What makes indigo so special, compared to other blue dyes?

Name: Kathy

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A Handbook of Indigo Dyeing,
by Vivien Prideaux

This book provides an excellent introduction to many basic methods of shibori dyeing, in addition to easy-to-follow illustrated recipes for using indigo.

Shibori: The Inventive Art of Japanese Shaped Resist Dyeing

This is the classic work on traditional Japanese shibori, a must for any serious shibori artist, though not intended to be a beginner's handbook.

Country or region: U.S.

Message: I am going to try dyeing some shibori. What blue color of Procion MX will approximate indigo most closely? What is it about indigo dye that makes it so special when you compare it to other blue dyes?

The most special thing about indigo is that, for thousands of years, it was the only good blue dye in existence. Until the discovery of Prussian blue in the mid-eighteenth century, indigo was the only good blue dye available in most of the world, though in the seventeenth century the introduction of logwood to Europe provided a less permanent alternative blue. The indigo dye molecule is produced by some fifty different plant species, worldwide; its unique ability to act as a permanent blue textile dye has been discovered repeatedly by different cultures around the world. Plant-derived indigo is still available, though the majority of indigo these days is synthesized from petrochemicals.

Unlike most natural dyes, indigo can be used without metal-based mordants such as alum, tin, or chrome, which stay in the fabric after dyeing, and which are toxic if ingested. Unlike mordants, the chemicals that are used with indigo are washed out completely after the dyeing process. Indigo is notable for being both more light-resistant and more wash-resistant than almost all other natural dyes. It's closely related to the famed Tyrian purple dye of antiquity, made by shellfish.

There are several different processes for applying vat dyes such as indigo, using fast-acting modern synthetic chemicals or slow old-fashioned fermentation vats, but all cause the dye to be chemically reduced to render it soluble, so that it penetrates inside textile fibers. Exposure to air then chemically oxidizes the dye, returning it to its insoluble state, so that dye molecules inside the textile fibers become permanently trapped. Until the invention of fiber reactive dyes, this was the best mechanism for attaching a dye to a textile fiber permanently. Other older types of dyes, the mordant dyes and direct dyes, are far more prone to quickly fading in the laundry.

Ironically, one of the most-prized properties of indigo in the fashion industry now is the result of inferior dyeing practices. If indigo is applied improperly, most of it will deposit on the outside of textile fibers, instead of penetrating further within them. Since much of the indigo in denim fabric is allowed to remain on the outside of the individual cotton fibers, in a dyeing fault known as "ring dyeing", it is easily worn off, so that a small amount of abrasion results in an exaggerated appearance of age. Unfortunately, some commercial denims, even those used in very expensive blue jeans, are so poorly dyed that a significant amount of blue dye is completely unattached, so that it crocks (that is, rubs off even when dry) onto other clothing, shoes, and purses, sometimes ruining them. An expert dyer can apply indigo so that crocking is not a big problem, but evidently some major clothing retailers do not care.

Fiber reactive dyes, such as Procion MX, are much easier to apply than indigo, and they have greater washfastness and, in most cases, equally good lightfastness. Vat dyes tend to have higher resistance to fading from light than other classes of dye, often ranking as high as 7 or 8 on a scale of 1 to 8; however, indigo is listed by a manufacturer as having a lightfastness rating of only 5, less than that of Procion blue MX-R. (See my page, "Lightfastness of Different Types of Dyes".) The main motivations for hand-dyers to use indigo are for historical reasons, and, for people who buy plant-sourced indigo, the satisfaction of using non-petroleum-based dyes.

The color indigo is a medium-bright blue. It is not nearly as bright as, say, the brilliant Remazol Blue R (reactive blue 19), because it absorbs light over a wider range of the visible spectrum, but it is brighter than some navy blue dyes. None of the unmixed ProcionMX dye colors exactly matches the hue of indigo. Procion Blue MX-7RX is too violet, while Procion Blue MX-G, Procion Navy MX-3R, Procion Blue MX-2G, Procion Blue MX-4GD, Procion Blue MX-3G, and Procion Turquoise MX-G are all too green. (See "Which Procion MX colors are pure, and which mixtures?".

I think that the closest match to indigo among the unmixed Procion MX blues would be Procion Blue MX-R, or reactive blue 4. Like indigo dye, it is a duller color than Procion Blue MX-G, and it is the only one of the Procion blue dyes that is neither greenish nor purplish. At a concentration of 4% OWD (that's four grams of dye per 100 grams of fabric), it looks very much like a medium indigo; at a weaker strength of 1.5%, Dharma calls it "sky blue", which looks like a paler dyeing with indigo.

For an even closer match from the Procion MX color range, you may prefer to choose a pre-mixed color, such as the "422N Indigo Blue" sold by PRO Chemical & Dye. Dharma's "PR130 Strong Navy" mixture has also been recommended as similar in color. Let me add that, much as I love true indigo dye, I like the color of Remazol Blue R even more, and also the cerulean blue Procion Blue MX-G. High concentrations of either of these unmixed dye colors produce fairly dark and yet glowing blues. They are both more blue than true indigo is, however.

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Posted: Friday - August 19, 2011 at 12:36 PM

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What makes indigo so special, compared to other blue dyes?

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