Keeping an Indigo Vat Active Between Uses

An Active Vat Is a Living Thing

Indigo rewards rhythm. A vat that gets dipped every few days, kept warm and fed a little at a time becomes far more dependable than one you haul out of the corner after a fortnight and try to coax back into life. The real pleasure of keeping a vat active is spontaneity: the moment an hour frees up, there's a bath waiting, already reduced and ready. That means treating it less like a jar of dyestuff and more like a sourdough starter — something you top up and watch over, even on the days you don't dye.

Whatever type you run — a fructose vat, a hydrosulphite vat, or a traditional fermented one — the underlying needs are the same: reduction, warmth, the correct pH, and as little oxygen as you can manage. Neglect any of those for long enough and the indigo drops out of solution, the bath turns a dull grey-blue and your first dip does precisely nothing.

Feeding Little and Often

It's far easier to keep a vat topped up than to rescue a starving one. After a solid session of dipping, add a small amount back: for a five-litre fructose vat, roughly a quarter to half a teaspoon of indigo powder, pre-dissolved in a little warm water and lye, along with a teaspoon of fructose. Chemical vats want their reducing agent replacing instead — a scant pinch of spectralite or thiourea dioxide, plus a drop of stock lye if the pH has slipped.

Resist the urge to dump in a big feed. Over-feeding leaves undissolved pigment sitting on the bottom, where it does nothing but sour the bath and confuse your readings. It's better to feed modestly, wait an hour, stir a corner gently and check the colour of the liquid beneath the surface film. If it's a clear yellow-green or amber, the vat is working. If it's still murky blue, it needs more time, not more indigo.

Keeping the Warmth Steady

Temperature is where most between-use vats fail, particularly in a British winter. Indigo bacteria and chemical reducers alike slow down sharply as things cool. Fructose vats are happiest around 30–40°C, hydrosulphite vats for wool often want 40–50°C, and silk vats rather cooler, around 25–30°C. A thermometer is not optional — your hand is a hopeless judge.

• Sit the vat on a seedling heat mat set to low, with a folded towel between mat and bucket so the base doesn't scorch.
• Stand the vat inside a larger tub of warm water; the water jacket holds temperature for hours and buffers sudden swings.
• Wrap the sides in an old woollen blanket, a sleeping mat offcut or bubble wrap — insulation matters as much as heat input.
• Keep the vat in the warmest room you have, away from draughty windows and cold stone floors.

Check the temperature once a day, ideally at the same time, and nudge it rather than blasting it. Big fluctuations stress the vat more than a steady slightly-low reading.

Checking pH and Other Health Signals

Alkalinity holds the indigo in its reduced, soluble state. Fructose vats generally sit around pH 10.5–11, chemical vats a touch higher, up to 12. Narrow-range indicator strips covering 9–13 are ideal, and it's worth keeping a dedicated set just for the vat so dye splashes don't skew your readings.

A healthy vat shows a coppery, bronze or steely film across the surface. Push the film aside and the liquid beneath should be yellow-green or amber, with fine bubbles gathering and a faint slippery feel between your fingers. Fructose vats smell sweetish or faintly hay-like; hydrosulphite vats smell of sulphur, which is entirely normal.

Reading the Signs of Trouble

• Blue film returning as soon as you stir: the vat is oxidising faster than it's reducing. Add a little reducing agent, close the lid and leave it be for an hour.
• Green but pale dips: the vat is reduced but short of indigo. Feed gently and re-test.
• Grey or murky with no colour: usually cold and exhausted. Warm it, add reducer, then wait — don't stir repeatedly, as that only drives in more oxygen.
• Brownish and sluggish despite feeding: check pH first. Too acidic and nothing will reduce properly.

Always stir slowly and only at the surface, and keep the lid on between checks. Every needless swirl of the bath undoes an hour of quiet reduction.

A Simple Weekly Routine

Once you have the habit, maintenance takes minutes. Each day: glance at the thermometer and top up warmth if needed. Every two or three days: check pH and the colour beneath the film. After any dyeing session: skim off any scum, feed lightly, replace the lid and leave the vat undisturbed overnight. Once a week, or if the vat looks tired, give it a small feed of both indigo and reducer and test with a scrap of cotton before committing a proper project.

Kept in this way, an indigo vat will happily stay workable for weeks, sometimes months, deepening in character as it goes. The reward is a bath that's always ready when the urge to dye strikes — no setup, no waiting, just cloth and colour.