Dyeing

Dyeing makes colour part of a fibre: the dye enters the yarn or cloth and stays through washing. Most plant dyes, such as madder and weld, give stronger and faster colours with a mordant, usually alum, while indigo is made soluble in a vat and turns blue again in the air.

What it is

Paint sits on the surface of a cloth and can rub off; a dye enters the fibre dissolved in water and stays through washing. Most dyes came from plants, a few from insects and sea snails.

On bare wool, madder root, the classic red, and weld, the classic yellow, give weaker colours that fade and wash out more easily. Dyers fixed them with a mordant, a metal salt put on the fibre first, to which the dye binds; for most of history it was alum. Indigo, the blue, does not dissolve in water at all and must be changed chemically in a vat before it can enter the yarn.

The plate shows a late medieval dyer’s yard: a madder bath in a copper kettle (2) over a fire (1), skeins lifted on a stick (3), the indigo vat (4), yarn drying on a pole (5) and dyestuffs (6). Magnified in the circle, a raw wool fibre (7) sits beside one mordanted with alum, still pale, and one mordanted and dyed (8).

How it works

Madder’s red comes from compounds in its roots, mainly alizarin and purpurin. Alone they give wool a weaker, duller orange that fades more readily. Simmered first in alum, a sulphate of aluminium and potassium, the wool takes up aluminium; in the dye bath the madder molecules bind to it and form a coloured compound that no longer dissolves, held inside the fibre. The mordant also sets the shade: madder gives red with alum and purple-browns with iron, weld bright yellow with alum and olive with iron.

Out of the vat the yarn is yellow-green; the air turns it blue.

Indigo forms from compounds in the leaves of tropical Indigofera plants and, in Europe, of woad. In an alkaline vat with no oxygen, the insoluble blue molecule gains hydrogen and becomes leucoindigo, a soluble yellow-green form that soaks into the fibre. Out of the vat, oxygen in the air turns it back into blue indigo, now trapped in the fibre. Each dip adds a layer; dark blues take many.

Before industrial chemicals, bacteria did the reducing. Woad or indigo was kept warm in water with an alkali, wood-ash lye or slaked lime, and bran to feed them; as they fermented, they used up the oxygen and turned the indigo into leucoindigo. Greens came from blue dyed over with yellow.

How to build

Madder, woad and weld: the red, the blue and the yellow of European dyers.

This outlines the traditional method for wool, not a tested recipe. You need wool yarn, alum, dried madder and weld, and stainless-steel or sound enamel pots, sticks and a thermometer kept only for dyeing.

Prepare the fibre. Wind the yarn into loose skeins tied in several places so the dye reaches every strand. Wash out the grease in warm water with mild soap. Always enter fibre wet, and change temperature slowly: heat shocks and rubbing felt wool.

Mordant. Maiwa, a Canadian supplier of natural dyes, gives alum at 15 per cent of the dry weight of wool. Dissolve it in hot water, add the wool, bring it slowly to about 90 °C, just below a simmer, and hold it there about an hour. Let it cool and rinse.

Dye red or yellow. Soak chopped madder overnight, then heat it gently with the mordanted wool for an hour or more, turning slowly. For European madder, Maiwa advises staying below about 72 °C: hotter baths dull the red towards brown. Weld is steeped overnight, heated and strained; the mordanted wool then spends about an hour in the liquid, also kept below about 71 °C, since more heat dulls the yellow. Cooling in the bath deepens the shade.

Dye blue. A traditional woad or indigo vat, set with alkali and bran, was kept warm for days until the yellow-green liquid showed a coppery skin and blue froth. The yarn goes in wet and slowly, without splashing, since air spoils the vat; after a dip that the method sets, often 20 minutes to an hour for wool, it is lifted, squeezed, left to turn blue and dipped again. Modern vats use other reducing agents, and wool needs a less alkaline bath than cotton (Maiwa gives pH 9–10 against 11–12): follow a tested method written for wool and check the pH.

Rinse and wash. After madder or weld, let the yarn cool in the bath, rinse it gently until the water runs clear and dry it in the shade; wash it with mild soap only later. After indigo, let the blue develop fully in the air, rinse in plain water, then soak the yarn in diluted vinegar to neutralise the alkali left in the wool. Wash it last, and check that no blue rubs off on a white cloth.

Safety. Hot baths scald: use a stable fire or stove and keep children and animals away. Alum irritates and must not be swallowed; iron salts are harmful in strong solution. Slaked lime and lye burn skin and eyes, and lye attacks aluminium pots; reducing agents can release toxic fumes, above all with acids. Wear goggles and gloves, and store these chemicals dry and labelled. Fermenting vats give off foul gases, so work outdoors or with good ventilation. Never use cooking pots or utensils, and never return dye tools to the kitchen. Chrome and tin mordants, once common, are toxic: do not use them. Spent baths are not harmless: mordant and iron baths and vat sludge hold metal salts or alkali. Never pour them into streams, wells or storm drains; ask the local waste service how to dispose of them.

History and evolution

Layers over 30,000 years old in Dzudzuana Cave, Georgia, held coloured fibres that the excavators read as flax dyed with local plants; the dyeing is an interpretation, and the flax identification has been disputed.

The oldest known indigo, identified by chemical analysis, colours cotton from Huaca Prieta, Peru, about 6,200–6,000 years old. The oldest confirmed madder colours an Egyptian leather quiver of about 2124–1981 BCE.

Each snail gave only a few drops of dye; commercial dyeing needed many thousands.

Sea snails gave the purple linked with Tyre: shell heaps show production on Crete early in the second millennium BCE and at Ugarit, in Syria, in the 15th–14th centuries BCE. Kermes, a scale insect, gave crimson; American cochineal, which reached Europe in the 16th century, gradually displaced it.

Medieval dyers formed guilds that guarded recipes and policed quality; London’s, recorded in 1188, gained in 1471 a charter to search workshops and fine bad dyeing.

In 1856 William Henry Perkin, an 18-year-old London student trying to make quinine, obtained mauveine from coal-tar chemicals, the first synthetic dye to succeed commercially. In 1868 Carl Graebe and Carl Liebermann synthesised alizarin, and madder growing soon dwindled; Adolf von Baeyer published a synthesis of indigo in 1880, and BASF put it on the market in 1897.