About 24 hours later, the liquid took on a blue-green colour with some bubbles and a shiny purplish cast on the surface of the liquid. Recognizing the right time when fermentation is nearing completion requires not only close observation but also intuition and extensive experience, which Aboubakar shared with us within that project. If you stop fermentation too early, indican will be incompletely extracted and the indigo yields will be low. On the other hand, if the fermentation process goes too far, the yields drop rapidly.
After the fermentation is complete, the plant matter is removed, set aside and later used for compost. Before the oxidation process begins, the remaining indoxyl-laden liquid is alkalized (giving the liquid a basic pH and the pigment more volume). Since indigo can also be formed without alkalizing, we have chosen to add only 1% calcium hydroxide (lime) to one half of the liquid (300 litres) and not to alkalize the other half (300 litres) at all in order to obtain the purest pigment.
The oxidation process is achieved by exposing the liquid to the oxygen in the air. This can be done in many ways. In India, for example, large tanks are aerated by a group of workers standing in the water and bringing oxygen into the liquid with powerful and coordinated kicks of their legs. In this process, the insoluble indigotin precipitates, settles, and is later filtered and finally dried to produce indigo powder. An indicator if oxidation process is going well is that the extraction liquid and foam at the surface turn distinctly blue in colour.