Indigo: Still Learning, and at last….. Indigo Fermentation

The indigo chapter of the book Joy Boutrup and I wrote, The Art and Science of Natural Dyes (Schiffer Press),  focuses on the use of quick reduction vats that use iron, henna or sugar, along with lime (calcium hydroxide) to reduce the indigo. Since the book was published I have been very interested to learn more about indigo fermentation. The concept seemed daunting and I was hesitant to begin.

Why did I want to make a fermentation vat? 

Initially, I thought that the indigo crocking issues that I described in the last post might be solved by making a vat that did not use large quantities of lime. 

The vats that use large quantities of lime (calcium hydroxide) also seemed to be presenting a challenge with “fading” issues. I had fabrics that were not exposed to direct light,  but the exposure to air itself seemed to make the indigo fade in a very unattractive manner. I consulted with another experienced indigo dyer who was having this same issue with her vats and we suspected that the amount of lime in the henna and sugar vats was part of the problem. Textiles dyed in an iron vat do not seem to exhibit this type of fading, despite the high volume of lime.


And finally, and maybe most importantly, fermentation seemed like the ultimate in understanding indigo. It’s the oldest, traditional process where bacteria is used to reduce the indigo.  I wanted to experience it.

At the end of last summer, I made the commitment to begin working with a fermentation vat. Hisako Sumi, a friend, colleague, and indigo dyer from Japan, has been encouraging and coaching me through the process. 

I began with a simple recipe for a fermentation vat that was published online by Cheryl Kolander of Aurora Silks. The vat was made with 

    • indigo pigment 
    • ground madder root, the source of fermentation 
    • soda ash, to achieve the correct pH 

It worked! It took several days of warm weather (or some applied heat) and patience. The blue dye from the vat was clear and strong. Thank you, Cheryl.

Since that first vat I have made and maintained many small (1 or 2 liter) experimental vats.  I have a 30 liter vat that I have using regularly for several months and am now preparing to make a 150 liter vat for use with larger textiles. 

The Indigo dye

A typical fermentation vat in Japan is made using sukumo. Sukumo is composted indigo (polygonum tinctorium) leaves. The sukumo is both the source of the indigo dye and the source of bacteria that results in fermentation of the vat. I do not have access to sukumo  although I still have plans to make a small batch of sukumo.  I have been using organic indigo pigment from Stoney Creek Colors for all of my fermentation vats. 


Fermentation vats do not require the high alkalinity that is necessary for the quick reduction vats, which perform best at a pH near 12.0. The fermentation vats require a pH between 9 -10.  I have made many fermentation vats since that first one, experimenting with various alkaline sources: wood ash lye that I leached from hardwood ash, soda ash, potash, and very small amounts of lime (calcium hydroxide) or lye (sodium hydroxide) to control the pH.  I have monitored the pH carefully.  In the first few days, when fermentation is beginning, the pH will go down.  

pH papers were not accurate enough to discern the pH fluctuations. I invested in a good pH meter, which I calibrate regularly. Because of the relatively low pH, the vat is suitable for both cellulose and protein fibers without fear of damaging the textiles. 

Organic material for fermentation

I used a very finely ground madder root powder from Maiwa in my initial experiments. I’ve been told that the freshness of the plant material matters for purposes of fermentation. Madder root was traditionally been used as a source of bacterial fermentation. Indigo dyers typically used madder root that had already been used for dyeing red. Once the red dyes have been extracted, the plant material is still a viable source for fermentation. 

Other sources of material for organic fermentation that I have used include: wheat bran (cooked for a few minutes in a little water) and dried, ground indigofera tinctoria leaves, which are sold as a hair dye (also called “black henna”). I was given a woad ball and added that to one of my vat experiments and it definitely speeded up the fermentation/reduction.

Traditionally, in England, woad balls were “couched”  or composted in order to extract both pigment and provide bacteria for fermentation – much like sukumo. When indigofera tinctoria was introduced from India, woad was used more often to boost fermentation, as opposed to being the primary source of dye.

Hisako encouraged me to use indigo plants from my own garden to to increase the source of bacteria for the vat. I grow small amounts of persicaria tinctoria, indigofera suffruticosa, and isatis tinctoria in the garden.  After grinding the fresh plant material, forming the balls or patties, they are dried for storage and added when needed to boost the fermentation of the vat.  I assume that once the plant material breaks down in the vat, it is also a minor source of indigo pigment but this is speculation. 

Natural fermentation vats have changed how my dye studio smells – no longer the sweet sugar or plant smell of the quick reduction indigo vat – but now the odor of true fermentation and rotting plant material. I’ve quickly gotten used to it and it is now the smell of a successful vat, though my husband finds the smell very offensive and avoids coming into the studio!


