Dyeing with Dried Japanese Indigo Leaves

The easiest way to save Japanese indigo is to dry the leaves. This is also the only option, really, when you grow a small amount of plants.

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In traditional Japanese dyeing with Japanese indigo, the harvested leaves were composted (fermented) in a very specific way, sprinkling the leaf mass with water and turning it over. The timing had to be just right, and Jenny Balfour-Paul writes in “Indigo, Egyptian Mummies to Blue Jeans” that the indigo farmers referred to the packing of the leaves as “putting the baby to bed”. Every time the leaf mass was turned over, sacrifices of rice wine were made to Aizen Shin, the god of indigo.

Composting Japanese indigo was serious business – a difficult and big undertaking. The composting process can only get going if the leaf mass is sufficiently large, on the order of 100’s of kilos (or several hundred pounds). The end result were composted leaves that contained a higher percentage of indigo than the fresh ones. This mass is known as sukumo.

People who only grow a few plants (like I do) have to find a different method. Having read about it on Deb McClintock’s page, I decided to dry my Japanese indigo leaves last year. And I did manage to do so after some trial and error.

The dry leaves look like this:

Dry leaves of Japanese indigo, 2016 harvest.

Some of the leaves look a bit blue, and that does make you think there’s indio. I’ve been wondering why drying the leaves would work (the must have been good reasons for the traditional sukumo method) and I’ve come up with the following story:

In living leaves there’s no indigo, only a precursor called indican. Here, the meaning of the word precursor is a molecule that can undergo some reaction(s) that produce indigo.

Indican production is thought to be a defense mechanism for the plant. In living leaves, the indican is primarily found in a compartment within the cell called the vacuole (shown by a Japanese team of researchers in this paper).

The plant cell also contains enzymes that are able to break down indican, producing indoxyl and sugar, but these enzymes are found in other compartments of the cell.

When you pick leaves and dry them, cell membranes will break because of the loss of water. So at some point, indican and enzymes from other parts of the cell will mix, and indoxyl is formed. When two molecules of indoxyl combine, blue indigo is formed.

I used Deb McClintock’s version of John Marshall’s method but I fiddled about quite a bit, finding my way to do it. The main change is that I didn’t discard the yellow dye, so I get a green-teal instead of blue.

Green-teal with dried leaves of Japanese indigo. From left to right, the skeins are 1st, 2nd and 3rd dip in a vat made from 50 g of dried leaves (3rd skein was naturally grey). The skein on the left was dipped 3 times in a vat made from 25 g of dried leaves. I’m knitting from the first skein already, the striped boy’s jacket in the background.

For my first attempt, I used 50 grams of dried leaves to dye 3 100-gram skeins of wool. The vat stopped working early on, so I added a bit of this, a bit of that. That lead to no recipe, but the result was completely fine.

My next attempt was made during an indigo workshop I taught a while ago, and I know it was hugely optimistic to bring such a difficult project. That vat only gave a slight hint of mint green, but at least we got a lot of brilliant blues from the ordinary indigo vats.

Afterwards, I started thinking that the vat may have gone wrong because the temperature was too low. This also makes sense when thinking about this failed experiment where I kept leaves lukewarm for a longish time.

High temperature during part of the vat preparation seems to be important, and that is a part of the method I ended up with for my third attempt:

First, I simmered 25 grams of dried leaves in water (enough to cover them) for 20-30 minutes. It wasn’t a rolling boil, but some bubbling going on.

To dye blue, the first water should be discarded and new water poured on the leaves. I did not do that, so I kept the yellows from the leaves.

I added 5 grams of sodium dithionite and about 1 tablespoon sodium carbonate. Check that pH is 9, and add more sodium carbonate if it isn’t.

Then, I simmered the vat for 15-20 minutes. It seems wrong to boil a vat after adding reducing agent and base, but in my attempt where I didn’t boil it at this step, it didn’t work.

I took the pot off the heat and added another 5 grams of sodium dithionite. I let it sit until the temperature was 40-50 C, then strained the leaves out. For my first attempt, I left the leaves in to get as much out of them as possible, but that is not a good idea. At this point, they are quite slimy and stick to the yarn.

When the temperature was 40-50 C, I put the pot on gentle heat to stay at that temperature. At this point, the vat is ready for use. I dipped a 100-gram skein of wool 3 times, and it turned a nice teal.

I’m impressed by the dye content of the leaves. 50 grams of dried leaves gave nice color to 300 grams of yarn, and 25 g gave a brighter color to 100 grams of yarn. My last vat was not exhausted, it had turned dark the next day because the indigo had been oxidized. I didn’t have more yarn on hand, but the vat could have given light shades on another skein.

Wool dyed teal with Japanese indigo, accompanied by fresh and partially dry leaves.

But I’ve saved the best for last: light fastness. I tested light fastness of the first skein from the 50-gram vat from July 1st to September 1st. The left side was covered and not exposed, right side was exposed to the light. I can’t really see any difference between them, and that means the light fastness rivals that of indigo blue. And that is quite impressive for a green-teal color!

Light test of Japanese indigo teal. Two months of sunlight did not affect the color.

PS: I’m growing Japanese indigo again this year. I harvested the first leaves on September 17. this year, and they are drying. They look even bluer than the ones from last year…

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