Indigo is the awkward one. The pigment in the tin will not dissolve in water, so you cannot simply stir it into a warm bath and dunk a shirt. To dye with it, you have to reduce it — change its chemical shape so that it dissolves — and that means building a vat. A vat is really a small, controlled chemical environment: alkaline, low in oxygen, and warm enough to work. Get it right and the liquid sits there looking yellow-green; lift your cloth out and it turns blue in the air in front of you, every single time, like a trick you never quite tire of.
This guide covers two vats that beginners can manage with supermarket and dye-supplier ingredients: the fructose vat, which is gentle and forgiving, and the thiox vat, which is faster and stronger. Read the safety section before you buy anything.
Indigo pigment is insoluble. In an alkaline, oxygen-poor solution, a reducing agent strips oxygen from the molecule and turns it into leuco-indigo, which is soluble and pale yellow-green. Fibre soaked in that solution takes up the leuco form. When you lift the fibre into the air, oxygen puts the molecule back the way it was, and the colour develops from green to blue.
Two things therefore govern everything: keep the alkali strong enough, and keep oxygen out. Almost every beginner's mistake comes down to one of those two.
Caustic soda and slaked lime are both alkalis, and both will burn skin and do genuine damage to eyes. Gloves and eye protection every time, even for a small top-up. Always add the powder to the water, never water to powder, and stir gently — caustic soda releases heat as it dissolves. Work somewhere ventilated, never in a kitchen where food is being prepared, and keep the vat out of reach of children and pets. Indigo is a fine dust, so wear a mask when you measure it.
When you finish with a vat, neutralise it with white vinegar until it stops fizzing, then check it with pH paper before pouring it away. Do not tip strong alkali down a drain in quantity. If alkali touches your skin, rinse with plenty of cool water; if it reaches an eye, rinse immediately and seek medical help.
Start here. Fructose vats run cool, smell mild and tolerate a certain amount of clumsiness. The classic ratio is 1 part indigo to 2 parts calcium hydroxide to 3 parts fructose. For a 2-litre vat that means 10 g indigo, 20 g calcium hydroxide, 30 g fructose and 2 litres of water at about 40 °C.
If it is still blue after twelve hours, add another 5 g of fructose, stir gently and wait. Fructose vats can take a day or two to get going, and patience is part of the method rather than a sign you have failed. Because lime is a milder alkali than caustic soda, this is also the vat to choose for wool and silk, keeping the temperature below about 40 °C and the dipping times short.
Thiourea dioxide reduces indigo quickly, which is useful when you want to dye the same afternoon. It uses caustic soda, so treat it with more respect. For a 2-litre vat: 10 g indigo, 20 g sodium hydroxide, 10 g thiourea dioxide, and 2 litres of water at 50–60 °C.
Keep the temperature steady. A thiox vat that goes cold slows to a crawl, and one that is too hot breaks the reducing agent down before it can do its job.
Wet the fibre out thoroughly in warm water and squeeze it. Dry cloth carries air into the vat and knocks it back. Lower the fibre in gently — no splashing, no bubbles — and move it around under the surface for two to five minutes. Squeeze thick bundles below the surface rather than lifting them dripping.
Lift the fibre out slowly, let it drain, and hang it up. Watch it: yellow-green, then green, then blue over 10–20 minutes. Do not rush this part. For deeper colour, dip again. Five or six short dips give a richer blue than one long soak, and the vat appreciates ten minutes to settle between them.
A vat rewards a little routine. Top it up with a spoonful of fructose, or a pinch of thiox with a little caustic soda, whenever the colour weakens or the surface goes dull. Keep it covered, keep it warm, and keep your hands and spoons out of it more than you need to. Treated that way, a well-behaved vat will keep working for weeks, and you will find yourself looking for things to dye.
Keep a notebook from the first vat. Note the quantities, the temperature and the number of dips, because the second vat is always better than the first, and the third better still.