The ideal temperature for fermentation is between 80-90°F (27-32°C). Think of the rising of bread dough! Warm climates are the natural environment to make and maintain these vats. In my North Carolina mountain dye studio (with no heat) it is more of a challenge. In Japan, I saw many large indigo vats wrapped in electric blankets. I use heating pads around my small one-liter and two-liter vats and a bucket warmer (used to keep honey in a liquid form) for the larger vat. To prevent the vat from getting too warm, I use a temperature controlling outlet with a thermostat and probe.  

Maintaining the Vats

I have been keeping careful records of these vats, sampling on a regular basis and documenting and recording any additions. Keeping good records is key to my understanding and confidence! 


Every few weeks the vats are “fed” with a small amount of cooked wheat bran or a small amount of indigo balls or patties. 

The quick 1,2,3 reduction vats using sugar, fruit, or plants have introduced many of us to non-chemical reduction processes and made it possible for us to dye with indigo on a regular basis. They are accessible,  easy to make, and can be ready within a few hours, thus making them ideal for workshops and experiments. These vats have taught us the principles of indigo reduction and dyeing and I am forever grateful to Michel Garcia for teaching us about them.

For the long-term serious dyer, I  believe that the fermentation vats are a viable alternative. They require more time and attention but I have been surprised at how well these vats have tolerated neglect when I am away from the studio – sometimes for several weeks. When I go away, I turn off any supplemental heat, cover them, and just let them be. When I return home, I check the pH and adjust if necessary. Then I dye a test sample. If the color is weaker than when I last tested, I will “feed” them, stir, add some heat and wait a day before test dyeing again.  

The fading issue seems to be solved with the use of fermentation vats. I can’t say that I have completely solved the crocking issue, though I think that yarns dyed with the fermentation vat are crocking less.  Maybe this is just the nature of indigo… 

37 thoughts on “Indigo: Still Learning, and at last….. Indigo Fermentation

  1. I’ve been wondering about the workshop you and Rowland Ricketts will be giving on March 11 . Will you have a vat to demonstrate? What will be discussed?

  2. Hi Catherine,

    I love your book. I used the recipe ‘ferrous acetate for printing’ with vinegar and soda ash-24B, yesterday. I left some in the refrigerator overnight and this morning see that it has separated out into layers. Can I keep the paste (without and dye powder added) for a day or so, or does it have to be used immediately?




  3. Thank you for sharing your findings with Indigo, very interesting piece. You have achieved some beautiful blues. I am not sure I’d feel able to manage the smell so I’ll stick to the sugar ones for now, but I feel encouraged to try sometime in the summer outside.

  4. Thanks so much for sharing your fascinating fermentation trials. I used my home-grown Woad last year to do a small fermentation test with Indigo, without any other reduction agent. It worked well, so on to larger trials later this year when I have more fresh material. I’d love to understand the chemistry of why a fermentation vat results in less crocking/fading. Is it mechanical – does fermentation somehow open up the fibres more, allowing more pigment to enter/be trapped inside them – or is it chemical – is there a stronger molecular bond that forms between the pigment and the fibres – or a combination of both? And does this apply to other fermented dyes (per Anne Rieger’s fermentation method). Perhaps a theme for a new book with Joy Boutrup?!

      1. FWIW a friend who was responsible for colour production at a pulp mill (paper) says cellulose fibres ‘open’ at high pH (>11 in the conversation, vs wool) to take up more dye.

  5. Fabulous post! You’ve inspired me to try a fermented vat after disappointing results with the lime fructose vat. Would like to know more about workshops!

  6. Weather here in the Central Valley of CA should warm up to the 70s and 80s next month. I will try my first fermentation vats. I have been using a variation of the 1 2 3 vat method for dyeing, so I am excited to see how this vat will work. Thanks so much for posting.

  7. Hi, just wondering which pH meter you use. I’m giving up on strips and need to switch to a meter.

  8. I am a huge fan of yours and read your experiments hungrily. They are scientific and so full of great research and experimentation. Thank you for all your work and information.

  9. Hi!! I just read your article and I feel totally identified. So far, I am using the indigo tub with fructose and what I see is that in a large amount of fiber although you do many dives of the fabric the color is not uniform, I also have samples in which the color goes with the weather. I want to bet on the Japanese fermentation tub. In October I will go to Japan to learn, meanwhile this spring I will plant persicaria. There is a very good study of Fibershed, in case you are interested. I don’t have sukumo now, but I want to make it. The tub with madder is not good for me because the madder is quite expensive to get in Spain, I have a few cultivated and they are not enough. I wanted to ask you if it is possible to make a fermentation tub with indigo powder with ash bleach and wheat bran. Thank you!!! I am writing to you from the Basque Country, in Spain

  10. Thanks for this, Catherine. It’s fascinating.

    You wrote: “The vats that use large quantities of lime (calcium hydroxide) also seemed to be presenting a challenge with “fading” issues. I had fabrics that were not exposed to direct light, but the exposure to air itself seemed to make the indigo fade in a very unattractive manner.”

