In the final step of the process, the insoluble indigotine precipitates, settles and needs to be filtered and finally dried to produce indigo powder. The liquid itself shows a red-brown layer of liquid at the top with a settled layer of indigo pigment at the bottom of the vat, and the foam has dissolved.
To separate indigotine from the rest of the extraction liquid, we undertook a series of filtration steps for concentration the pigment to a slurry. We filtered the liquid to which lime was added with a fine cloth (silk or fine cotton). For the pure pigment, we also carried out tests on paper filters, which are also used for filtering wines, beers, juices, etc. After several days of draining, the filtered indigo paste was ready for final drying.
