Step 04 — The Science of Colour and Fibre

You don’t need a chemistry degree to dye fabric, but a little science goes a long way toward understanding why some colours last and others fade, why wool takes dye so eagerly while cotton resists it, and why adding a splash of iron changes a golden yellow to army green. This note gives you the conceptual scaffolding without the jargon overload.

What Is Colour, Physically?

Colour is what our eyes perceive when molecules absorb certain wavelengths of light and reflect others. A yellow onion skin looks yellow because its pigment molecules (called flavonoids) absorb blue and violet light and bounce back the yellow. The specific molecular structure of a dye determines which wavelengths it absorbs — and therefore what colour we see.

When that molecular structure breaks down — through UV exposure, washing, or oxidation — the colour changes or disappears. This is why lightfastness matters: it describes how resistant a dye’s molecules are to that breakdown.

Why Fibre Type Matters

Textile fibres fall into two broad categories, and they behave very differently in a dyebath.

Protein fibres — wool, silk, alpaca, and other animal-derived materials — are made of keratin or fibroin, proteins built from amino acids. Those amino acids carry electrical charges that bond readily with dye molecules and with mordants like alum. This is why protein fibres are the easiest to dye deeply and evenly.

Cellulose fibres — cotton, linen, hemp, bamboo — are made of plant-based carbohydrate chains. They carry fewer bonding sites for most dye molecules, so they absorb colour less readily. Dyers compensate by pre-treating cellulose fibres with tannin (which adds bonding sites) or by using higher mordant concentrations. You’ll find the practical steps in Step 05 — Natural Fibres and Fabric Preparation.

The Role of the Mordant

A mordant (from the Latin mordere, “to bite”) is a metallic salt or other substance that forms a chemical bridge between the dye molecule and the fibre. Without a mordant, many dyes sit loosely on the surface of the fibre and wash out quickly. With one, the dye is locked in at a molecular level.

Alum (potassium aluminium sulphate) is the most common mordant — safe, effective, and widely available. Iron, copper, and tannins also act as mordants, and each shifts the final colour slightly. Step 06 — Mordants — Making Colour Stick covers the full range.

pH and Colour Shifting

Many natural dyes are pH sensitive, meaning their colour changes in acid or alkaline conditions. Anthocyanins — the pigments in red cabbage, purple elderberry, and black hollyhock — are a vivid example: they turn pink in acid, purple in neutral, and green or yellow in alkali. Dyers exploit this deliberately by adding vinegar (acid) or washing soda (alkali) to shift hues. Step 11 — Colour Modifiers and Afterbaths is where this becomes a practical tool.

Heat and Exhaustion

Heat opens up fibre structure, allowing dye molecules to penetrate more deeply. Most dyebaths are held at a simmer (around 80–95 °C for protein fibres; lower for delicate silk). As the dye bonds to the fibre, the bath gradually loses colour — dyers say the bath is exhausting. A fully exhausted bath is a sign of efficient, even uptake, and the near-clear liquid can often be disposed of responsibly down the drain.