Precipitated indigo is something we have made many times. It is a method of settling and storing indigo pigment and is also used to make indigo paste and dried indigo cakes.
At Hitotsuya（ひとつ屋）, we can now make it with reasonable consistency, but we still change the conditions and continue testing in search of a deeper and more stable blue.
This article introduces the basic method we repeatedly use with Japanese indigo, from harvesting through to storage. Temperature, water temperature, plant growth and quantity all affect the result, so the times and amounts given here are working guides rather than fixed answers.
What is precipitated indigo?
Fresh indigo leaves are soaked in water to release the compounds from which the blue pigment develops. Slaked lime is then added to make the liquid alkaline, and air is incorporated through agitation so that the blue pigment becomes insoluble and settles.
The method is widely associated with tropical indigo plants, but it can also be used with Japanese indigo.
The leaves do not contain ready-made blue pigment. They contain indican, a colourless precursor. As the leaf tissue breaks down in water, enzymes and related processes produce indoxyl, which is oxidised by oxygen in the air to form pigment consisting mainly of indigo.
In this article, we use the customary indigo-making term “fermentation” for the soaking stage.
A Japanese-language conference paper from the Japan Society of Home Economics, Study of precipitated indigo formation from Japanese indigo and its dyeing conditions, also examines soaking time, lime addition, pH and agitation conditions.
In that study, soaking for 48, 72 and 96 hours at about 30°C was compared. The yield increased at 72 hours compared with 48 hours, while 96 hours produced little further increase and made the work more difficult because the leaves had decayed further. These findings apply to the materials and conditions used in that trial and are not universal.
▼ Watch the precipitated indigo process
The complete sequence is also introduced in the Hitotsuya YouTube video How to make precipitated indigo.
1. Harvest the Japanese indigo
The material is Japanese indigo grown at the Hitotsuya textile farm. We have mainly used the first harvest, taken around early July after the traditional Hangesho period, while the plants are leafy and before flowering.
For sowing, raising seedlings and harvesting, see our Japanese-language article How to grow Japanese indigo.
Whole stems and leaves can be soaked together, but at Hitotsuya we remove the leaves and use them on their own. This takes more work, but concentrates the process on the part containing the compounds needed for blue pigment.
2. Soak and ferment the leaves
Place the leaves loosely in a large lidded bucket. Add enough water to cover them, use a drop lid or similar weight to keep them below the surface, and leave them outdoors for two to three days. The lid should keep out debris and insects but must not seal the vessel completely.
The required time varies greatly with air temperature, water temperature, leaf quantity and leaf condition. Under midsummer sun, the condition shown below was reached after about 48 hours.
▼ Indigo leaves after about 48 hours in summer heat
After about 60 hours, the leaves had turned brown, bubbles and a purplish film had appeared on the surface, and the liquid had become slightly viscous. At Hitotsuya, this is our practical sign that fermentation is complete.
Longer soaking is not always better. If decay progresses too far, the yield may not increase substantially, while the leaves break down and become harder to filter.
At this stage, the characteristic indigo smell is joined by a strong decomposing odour. Work outdoors or in a place with ample ventilation.
▼ Fermentation liquid after about 60 hours
3. Filter the leaves and add slaked lime
Filter the leaves carefully from the liquid. Plant fragments left behind can remain in the finished paste, so use a mesh, cloth or similar filter.
At Hitotsuya, one trial used about 30 g of slaked lime for 20 L of fermentation liquid. This is only one example. The required amount varies with the quantity of indigo, liquid condition, water quality and lime condition, so adjust while checking the well-agitated liquid. Our practical target is around pH 10–11.
Slaked lime is only slightly soluble in water. Mix about 30 g with about 300 mL of water in a separate vessel to make a white suspension, or lime milk, rather than expecting a clear solution. Add it gradually while agitating the fermentation liquid and observe the foam, liquid colour and pH.
