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I have bought indigo powder to color my soaps blue (cold/hot process saponification). Unfortunately I received something which color is closer to match than blue jeans...
After some internet research I found that what I have is unfermented indigo powder, while the blue colorant is made with fermented indigo; very detailed information here.

My question: does someone have a recipe to ferment my green powder to get something blue I can use in my soaps? I found a lot of vat recipes, but that's to dye fabric and I don't think it will work in soaps.

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    It seems the green indigo needs to oxidize. Have you tried mixing it with water and dying cloth with this mixture, to see if it oxidizes while drying? – Joachim Nov 14 '19 at 11:22
  • That article says that blue indigo is made by oxidation, while fermentation does the reverse, reduction. – Buzz Nov 14 '19 at 22:47
  • I did not try to dye cloth with it, but it is exposed to air when the soap dry. However the color did not change. – Oneira Dec 24 '19 at 9:27
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Try mixing the unfermented indigo powder with water in a bowl, and 'beating' it, using a smaller bowl to scoop it up and pouring it back until the mix turns blue.

Consider adding lye [NaOH] or slaked lime [Ca(OH)2] to help the fermentation process. The effectiveness might depend on how the unfermented powder was produced, though.

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