Dyes & pigments

Your jeans are blue, your curry stains the wooden spoon yellow, and the wall of your room carries exactly the colour that was in the bucket before. Behind all these colours are chemical substances that fall into two big families: dyes and pigments. The difference sounds small, but it decides whether a colour sits inside a fabric, covers a wall, or says goodbye in the wash.

Dye or pigment?

A dye dissolves: its individual molecules spread through the water like sugar in tea, the liquid stays clear and becomes evenly coloured. A pigment does not dissolve: it consists of tiny solid particles that merely drift about in the liquid. You know this from the topic on mixtures: dye plus water is a solution, pigment plus water is a suspension. Leave pigment paint standing and the particles slowly settle to the bottom. That is exactly why paint cans say: stir before use.

Why things have colour at all

White light contains all the colours of the rainbow mixed together. When it hits a dye or a pigment, the molecules swallow some of these colours, and the rest is reflected and lands in your eye. A red T-shirt swallows green and blue and reflects red. The indigo dye in your jeans mainly swallows yellowish and red light, leaving the typical jeans blue. So the colour of a thing that does not glow by itself is always the leftover light it did not keep.

Dyeing with plant power

Long before chemical factories existed, people took their colours from nature: indigo for blue from the leaves of the indigo plant, bright yellow from the turmeric root, red from the madder root. To dye fabric, you dissolve the dye in a warm dye bath, put the fabric in, and let the dye molecules travel into the fibres, where they hold on. How well they stick varies a lot: turmeric yellow holds only weakly and washes out quickly. Indigo does not dissolve in water by itself and has to be made soluble chemically for the dye bath. Inside the fibre the air turns it back into tiny insoluble particles that sit trapped in the weave. That is why jeans stay blue through many washes.