Indigo Pigment Processing

During October 2022, our team harvested and processed one of our beds of Japanese Indigo for pigment. The process was led by Isa Rodrigues and assisted by Griselda Pena Candelario and Pearl Wood, Pratt work study students. Below is a summary of the process.

Day 1: Indigo Harvest

The garden team harvested the indigo in early October - about 2.5 kg. Since this was our first time processing our indigo, we wanted to experiment with different methods. The leaves of half of the harvest were stripped and soaked in water, and the rest of the indigo (whole plant)  was soaked in water. We started with room temperature filtered water, and the soaking buckets were covered and left outside.

When harvesting, we were careful to not bruise or damage leaves during stripping.

Of the 2.5 kg of indigo what was harvested, 1.25 kg was stripped and leaves were soaked in a 5 gallon bucket; the other 1.25 kg was split into two bundles and soaked in 5 gallons each. We used filtered and tap water at 70F.

Day 2 - 5: Fermentation

The indigo was left to ferment for 5 days. Since temperatures in NYC were already cold, the soaking buckets were moved indoors to a sunny spot on day 3. By day 5, the leaves were still very green, but the solution had only a very light teal color. Extraction will be complete when the solution is a teal color (indoxyl is being extracted), small bubbles form at the surface and the vat has a fruity smell from the fermentation process.

Day 5 - 6: Alkalization + Aeration

Since the indigo fermentation was slow , after consulting with Liz Spencer (The Dogwood Dyer) we decided to try the hot water extraction method. The soaking solution with the indigo leaves was heated up to 50C for 2h. After 2h the solution showed a mermaid teal color and it was removed from the heat source. The leaves were removed, and calcium hydroxide was added  to the solution, starting with 2 TBS at the time, until the pH reached 10. Then, the solution was poured from a bucket into another repeatedly for about 10 minutes (about 50 times) to  incorporate oxygen and encourage the indoxyl to transform into indigotin. The solution started to look dark and and murky from the pigment precipitation. 

The soaking solution of the indigo stems was discarded, the extraction was restarted with hot water (50C) and left to soak for 24h. After removing the indigo plants, the same alkalization/aeration process was repeated with the extract solution from the indigo whole pant.

Day 7: Straining 

Both extract solutions were let to rest for 24h, until the indigotin pigment is settled in the bottom. After that, solution was strained through a coffee filter, to obtain the indigo pigment. Line a fine mesh strainer with dense silk or cotton fabric, and place it securely on top of a bowl/jar. Then, pour the solution into the strainer - this will take a few hours.

At the end, pour some rain water or distilled water to rinse the pigment. Let it completely dry.