Previously, we made precipitated indigo from Japanese indigo that we had grown ourselves.
The full process is introduced in our Japanese-language article How to make precipitated indigo, indigo paste and dried indigo cakes.
This time, we use a dried indigo cake made by completely drying the precipitated indigo and dye cloth in a hydrosulfite vat prepared with caustic soda and sodium hydrosulfite.
Making precipitated indigo from home-grown Japanese indigo
The dye material came from Japanese indigo grown in the dye garden at Hitotsuya（ひとつ屋）.
The sequence from sowing and raising seedlings to planting and harvesting is introduced in our Japanese-language article How to grow Japanese indigo: sowing, seedlings, planting and harvesting.
▼ Japanese indigo grown in the Hitotsuya dye garden
The harvested leaves were steeped in water to release the compounds from which the blue pigment develops.
Lime was then added, and the liquid was thoroughly agitated so that the blue pigment would form and settle.
At Hitotsuya, the water-rich material is called precipitated indigo. After more water is removed, the thick paste is called indigo paste, and after complete drying it becomes a dried indigo cake.
▼ Making precipitated indigo from harvested Japanese indigo
▼ The next stage in making precipitated indigo
The detailed method is available in our Japanese-language article How to make precipitated indigo, indigo paste and dried indigo cakes.
The work can also be seen in our YouTube video.
Drying precipitated indigo into a solid cake
The finished precipitated indigo is drained and dried completely to make a solid indigo cake.
Removing the water makes the amount of indigo easier to weigh accurately without variations caused by moisture content.
▼ A dried indigo cake made by completely drying precipitated indigo
Preparing a hydrosulfite vat with a dried indigo cake
For this trial, we used 10 g of dried indigo cake.
About 100 mL of water was added to 10 g of the indigo cake, and the mixture was stirred thoroughly until no solid pieces remained.
We then added 7 g of caustic soda and 10 g of sodium hydrosulfite, followed by about 1,000 mL of water, and mixed until the chemicals had dissolved. The liquid can be warmed gently if they are slow to dissolve.
The prepared concentrate was added to about 5,000–6,000 mL of lukewarm water.
The cloth was first wetted with water and gently squeezed, then placed in the vat for about five to six minutes.
※ Handling caustic soda and sodium hydrosulfite
Check the product instructions and safety data sheets, wear protective gloves and safety glasses, and work in a well-ventilated place.
These quantities are one example based on the indigo cake and dyeing conditions used when this article was written. Results vary with pigment concentration, material and bath volume.
▼ Dyeing cloth in a vat made with indigo cake, caustic soda and sodium hydrosulfite
Exposing the cloth to air to develop the blue colour
After the cloth is removed from the vat, it is opened out and exposed fully to oxygen in the air.
A cloth that does not yet appear blue in the vat gradually develops an indigo-blue colour as it oxidises.
After oxidation, rinse it thoroughly until the water is nearly clear, arrange its shape and allow it to dry.
▼ The finished cloth after oxidation and thorough rinsing
Achieving a deeper indigo colour
This method uses reducing agents such as sodium hydrosulfite and caustic soda and is generally known as a hydrosulfite vat or chemical reduction vat.
To obtain a deeper blue or navy colour than the example shown, the pigment concentration during indigo preparation, the amount of indigo cake, the condition of the vat, and the number of dyeing and oxidation cycles can be adjusted.
Rather than trying to reach a dark shade in a single immersion, colour can be built gradually by repeating the dyeing and oxidation stages.
Hitotsuya is also continuing to study a method that does not use chemical reducing agents but is easier to manage than a full traditional wood-ash lye fermentation vat.
From growing the plants to making the dye and finally colouring cloth, we record each stage while searching for a more approachable way to work with indigo.