Category Archives: about dyes

Extracting indigo dye from denim

Name: Marc
Country or region: Philippines
Message: Greetings Dr Burch! Our team is conduction an undergraduate thesis that will extract indigo dye from jeans/denim. We would like to ask if you have any word of wisdom or advice about this. Also is it okay if you can tell us the skeletal formula of cotton/cellulose-indigo dye? We would like to verify our work with yours. Thank you so much.

Hi Marc! This is a nice question.

Interestingly, there IS no skeletal formula for the indigo dye-cellulose bond, because there is no bond. Vat dyes (see “About Vat Dyes”) do not form bonds to cellulose, the way a fiber reactive dye forms covalent bonds to the cellulose. Instead, the vat dye, such as indigo, is rendered soluble in water by reducing it chemically. There are several different ways to do this. Once the vat dye is in its reduced “leuco” form, its solubility means it can enter or exit the fiber, along with the water it is dissolved in. When we dye cotton with indigo, we first reduce the indigo in the dyeing vat, then dip the cotton in the indigo dye vat, then pull the cotton out and expose it to air, so that the oxygen in the air can oxidize the indigo back to its colored form. The oxidized indigo that is stuck inside the fiber at this point remains there; it can’t come out until it is reduced back to the soluble form.

Good dyeing practice involves repeated dippings, rather than a higher concentration of indigo in the vat, because putting too much indigo in the vat results in a lot of dye accumulating on the outside of the cotton fibers, rather than penetrating inside. When dye sits on the outside of the fiber, rather than on the inside, it is subject to wearing off; this is a fault called “ring dyeing”, because a microscopic examination of a cross-section of the dyed fiber will show a ring of color on the outside of the fiber and less dye inside, rather than a smooth penetration of color throughout. Amusingly, this ring-dyeing fault has become popular in denim used for jeans, because new jeans that have the look of being old and already having been worn a lot are very popular, so having the dye perform poorly and rub off becomes a plus, as far as marketing is concerned. Unfortunately a side effect of this is “crocking”, in which the dye rubs off onto other things; I’ve seen many complaints about poorly-dyed (but often expensive) blue jeans ruining a light-colored couch that the wearer sat on, or a light-colored purse that bumped against the jeans.

What you want to do to solubilize the indigo in your sample is to soak the dyed fabric in a reducing bath. You should follow the instructions in a good recipe for indigo dyeing to do this, with the obvious exception that you will omit the dye from the recipe. Once you have extracted dye into water, I expect you will be able to return the extracted indigo into its insoluble form, by exposure to air, and filter it out onto filter paper.

You may feel confused at the idea of having a vat (i.e. a bucket, or a beaker) of a reducing bath, when there is air touching the surface of the liquid in your vat all the time. Why doesn’t the oxygen in the air above the vat oxidize all of the dye in the dyebath? It is important to avoid stirring the bath enough to introduce a large amount of air into the water. Your reducing bath will contain an excess of the reducing substance, enough to deal with the small amount of oxygen that is introduced through the surface of the dyebath, assuming that you are careful to follow the instructions about not stirring the liquid too vigorously.

My web page “About Vat Dyes” contains links to a number of online sources for vat dyeing instructions, as well as to books with instructions which may be more detailed. Note that there are several entirely different ways to reduce the vat dye, including using thiourea dioxide, sodium bisulfite, or sodium hydrosulfite (which is an old name for sodium dithionite). Metal ions such as zinc can be used, for example in the zinc-lime indigo vat, but this can be hazardous to the dyer and it leaves you with hazardous waste to dispose of, so the zinc-lime bath is not something I recommend for your use. In natural fermantation dyebaths, which were used for all indigo dyeing before the nineteenth century, other substances are converted to reducing agents by microbial action. Many versions of the natural fermentation vat use large quantities of aged human urine, while other fermentation vats can be based on “reducing sugars” (check the Wikipedia page on that phrase), as some sugars can be used as reducing agents. The natural fermentation vats have the appeal of using less hazardous substances, but they are far more finicky, time-consuming, and difficult to get to work right, so I expect you will use a chemical reducing vat, probably with sodium dithionite (usually referred to be dyers under the name sodium hydrosulfite), which is inexpensive and easy to find. In my area, the United States, it’s sold in every fabric shop as “Rit Color Remover”.

