A Cautionary Tale: Indigo and Reducing Chemicals

The first indigo vats that I made in the 1970’s were reduced using sodium hydrosulfite. It was the only way I then knew to reduce the indigo. A few years later I began using thiourea dioxide as the reducing agent. I was never comfortable with these vats and, as a result, I never did much indigo dyeing. The vats smelled bad and I felt that I never truly understood them.

My current, and very positive, relationship with indigo began after meeting Michel Garcia in 2008. Michel introduced me to indigo vats that are reduced with natural materials: sugars, plants, or minerals. Since then I have worked hard to learn as much as I could about these vats, and have maintained them constantly in my studio. Indigo dyeing has become an integral part of my studio work and experimentations.


I know many dyers who use either sodium hydrosulfite (“hydro”) or thiourea dioxide (“thiox”) to reduce their vats. These chemicals work efficiently and predictably and can now understand why a dyer would make the choice to use them. But I’ve recently been asked about the safety of these reducing chemicals. This is not a question that I felt qualified to answer myself. I was with Joy Boutrup last week (we were teaching together in Iceland!) and I asked her to address this question.  

To paraphrase Joy:

Both sodium hydrosulfite and thiourea dioxide are sulfur based and will release hydrogen sulfide. This is what causes the bad smell and can affect human health when the gas is inhaled.  It can be irritating to the nose, throat, and airways and potentially paralyzing to breathing if inhaled in large quantities. Dyers with asthma should be especially cautious when using these reducing chemicals.

Both “hydro” and “thiox” also release sulfur dioxide, which is considered to be damaging to the brain. It is a chemical that has been used in photography and some photographers have suffered from long term exposure to this chemical. 

When using these chemicals dyeing must be done outdoors or in a very well ventilated area! Occasional dyeing using these types of vats may not result in noticeable effects but if you stand with your head over an indigo vat day after day, then serious breathing problems could result. Joy was recently consulted by a group in Denmark, who have been using these chemical vats for many years. Some of the dyers are experiencing a variety of health problems as a result of the exposure to the chemicals. She recommended that they begin using the natural/organic vats as an alternative. 

The other issue is disposal. The chemicals are very reductive and cannot just be thrown out. The reduction must be stopped before disposal but it’s not easy to keep the sulfur products from being reductive. Other chemicals can be used to stop the reduction but they are just as damaging. Some dyers simply keep their vat, adding additional indigo and reduction chemicals as necessary. Continuing to use the chemicals in the vat is preferable to uncontrolled disposal.

One another note: I have a friend who does production dyeing. She uses an indigo vat reduced with thiourea dioxide but was having trouble achieving pale blue colors. Dyers will often just do a quick 10-30 second dip in the vat to get light blues, but this doesn’t give the indigo time to penetrate the fiber. As a result, the dye is less fast to washing and crocking and is not likely to be even. I suggested that she try my approach to achieving pale blues: use only a small amount of indigo in the vat (1-2 grams per liter). 

When she told me that the small amount of indigo still resulted in a deep blue color, I did some tests myself using both “thiox” and sugar as reduction agents. 

I was surprised by the results. The organic sugar vat resulted in the pale blue color that I expected, while the vat reduced with the thiox was a very deep blue, although only a small amount of indigo had been use. 

Conclusion: the reduction chemicals will reduce the maximum amount of indigo, making it impossible to achieve the pale colors, while in the sugar vat, only part of the indigo is reduced at a time.  This is why a sugar or other natural vat can continue to be used over many days, weeks, or months. 

These reduction chemicals are used in industry and efficiently maximize the dye that can be applied to warps for blue jeans. But even industry has been concerned about the longterm effects of chemical reduction and disposal. Some industrial users have begun to use electrical reduction instead of chemicals for all vat dyes.  But artisan dyers are not industry…. The naturally reduced indigo vats are not only safer to use, but they allow us better control over our color and color mixing.

chemical vs sugar
Wool yarns were dyed in a vats, each made with 2 grams indigo/liter. Yarns had not been neutralized when this photo was taken.

46 thoughts on “A Cautionary Tale: Indigo and Reducing Chemicals

  1. Excellent post, when using the thio for reducing dried indigo leaves you never bring it to a boil or you have the fume effect. I really prefer not using thio due to the smell. Now you given me even more reason to not use it except with my dried indigo with caution.