The study cited above reported a larger yield under a condition in which the liquid was adjusted to pH 11.6 and the lime water was added at once. At Hitotsuya, we add it while observing the liquid to avoid excess. Neither value nor method should be treated as a fixed answer when materials and equipment differ.
Too much slaked lime can leave more white mineral matter in the precipitate and make the finished paste look pale. Slaked lime also absorbs carbon dioxide from the air and gradually changes, so store it sealed and dry and check its condition if the pH no longer rises as expected.
※ Handling slaked lime
Check the product label and safety data sheet, and use gloves, safety glasses and a suitable mask. Avoid breathing the dust or allowing it to contact the eyes or skin. See the Japanese calcium hydroxide safety data sheet for details.
▼ Add slaked lime mixed with water a little at a time
4. Agitate while incorporating air
After adding the lime, agitate vigorously so that plenty of air enters the liquid. Rather than stirring gently, lift and drop the liquid and expose it to air while creating foam.
The foam is pale at first and gradually turns blue. At the same time, the strong smell becomes weaker. We have repeatedly observed this change during our own work.
▼ White foam turns blue during agitation
With further agitation, the blue foam becomes larger and eventually disappears quickly. The necessary amount of agitation varies with liquid volume, vessel shape and working method. Judge by the colour, size and persistence of the foam and by changes in the liquid rather than by a fixed count.
▼ Finish when the blue foam grows and then disappears quickly
5. Let the blue pigment settle
Cover the vessel to keep out dust and leave it undisturbed for about two days. The blue pigment, mainly indigo, settles at the bottom and separates from the water above.
Remove the clear upper liquid without disturbing the sediment and transfer the material to a dedicated container if needed. Do not use a container that could be mistaken for drinking water, and label it with the contents, date and materials used.
In this article, the water-rich blue material obtained by this settling method is called precipitated indigo.
※ Disposing of the upper liquid
The upper liquid may still be alkaline. Check its pH and follow the product safety information and local disposal rules.
▼ Blue pigment settled at the bottom of the container
The difference between precipitated indigo, indigo paste and dried indigo cakes
The meanings and moisture levels associated with these names vary by region, period and maker. At Hitotsuya, we use them as follows to distinguish the condition in which the material is stored.
- Precipitated indigo: blue pigment obtained by settling, with a large amount of water still present
- Indigo paste: material from which more water has been removed until it becomes muddy or paste-like
- Dried indigo cake: indigo paste dried further into a solid or dry state
These are working definitions used at Hitotsuya. Other regions and makers may use the same terms differently.
▼ Indigo paste after reducing the water content
▼ A dried indigo cake after further drying
Breaking the dried cake into smaller particles with a mortar makes it easier to measure and disperse in water or another medium. The pigment does not dissolve in water but remains as fine particles, so use a mask and safety glasses when crushing it.
The indigo in precipitated indigo, paste and dried cakes is not water-soluble in its normal state. To dye fibres in the usual way, the indigo must be reduced in an alkaline vat so that it becomes soluble.
A practical example is introduced in our Japanese-language article Dyeing with indigo paste, precipitated indigo and dried indigo cakes in a hydrosulfite vat.
For paint or pigment dyeing, a suitable binder and fixation method are required for the intended use.
We also store the material as a paste in dedicated labelled containers away from direct sunlight and high temperatures. During storage, check the smell, mould, separation and pH, and do not use material that shows an abnormal change.
Adjust the process by observing the indigo
This is the basic method used at Hitotsuya for making precipitated indigo, indigo paste and dried cakes from Japanese indigo.
Even with the same quantities, results vary with harvest season, leaf condition, air and water temperature, vessel, water quality, soaking time, lime amount, pH and agitation method.
Rather than following numbers alone, observe the leaf colour, viscosity, smell, foam, pH and sediment and find the conditions suited to that year’s indigo. We will continue our records and experiments in search of a deeper and easier-to-use material.
Other Japanese-language dyeing methods are collected in Natural dyeing how-to videos and recipes. Those wishing to learn in person can also see the Hitotsuya natural dyeing classes.