There are a few tricks that can give you a more powerfully blue result when using indigo.
First, indigo root powder is actually a dull loden green when it is ground from dried roots. The addition of an alkali and water cause it to turn a greenish-teal liquid, often with a glimmery copper sheen on the top of the kettle it is brewing it. When you dip cloth into the vat of dye solution it also turns a greenish-teal shade initially, until you pull it out of the vat and watch it transform to a lovely indigo blue. We call this a ‘vat’ of dye. This vat can be reduced and the concentrated solution then dried into cakes and even ground up into powder. This indigo powder behaves similar to baking cocoa however, it is slightly resistant to cold water and takes several hours to fully dissolve.
A few years ago dyers overseas developed another method of reducing indigo for resale – Freeze Dying! It is just like instant coffee crystals, only deep indigo blue. It dissolves best in plain hot water though. Below is a photo of indigo crytals added directly to a cold, previously prepared lye solution instead of directly to the water required to dissolve the lye. You can see how the crystals want to just stay undissolved, giving a weak colouration to the lye solution. This will also result in ‘bleeding colour spots’ in your batch of curing soap. The concentrated, undissolved specks will ‘grow’ in the bar as it bleeds from the center of the undissolved speck. Probably not what you are going for!
Indigo Crystals in Pre Made Lye Solution resist dissolving
You will achieve best results by adding your indigo crystals or powder directly to the measured water for your soap formula, THEN adding your measured amount of lye beads/flakes to this container. Stir well, and let sit overnight or until the lye solution is cool. Remember to use great care when handling any lye.
Fully Dissolved Indigo Crystals
You can see in the photo above how intense the indigo blue is when the crystals are properly dissolved first in water. The addition of the lye (sodium hydroxide) creates an alkali environment which extracts the full saturation from the indigo crystals. In the 1 gallon Rubbermaid pitcher above, I have 1 teaspoon of indigo crytals dissolved in 10.7 oz of warm water. After the crystals are hydrated and the water cool I added 3.9 oz of sodium hydroxide. Remember to STIR WELL. The sodium hydroxide will cause an exothermic reaction, meaning the water will heat up to about 240 degrees. Cover and set aside until cool.
Indigo Soap at Light Trace
In this photo we have our indigo water and lye solution added to the previously prepared soap oils. This has been blended with a stick blender, also called and immersion blender, until it is at a light trace stage. Light trace would be like thin crepe batter. This is the early stage when the oils and lye solution are holding together without separating. Medium Trace is next, this is when soap mixture in the pot lets you see trails of the soap dribbled from the spoon for a fraction of a second. Full trace is when your soap mass is similar to cake batter.
Pouring into Molds, Slightly Duller Shade
By the time you reach medium trace (my favorite place to pour into the mold, so I retain flat tops on my bars) your indigo soap will have lost some of that initial vibrant blue and now be a more grey blue shade. This is normal. Indigo is still the truest blue you can achieve without using synthetic dyes or lab created cosmetic pigments (such as ultramarine blue).
Curing Indigo Soap – Darkening
Keep in mind that whatever shade of indigo blue you see in your soap pot, that as your batch cures it will deepen and become darker. I used a highly concentrated amount of indigo the first time I used it. The bars bled in the shower as if I had a brand new pair of cheap denim jeans on. It was a disaster! To save that first batch, I sliced the soap into very thin slices, used cookie cutters to create star shapes, and embedded those pretty dark blue stars into a batch of plain white soap. It was a spectacular save that let me use some new artistic methods with cold process soapmaking.