Mastering Indigo Fermentation: Lessons from a Five‑Year‑Old Vat

Over the years I’ve built, used, and discarded countless indigo vats. Now my 5‑year‑old, 100‑liter henna vat is officially finished. After repeatedly adding indigo pigment, lime, and more henna, it still produces dye, but the usable space above the sludge has become almost negligible.

For the past year I’ve devoted myself to understanding these fermented vats. I followed a rigorous protocol: each vat began with a precise amount of indigo pigment, an alkalinity source (soda ash or wood‑ash lye), and plant‑based starters such as wheat bran, madder root, and dried indigofera leaves. Throughout the year I added only modest amounts of lime and bran to keep the pH steady. I never introduced additional indigo.

In May I attempted to produce a spectrum from the palest to the darkest blues. To my disappointment, every vat yielded too dark a shade for my pale targets. I had expected the indigo content to wane with use, but no matter how many dips, the color never lightened.

Now, a year after the vats were first made, I can see measurable progress.

Indigo on cotton cloth, same vat: 1‑24 ten‑minute dips. February, 2021

Key observations:

This is a long process…

Two dips in May 2020 produced the same shade as five dips in February 2021.

The deep blue achieved after 12 dips in May 2020 was not reached even after 24 dips in February 2021.

Subtle differences in the darkest shades are hard to spot in the photos – but they exist.

I now understand the value of maintaining a range of vats—from old to new, weak to strong. Michel Garcia has said this before, but the data speak for themselves.

This spring I won’t discard my weakening vats. Instead, I’ll add a new vat dedicated to producing the deep blues I need for building black tones on my woven cellulose fabrics.