Crafting Natural Blue Pigments from Plants

Kathryn Reichert provides a step-by-step guide to extracting natural indigo pigment through fermentation, a process that can be used to create vibrant anthotypes later. Kathryn shares the science behind pigment transformation, key techniques for maximizing pigment quality, and practical tips for producing a rich, natural color for use across various artistic mediums.

Writer and photography / Kathryn Reichert

“My journey into pigment extraction began with a simple curiosity: I wanted to learn how to draw vibrant colors from plants to use in inks, paints, and dyes. Indigo, with its iconic hue, seemed like the perfect starting point.”

While reading about indigo dyeing for fabric, I also discovered how the technique could beautifully lend itself to creating anthotypes, connecting the world of botanical dyes with photographic prints.

Anthotype made with two coats of indigo paste on Hahnemuhle Platinum Rag 300 gsm, exposed for 3 weeks. The print shows water damage from rain seeping into a cracked contact frame, causing discoloration and red bleeding from the fabric backing along the edges.

As I delved deeper into the extraction process, I realized it required a careful, multi-step approach. Japanese indigo leaves (Persicaria tinctoria) contain a colorless compound called indican, which is the precursor to indigo. Through hydrolysis—a process where water breaks down a compound into smaller parts—the indican transforms into indoxyl. During fermentation, this indoxyl takes on a vibrant blue-green color. By adjusting the pH and adding oxygen, the indoxyl changes into indigotin, the deep blue pigment indigo is known for. Without this transformation, crushing the leaves would only release the green color from the chlorophyll, rather than the desired blue.

Several factors can influence the speed and success of pigment extraction. Ideally, a good extraction will result in a strong, deep blue color, which indicates that the indigo has fully developed. Key factors to consider include the ambient temperature, the type of water you use (e.g., tap versus distilled), and the condition of the leaves. For the best results, harvest the leaves in the morning, when they have higher water content, and select leaves that are in good condition—damaged leaves can lower the quality of the pigment. Additionally, be careful when handling the compounds involved (indican and indoxyl), as they are delicate and can degrade quickly if not processed promptly.

Harvest and Prepare the Indigo

Harvest a bunch of indigo plants. Immediately after cutting, place the stalks in a vase filled with cold water.
Carefully strip the leaves from the stems without crushing them, and place the leaves into a large vessel half-filled with warm water.
Ensure the leaves are fully submerged by gently weighing them down with a clean, heavy object. Cover the vessel and place it somewhere undisturbed but still exposed to some sunlight.

For this batch, I harvested a standard vase full of mature indigo stalks. It's best to harvest in the morning when the water content is high, choosing only healthy stems, and for maximum pigment, be sure to harvest before the indigo flowers. For fermentation, I used a large plastic planter, though any sizable vessel will work. — For this batch, I harvested a standard vase full of mature indigo stalks. It’s best to harvest in the morning when the water content is high, choosing only healthy stems, and for maximum pigment, be sure to harvest before the indigo flowers. For fermentation, I used a large plastic planter, though any sizable vessel will work.

Begin Fermentation (Hydrolysis Stage)

Let the leaves sit in the water, checking several times a day. You’re waiting for the water to turn a vivid antifreeze blue-green color, which indicates the presence of indoxyl. The water should also develop a sweet, almost pungent aroma and have a purple sheen on the surface.
In warmer climates, this process could take a day; in cooler climates, it may take up to a week. Check every few hours until you understand the timing for your specific environment.

Test for Readiness

Once you suspect the mixture is ready, test it by collecting a small amount of the water in a glass jar.
To alkalize the water, gradually add small amounts of calcium hydroxide, stirring it in until the pH reaches 10. Calcium hydroxide acts as a flocculant, causing indigo pigment particles to settle at the bottom of the jar.

Notice the aqua color of the water and the iridescent purple sheen on the surface. The sweet aroma indicated that fermentation was complete, signaling it was time to move on to alkalizing and oxidizing.

Alkalize (Indoxyl to Indigotin Conversion)

Pour the jar of treated water back into the large vessel. Continue to add calcium hydroxide in small amounts (about ½ teaspoon at a time), mixing thoroughly to break up clumps, and then return the mixture to the large vessel.
Regularly check the pH of the vessel, aiming to reach a pH of 10. Add the calcium hydroxide slowly to avoid degrading the pigment.

Oxidize and Settle the Pigment

Agitate the entire vessel to oxidize the water. Depending on the vessel size, you can stir vigorously or use a paint mixer attached to a drill. This process may take up to 20 minutes.
As you agitate, you’ll see white bubbles on the surface turn blue. When this happens, stop oxidizing and allow the pigment to settle. The blue pigment will sink to the bottom, leaving the liquid on top yellowish or clear.
After the pigment has settled, carefully decant the liquid off the top without disturbing the settled pigment. This process can take several hours.

Calcium hydroxide acts as a flocculant, causing the indigo pigment in the extraction water to separate and settle at the bottom of the jar.

A clear distinction between layers appears after the pigment has settled. The top layer is ready to be decanted, and using a turkey baster is an easy way to gently siphon out the water.

Making a pigment lake or lake pigment — After decanting as much water as possible, pour the remnants into a coffee filter to strain out the remaining water. Ensure the setup allows for airflow to prevent mold formation.

Decant and Clean the Pigment

Once most of the water is decanted, transfer the pigment to a smaller container. Neutralize the decanted water by adding something acidic before discarding it. You can also use this water for experimenting with other anthotype colors or dyeing.
Repeat the decanting process until as much water as possible has been removed. You can then add warm, fresh water to the pigment to clean it, which will enhance the color depth. After settling again, the water on top should become clearer each time. Avoid repeating this more than a couple of times, as it can degrade the indigotin.

Straining and Storage

After the final decanting, pour the mixture through a coffee filter or cheesecloth to strain out any remaining water. The resulting paste can be used to make your anthotype.
For any leftover pigment, allow it to fully dry in the filter, ensuring airflow underneath. Once dry, crush the pigment into powder and store it in a container out of direct sunlight. This powder can be stored indefinitely and reconstituted by adding water when you’re ready to make more anthotypes.

You’ll be left with an indigo paste, perfect for creating anthotypes. If you only need a portion, allow the remaining paste to continue drying in the filter.

Only a small amount of pigment is typically obtained from the plants. Once fully dried, the pigment can be ground and stored for future use.

In exploring pigment extraction through fermentation, I’ve come to appreciate how each step—however intricate—unlocks new possibilities across art forms, from vivid dyes to ethereal anthotypes. This process continues to reveal the hidden potential within plants, turning each experiment into a new way of seeing color and creativity.

Kathryn Reichert is known for her photography that blends digital and analog media with alternative processes. Her work explores psychological landscapes shaped by experiences and memories. Since 2015, her art has been shown in galleries across the U.S. and Europe. She holds degrees in business and fine arts and lives in New Jersey.