A working indigo vat is not blue. It is a dull yellow-green with a coppery film on the surface. Cloth lifted out of it is green too, and turns blue while you watch, over ten or twenty seconds, from the edges inward. There is no blue pigment in the leaves that started the process and none in the liquor. The colour is made in the cloth by the air. That reversal — colour as something that happens after dyeing rather than during it — explains nearly everything distinctive about indigo cloth, from the number of dips to the way it wears out.
Where the blue actually comes from
The leaves of dye plants used in China — Persicaria tinctoria, woad, Strobilanthes cusia among them — contain a colourless glycoside called indican. Soaking and fermenting the leaves splits off the sugar and leaves indoxyl. In an alkaline vat, and with the oxygen depleted, that indoxyl is reduced to leuco-indigo: soluble, and nearly colourless or pale yellow. Soluble is the important word. Only in this reduced form can the colour precursor get inside a cotton or silk fibre at all.
Then the cloth is lifted out. Oxygen in the air oxidises leuco-indigo back to indigo, which is insoluble, and the blue precipitates as fine particles inside and on the fibre. Nothing was transferred from leaf to cloth in the way a modern dye transfers. A soluble precursor went in and an insoluble pigment was made in place. The vat has to be kept reduced and the cloth has to be exposed to air afterwards; skip either half and you get no blue.
A pigment pretending to be a dye
Because indigo does not dissolve in water and does not form a chemical bond with cellulose, it is a pigment mechanically trapped in the fibre rather than a true dye. Two consequences follow, and both are visible in any old blue garment.
First, one dip deposits very little. Deep blue is built by dipping, airing, and dipping again — a dozen, twenty, sometimes more cycles, each adding a thin layer. Traditional recipes that specify a number of dippings are doing concentration arithmetic, not ritual. The depth of the colour is a count of repetitions, which is also why a dyer can stop at any point and get a repeatable shade.
Second, much of the pigment sits near the surface. Indigo crocks: it rubs off on hands and on pale fabric, and it fades first at cuffs, collars, seams and fold lines, exactly where the cloth is handled and creased. This is usually described as a defect, and in technical terms it is. It is also the reason people keep wearing indigo after almost everything else in the wardrobe has been replaced. The colour ages at the same rate as the garment and in the same places. A blue that stayed uniformly new would record nothing.
Keeping the vat alive
A fermentation vat is a small ecosystem with a job to do: consume the oxygen so the leuco form survives. Plant matter, lime for alkalinity, and a feed such as rice bran, wheat bran or molasses go in; bacteria do the reducing; the vat is kept warm. A healthy vat has a particular sheen on the surface, a particular smell, and a particular way of sheeting off the hand. If it takes up too much oxygen it is said to have gone off, and it will not dye again until it is brought back to a reduced state — fed, warmed, left alone. Modern vats shortcut all of this with sodium hydrosulphite, a reducing agent that works in minutes and is far more predictable, but the chemistry at the fibre is unchanged: soluble precursor in, insoluble pigment out.
The dyer's judgement is mostly about the vat, not the cloth. Too little alkali and the leuco form will not stay in solution; too much, or too hot, and the fibre is damaged or the colour goes dull. A vat is read by its surface film and the colour of the liquor on a lifted finger. It is one of the few crafts where the tool you spend the day adjusting is a liquid you cannot see through.
The pattern is the part you kept out
Since blue appears only where the liquor reached the fibre and the air followed, every resist technique is a way of excluding liquid. Wax painted or stamped on, threads tied tight, boards clamped over folded cloth, or a paste of lime and soybean flour pushed through a cut stencil onto cotton — the last is the basis of Chinese blue-and-white printed cloth. In every case the design is negative space. Nothing blue is ever applied to a pattern; the pattern is what was protected from the blue.
The character of each technique follows from how good the exclusion is. Wax cracks, so fine lines of blue creep through and the result has a characteristic web. Tied threads let a little liquor seep at the boundary, which gives the soft halo instead of a hard edge. Clamped boards give mirror symmetry because the cloth was folded before it was clamped, and the repeat is a property of the folding rather than the carving. And because each dip darkens the exposed areas by one step, a single resist can produce several different blues if parts are uncovered between dips. The whole palette is made from one vat and a sequence of permissions.
Synthetic indigo did not change the molecule
Industrial synthesis arrived at the end of the nineteenth century and made the same molecule cheaply, uniformly and in enormous quantity. Natural indigo cultivation collapsed within decades in most places. It is worth stating plainly what that means: the finished molecule is identical, and you cannot tell a naturally dyed cloth from a synthetically dyed one by looking at it. What differs is the small freight of other compounds in plant-derived indigo, which shifts the hue very slightly, and the entire practice around the vat. The romance of natural indigo lives in the process, not in the colour chemistry. A workshop that claims its blue is chemically different is saying something that is not true, and a workshop that claims its blue is worth more because it took twenty dips and a living vat is saying something that is.
This is also why the old recipes survive as technique rather than as secret ingredients. The knowledge is in reading a reduced vat, judging a dip, and knowing when to stop.
Reading a blue cloth
Look at the inside of a hem, the underside of a collar, the fold lines where liquor reached less freely. Those paler places are not fading; they were never fully dyed. Look at the boundaries of a resist for the halo that says a thread was tied rather than wax applied. Look for the step between two blues that tells you a section was uncovered between dips. Each of these is a record of a decision made at the vat, preserved in the object.
The leaf had no blue in it. The vat had no blue in it either. What you are holding is a log of how many times the cloth was allowed to meet the air — and, after that, of how many times it was worn.