Why mordants are the quiet backbone of a dye garden

A mordant is the bridge between your cloth and your colour. Without one, many dyes simply rinse away within a few washes — the pigment has nothing to hold on to. With one, the same pot of onion skin or madder root can give you soft apricot on one piece of cloth and deep terracotta on another, simply because you changed the chemistry rather than the plant.

In a home setting, mordanting is not complicated, but it does reward care. Three mordants will carry you through most projects: alum, iron and tannin. Learn how each one behaves, and how to measure, ventilate and dispose of them sensibly, and you have a calm, repeatable dye practice you can return to for years.

Alum: the reliable, colour-clear base

Alum (potassium aluminium sulphate) is the mordant most dyers reach for first. It is gentle on wool, works on silk, cotton and linen, and shifts colour very little — which makes it perfect when you want the dye itself to do the talking. It also acts as a brightener, particularly on protein fibres.

A common starting point is 10–15% of the weight of your dry fibre, plus around 6% cream of tartar if you are working with wool and want a softer handle. So 100g of dry wool means roughly 10–15g of alum. Dissolve the powders in a little hot water first, top up the pot with enough warm water to let the fibre move freely, then raise slowly to around 80–85°C and hold for about an hour. Let it cool overnight in the bath if you can — the fibre will take up the mordant more evenly.

Alum is forgiving, but it is not tasteless or harmless dust. Treat it as a chemical, not a seasoning.

Iron: the smallest amounts, the biggest shifts

Iron (ferrous sulphate) is the saddler of the dye pot. It saddens and deepens colour: madder turns from brick to aubergine, brown onion skins move towards charcoal, and yellows become olive and moss green. It is also the easiest mordant to overdo.

Work at 1–2% of the weight of the fibre — that is 1–2g for 100g of wool, weighed on accurate scales, not guessed with a spoon. Anything more and the fibre can become brittle, harsh and prone to rotting over time, especially on cotton and linen. Iron can be used as a pre-mordant at cool temperatures (30–40°C), or, more controllably, as an after-bath: dye first, then dip the rinsed, wet fibre into a weak iron solution for a few minutes and watch the colour change before your eyes. Pull it out sooner than you think you should.

Keep iron in its own dedicated pan. It leaves a residue that will dull and grey every other batch.

Tannin: the plant-based bridge for cellulose

Plant fibres — cotton, linen, hemp — have little natural affinity for most dyes, which is why they often need two steps. Tannin is the first. Oak gall, sumac, myrobalan, acacia, chestnut and even strong black tea all supply tannins that cling to cellulose and give the alum something to bond with.

Use 5–10% of the weight of the fibre, simmer gently for 45–60 minutes at around 60–80°C, then rinse. Tannin darkens whites slightly, so if you are aiming for a pale, clear result, keep the percentage low or skip it. On wool and silk, tannin is optional but can add depth and improve lightfastness.

A neat trick: many common plant dyes — tea, coffee, pomegranate rind, avocado stone — contain tannins themselves. If you are dyeing cotton with one of those, you may already have done half the work.

Measuring, ventilation and a calm worktop

The single biggest cause of messy, disappointing dye sessions is vagueness. A few habits will protect both your results and your lungs:

• Weigh everything. A set of 0.1g scales costs little and makes your recipes repeatable.
• Wear a dust mask when scooping powders, and gloves whenever you handle a bath.
• Work near an open window or door, with a fan drawing air away if you can. Never mordant in a sealed, steamy bathroom.
• Keep dedicated pots, spoons and jugs, clearly marked and never used for food.
• Note the pH if you can — a cheap strip test explains many colour surprises.
• Wipe spills immediately with a damp cloth; dried powders become airborne again when swept.

Disposal, storage and a tidy finish

Spent alum and tannin baths can usually be diluted heavily and poured away in a domestic drain, but always check your local water authority's guidance first, and keep volumes small. Iron baths should be exhausted (pale, not amber) before disposal and also heavily diluted. Never pour copper, tin or chrome mordants down the sink — take them to a household waste recycling centre.

Store powders in sealed, labelled jars away from food, out of reach of children and damp air. Write the date on the label. Rinse your pots the same evening rather than leaving them to crust overnight, hang your cloth to dry somewhere airy, and jot down what you used while it is fresh in your mind. A tidy dye day ends with clean tools and a short, honest note — and that is what turns a hobby into a craft you can trust.