
In temperate climates like Austria, Persicaria tinctoria can be harvested twice a year, once in early to mid-July and once around 5 weeks later, in mid-to-late August. A third harvest may be possible in autumn, but as autumn in Austria can quickly set in, along with colder temperatures, the indigo concentration may be very low. It is therefore more advisable to leave the plants to go to seed for next year’s crop, as the cycle starts again. Seeds may mature and can be ready for harvest by late October or early November.
INDIGO PIGMENT EXTRACTION
The process of extracting indigo pigment involves first soaking the leaves and stems in water and allowing it to ferment for up to 24 hours, depending on the outside temperature. The plant residue is removed and used as fertiliser and the remaining liquid is then mixed with oxygen. Both processes, fermentation and oxidation, require close supervision by experienced hands, carefully controlling the surface of the water during the fermentation process as well as the amount of air being mixed into the fermented liquid. At the end of the process, the pigment settles at the bottom of the vat as a blue paste. The paste is finally drained and filtered, and then dried and grinded for easy storage.
Dyeing with indigo is a time-consuming process, because unlike most dyes, indigotin is not soluble in water and requires a vat dyeing process in which chemical reactions occur. To be dissolved it first needs a reduction in an alkaline, oxygen-reduced solution to temporarily convert the insoluble indigo pigment into a water-soluble form. The liquid in the vat changes its colour, becoming yellow. At this stage the indigo is referred to as indoxyl, also called leuco-indigo or “white indigo”. After that the submerged textile material needs an oxidation by exposure to air to let the white indigo revert back to the insoluble original state. The indigo molecules expand again and stick to the fibres thus providing its long-lasting blue. When textiles are removed from the indigo vat and absorb oxygen from the air, they change from yellow to green to blue. With every dip in the indigo vat, the colour can be built up from what Aboubakar Fofana calls a light “Blue of Nothingness” to a very deep, dark blue.