  2. Thank you for this very informative information. I would like to switch to using a natural process for indigo.

    Where do you suggest I get started ? I would like to take a workshop.

    Thanks for your time.

    Regards, Kim Meuli Brown

    Sent from my iPhone – Please excuse any typos


    1. Joy and I include these vats in our book: The Art and Science of Natural Dye and they are also included in the Slow Fiber Studio DVD’s with Michel Garcia. It’s always good to take a class though, so that you can actually go through the process with someone who knows the process. Not sure where you live, but I’m teaching a class on woven shibori and the various indigo vats at Red Stone Glen in PA this July https://redstoneglen.com/classes/woven-shibori-and-the-organic-indigo-vat

  3. Thank you for the detailed post! I’m about to start/restart my vat after moving my studio, and had planned to go back to Thio after having issues with my 1-2-3 Henna vat. My main issues were the sediment making it necessary to dump the vat at certain point, wasting the water (where with Thio I can use the same water pretty much indefinitely), and the electrical energy required to keep the henna (1-2-3) vat hot enough to maintain a reduced state (I’m using 2 aquarium tank heaters).

    I find myself torn – my area (Colorado, USA) is recovering from a drought, and most of our electrical energy grid is fossil fuel driven… I want to make the best choice for my health, *and* the environment, but it seems nearly impossible to consider all the impacts!

    Your article is tempting me to try my hand at 1-2-3 again… it IS summer here now, so perhaps the energy issue will be much less problematic?


    1. Also – I see “mineral” reduction mentioned. I’ve only used plant reduction methods as an alternative… could you point me to a reliable source of info on mineral based reduction? I do have Michel Garcia’s DVDs, maybe I missed his reference to this?

      1. I’m not sure if Michel addresses the mineral vat in any of the DVD’s but Joy and I do include it in our book: The Art and Science of Natural Dye.

    2. If the sediment builds up too much in the vat, you can always just scoop some of it out and discard it. I do that occasionally with my own vats. The henna vat only needs hot water to get started. For the most part, I use it at room temperature after that.

      1. Can you let the vat cool and then re-warm/reduce it again like you can with thiourea dioxide vats?

      2. Yes, you can, but I don’t typically use my vats warm – just at room temperature. The thiourea dioxide vats require a higher temp but once the natural vats are in reduction they don’t need additional heat.

  4. Excellent article! I too used thiox for years in a university setting, and became aware of its ill effects. It was awkward to where a respirator while teaching. Then I also met with Michel Garcia in France in 2008, and I too changed my way of working with indigo using only mineral or plant based reductions from then on. Thank you Catharine for your, always clear, explanation of this issue and for sharing the science.

  5. Wonderful information. THANK YOU for this. This helps me to explain how indigo in denim looks vastly different than the indigo dyed goods that I produce to people. I’ve had nothing but the best luck with my 1-2-3 vat. I get virtually no fading. But I do have the advantage of dying fabric and letting it sit for months before I use it for garments which helps with the colorfast I’ve found. It becomes ‘one’ with the fabric. I say, dye now, put it away, focus on other projects, and use the indigo fabrics later if possible. If you plan out a rotation, you will always have colorfast indigo dyed fabrics to use in your crafts.

  6. Hi was going to write and ask something about your last newsletter but was too busy and see this one, so am writing wo planning. Was thinking was a snob”to use mostly natural (and safe) stuff, didnt like smell of most perfumes,lotions when a kid. Then had trouble breathing after ate apricots at lunch. Realized when was home from school, Mom said it was the sulfur dioxide. In college had a little trouble when worked in photo lab with the fixer (sodium hydrosulfite). Some have trouble with it and found if up my B12 and a few other nutrients, will keep breathing. Very interesting is your experiment. Had tried to bring my vat back(sat in garage a few yrs), but it stayed blue most of time. Did once get it to be green. Funny thing, it dyed fabric blue anyway and stayed on thru laundry so far. Have tried painting with it and maybe add to clay when plaster items. Indigo is so awesome. Thanks for all your experimenting and dyeing.

  7. thanks for writing about this, the planet will be more healthy as well as us if we all used the organic vat way.

  8. Thank you Catharine for your informative article. Thought readers might be interested in a further discussion about TUD.

