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!