Using only urine (ammonia) and natural indigo, the sig vat is perhaps the least polluting, most environmentally-sound of all the indigo vat recipes, but the need to collect and store urine, the smell, and the need for multiple dips to obtain darker blues meant the dyeing industry welcomed development of new methods of dyeing with indigo. By the late 20th century the (extremely convenient) sodium dithionite/hydrosulphite vat (the zinc-lime vat developed in 1845 is a type of hydrosulphite vat) was responsible for the bulk of indigo blue on textiles, but concerns about pollution associated with the process – toxic sulphites, sulphates, and sulphides are produced as degradation products – was increasing. In the late 1970s thiourea dioxide (effective at only 10–12% the usual concentration of sodium hydrosulphite), was being proposed as a more environmentally-sound reducing agent. Now, reducing sugars are under investigation.

Reducing sugars contain or generate aldehyde groups that, activated by alkalinity and heat, will absorb oxygen from a solution. Fructose and glucose found in ripe fruit are reducing sugars; ordinary sugar – sucrose – is not a reducing sugar. So it’s possible to create an vat from indigo, lime (calcium hydroxide) and over-ripe fruit such as bananas, or dates that relies on the chemistry of the sugars rather than fermentation of the fruit. (There’s a blog post about a banana vat here.) But it’s easier to prove the principle using fructose from the supermarket shelf in Michel Garcia’s 1-2-3 recipe. It’s very simple: One Part indigo to Two Parts lime to Three Parts fructose, plus warmth.

I warmed roughly 1 litre tap water (for the record our tap water is alkaline, roughly pH8.4) to 50–60°C in a saucepan. I dissolved 15g indigo – for this I used Tamil Nadu indigo – in warm water, added it to the saucepan, then did the same to 30g lime, then gently stirred in 45g fructose. The Tamil Nadu indigo is markedly ‘blue-er’ in solution than the ordinary indigo I’ve used up to now; the solution was a vivid deep blue.

I left this to work for about an hour, at the end of which it showed all the signs that indigo had been reduced and the vat was ready.

Vat ready for use. Note the flor, the metallic copper/purple/blue skin of indigo that forms where the reduced indigo oxidised on contact with air. In this vat there were metallic bubbles, too. Beautiful! Scrape the flor to one side with a spoon before adding materials to the vat, or remove it altogether and replace when you’ve finished.

Beautiful, but the strong reduction is an indication of high pH and indeed, the pH paper read 11 or higher. I added a skein of handspun silk, DRY, because I wanted to see what happened if I did so. The silk instantly went blue, as did the vat; wearing rubber gloves I squeezed air out of the silk until it sank, then reheated the vat to 50°C and left it to work, theorising that there was sufficent fructose to re-reduce the solution.

45 minutes later I removed the silk. Beautiful!

I thought about a second dip, but worried about the effect of longer exposure to high pH and decided to wait to see what colour this became. After all, I have all summer to make things blue.

Three silks for comparison: on the left, 45 mins in the fructose vat; in the middle, about 12 hours (two dips) in the sig vat; on the right, 90 minutes in the sig vat. Interesting!

We left our trusty sig vat on May 15th, hoping for a warm day that didn’t occur for another two days. But by the 17th the liquid was very definitely yellow-green although there was a lot of sediment and  particulate matter adhering to the glass of the jar. I think this is a combination of vegetable and mineral matter in the indigo powder – the actual indigo content of ‘ordinary’ indigo may be well under 30% by weight – and some debris from the urine itself. At any rate, Sunday the 17th was forecast warm and sunny, so on Saturday I de-gummed 2 smallish skeins of handspun silk, neutralised the washing solution with a vinegar rinse, then rinsed them thoroughly to remove the vinegar, and left them soaking in water ready for dyeing. Remember that the soluble leuco-indigo needed to dye material converts to the insoluble, non-dyeing blue form in the presence of oxygen: it is generally advised that fibre be thoroughly wetted to minimise the amount of oxygen it introduces into the vat. If you’re dyeing in a urine vat, urine is the obvious fluid to use for wetting, as it adds nutrients for the fermentation.

Sunday morning I cracked open the lid of the jar in the greenhouse and the smell of the vat rose to greet me. A powerful aroma, now with some ammonia in the mix. I put both skeins in the jar, which on reflection was a bad move: there really wasn’t enough room for both. Also, as they were loose in the jar, they came into contact with the sediment.

My handspun silk in the urine vat in the greenhouse with the Japanese Indigo plants.

I couldn’t wait to check whether it was working. Literally. The timestamp tells me this photo was taken only three minutes after the skeins went into the vat. How sad is that?

But look! It’s going blue! I pushed it back into the jar to go bluer.

I removed both skeins from the jar after 1.25 hours and hung them to dry on an improvised rack to oxidise. Note how turquoise the yarn appears at the beginning of the oxidation.

Now, at this point I digress to explain that throughout the afternoon my husband was walking back and forth from house to garage past the greenhouse and, eventually, the yarn from the vat. When he first encountered the smell of the vat he commented that it was a bit whiffy. I quickly shut the greenhouse to trap the odour. But by 1530, with the yarn out to oxidise, his comments were more forceful, along the lines of “The birds are falling dead from the trees!” The smell does fade from the dyed material; it’s much less noticeable when the material is dry, and I find that after washing with scented detergent and hanging to dry outside on the rosemary bush, I can only smell it if I press the fabric to my nose. The distinctive scent of the urine vat was once regarded as an indicator of good cloth well-dyed: when chemical vats were introduced in the second half of the 19th century, cloth-buyers would check for the smell of the urine vat. Some cloth merchants used commercially-available fake urine vat scent to mislead them!