    I was introduced to TUD (thiox / thiourea dioxide) by Patricia Black who cautioned on the health risks insisting that participants use a full respirator mask. So inspired by the process of discharge Patricia taught, I explored it with Shibori for several years. Interestingly, after bringing back a variety of black silks from a trip to Chiangmai Mai, I discovered TUD could strip back to silver, grey, yellow, mustard and even pink. Of course it was not so much the variety of silk, but the differences in the black dye originally used. Such stunning effects that it was hard to leave it behind, but with health and environmental concerns always on my mind, I felt it was not good studio practice, particularly as the discharge process required boiling, steam, fumes and consequently the use of a respirator.

    My directions changed with my enthusiasm for indigo Shibori and so too my tolerance for TUD. For the sake of blue beauty I compromised my principles for a while.

    I began using it again in my own practice and started teaching with it. The facility I have used for a number of years was a community college and I only had access on weekends and was not able to leave a vat there over any length of time. Sometimes I packed all my studio equipment in a little box trailer and traveled to a group of enthusiasts. Each time disposing of the vat at the end of the weekend. With most classes limited to 1 or 2 days, this is definitely the most convenient method of reduction.

    HOWEVER I still had an unease about TUD

    I attended the International Shibori Symposium in France in 2008 and Michel Garcia’s natural indigo workshops. Along with Catharine, I too was inspired, and since then have been enthusiastically exploring variations of his 1:2:3 indigo processes. I favour fructose and lime.

    I now teach that from my own studio space with good results. I caught up with Catherine again at the International Festival of Plants, Ecology and Colour in Madagascar where chemists, botanists, conservationists, traditional artisans and contemporary artists gathered to share knowledge and enthusiasm for natural textile techniques. We worked with local Malagassi women in the forest to dye raffia in a boiling indigo dyepot using fresh leaves from their local environment.

    My research has seen me peer into the blue of deep generational indigo vats in Japan, Taiwan, Thailand, and with women Kumba Pua Weaver’s of the remote Rumah Garie longhouse in Sarawak Borneo. A masterclass with Ababoukar Forfana sealed my resolve to use plant and minerals rather than chemicals.

    I have made major changes to my own artistic practice. My classes are conducted at home, restricting numbers to 6 people, taking more time, necessitating patience. But no more chemicals.

    I guess I’m relating this story to demonstrate that changes are taking place. With the loss of one of our beautiful weavers, Kay Faulkner, it is timely to appreciate the generational significance of such changes. Kay’s philosophy was to “weave beautiful textiles that industry cannot achieve”.

    At a time when the environment of our planet is suffering the onslaught of the consequences of fast fashion, pollution from the textile industry flows into rivers and oceans. Industry has much to learn from artists and traditional dyers.

    The last 15 years has seen a rise in global enthusiasm for all things natural and an explosion of the modern love of indigo.

    We artists have been leading the way. The Natural dye movement is dynamic – it’s dynamism is irreversible. Artists such as Catharine Ellis and Ababoukar, Forfana enterprises such as Maiwa, Botanical Colours and Aryana, botanical chemists such as Michel Garcia and Joy Boutrup and many, many traditional dye practitioners are such an inspiration. Cheers Susan

  9. Hello
    I’ve got a question about pre reduced indigo 60%. . Our group of dyers are using 60% reduced indigo and following your instructions for using it which were published on the net by Earth Guild. I was wondering if your disposal advice re neutralising with vinegar and lots of water still applies .
    I might add that the group love using this indigo as we can get the pot going in no time and do not have to heat it .
    Cheers Jane

    1. There is much that we do not understand about how pre-reduced indigo is made or stabilized, but if using it, I think this approach is still OK – or at least the best approach we know of.

  10. I too struggled with thiox vats and had to quit using indigo. It made me sick to be around the fumes. It certainly presented problems using natural dyes. I wasn’t able to access the color range using indigo. At some point about 8 years ago I made the decision to do some research and found the MSDS for thiourea dioxide online. https://prochemicalanddye.net/downloads/dl/file/id/61/product/0/thiox_sds.pdf. If you scroll down to Section 10 it states: “Hazardous Decomposition Products: Thermal oxidative decomposition of Thiourea Dioxide can produce toxic fumes of carbon monoxide, carbon dioxide, hydrogen cyanide, nitrogen oxides and sulfur oxides.” I consulted with a friend doing quantities of indigo dyeing for a company in Maine. She always felt a bit ill standing over a vat all day. I told her to have the evnironmental guy in the plant check the CO levels which he did. The carbon monoxide meter did not sound alarm but it did indicate there was some CO in the room. So it registered on the dial.

