Choosing the Correct Indigo for Natural Blue Soap

Many soap makers struggle with obtaining a true natural blue in cold process soap because “indigo” is often used as a generic term for blue colorants, when in reality not all indigos are the same. Different “indigo” powders can produce dramatically different results in soap, ranging from soft denim blues to muted grays and charcoal tones.

There are several different indigo options commonly sold to soap makers, but the two I’m going to focus on here are:

• Indigofera tinctoria
• Baphicacanthus cusia (Nees) Bremek powder

These two botanicals behave very similarly in soap, but produce very different blues. Understanding these differences can save you a lot of frustration when trying to achieve a true natural blue.

Indigofera Tinctoria

When soap makers are showing off the amazing true blue indigo soap, they are probably used Indigofera tinctoria. This is the indigo I personally prefer and use most often in my own soaps and classes because it produces reliable, beautiful blue tones in soap.

However, Indigofera tinctoria is commonly sold in two very different forms:

• fermented Indigofera tinctoria powder,
• and unfermented Indigofera tinctoria powder.

Fermented Indigofera tinctoria

Fermented Indigofera tinctoria is the version used to create blue soaps. This powder is deep blue in color and is traditionally processed for dye purposes. It produces the beautiful natural blues most people associate with indigo soap.

This is the form I use throughout my Year of the Rainbow Blue Month lessons and inside the Indigo Master Class.

I’ve had consistently beautiful results using indigo from Anne George, which has become my personal go-to source for natural blue soapmaking.

Unfermented Indigofera tinctoria

Unfermented Indigofera tinctoria is completely different in appearance and use. This powder is green and is commonly sold for natural hair dye and beard dye applications, especially for achieving darker brown and black tones in hair.

Soap makers sometimes accidentally purchase the unfermented version expecting blue soap results and are understandably confused when the colors do not behave the same way.

Here are photos from the Indigo Master Class where we explored both the fermented and unfermented Indigofera tinctoria powders side-by-side. You can see the beautiful blue tones created by the fermented indigo, alongside the warm amber and earthy tones created by the unfermented green powder.

What About Woad?

Woad contains the exact same primary blue colorant found in Indigofera tinctoria. The difference is not the final blue itself… it is the amount of colorant contained within the plant material and how difficult it is to extract.

Woad contains less of the blue color compound, and extracting it tends to be more labor intensive. Because of this, woad blue is often significantly more expensive and harder to source consistently.

For my own soapmaking, Indigofera tinctoria simply makes more sense because it is:

• easier to source,
• more cost effective,
• and consistently produces beautiful blues in soap.

Why Some Indigo Powders Turn Gray

Inside my classes, we tested this directly using a commonly sold indigo powder: Baphicacanthus cusia (Nees) Bremek powder.

This powder is frequently sold as a natural indigo for soapmaking, but in my own testing it consistently leaned much more gray than blue when compared side-by-side with fermented Indigofera tinctoria.

The Bremek powder created:

• muted gray-blues,
• slate tones,
• and softer charcoal shades

while fermented Indigofera tinctoria produced much clearer blue tones in identical soap recipes. Neither result is necessarily “wrong,” but they are very different color experiences. If your goal is a true natural blue soap, understanding which indigo you are purchasing matters tremendously.

How to Use Indigo in Handmade Soap

Indigo can be macerated with either oils or water before being added to soap.

• When indigo is macerated with oils, soap makers commonly refer to this as an oil infusion.
• When working with water, you’ll create a decoction. This is often called “indigo ink”.

Both approaches work well, and choosing between them often comes down to personal formulation preferences and workflow style rather than one method creating “better” blues than the other.

You’ll often hear me say that one method is my current favorite, but honestly, that tends to change every time I start making blue soaps again. I fall back in love with whichever indigo method I’m currently working with because they each bring something a little different to the soapmaking experience.

Some soap makers also disperse indigo powder directly into soap batter. Personally, I strongly advise against this approach because indigo is extremely concentrated and tends to leave behind dark blue speckles if it has not been fully macerated beforehand.

Proper maceration allows the indigo to disperse more evenly throughout the soap batter and creates a much smoother final appearance.

Inside my Extracting Color From Nature Master Class series and Indigo Master Class, I go much deeper into:

• oil macerations,
• water decoctions,
• concentration testing,
• layering techniques,
• and troubleshooting indigo in cold process soap.

More Indigo Is Not Always Better

At lower amounts, indigo can create beautiful soft blues similar to faded denim, dusty blue, and baby blue tones. At very high concentrations, however, indigo can:

• accelerate trace,
• create false trace,
• increase soda ash,
• stain washcloths,
• tint lather blue,
• and occasionally turn your sink, hands, or skin a little Smurfy. 😄

Learning how to balance concentration is one of the keys to working successfully with natural blue colorants.

Sometimes depth comes not from adding more indigo, but from layering colorants thoughtfully. For example, combining indigo with a very small amount of activated charcoal can create deeper navy tones without overloading the soap with indigo itself.

My Favorite Indigo Resources

If you’d like to explore indigo in more depth, I have a few additional resources that walk through the process step-by-step.

Free Course: Know Your Indigo

This free course covers:

• common indigo types,
• fermented vs unfermented indigo,
• why some indigos turn gray,
• and foundational concepts for working with natural blue colorants in soap.

Indigo Master Class

Inside the Indigo Master Class, I dive much deeper into:

• infusion methods,
• decoctions,
• color comparisons,
• concentrated testing,
• troubleshooting,
• and advanced blue soap techniques.

You’ll also see side-by-side testing results using different indigo powders so you can better understand how dramatically the source material changes the final soap color.

Final Thoughts

If your natural blue soaps haven’t turned out the way you hoped, don’t immediately assume you “did something wrong.” Very often, the issue starts with the indigo itself.

Understanding which indigo you’re using, whether it is fermented or unfermented, how it was prepared, and how much you’re using can completely change your results.

Natural blue soapmaking has a learning curve, but once you understand the personality of indigo, it becomes one of the most rewarding natural colorants to work with.

Happy soaping. 🧼