Why Does Dye Wash Straight Out of Some Cloth? A Chemical Anchor

A dye has to do two things, namely produce a colour and stay in the fibre through washing, light and wear, and most natural colourants do the first perfectly well and fail completely at the second. The pigment molecules sit in the fibre without any strong attachment, so water carries them away. A mordant is a substance, usually a metal salt, that attaches to the fibre and also attaches to the dye, forming a bridge that holds the colour in place. Without one, a cloth dyed with most plant material fades or rinses out within a few washes, which is why the treatment is not an optional refinement but the thing that makes the whole craft possible.

Traditional practice relies on a small number of metal salts:

• •Alum, the most used by far, giving clear bright results and low toxicity
• •Iron, which darkens and dulls every colour it touches and is called saddening
• •Copper, which shifts colours towards green
• •Tin, which brightens reds intensely and weakens the fibre if overused
• •Chrome, historically important and now largely abandoned as too hazardous
• •Tannins from oak galls and other plants, used alone or alongside a metal

The bridging metal becomes part of the coloured structure rather than sitting beside it, which is why the same dye bath produces different results depending on what was used. Madder root treated with alum gives a clear red, with iron gives a purple brown, and with tin gives a brilliant scarlet. A single plant can therefore yield a range of shades, and traditional dyers exploited this deliberately, treating different skeins differently before dyeing them together. The order matters too, since the material can be treated before, during or after the dye bath, each producing a different result. Modifying the finished cloth by dipping it in an iron solution shifts the whole thing at once, which is how many historical greens and blacks were achieved.

The same treatment and the same dye give quite different results on different materials, and the reason is chemical. Wool and silk are proteins, carrying many chemical groups along their chains that metal salts attach to readily, which is why they take these treatments well and hold colour strongly. Cotton and linen are cellulose, offering far fewer sites, which is why they are notoriously difficult and why traditional practice for them involves elaborate multi-stage preparation, frequently including tannins, oil treatments and repeated steeping over weeks. Synthetic fibres reject natural dyes almost entirely and require dyes designed for them. A dyer therefore chooses the whole procedure by the fibre first, and a recipe that produces a strong colour on wool may give almost nothing on cotton.

A small and important group of dyes bind to fibre without help, and their history reflects that advantage. Indigo works by a completely different mechanism, being reduced to a soluble form in an oxygen-free bath, absorbed by the fibre, and then reoxidised in air to an insoluble pigment trapped inside, which is why the colour develops as the cloth is lifted out and why it rubs off over time rather than washing out. Shellfish purple works similarly. Walnut hulls and a few tannin-rich materials bind directly. Everything else needs the bridge, which is why the discovery and trade of alum was economically significant enough to fund wars and to be the subject of papal monopolies in the fifteenth century.

Most natural colourants sit loose in the fibre and rinse out, so a metal salt is used to form a bridge attaching to both. Alum, iron, copper and tin are the traditional choices, and each changes the resulting colour because the metal becomes part of the coloured structure. Indigo and a few others bind without help, which is why alum was valuable enough to be worth monopolising.