    At the same time I found reference to Michel Garcia’s 1-2-3 recipe. It seemed almost too simple but I decided to give it a try for some friends coming over to dye. It worked. No fumes or headaches. Shortly after I was able to attend his workshop in Philadelphia. I’ve never looked back.

  11. I actually have had the opposite result when dyeing with a thiox reduced indigo vat. For some time I was only able to get very pale shades of blue. I believe this was due to the concentration of dye stock to water as I was using 1/2 cup to a large stockpot of water. Even with multiple dips the color remained light blue. This spring I dyed with a friend who keeps an indigo vat going all year long. She used a much greater concentration of both indigo and thiox and was able to achieve midnight blue shades. Although I am interested in trying a natural vat, for my purposes the thiox reduced vat works well. I only dye with indigo once or twice a year and work either with a strong exhaust fan or dye outdoors. I dye wool fabric for traditional rughooking and once it is rinsed and neutralized there is never any smell attached to the fabric. Thank you Catherine for your posts which are always so informative and thought-provoking.

  12. Greetings Catharine, Gale here. Thank you for this most valuable info. . And, I love your new book. Congratulations !

    Sent from my iPad


    1. Thanks Gale! Joy and I are teaching right now at Penland and continuing to work on new solutions to old problems. The book is like a lifeline for the students in the class to talk home.

  13. Great post! It was exactly the same when we did natural indigo dye, bad smells and sometimes bad results too. Let’s try organic solutions immediately. Thank you very much for sharing this information.

  14. Hi, I’ve just spent a week with Michel and Yoshiko in La Faouet and you might be interested to know that he has made some some quite big changes to how he dyes both indigo and other natural dyes. Mordanting is done with a quick dip in a very small amount of solution and for indigo he is now mixing all three ingredients dry and then creating a vortex to add them to.

  15. Congratulations on a book I’m sure will become a classic, wish I had had it years ago. Started a fruit vat 6 weeks ago after receiving the book and am thrilled with how well it is working. Unfortunately the coil binding and pages have separately completely from the cover and if I hadn’t woven the coils together with strong linen thread all the pages were coming off the coils – I have sent an email to Schiffer as I believe this is their responsibility. btw. I took a workshop you taught many years ago in Vancouver, Canada, have been weaving shibori ever since.

    1. So sorry to hear of the binding issue. I’m glad that you contacted Schiffer directly. I will have a conversation with them as well. And congrats on your fruit indigo vat!

  16. Hi Catherine! I’ve a question. Do u think fabrics that have been dipped in Indigo vats that have been reduced using sodium hydrosulfite vs those that have been dipped in organic or fermentation vats are similar in quality— let’s say in their colourfastness or was fastness? Is there an obvious difference? I’ve been wondering about it. I’ve been exposed to the hydrosulfite vat, and have experimented with the fruit vat ( using bananas/ mangoes mostly)…still have to try the other organic vats. Thank you very much for your blog! I’ve also ordered it book! Kat

    1. Once the indigo has been successfully applied to the textile, I think that they would be of equal quality. The process of making the reduction vat is the only difference.

  17. This is wonderful information. My own personal philosophies align much more closely with the use of natural sugars and minerals for indigo reduction. The problem is that I just can’t get the 1-2-3 vat to work reliably. I have read and studied, watched videos, and measured carefully and precisely and I almost never get a good reduction. I can easily get the pH in the right range, but have never gotten a healthy flower. After weeks of tinkering, reading and experimenting, I inevitably get so frustrated by the process that I give up and go back to Thiox. I’ve probably thrown out 10 vats full of perfectly good indigo in frustration. Everything I read and watch makes it seem easy and straightforward- but it just isn’t. Unfortunately there isn’t anyone in my area to go to for advice. So, sadly, back to the thiox or hydrosulfite reduction vat it is for me.

    1. Hi Kimberly. Frankly, I’m a bit baffled as to why your vats have not worked. How large a vat are you making? The flower is not the most important thing – but the reduction that takes place beneath the surface. That’s hard to read from the top sometimes. Are your vats dyeing?? Are you getting a good blue color? You might consider making small vats – a liter or two in a glass jar. That way you are not investing a lot in materials but can monitor what is happing with the color, sediment, while dyeing samples. Where are you located? If you could locate a dyer in your area who is using the natural organic vats, perhaps that would help.

