Natural dyeing

Mordanting: the natural-dye step people skip

Most disappointing natural-dye results are not dye problems. They are mordant or scouring problems, and both happen before the dye pot. Mordanting is the unglamorous step that decides whether your colour comes out strong, even and still there in a year.

Why a mordant is needed at all

Very few natural dyes attach to fibre on their own. Most are adjective dyes: the colourant has no strong affinity for cotton or wool and rinses straight back out. A mordant is a metal salt, usually an aluminium compound, that fixes to the fibre and then forms a complex with the dye molecule, bridging the two.

A few differ: indigo is a vat dye that becomes insoluble inside the fibre, and walnut hulls are tannin-rich enough to work unmordanted. Everything else, from madder to weld to cochineal, wants a mordant.

Weight of fibre is the unit

Every quantity in natural dyeing is a percentage of weight of fibre, written WOF. Weigh your yarn or cloth dry, before wetting and before scouring, and write the number down. If you have 100 g of wool and a recipe says alum at 12% WOF, that is 12 g of alum. Dye quantities work the same way. Water volume does not; the bath only needs to be big enough for the goods to move freely.

Scour first, properly

Unscoured fabric dyes patchily, and the blotches follow whatever is still on the cloth: spinning oils, sizing, mill finishes, natural waxes and handling grease. Nothing in the mordant bath removes them, so they act as resists.

Cellulose (cotton, linen, hemp)

Cellulose needs a hot alkaline scour: a large pot of water with soda ash at around 2% WOF plus a little neutral detergent, held at or just below a simmer for one to two hours, then rinsed until the water runs clear. New commercial cotton often needs this twice.

Protein (wool, silk)

Protein fibres are damaged by alkali, so no soda ash. Wash gently at around 50 to 60 °C with a pH-neutral detergent, and rinse at the same temperature. Wool felts in response to heat change plus agitation, so heat and cool it slowly.

Alum on protein fibres

For wool and silk, potassium aluminium sulphate (ordinary alum) is used at roughly 10 to 15% WOF. Many dyers add cream of tartar at about 6 to 8% WOF for wool, which brightens the colour and keeps the handle soft; silk is usually mordanted without it.

Dissolve the alum fully in hot water first; undissolved crystals against wool make dark spots. Add the solution to a bath of warm water, enter the goods already wetted out, raise slowly to about 80 to 90 °C, below a rolling boil, and hold for 45 minutes to an hour. Let it cool with the fibre in it. Silk can be cold-soaked overnight instead, and mordanted fibre can be dyed at once or dried and stored.

Cellulose needs tannin as well

Wool and silk are protein, with amino and carboxyl groups that aluminium ions bind to readily. Cotton and linen are cellulose, which offers little but hydroxyl groups, and aluminium holds onto those poorly. Hence the extra step: tannin deposited first provides the binding sites cellulose lacks.

The traditional sequence is scour, then tannin, then alum. Tannin sources include gallnut, myrobalan, sumac and oak galls, generally in the range of 8 to 15% WOF; gallnut is close to colourless, myrobalan adds a yellow cast. Follow with alum at a higher percentage than you would use on wool, in the region of 15 to 20% WOF, repeating both steps for maximum depth.

Aluminium acetate, usually around 5 to 8% WOF, is the simpler modern alternative and is often used on cellulose without a tannin step: one bath instead of two, at a higher price.

Iron is a modifier, not a mordant

Ferrous sulphate has some mordanting action, but in practice it is a modifier: an after-bath that saddens and darkens colour rather than fixing it. Yellows go olive, madder reds shift towards brown, and tannin-rich dyes head for grey and near-black.

Use it sparingly, around 1 to 2% WOF, in a separate warm bath, dipping already-dyed goods for a few minutes and watching closely, because it acts fast and looks darker wet than dry. Rinse well afterwards. Too much iron makes wool harsh and weakens cellulose over time, which is why iron-dyed blacks in old textiles are often the parts that have rotted. Iron also contaminates, so keep a dedicated bucket and tongs.

Safe handling

Keep dye and mordant equipment permanently separate from kitchen equipment: pots, spoons, sieves, scales and buckets. Stainless steel and enamel are neutral; iron, copper and aluminium pots act on the dye themselves, which can be useful but is not controllable. Wear gloves, wear a dust mask when weighing powders, and label every container.

Exhaust baths as fully as you can before tipping anything away, and check your local rules. Do not pour concentrated iron solutions into a septic system, where they disturb the bacteria the tank depends on. Keep iron water in a lidded bucket instead; it lasts for years.

Kitchen dyes behave differently

The popular beginner materials are not typical. Avocado pits and skins carry their own tannin, which is why they give pinks and rusts on unmordanted cotton, though alum still makes them stronger and more even. They brown if cooked hard, so keep that bath well below a simmer. Onion skins are high in tannin and flavonoids and give strong golds quickly, shifting to olive with iron.

Black beans are the cautionary example. The cold soak water gives blues and violets that are notoriously fugitive, often turning grey or tan within weeks in daylight, and turmeric behaves the same way. A mordant is not only about getting colour to take; it is a large part of whether that colour survives washing and light. If a technique needs no mordant, ask how the sample looked six months later.

A natural dye box covering scouring, mordanting and a first dye bath is in development, alongside the rest of what is planned here.