You can choose to reduce with AZ special (strong reducing agent) or with tree ash. We will explain the ways you reduce the Sukumo.
The former is easier and suitable for beginners. The latter is 100% natural and traditional but very difficult.
So, I recommend that for the first time please use AZ special and when you are familiar with the Sukumo, then please use Oaktree ash.
1. Using AZ special (for 6 liters of the liquid)
Please do this in a place with good ventilation. AZ Special is invented in 1994 by Aikuma as a one of the best reducing agents in the world. It can reduce any indigo without the use of other reducing agents and keep five times the reducing power of Hydrosulfite. So, this is very easy reducing agent for sukumo and involves the following three steps.
① Put 200 g of the sukumo indigo in 2 liters of water and let stand for about 30 minutes, then please discard the brown water with a filter cloth. After this process is repeated twice, add 2 liters of water again and leave it for one night. Discard the brown water with a filter cloth again and mix well to make a sludge-like mixture.
② Add 1.2 liters of hot water and 90 g of AZ Special to ① and mix well until the mixture becomes sludgy, allowing it to cool for about 1 hour, then add 4.8 liters of cold water, mix well, cover with plastic wrap, and let stand for a day.
③ During this time, the liquid should be stirred two times. After that, you can get the sukumo indigo liquid.
2. Using Oaktree Ash (for 10 liters of the liquid)
Indigo dyeing with a indigo liquid made from Sukumo and tree ash is called "発酵建て" (fermentation reducing) or "地獄建て" (hell reducing) in Japan. The whole process is a bit difficult as it takes about 10 days but all the ingredients in this process are natural and traditional. It involves the following four steps.
① Place 1 kg of Sukumo in a large bucket and add 500 g of oak tree ash into it.
Pour 10 liters of hot water (70-80°C) and stir the bucket with a stick for 5 min.
② The surface of the liquid foams up, but the color of the liquid is brown then.
③ Stir the liquid every day for a week. After that, the color of the liquid will become closer to blue.
④ If large bubbles form on the surface of the liquid, you can dye with it. The liquid can be used for up to six months.