Here’s a piece I wrote a decade ago about the comparative safety of different types of indigo dye vats: “Safety of auxiliary chemicals for indigo”. I now feel that I didn’t make enough mention of the fact that even moderate exposures to sulfur-containing reducing agents can cause serious problems for people who have asthma. Be sure to use appropriate methods to prevent exposure, such as a well-fitted respirator equipped with acid gas cartridges, or the use of a fume hood in a laboratory. I expect that, as science students, you are already aware of such precautions.

There is additional discussion of the different sulfur-containing reducing agents in the “Reductive Discharges” section of my page on “What chemicals can be used to remove dye?, which includes synonyms for different reducing chemicals, useful given the non-standard names often applied to these chemicals.

I hope you find this helpful.

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Pre-reduced indigo: natural versus synthetic

Name: Jen

Country or region: US

Message: I’m doing research on pre-reduced indigo crystals. I’m finding that they’re being called everything from freeze-dried to instant and claims of them being ‘natural’, extract, etc. This flies in the face of what I’ve also been told directly by an importer- if it comes from India it’s all synthetic. Even the stuff one seller is claiming produced in Japan has an import label from India! So you can see confusion.
Any information or insight you’d be nice enough to share is most appreciated.

Hi Jen,

I believe that I’ve NEVER encountered pre-reduced indigo that was not synthetic. Those who wish to use only plant-derived indigo must do more work to reduce their indigo themselves.

I suspect that there is no difference at all between pre-reduced indigos that are listed as being freeze-dried, dried, or instant, merely different ways of describing the same product. Indigo “extracts” appear always to be the normal oxidized form which must be reduced by the dyer before use. Note that pre-reduced indigo is not 100% reduced; the various retailers all specify that their pre-reduced indigo is 60% reduced. The remaining 40%, which is in the oxidized form, is unusable unless the dyer chemically reduces it in the indigo dyeing vat. (Both synthetic indigo vats and natural fermentation indigo vats produce chemical reduction.)

Often a retailer’s advertising copy reads as though their pre-reduced indigo is natural, but if they are both reputable and careful you can see that it is not. For example, Dharma Trading Company, which is good about what claims they make, says:

As an alternative, try this Pre-reduced Indigo. Synthetic Indigo (chemically identical to natural Indigo) has been pre-reduced chemically, then dried, so it dissolves in water right off the bat….This all makes it have a lower environmental impact for the dyer than taking the original natural or synthetic Indigo from start to finish with all the chemicals involved.

Jacquard Products starts off saying “This natural vat dye exists in plants all over the world”, but concludes clearly with “Jacquard’s indigo is a synthetic organic and comes pre-reduced 60% for unprecedented ease of use.” (source)

“Organic” is a difficult word for the dyer, because an organic chemical is any chemical that contains carbon; it has nothing to do with organic farming. All dyes, whether natural or synthetic, are organic chemicals, aside from a few mineral colors that are not really of interest. However, many dyers mistake claims that a dye is organic, thinking it means it is a plant-derived dye when in fact it is not. Dye sellers sometimes purposefully exploit this confusion.

Jacquard pre-reduced indigo is available through Amazon, as well as other sources; a supplier on Amazon currently selling Jacquard pre-reduced indigo, “The BT Group”, provides misleading information claiming that what they are selling is naturally occurring. There’s an issue with resellers on Amazon, which can post wrong information about their products; it’s likely that The BT Group doesn’t know anything about what they are selling, and merely selected several phrases from the Jacquard copy without paying attention to how they have changed the meaning by omitting a key word. Similarly, Etsy sellers give blatantly false information about whether the pre-reduced indigo dye they sell is from natural sources, when what they are selling is obviously synthetic, given their suppliers.

Earth Guild makes an error, showing instructions for using natural indigo on a page whose title is “PRE-REDUCED INDIGO INSTRUCTIONS”, but I don’t think that the title of a web page amounts to a claim, given that many people who edit web pages don’t even know how to alter the titles of their pages. (source) In corroboration of this interpretation, they have a page on Lanaset dyes with this same webpage title, so it’s just a mistake.