As the oxidation proceeded I was able to see just how much gunge was stuck to the skeins, but could reassure myself that it will wash off (and it mostly did, even in the quick post-dyeing wash). I put the vat to bed (as it were), topping it up with fresh urine and replacing the bag of indigo.

Once washed and dried the skeins proved paler than I’d hoped, so I tested one of the characteristics of the urine vat. With pH paper. Most indigo vats are strongly alkaline (my post on Indigo chemistry explains why), running at pH11 or even higher. This alkalinity degrades and damages protein fibres such as silk, so repeated or lengthy dips to get greater depth of colour are a bad idea. The urine vat, however, is much less alkaline: this one, yellow-green and ready to dye, tested pH9 on the 24th May. I decided to see what happened if I put a dry skein of silk for a longer period (dry because I had read that putting the fibre in dry could give a greater depth of colour). I pushed a skein into the jar late in the afternoon, took it out to oxidise for a couple of hours in the early evening – it was noticeably darker – and then replaced it in the jar overnight. End result: a significantly darker indigo.

It’s still a little bit turquoise, but obviously much darker than the skein next to it. Which is in fact a bit darker than it looks in the photo: light reflecting off the silk makes it paler.

Conclusions

Pros: This is the easiest and least expensive of the vats I’ve tried so far (more than I’ve written about so far), and it’s kinder to protein fibres such as silk and wool than the other vats. The results are good, but I think they’ll be better in a different vat. I think a higher grade of indigo (containing less waste material) will give me darker colours, and a larger, deeper vat will make it possible to avoid any sediment, perhaps by placing something like a colander or strainer at the bottom of the vat to keep the material off the bottom. So I’ve ordered a better grade of indigo and I’m about to start collecting more urine.

Cons: This vat has a strong, distinctive, and – to some people at least – highly unpleasant odour. While the odour from the vat can be controlled and contained, the smell coming off the dyed material as it oxidises is less easy to contain if it’s hung out to air. However, it is possible to oxidise the dyed material in well-oxygenated water: allow the cold water (cold water holds more oxygen than hot) to plunge into a container in such a way as to generate lots of aeration bubbles. Immerse the fabric in that water, swirl it around, keep the tap running to refresh the oxygen. This is not a technique to use with warm wool, but I have tried it with silk and found no damage, although the skein could have done with more ties. In theory it should not remove indigo that wouldn’t be lost in the normal washing process, but I have no real way to test that theory at the moment.

Oxford English Dictionary: Also seg, sigg, zig(g). Of obscure origin, the form does not correspond to older Flem. seyck, G. seiche, in the same sense. Urine.

Anyone horrified by the idea of working with urine should leave the room now…

Urine has been a valued ingredient in the dyeing process for many centuries: dyers had containers on the street to collect contributions from passing members of the public. Fresh urine is sterile, or nearly so (If you can’t get clean water to wash out a wound, the next best thing is to pee in it. Honest!) but bacteria soon begin breaking down the proteins, sugars and other compounds in the urine to produce ammonia, which gives stale urine that pungent, penetrating odour. Ammonia is a very useful alkali and, just as important for working with indigo, the bacteria that create it consume oxygen as they work. The resulting low-oxygen, alkaline environment is ideal for converting indigo into its soluble form for dyeing – and all that’s needed is pee! Incidentally, the first pee of the morning is better than any other, as it’s richer in nutrients for the bacteria. The urine of children is better than that of adults, as their higher metabolic rates mean more nutrients in the urine. The sugar-rich urine of diabetics is good, too. But beware the urine produced by those indulging in diuretics such as beer: it contains fewer nutrients as the body tries to flush out the toxins with higher volumes of fluid. Some urine vats fail to ferment for no obvious reason; it’s possible that some medications interfere with the bacterial action.

The simplicity of this process has interested me since I first read about it, and it’s an obvious candidate for inclusion in my Summer of Blue. So, as the teaser image on the first woad vat post suggested, I made some preparations.

That’s a 3 litre jar, half-full of our urine, with a bucket containing thermal insulation to keep the fermenting urine warm. I collected more urine over the next day. All that’s needed for a urine-based indigo vat: urine, natural indigo, and a twist of fine cloth to contain the indigo powder.

The cloth with the indigo is tied tightly and dropped into the jar. At this point the urine has almost no smell at all, certainly nothing offensive.

The bag of indigo is squeezed gently every day to release more indigo into the fluid, which will be blue until bacterial action – fermentation! – creates the high pH, low oxygen environment in which the blue indigo converts to the form in which it dyes, the yellowish soluble ‘white indigo’ or leuco-indigotin. Fermentation requires gentle warmth; I was going to put the jar in that bucket on a hot water bottle surrounded by insulation, but the warmth of the sun persuaded me to try an easier solution:

I simply sat it on the warm soil in the pop-up greenhouse that is sheltering my Japanese indigo plants. That was May 13. On May 14 the temperature plummeted and we had rain; the 15th had slightly more sun, but was still cold. Today, the 16th, is warm again. I foolishly didn’t take a photo of the fluid surface before I squeezed the indigo, so I can’t show you the tiny patches of metallic scum, a sign that it is working. But in this photo you might be able to see the greenish rather than bluish colour of the liquid. More important is the smell: it has the rich organic odour of stable manure or something similar. Nothing like the acrid ammonia of stale urine.

And look at the change in pH over the course of three days! No lime needed here:

If tomorrow is as warm as today, I might put some wool in that jar tomorrow evening, just to see what happens. The urine fermentation vat works at a far lower pH than the vats reduced with thiourea, so they’re much kinder to protein fibres. At pH 9 wool and silk can sit in the vat for several days without damage, or they can be dipped repeatedly, both processes yielding darker shades of blue. Blue!