  18. Hi Catherine, thank you for your wonderful blog. It has been so informative. Looking forward to reading the book.

    I’ve been experimenting with the fructose VAT for a couple of months, and whilst initially it seemed to work well, I’ve noticed that some of my shibori test pieces have faded and yellowed a lot, over the course of a week! The white parts of the shibori have also started to turn yellow. I dipped them 6/7 times for 15 minutes per dip, and gave them an acid bath. The shade also seems to be more greenish blue even after 7 dips.

    What could the reason be for the rapid fading? They’ve not been in direct sunlight at all. I’m currently considering going back to chemical reduction to avoid this issue, unless I can figure out how to solve it. I started using an Indigo from a different supplier, so perhaps that could be the problem? Any pointers would be much appreciated!

    1. I can’t emphasize enough how important the rinsing/finishing is after dyeing with indigo. I think that the large amount of lime in the fructose vat can cause the problems that you are describing.
      Try a longer rinse soaking time. Allow the textile to oxidize overnight and then soak it in water for several hours or overnight. After that, neutralize in a mild vinegar/water solution. Then finish with a hot wash. You don’t mention what the fiber is that you are dyeing. If it’s cotton, then boil it for 10 minutes. If it’s silk or wool, then put it in very, very, hot water for about 10 minutes. Then rinse.

      1. Hi Catherine, re this hot rinse at the end of the process, I am worried that putting un spun wool or silk fibre in very hot water will cause a felting problem and how to stop the shock to the fibres? My 1.2.3 Fructose Vat is not hot, it is only hand hot and has always worked perfectly. I have been told that there is no need for vinegar at all especially when using Cotton fibre. Please can you clarify. Thank you.

      2. Agitation is what causes felting, not temperature. Bringing a “boil out” bath up to a very warm temperature and then maintaining it for 10 minutes or so will not damage the fibers.

        The fructose vat uses quite a lot of lime. That can be problematic to the textile. Unless the lime is neutralized/removed, it can cause havoc with the indigo blue, causing it to fade and lighten where exposed to the air – like on the edge of a folded textile.

        Lime is difficult to remove. I not only use a mild vinegar soak, but also soak it in water overnight beforehand. Then I finish with a boil, as indicated in our book The Art and Science of Natural Dyes. During the dyeing process itself it is very important to keep the textile from the loose lime that settles in the bottom of the vat.

  19. Hi Catherine,
    Thank you so much for your valuable post! We are recent transplants to the Blueridge, and I’m thrilled to learn about Penland. I will keep your name in mind when it’s safe to take classes again. I attended Haystack Mountain School of Craft while growing up on the coast of Maine and can’t wait to dive back into traditional textile craft. I wondered – when using sugar, what is the ratio between pre-reduced indigo, soda ash sugar? Thank you so much!

    1. Welcome to the Blue Ridge Mountain area! The pre-reduced indigo is not recommended and you don’t need that when using sugar. Use and indigo pigment (I recommend getting it from Stoney Creek Colors), fructose, and lime (calcium hydroxide). Soda ash is not alkaline enough for this vat. Joy and I go into this in detail in our book, The Art and Science of Natural Dyes, but you can also find instructions on the Stoney Creek, Maiwa, or Botanical Colors websites.

      1. Thank you so much! Oh, I am familiar with your book, now putting the name with the title. Very neat. I look forward to experimenting with indigo, and will check out Stoney Creek Colors – was not familiar with that resource, so thank you again!

  20. Do the organic vats release any toxic fumes at all?
    Thx for this info! I have been indigo dyeing w thiorea dioxide the last couple of days and I do notice my breathing is affected. I also have asthma which is another reason to switch but I am curious if the organic vats really smell much better or are much less toxic.

    1. No toxic fumes. All natural ingredients. The quick reductio vats (fructose, henna, iron) really don’t have much odor at all. Fermentation vats do have an odor but it is from the breakdown of organic materials. If you have asthma, then I think that the chemical vats would be a bad choice for you.

      1. oh thank you so much for this info! I’d like to try the banana mush vat but maybe the fructose one would last longer without going too off? The one good thing about the vat I am using now is that it doesn’t go off and I can re-use it in a few months.. but the fumes hurt my lungs it’s not worth it. I feel bad too because I read thiorea dioxide is a marine pollutant so I need to find a way of disposing it safely agh..

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