Alum, Iron, and Ash: How Natural Dyes Are Made to Last

Pull a naturally dyed textile from a chest that has sat untouched for forty years and you may find the colour still intact — a warm saffron, a bruised indigo, a soft iron grey — as vivid as the year it was woven. Reach instead for a piece that was skimped on somewhere in the dyeing process and the same four decades will leave you with a ghost: a faint impression of what the colour once was, bleached by light and laundering into near-nothing. The difference, more often than not, is not the dye plant. It is the mordant.

What exactly is a mordant?

The word comes from the Latin mordere — to bite. A mordant is a mineral salt, a plant extract, or an earth that grips both the dye molecule and the fibre at once, forming a three-way bond that neither washing nor sunlight dissolves easily. Without it, most natural pigments simply sit on the surface of the thread rather than bonding to it; they colour the rinse water a little more with each washing and eventually surrender altogether.

In traditional Thai dyeing, mordanting is not a refinement or a shortcut. It is the hinge on which every other step turns. A dyer who knows her dye plants but skimps on mordanting will produce work that disappoints in a year. A dyer who understands mordanting can wring a dozen distinct colours from a single plant.

How does alum produce the clearest colours?

Among mordants, alum — potassium aluminium sulfate, known in Thai as สารส้ม (sara som) — is the most trusted in textile work and the most commonly used in Thai silk dyeing. It gives colours their clearest, warmest expression: a mango-bark yellow sings with alum where it might go muddy with iron; lac, the resinous red harvested from scale insects on rain-forest trees, blazes carmine and magenta when alum has prepared the ground.

For silk, which is a protein fibre like wool, the mordanting process is relatively direct. The skeins are simmered in an alum bath — often preceded by a mild acid pre-soak in tamarind water to open the silk’s surface — and then moved while still warm into the dye bath. The aluminium ion forms a coordination complex with the dye molecule, anchoring it inside the fibre rather than merely on it.

Why does cotton need a different approach?

Cotton behaves differently. Where silk and wool are protein fibres, cotton is cellulose — and cellulose has very few sites where an aluminium mordant can grip. A weaver who treated cotton the same way she treated silk would find the dye bleeding with the first wash.

The solution is an intermediate step: tannin. Bark from mangrove trees, dried myrobalan fruit, the husks of certain forest nuts — all of these are rich in tannins, astringent plant compounds that bond readily to cellulose and in turn attract and hold the metallic mordant. A cotton skein is first steeped in a tannin bath, then moved into alum, and only then introduced to the dye. The tannin acts as a bridge: cotton to tannin, tannin to mordant, mordant to dye. Remove any link in that chain and the colour is provisional.

What does iron do that alum cannot?

Iron mordants — drawn traditionally from iron-rich river mud, the rust of aged iron vessels, or water left to stand with iron filings — occupy a different register entirely. Where alum gives clarity, iron gives depth and gravity. Dyers in southern Thailand have long used iron-laden mud to shift and deepen natural colours: a golden yellow becomes olive, then forest green; a warm red tips toward brown; a pale indigo can deepen to near-black. The effect is called saddening, a term that suits the shift perfectly — the iron pulls brightness toward shadow.

Iron mordanting requires restraint. Overdone, it weakens the fibre by forming compounds that attract moisture and, over time, cause silk to shred along the dye lines — a fault that has destroyed more than a few antique pieces. Thai dyers working in the iron tradition measure by experience, not by formula.

What is the role of ash and lime?

Before either alum or iron touches the thread, many dyers begin with an alkaline wash — wood ash dissolved in water, or a solution of lime. This pre-treatment does several things at once: it opens the surface of the fibre, removes oils and sizing that would resist the mordant, and in the case of indigo dyeing creates the alkaline condition the vat requires to work at all. Banana ash was historically used in some northeastern workshops; rice-straw ash in others. The specific ash matters, because its mineral content varies, and that variation shifts the pH and the character of the final colour — a sensitivity that most mass-produced textiles never engage with at all.

Why does mordanting reveal a dyer’s skill more than the dye itself?

The dye plant is, in a sense, the easy part. Once a dyer has found a reliable stand of jackfruit root or a supplier of quality shellac, the plant’s colour potential is fixed. What is not fixed is how much of that potential she will coax from it — and that depends almost entirely on preparation. The same batch of pomegranate rind will give a bleached-out tan on unmordanted cotton and a glowing amber on cloth prepared with alum and tannin. The same lac will give a dusty pink or a saturated rose depending on the acidity of the pre-bath.

That is why, when you are evaluating a naturally dyed piece, the mordanting history is the part you cannot see but will eventually feel. A textile dyed with proper mordanting is something you notice after a decade of use, when the colour is still present, still distinct, still speaking. It is the invisible patience in the process — less dramatic than binding silk for mudmee, less photogenic than lifting a skein from an indigo vat, but no less essential.

To care for these textiles well once they are yours, the guide to washing naturally dyed handwovens covers the practices that protect the mordant bond over time. And if you want to test whether a piece was genuinely indigo-dyed or merely printed to resemble it, the signs described in how to read real natural indigo often begin with exactly this kind of process knowledge.

The colour in a well-made naturally dyed cloth is not printed on the surface. It was built into the fibre, mineral by mineral, plant by plant, long before the thread reached the loom.