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!