Japanese indigo grows vigorously under the strong summer sun. Once the plants are ready to harvest, the leaves can be processed using a precipitation method to extract the indigo pigment and make what is known in Japan as chinden-ai, or precipitated indigo. When the pigment is stored as a thick, wet paste, it is also called doro-ai, meaning “mud indigo.”
Japanese indigo, or Persicaria tinctoria, is commonly used for fresh-leaf dyeing while the leaves are still green and newly harvested. Drying or freezing changes the condition of the compounds and enzymes inside the leaves, so the same fresh-leaf dyeing method may no longer produce the same result.
This does not mean that dried or frozen indigo leaves can no longer be used. Over time, people have developed several ways of preserving and processing indigo so that it can be used beyond the short summer growing season.
Two important forms of processed indigo in Japan are sukumo and precipitated indigo.
Sukumo is closely associated with Awa indigo from Tokushima Prefecture on the island of Shikoku. Dried Japanese indigo leaves are repeatedly moistened, turned and carefully composted over an extended period. The resulting fermented leaf material becomes the raw material used to prepare a traditional Japanese indigo vat.
Before sukumo can dye cloth, the insoluble indigo pigment must be converted into a water-soluble form through reduction. This process, known in Japanese as ai-date, or “building an indigo vat,” traditionally uses an alkaline liquid made with wood-ash lye and relies on the activity of microorganisms. Making sukumo and maintaining the vat both require time, experience and careful control.
In tropical and subtropical regions, indigo has also traditionally been prepared by soaking the stems and leaves of plants such as true indigo and Ryukyu indigo in water. Lime is then added to the extracted liquid, which is vigorously aerated so that the blue pigment forms and settles at the bottom.
The resulting material is generally described in English as precipitated indigo or indigo paste. The same method can also be used with Japanese indigo.
The leaves themselves do not contain blue indigo pigment in its finished form. Instead, they contain a colourless precursor called indican.
When the leaves are soaked in water, enzymes break down the indican and produce a compound called indoxyl. When the liquid is made alkaline and充分 air is introduced through vigorous agitation, the indoxyl oxidises and forms blue, water-insoluble indigo pigment.
When this pigment is allowed to settle and is collected as a thick paste, it becomes precipitated indigo. The process looked possible to carry out on a small scale, so I decided to try it myself.
Detailed Instructions for Making Precipitated Indigo and Indigo Paste
1. Place the harvested Japanese indigo leaves in a large container and add enough water to keep them completely submerged.
The leaves naturally float, so place a drop lid, board or suitable weight over them to hold them beneath the surface of the water.
Cover the container sufficiently to keep out insects and debris, but do not seal it completely airtight.
In Japanese indigo-making, this soaking stage is often referred to as “fermentation.” The term is used here in keeping with traditional workshop language, although several enzymatic and microbial processes may be occurring at the same time.
The required soaking time varies greatly according to the air temperature, water temperature, quantity of leaves and condition of the plants. During the height of summer, the leaves shown below reached this stage after approximately 48 hours.
▼ Japanese indigo leaves after approximately 48 hours of summer soaking
2. As the soaking continues, the condition of the leaves and liquid begins to change, and a strong, distinctive smell develops.
The leaves gradually turn brown. The liquid may become slightly viscous, and a purple or blue film may appear on the surface.
In this trial, the liquid developed an odour similar to decomposing plant matter. The exact smell, colour and timing will vary depending on the type of indigo, the temperature and the condition of the harvested plants.
If the leaves remain in the water for too long, decomposition may progress too far and affect both the amount and quality of the pigment recovered. The decision to proceed should therefore be based on the condition of the leaves and liquid rather than on time alone.
3. Remove the leaves and stems from the extraction liquid.
Strain the liquid through a sieve or cloth as necessary so that no large pieces of plant material remain.
4. Add hydrated lime to the extraction liquid and agitate it thoroughly so that plenty of air is incorporated.
Hydrated lime, also known as calcium hydroxide or slaked lime, should not be added all at once as a dry powder. First mix it with a small amount of water to make a smooth lime slurry, then add it gradually while observing the colour of the liquid and the condition of the foam.
The lime does not simply separate the pigment from the water. Its main role is to raise the pH of the extraction liquid. Together with oxygen introduced during agitation, this alkaline condition helps the indoxyl oxidise and form water-insoluble indigo pigment.
The appropriate amount of lime depends on the volume and condition of the extraction liquid. Excess lime may remain in the finished paste and can affect the concentration and quality of the indigo, so it should be added gradually rather than according to a fixed amount.
Safety precautions when handling hydrated lime
Hydrated lime is strongly alkaline. Wear protective gloves and safety glasses, avoid breathing the dust, and work in a well-ventilated area. Prevent the powder or liquid from coming into contact with the eyes or skin.
Do not substitute dolomitic lime, ordinary garden lime or other agricultural lime products, as their composition and behaviour differ from hydrated lime.
5. Continue aerating the liquid until the liquid and foam gradually turn blue.
Simply stirring the liquid gently is not enough. Lift and pour the liquid back into the container, beat it vigorously, or use another method that introduces plenty of air and creates foam.
As oxygen enters the liquid, indigo pigment begins to form. The colour of the liquid and foam changes from greenish or brownish tones to blue and eventually to a deeper indigo colour.
In this trial, the strong smell present before the lime was added gradually became less noticeable. However, a change in smell alone is not a reliable indication that oxidation is complete.
Continue observing the colour of the liquid and foam, as well as the size and behaviour of the bubbles, while充分 aeration proceeds.
6. Stop agitating the liquid and leave it undisturbed so that the indigo pigment can settle.
In this trial, the liquid was left to stand for approximately two days. The water-insoluble indigo pigment and other fine solids gradually sank to the bottom, separating into a clear or yellow-brown upper liquid and a blue sediment below.
The blue material at the bottom is precipitated indigo. Once the upper liquid has been carefully removed and the remaining material has been concentrated into a thick paste, it is commonly called indigo paste, or doro-ai in Japanese.
The indigo pigment in this paste is insoluble in water, so fabric cannot be dyed simply by placing it directly into the paste.
Before it can be used for conventional indigo dyeing, the pigment must be reduced to its water-soluble form in an alkaline indigo vat.
That is as far as the process went on this day. Will this home-made indigo become a usable dye? The next stage will reveal the answer.
▼ The complete process can also be seen in the YouTube video “How to Make Precipitated Indigo”. The narration is in Japanese, but the video shows the sequence of the work visually.