George Weil helpfully says that pre-reduced indigo is used as “an alternative to Natural Indigo”. (source)

There are blogs that have instructions for using pre-reduced indigo which assert that the dye is natural, but in every case it seems that the blogger was insufficiently knowledgeable. Since they’re not selling the dye, they’re not legally liable for their claims in the same way that someone who is selling a pre-reduced indigo is.

If you find any additional information, I will be interested in learning about it!

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looking for a light-fast dye or pigment with peak absorption centered between about 570-590 nm

Dear Paula,

I came across your website on dyes, it’s great! Currently, I’m experimenting with an idea to understand how insects see color in plants and how it may relate to a decreased use of pesticides. Something that we all may benefit from. In this, I’m looking at a light-fast dye or pigment with peak absorption centered between about 570-590 nm. The narrower the absorption band the better, and the lower the fluorescence quantum yield the better (for a controlled experiment on absorption).

The material substrate is polyurethane or other plastics. Would you know some dyes that would fit the mold?

Thank you!


Hi Keenan,

You want a dye that absorbs in the orange range, which means you want a dye that looks blue. To help with visualizing this, let’s look at a graph I happen to have handy, which shows the absorption spectra of a number of dichlorotriazine dyes that are popularly used for hand dyeing:

absorption spectra of a number of dichlorotriazine dyes that are popularly used for hand dyeing CLICK TO SEE FULL SIZE

(Image provided by Olli Niemitalo.)

The closest of this range (which is not suitable for your substrate; I’m using it only because it is handy) is Procion Blue MX-7RX, which is Colour Index Reactive Blue 161. Its peak is not as sharp as you’d like, anyway, as it absorbs pretty significantly from 530 to 640 nm. Procion Blue MX-7RX is notable for a deep violet-blue color. It is also noted for being quite poorly lightfast when used on cotton, though its lightfastness is significantly greater when it is used with an acid dye recipe on silk.

Dyes that have narrow absorption ranges are unusually clear and bright in color. Wider absorption ranges result in duller colors. A dye that has a very narrow absorption range cannot be used to produce a dark color, no matter how high a concentration of it is used, because it allows most colors of wavelengths to be passed freely. Your desired dye will be a very bright, clear blue, on the violet side of blue, since it will not absorb any wavelengths in either end of the spectrum. You do not want a dye whose color is described as a navy blue, because navy blue dyes always absorb over a wide range of the visible spectrum. You may want to look for a blue dye whose name includes the words “brilliant” or “bright”. There are many dyes that are of a medium royal blue color or of a cyan color, or of a reddish violet, but violet-blue dyes are far rarer. Of course, it is no use to look at dyes that are composed of mixtures of other dyes, as these will always tend to absorb a winder spectrum.

The one person I know who has made a great study of blue dyes is Dr. Steve Mihok, who looked at dyes that attract tsetse flies when used in fly traps. You can see his descriptions of many blue dyes using the Internet Archive (his original site appears to have been taken over by spam and no longer contains useful information). See his page “Blue Dyes” at the Internet Archive, captured in January of 2012.

Mihok concentrated on metal phthalocyanine dyes, which tend to absorb in a turquoise-blue range, I’d say around 600 to 700 nm. My one example dye which absorbs near your target, Colour Index Reactive Blue 161, has as a chromophore a triphenodioxazine structure, I believe. It would make sense to look for a dye with this same chromophore. Following is an image of Direct Blue 106, as an example of the triphenodioxazine structure:

an image of Direct Blue 106, as an example of the triphenodioxazine structure

Unfortunately, the large size of this chromophore may make it unsuitable for use in plastics. Disperse dyes are typically relatively small molecules. The 2003 book “Industrial Dyes: Chemistry, Properties, Applications”, edited by Klaus Hunger, says, “Like phthalocyanine dyes, triphenodioxazine dyes are large molecules, and therefore their use is restricted to coloring the more open-structured substrates such as paper and cotton.” (page 112) I suspect that this leaves you having to find an antraquinone dye. Anthraquinone dyes include many popular blue dyes, such as Procion Blue MX-R (reactive blue 4) and Remazol Brilliant Blue R (reactive blue 19), and many violet dyes, as well. This is disperse blue 3, an example of an anthraquinone dye:

disperse blue 3, an example of an anthraquinone dye

It is important to consider what class of dye you need. Different materials require different dyes; for example, a dye that works on wool is unlikely to work on cotton, and dyes that work on either cotton or wool will not work on polyester, but wool dyes will work on nylon. You say that you want to color polyurethane or other plastics. Polyurethane is quite different from PET plastic, in its dyeing properties, which in turn is quite different from nylon plastic. Although no acid dyes work on polyester (which includes PET), acid dyes can be used to dye polyurethane, though with varying degrees of washfastness. The most washfast acid dyes to use on polyurethane would be metal complex acid dyes. Disperse dye, which is used on most synthetic fibers and is the only option for dyeing polyester, can be used to dye polyurethane, but the washfastness is poor, and the heat required may damage the polyurethane.

It is probably best to dye polyurethane in liquid form before fabricating it into objects. Solution-dyed plastics tend to be more resistant to both light-faging and wash-fading than fabrics dyed after manufacture. This is the explanation of why, for example, Sunbrella brand acrylic fabric is so resistant to fading that it can be used in outdoor furniture that retains its color even after extensive exposure to run and rain. I imagine that you would use a solvent dye for coloring the liquid plastic before using it in manufacturing. Solvent dyes are not soluble in water, but are soluble in organic solvents. Many disperse dyes, which are used with special carrier chemicals to dye polyester fiber after manufacture, are actually identical to solvent dyes.

To color the widest range of already-manufactured plastics, I’d suggest you concentrate on disperse dyes, especially since washfastness is not as much of an issue for you. They will work on nylon, polyester, and polyurethane. When dyeing polyester it is generally necessary to use boiling temperatures and an additional carrier chemical; when dyeing other plastics, the carrier chemical should be omitted, and lower temperatures may be adequate, though the water must still be very hot, at least abot 60 degrees C. Nylon can be dyed at lower temperatures than polyester, and I think the same is probably true of polyurethane as well.

Sourcing your blue dye is an issue. You will note that many textile dyes are sold in the form of in-house mixtures, whose constituents are not made public. Dyes obtained from chemical suppliers tend to be much more expensive per gram, perhaps with greater purity, prohibitive for dyeing large quantities. Textile dye suppliers such as PRO Chemical & Dye and Aljo Manufacturing sell dyes in useful quantities for hand dyers, often at much better prices, but in some cases the generic identity of dyes is not made clear, and many dyes are sold as mixtures in order to produce specific colors. I have listed a number of disperse dyes that are used for hand dyeing in a large chart on my page, “About Disperse Dyes”, at , including disperse blue 3, which is described a sky blue and is sold by Aljo, and disperse blue 56, sold by ProChem, which is described as royal blue; disperse blue 60 is described as turquoise, so its absorbance is probably at a greater wavelength than you want, and disperse blue 281 is described as navy blue, which would have far too wide an absorption band to suit you.
Sigma Aldritch sells Disperse blues 1, 3, 14, 27, 35, 56, 60, and 124. All of the ones whose structures are indicated on their web site are anthraquinone type dyes. I suggest that you contact Sigma Aldritch and ask about the absorption spectra of all of these dyes.

I’ve been discussing only dyes, but you did mention pigments as an alternative. Pigments differ in that they do not bond directly to a substance, but instead are glued to it by some sort of binder, or they can be incorporated directly into some materials. Pigment dyes are pigments whose binder systems allow them to be applied in much the same way as dyes. Unless a pigment is sold with a Colour Index number, it probably consists of a mixture of more than one colored substance. The only pigments I would recommend you look at would be those sold by a chemical supplier such as Sigma Aldrich.

I am very interested in your project and would appreciate it if you would let me know more about it in the future.

For more information, see the following pages:

About Disperse Dyes

Dyeing Polyester with Disperse Dyes

What to Use to Dye White Polyurethane Foam, December 19, 2007

Steve Mihok’s Blue Dyes, captured by the Internet Archive on January 24, 2012

Sigma Aldrich

Lightfastness of Different Types of Dyes

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