    1. The fading is indeed an issue; lots of pieces of mine have faded/yellowed. However, I’ve been trying to keep my vat at 10-11, and so far pieces I’ve dyed in that pH range haven’t crocked since I’ve been following your practice of 5 or so dips at 15 mins per. I also beat the fiber on roadway or concrete (post or walkway) and that evens out the color amazingly. One down side is that the vat’s pH drops over night to 9 or less sometimes. Added calx raises it temporarily.

    2. Is it possible to transform a calx vat (indigo, henna, calx nourished with fruits like pawpaw and banana—but loved pawpaw best) into a fermentation vat or is it necessary to start from scratch?

    1. I love the image of you beating your cloth on the roadway! I am not optimistic about transforming a calx vat intern a true fermentation vat. You will still have all that lime/plaster to deal with in the vat. In theory, I had always assumed that the quick fermentation vats became fermentation vats over time, but I find that they do need a much higher pH than the true fermentation vat.

      1. Thanks. It looks like I’ve to find a new container for a true fermentation vat. Now, I’m wondering how the fabric will fare if it’s dipped in the paw-paw-loving vat and then has two final dips in the fermentation one. I must try that to see the color result and whether the fade will occur.

  11. I’ve been finishing indigo dyed wool and came up with a question: after the vinegar/acid bath, you recommend (chapter 9) simmering for 10 min. What if the wool will be over dyed, and simmered for >10 minutes as part of the over dyeing process? Would the heat involved with overdyeing accomplish the same end? (Just trying to conserve resources, including the energy of the dyer;-))


  12. I can’t help wondering if a fermentation vat can be tasted. I used to taste my henna vat, and that gave a good indication of the health of the vat.

  13. Hi Catherine
    Can I use other plant material to feed the vat in the manner you describe with madder, after dye has been extracted such as weld?

    1. HI Joyce, I have never used weld or heard of it being used. Madder is an anthraquinone and has traditionally been used for indigo vats. I’m sure that there are other plants that could be used but I certainly don’t know them all, though I did recently use cassia seeds successfully. I’m guessing that weld would only have cellulose left after the dye extraction. I’m using this quarantine time at home to further investigate the fermentation vats.

      1. thanks, I have read there is some enzyme in madder that is beneficial but I wondered if it was only madder that could be used.Weld to the compost pile then! Good luck with further investigations. Keep well.

  14. Thank you for sharing this information, Catharine! Summer break just started and experimenting with a fermentation vat is at the top of my to-do list. Your thorough investigations and note-taking are inspirational as always!

  15. I did not find any crocking with an urine vat. At all. I use a small urine vat for projects that don’t require a big quantity of yarn. However, when I tried to make a big one for larger quantities of yarn, I found working with it rather unpleasant.

  16. Catherine, you said: “I can’t say that I have completely solved the crocking issue, though I think that yarns dyed with the fermentation vat are crocking less.”

    The answer to that may lie in this book, “Nuno ni odoru hito no te: Chugoku Kishu Myozoku senshoku tanbo 18-nen”, aka “Imprints on Cloth” by Sadae Torimaru, at Also, in the notes on indigo at Asian Textiles, this: “To make black, some weavers add tannin plants to their indigo vats, or overdye the indigo with tannin and iron.” From these two sources, I gathered that the addition of possibly glutinous animal protein (the Miao use boiled extract of water buffalo belly skin as well as egg whites or pig’s blood) or tannin may prevent crocking. A particular favorite seems to be tannin from morinda (noni tree). Happily for me, the yard of my father’s house has such a tree. An alternative might well be the addition of walnut leaves.

    I’m going to try using boiled pig’s belly skin (a cousin is butchering a couple of his pigs this weekend) and pig’s blood plasma (provided I can get the blood because it’s sold to make black pudding).

    1. There are many treatments that help to prevent crocking – a simpler one is to use an application of fresh soy milk which would work similarly to bind the dye to the textile to prevent crocking.

  17. Still trying to get my vat to come to order again. It’s in a five gallon bucket, and it has two seedling mats wrapped around it. This seems to work to keep it at around 90 degrees. The pH is 9.5. I also added some cooked bran. But no fermentation. I wonder if I need to add some more madder. I can’t figure out what’s wrong with this vat. It was working fine in July.

    1. I have three large vats that are going strong after 2+ years. I had another ( a combination of sukumo and indigo pigment) that I began last fall that was dyeing beautifully and it just quit after about 5 months. I never really understood why.

      1. When they quit, do you discard them, or add more stuff to that vat to try to get it going again? I was thinking of maybe adding more madder or indigo to the vat that won’t reduce, but maybe I should just pour it out.

      2. Starting to wonder if maybe this vat got too hot. One time it got to over 100 degrees. I wonder if that kills the bacteria? It wouldn’t kill most bacteria, but maybe the indigo bacteria are more particular about temperature. Also mine doesn’t have any smell at all that I can detect. I am going to keep trying for another week or so. If it doesn’t reduce after that, I may start over.

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