There is a particular disappointment that comes from pulling a freshly dyed skein of wool out of the pot, only to watch the colour rinse away down the sink. Usually, the fibre was never properly prepared. Protein fibres — wool, alpaca, mohair, silk and other animal-derived materials — will accept natural dyes beautifully, but only if you give them a clean surface and a welcoming chemical doorway. That is what scouring and mordanting do.

Why Protein Fibres Need Preparation

Wool straight from a fleece carries lanolin, sweat salts, dust and vegetable matter. Even commercially spun yarn, which looks perfectly clean, retains spinning oils, machine residue and the sizing added during processing. These sit on the surface of the fibre and form a barrier. Dye molecules that cannot reach the cortex of the fibre will simply sit on top, where they rub off on your hands, fade in sunlight and bleed in the wash.

There is also a chemical issue. Protein fibres are made of keratin and fibroin, both built from amino acid chains. These chains carry carboxyl and amino groups that can bond with dye molecules, but only if those groups are available and the fibre surface is fully wetted. Mordants — usually metal salts — act as a bridge, binding to the fibre at one end and to the dye at the other. Without that bridge, many dyes have nothing to hold onto, particularly the madder, weld and cochineal that so many British growers want to use.

Scouring Wool, Silk and Other Protein Fibres

Scouring is simply washing with purpose. For most home dyers, a gentle alkaline bath followed by careful rinsing is enough. Use a pH-neutral or slightly alkaline wool detergent — the sort sold for hand-washing fleece — rather than washing-up liquid, which is too harsh and often contains additives that cling to fibre.

Fill a large pot with hot water, around 50°C for wool and 60°C for silk. Add the detergent at roughly 5 ml per litre, or about a teaspoon per 5 litres. Add the fibre, keeping it loose and never crowding the pot. Let it soak for 20 to 30 minutes, stirring gently with a spoon rather than your hands, and resist any urge to agitate. Wool felt is made by agitation, heat and moisture, and you are supplying two of the three already.

Drain the water and repeat if the rinse water is still cloudy. Follow with two or three rinses at the same temperature, cooling gradually. Sudden temperature changes are the second great enemy of protein fibres and will cause shrinkage or a matted surface.

• Wool fleece: scour twice, the second bath with a little less detergent. Expect a surprising amount of dirt.
• Silk: hotter water is fine but keep it brief. Silk loses lustre if left in alkali too long.
• Alpaca and mohair: treat as wool, but handle even less. These fibres felt quickly.
• Recycled or second-hand yarn: scour more thoroughly. You have no idea what has been applied to it.

Choosing and Using Mordants Safely

Alum — potassium aluminium sulphate — is the mordant most natural dyers reach for first, and for good reason. It is comparatively low in toxicity, works well on protein fibres and gives clear, bright results. A typical mordant bath uses alum at around 10 to 15 per cent of the fibre's dry weight, plus a small amount of cream of tartar at 6 per cent to soften the effect and improve the take-up of colour.

Dissolve the alum and cream of tartar in a little hot water first, then add to a pot of warm water. Enter the scoured, still-damp fibre, bring the bath slowly to 80 to 85°C, and hold it there for an hour. Do not boil protein fibres. Then let the pot cool overnight. The fibre can go straight into the dye bath while still damp, which helps evenness.

Other mordants — copper, iron, tin and chrome — are used in far smaller quantities and demand real care. Iron will sadden colours to greys and olives, but it is corrosive and can make fibre brittle if overused. Copper produces greens and deepens browns, yet it is toxic to aquatic life, so never pour spent baths down the drain. If you use these, keep dedicated pots and utensils that never touch food, wear gloves, work in a ventilated space, and dispose of the spent liquor responsibly through your local household waste facility.

Tannins as a Pre-Mordant for Cellulose-Free Clarity

Protein fibres mordant willingly, but some dyers like to add a tannin step first. A bath of oak gall, sumac or even strong tea introduces tannic acid to the fibre, which improves the depth of colour and helps certain dyes — walnut, madder, cutch — bond more firmly. This is optional for wool and silk, but it is a helpful trick if you are dyeing with hard-to-fix plant material or aiming for very dark shades.

Make a tannin bath by simmering the plant material for an hour, straining, then soaking the scoured fibre in the cooled liquor for a few hours or overnight. Rinse lightly before mordanting. It is a small extra step that pays dividends in longevity.

Practical Tips for Clearer, Lasting Colour

The difference between a dye that lasts and one that fades is often in the detail. Use soft or rainwater where you can, because hard water minerals can dull colours and interfere with mordanting. Weigh your fibre dry before scouring so that you can calculate mordant quantities accurately — guessing causes patchiness. Keep your fibre moving gently but never roughly, and always enter the pot wet rather than dry so that the mordant or dye penetrates evenly.

• Record everything: fibre weight, mordant percentages, temperatures and timings.
• Use a dedicated dye pot, never one used for food.
• Wear gloves and an apron, and keep children and pets away from the work area.
• Cool baths slowly, and rinse in water of a similar temperature to avoid felting.
• After dyeing, dry in shade. Direct sun can shift colours before they have fully set.

Prepared properly, a British wool fleece dyed with home-grown weld, madder or woad will hold its colour for years. The work is in the preparation, and it is work worth doing.