Did You Know... About Indigo?

Did you know that indigo is one of the oldest dyestuffs in the world? For millennia this dye provided the only colorfast and lightfast clear blue available for fabrics. It was highly valuable for trade and recipes were often closely guarded secrets. It has been used all over the world to make everything from intricate Japanese kimonos to Levi jeans for the original 49ers.

What makes indigo special is that in its normal state the dye molecules are insoluble in water — they will simply sink to the bottom of the vat unless a chemical reducing agent is added. Reducing agents steal a hydrogen atom from the dye molecule, allowing it to form loose bonds with the hydrogen in the water so it dissolves and creates the vat. There are all kinds of reducing agents that can be used, from yeast or live bacteria in a fermentation vat to more dangerous chemicals like lye (sodium hydroxide). We have a somewhat safer and effective recipe using Sodium Hydrosulfite and Soda Ash. In Europe, one of the oldest and cheapest reducing agents was stale urine. Oral dye history is full of stories about European dye houses that would put out pots for people to "donate" to, and dye houses that were built next to taverns for a steady supply. Due to the smell, dye houses were often restricted to the outskirts of town.

This amazing dyestuff naturally occurs in indigo-containing plants of the tropical genus Indigofera, which grows in many countries, as well as in a less concentrated form in a plant called Woad, native to temperate Europe. The most well known, Indigofera tinctoria, was native to India — likely the earliest major player in the lucrative indigo trade. The compound that makes up the dye is called indican. It is extracted from the plants by crushing the leaves and soaking them until they ferment and release the indican, which was then precipitated and dried into cakes, and ground to a powder by the user.

Designs on indigo cloth are achieved with various tying techniques such as traditional shibori and tie-dye. Other types of resist such as rice paste and wax are also used to protect areas of the fabric and create intricate patterns. Indigo can also be used for printing and painting with the use of different chemicals like arsenic trisulfide or iron sulfate, combined with thickeners.

When indigo is in solution it is a yellow-green color, though the top layer of the vat is often blue because it is exposed to air. Indigo is sometimes called a magic dye because of the way fabric changes color from yellow-green to deep blue as the air oxidizes the dye. As the dye molecules oxidize they become insoluble in water again, so they don't wash out of the fabric. The fabric is dipped in the indigo vat to soak up dye, then hung in the air to oxidize and turn blue. Depth of color can be built up through repeated dippings — successive dippings and airings give you darker and darker blues, ranging from pale sky blue all the way to deep navy.

We are all familiar with the characteristic way indigo fades as fabric is used and worn — the fade lines on your favorite jeans, for example. This is because indigo does not actually chemically bond to the fabric. Instead, it becomes insoluble in water when it reacts with air and becomes lodged in the small spaces within the fiber. Over time, as the fibers are rubbed with wear, some of the dye is worked loose, creating those faded lines.

Indigo-dyed fabric can also "crock" — rubbing off onto things and people — if not properly laundered before use. If you don't want to try out for the Blue Man Group, be sure to wash newly dyed or store-bought indigo-dyed fabric in hot water with Dharma Dyers Detergent. Our Dharma Dye Fixative or Retayne in a hot soak can help the fabric retain more indigo before washing, resulting in less fade on the first wash and no crocking after.

Since indigo was one of the only natural blue dyes — and the best — it was often used to overdye yellow fabrics to make greens. In museum tapestries you will often notice that many plants appear very bluish; this is because natural yellow dyes are more sensitive to UV fading than indigo, so over time the indigo has become the predominant color in the fibers.

These days most indigo dye in commercial use is made synthetically, but you can still enjoy the magic of true indigo with our wonderful natural indigo. It is a little more work than pre-reduced synthetics, but it also has greater depth and variation — which is all part of the magic.