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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Can I use soda ash in my front loader washer to soften my hard water?

Name: Mary
Country or region: United States

Message: Hello: I am wondering if I can use soda ash in my front loader washer to soften my hard water. I am a beginner dyer and had to purchase some soda ash and wondered if I can use the same product for dyeing and as a water softener.

I have also seen washing soda (by Arm & Hammer) used as a water softener. Is that better? If possible, I’d like to use one product for dyeing and water softening. Please advise. Your comments/suggestions would be most appreciated. Mary

Soda ash is not a good water softener for dyeing.

What works really well as a water softener for dyeing is sodium hexametaphosphate (also known as Metaphos, and formerly sold under the name of Calgon). Here in the US, I recommend that you order Water Softener from Dharma Trading Company, or Water Softener from Colorado Wholesale Dye (they have really good prices!), or Metaphos from PRO Chemical & Dye, or buy Jacquard Products brand Calgon from an art supply store that sells dyes from Jacquard Products. (See Sources for Dyeing Supplies Around the World for contact information for these and other suppliers.) Do NOT buy Calgon that is not specifically labeled as being Jacquard brand or sodium hexametahosphate; although “calgon” used to mean sodium hexametaphosphate, the Calgon company now sells entirely different products, such as sodium citrate or polycarboxylate, under the same name, which can cause real problems in dyeing. As long as you buy a product with “hexametaphosphate” in the fine print, you’ll be fine.

Water softening is the removal of hard water metal ions, specifically calcium and magnesium. Calcium forms complexes with some unattached dye molecules that are difficult to wash out, which results in some slowly-released unattached dye that bleeds in the laundry; it can also produce unwanted spotting as the dyes attach to the fabric. Sodium hexametaphosphate binds to the calcium and magnesium, making them water soluble, and removing the problems caused for dyeing by hard water.

We frequently use sodium carbonate to increase the pH so that cellulose can react with fiber reactive dyes. Although sodium carbonate is listed as a water softener for some purposes, using sodium carbonate does not solve the calcium problem when dyeing. It’s just as useless for this purpose in a front-loading machine as in a top-loader. Sodium carbonate reacts with the calcium in hard water to form insoluble calcium carbonate, which is deposited not only on the sides of a sink or bathtub, but also on the fabric, interfering with the ability of dye to evenly reach the fiber that you are dyeing. The water in the dyebath is, technically, softened by this reaction, because you end up with less calcium in the water; however, putting insoluble calcium carbonate onto your fabric does you no good at all.

A household water softening device uses a resin to replace the calcium ions with sodium ions, but this is not possible without the water softening equipment. You can’t do this in a washing machine.

Washing soda, including that sold by Arm & Hammer, is nothing more than sodium carbonate with a few extra water molecules complexed to it. It is not better or worse as a water softener than soda ash is; they act exactly the same, which is to say, neither is useful as a water softener for dyeing, and both work very well as a fixative for fiber reactive dyes such as Procion dye. You can always use washing soda (sodium carbonate decahydrate) as a substitute for soda ash (anhydrous sodium carbonate), or vice versa, if you remember that washing soda has more volume and more weight per a given amount of sodium carbonate than soda ash does. You need to use 2.7 times as much washing soda as a substitute for soda ash, if measuring by weight, or 4.6 times as much if measuring by volume, to get the exact same number of sodium carbonate molecules. However, most of our dyeing recipes include a comfortable excess of sodium carbonate, so it’s usually not all that important if you forget to increase the amount you use.

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Shortages of Procion Turquoise MX-G and Blue MX-G

My old cat, Isis, on a bedspread dyed with turquoise MX-G, blue MX-G, and yellow MX-8G.

Last month, Dharma Trading Company sent out an important and alarming email about recent dye shortages:

Yes, it is true. The supply of our most important and most beautiful blues, #23 Cerulean Blue and #25 Turquoise, has become problematic. It is even possible that Cerulean Blue won’t be made anymore, due to a lack of the chemical precursors used to make it. This seems to be a little up in the air, so we are keeping our fingers crossed.

#23 Cerulean Blue
We were able to get a couple of shipments of #23 Cerulean before our manufacturer ran out. Right now, we have enough Cerulean Blue to sell up to 25 lbs per customer, but no more. As we run low, we will have to ration it further.

#25 Turquoise
Right now we are having to limit orders of Turquoise to 2 oz. per customer, and are unable to package any more. What we have in stock is it for now. We have a teeny tiny shipment coming in around 12/11 or so, so we hope to be able to be able to sell 2 and 8 oz jars, one per customer, until we get some more. But, we are assured by several importers that they will get more Turquoise, it is just a matter of when.

Mixes containing these colors are all still in stock, but as they run low,
some may have to be rationed as well.

Rest assured that we are doing everything we can to resource these colors for you as we know how important it is to people’s livelihoods. We will keep you informed when any new information comes to light. Thank you for your patience!

As of this writing, Dharma’s website says,

ATTENTION: there is currently a worldwide shortage of 2 dye colors, #23 Cerulean Blue, and #25 Turquoise. For now, we have Cerulean Blue, so at least temporarily, that gorgeous color is in stock, but we have to limit it, and so cannot allow large orders. Turquoise – we currently now have 2 and 8 oz jars, 1st come, 1st serve, 1 per customer please. We are expecting a super huge shipment of Turquoise around mid January. We will keep you updated as we learn more.

Meanwhile, ProChem’s website says,

Procion Turquoise MX-G


ProChemical and Dye currently has both of these dyes in stock, so I’m sure Dharma’s expected big shipment of turquoise will be in stock soon. Whew! Big sigh of relief.

What would we do if these two dyes were to become discontinued altogether? We would have to turn to another class of fiber reactive dyes. There are equally brilliant turquoise and blue dyes available in other types of fiber reactive dyes, though they are a little more expensive than our Procion dyes, and none of them are currently available in the wonderful variety of pre-mixed colors that the Procion dyes are. We won’t have to give up using these glorious colors in our work. This would still be an awful thing to have happen. It would be a tremendous headache for everyone who relies on existing recipes for mixing dye colors. It must be a nightmare for the people who mix colors at Dharma, Prochem, Jacquard, and our other suppliers.

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