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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