Category Archives: vat 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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Where can we order Vat dyes in Germany?

Name: Karin
Country or region: Germany
Message: Hello and sorry for disturbing, we want to order Vat Dyes and didn’t know where. Can you help us?
Thanks and best regards

Hi Karen,

Vat dyes are not as difficult to find as Naphthol dyes (yesterday’s question). A good retail source for vat dyes in Europe is Granat Farvekompagniet in Denmark. Look for their page of Granat Kypefarver/Batikfarver.

Vat dyes are widely available in many countries, including countries in Africa in which other classes of dye can be difficult to find. North American dyesellers from which vat dyes can be ordered include Aljo Manufacturing, PRO Chemical & Dye, and Maiwa Handprints. In Australia, Batik Oetoro sells Indanthren Vat dyes. (More information for each of these companies is available on my page, Sources for Dyeing Supplies Around the World.)

Vat dyes are not as suitable for beginners as Fiber Reactive dyes, but they are neither difficult nor dangerous to use, assuming normal safety precautions with safety goggles and gloves. The dyes are usually purchased in an insoluble oxidized form, and must be chemically reduced in order to solubilize them and get them inside the textile fiber; when the fiber is then exposed to air again, the dyes re-oxidize and become insoluble, so that the dye inside the fiber becomes fixed in place.

Vat dyes are particularly interesting for printing on fabric that has been dyed with dischargeable dyes, such as Remazol dyes or most Procion dyes. The same chemicals that are used to solubilize and reduce the vat dye will remove the existing color from the dye on the fabric, allowing the brightness of the vat dye in your design to contrast sharply with the background color. Granat Kypefarver/Batikfarver’s range of Vat dyes has been especially selected for being suitable for illumination dyeing on backgrounds made with Fiber Reactive dye.

Another useful property of Vat dyes is that most of them are less susceptible to fading than other types of dye. Not all Vat dyes are equally light resistant, but many are significantly more light-resistant than Fiber Reactive dyes or Direct dyes.

A specialized category of Vat dyes is light-sensitive pre-reduced Vat dye, which can be used to make single-color photographic prints on fabric. There are two brands of this amazing product available, with slightly different colors. Jacquard Products makes SolarFast light sensitive dyes, in fourteen different colors, and Lumi makes Inkodye, available in nine colors. Unlike sun-printed fabric paints such as Setacolor, the light-sensitive dye does not change the feel of the fabric, and it wears better than fabric paint since the dye penetrates the fiber. Since it is actually visible light that fixes the dye, rather than the heat from the infrared in sunlight as for fabric paints, inkjet transparencies with photographs or drawings can be used to make quite detailed designs.

For more information about Vat dyes and their use, see my page “About Vat Dyes”.

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