Choosing Mordants for Vibrant Natural Dye Colours

What a Mordant Actually Does

Natural dyeing has a reputation for soft, faded colours, but that is almost always a mordant problem rather than a dye problem. A mordant is a metal salt that binds to both the fibre and the dye molecule, forming a bridge that locks colour into the cloth. Without one, most dyes rinse straight out of wool and barely cling to cotton at all. With the right one, a single madder bath can give you coral, brick red or deep aubergine depending on what you added beforehand.

Mordants do more than fix colour; they shift it. The same dye on the same fibre will look completely different depending on whether you used alum, iron or tannin. That is the real pleasure of natural dyeing — not one colour, but a family of them from a single pot. Everything below is measured as a percentage of the weight of fibre (WOF), so 10% alum on 100g of dry wool means 10g of alum.

Alum: The Dependable Base Coat

Potassium aluminium sulphate — plain alum — is the mordant most dyers reach for first, and for good reason. It is forgiving, it keeps fibres strong, and it gives clear, bright colours that hold up to light. It is also the safest of the common mordants to handle.

For wool and silk, use 10–15% WOF alum with about 5–6% WOF cream of tartar, which brightens the final colour and softens the hand of the fibre. Dissolve both in hot water, add to a pot with enough water for the fibre to move freely, and bring slowly to around 60–80°C. Hold for an hour, then let the fibre cool in the bath — ideally overnight. Wool felts if you rush the temperature, so patience here pays off.

Cotton, linen and hemp need more help. Use 15–20% WOF alum, and always treat cellulose with a tannin first (see below). Alum on its own will give you pleasant but pale results on plant fibres.

Iron: Depth, Dusk and a Word of Caution

Ferrous sulphate — iron — is the mordant that turns a cheerful weld yellow into olive, and madder red into a rich aubergine-brown. It is usually used as a modifier after dyeing rather than a pre-mordant, dipped briefly into a weak solution until the colour shifts.

Restraint is everything. 1–2% WOF is plenty for most projects, and anything above 4% starts to damage fibre: wool goes brittle and cotton can develop weak spots over time. Keep dips short — a few minutes is often enough — and rinse thoroughly afterwards. You can also make a gentle iron water by soaking rusty nails in a vinegar-and-water solution for a fortnight, which gives you a milder, more controllable liquor for shibori work where you want subtle gradations rather than a flat shift.

Tannins: Essential for Cotton, Beautiful on Wool

Tannins are plant-derived and act as both a mordant and a colour in their own right. Oak gall, sumac leaf, pomegranate rind and strong black tea all work well. They bond readily to cellulose, which is exactly why cotton needs them before alum.

Use 5–10% WOF tannin, simmer gently for an hour, and let the fibre cool in the bath. Gallnut gives a cool, slightly grey-beige base; pomegranate rind is warmer and yellower. On wool and silk, tannins add depth and can push colours towards muted, antique tones that many dyers find more interesting than the bright alum versions. Be aware that tannin-rich fibres can darken over time when exposed to light, so store finished pieces away from direct sun.

Layering and Local Mordanting for Shibori

Because mordants determine how much dye a fibre takes up, they are a wonderful tool for pattern. Paint an alum solution onto cloth with a brush, let it dry, then dye — the painted areas grab colour while the rest stays pale. Dip just the bound tips of a stitched nui shibori bundle into iron water after dyeing and you get dark, smoky edges against a lighter ground.

• Itajime (folded and clamped): mordant the whole piece evenly, then dye, and let the clamped folds create the resist.
• Arashi (wrapped around a pole): a tannin pre-treatment followed by alum gives crisper contrast on cotton.
• Stitched resists: dip finished bundles briefly in iron for shadowed outlines, but rinse immediately to protect the fibres.

Making It Repeatable

Small variables change your results more than you would expect. Hard water pushes colours duller, so if your tap water is chalky, use rainwater or filtered water for both mordanting and dyeing. Keep dedicated stainless steel or enamel pots — never aluminium, and never the pans you cook with. Weigh your fibre dry, write down every percentage, and note the temperature and timing.

Lightfastness comes from the combination, not the mordant alone. Alum with madder, weld or cochineal gives excellent fastness. Iron deepens and generally improves lightfastness but can continue to darken for weeks, so let pieces rest before you judge them. Tannin-plus-alum on cotton is the most reliable route for plant fibres. Get those three working together and you will find that natural dyeing is not muted at all — it is simply asking you to choose your colours twice: once with the dye, and once with the mordant.