Step 06 — Mordants — Making Colour Stick

A mordant is the substance that locks dye into fibre. Without it, most natural dyes wash straight out within a few laundering cycles. The word comes from the Latin mordere — “to bite” — because mordants were once thought to make the dye bite into the cloth. The chemistry is more nuanced than that, but the image is useful: mordants create a tight grip between molecule and fibre.

How Mordants Work

Most mordants are metallic salts. When dissolved in water and absorbed into a fibre, they form coordination bonds with both the fibre’s protein or cellulose structure and the dye molecule simultaneously — acting as a molecular bridge. The choice of mordant affects not just how well the colour holds, but what colour you actually get, because different metals shift dye molecules’ light-absorption properties.

For the underlying chemistry of why fibres accept mordants differently, see Step 04 — The Science of Colour and Fibre.

The Main Mordants

Alum (potassium aluminium sulphate) is the workhorse of natural dyeing — safe to handle, widely available from dye suppliers or even grocery stores (as pickling alum), and it produces clear, true colours without shifting them too drastically. Use at around 15–20% weight of fibre (WOF) for protein fibres, a little higher for cellulose.

Iron (ferrous sulphate) is a colour saddenner — it deepens and darkens colours, shifting yellows to olive and greens to almost black. A small amount goes a long way; too much iron can make fibres brittle over time (called iron damage). Use at 2–4% WOF or as a very dilute afterbath. Iron is also explored in Step 09 — Iron, Tannins, and Mineral Dyes.

Copper sulphate shifts colours toward green and teal. It’s effective but more toxic than alum and requires careful disposal — not recommended for beginners without proper safety measures.

Tannins are plant-based mordants (rather than metallic salts) that work especially well on cellulose fibres. Oak galls, sumac, black tea, and pomegranate all provide tannin. They serve double duty: bonding to the fibre and providing their own colour contribution.

Cream of tartar (tartaric acid) isn’t strictly a mordant but is often used alongside alum to brighten colours and keep wool fibres soft and lustrous.

Mordanting Methods

Pre-mordanting — treating the fibre before dyeing — is the most common approach and gives the most even colour. Co-mordanting (mordant in the dyebath) simplifies the process. Post-mordanting (mordanting after dyeing, sometimes called an afterbath) can create surprising colour shifts, which is explored more in Step 11 — Colour Modifiers and Afterbaths.

Safety and Disposal

Alum is the safest choice for home use — it’s used in food preservation and poses minimal risk. Iron is also low risk in small quantities. Copper and chrome (another historic mordant) are more hazardous and should be treated as chemical waste rather than poured down the drain.

Always use dedicated pots, spoons, and measuring equipment for dyeing — never return them to kitchen use. Wear gloves when handling mordant solutions. Good ventilation is always sensible.