Same plant. Same yarn. Different mordant. Completely different colour. Here’s how the chemistry of natural dyeing really works.

If you’ve ever wondered why two skeins dyed with the same plant can come out completely different colours — one a soft blush, one a deep burgundy — the answer lies in the mordant. Mordants are the unsung heroes of natural dyeing: the substances that chemically bond to the fibre and the dye molecule simultaneously, locking colour in place and fundamentally altering the shade it produces. Without a mordant, most natural dyes simply wash out. With one, colours that have survived thousands of years in Egyptian tombs can still be read clearly today.

The word comes from the Latin mordere — “to bite.” A mordant bites into the fibre, creating attachment points for the dye that would not otherwise exist. It is a beautifully simple concept with complex and often surprising results.

A Brief History of Mordants

The use of mordants appears to be almost as old as dyeing itself. Egyptian dyers scoured linen in hot alkaline baths to remove the plant’s natural waxes, then used iron and aluminium compounds so that dye could bond to the fibre — plant fibres like linen and cotton otherwise resist colour. Madder-dyed textile fragments found in Tutankhamun’s tomb are over 3,300 years old and still read as red.

In Anglo-Saxon England, a surprisingly effective mordanting agent was used: human and animal urine. The ammonia in urine acted not only in the indigo vat (as a fermentation agent) but also as a general dyeing assistant across many natural dye processes. Tannin-treated textiles have been found in Anglo-Saxon burial sites, showing that mordanting was standard practice even in early medieval Britain. The sophistication of early dyers is consistently underestimated.

The Five Key Historical Mordants

The five mordants most used throughout history each produce distinct effects on the same dye:

Alum (aluminium sulphate): The most widely used mordant throughout history. Gives bright, clear colours — it is the default choice for most natural dyeing and produces the “true” colour of most dyes. Safe and relatively easy to work with.

Iron (ferrous sulphate): Produces darker, “sadder” tones — deepening greens, greys and dark colours. The same madder dye that gives brick red with alum will give brownish-purple with iron. Iron is a dye modifier as much as a mordant, and must be handled with care: it can make fibres brittle over time (known as “saddening” or “iron poisoning” of cloth). Keep iron baths separate from alum baths.

Copper (copper sulphate): Shifts colours into blue-green tones. Weld with copper, for instance, can give a lovely teal-green. Copper is effective but toxic and must be handled carefully, with run-off treated as hazardous waste.

Tin (stannous chloride): Produces especially bright, vivid colours, particularly on wool. Used historically to achieve brilliant scarlets with cochineal. Tin can be harsh on fibres at high concentrations.

Tannin: A plant-based pre-treatment used primarily for cotton and other plant fibres, which otherwise struggle to bond with many natural dyes. Oak galls, sumac, black tea, and walnut hulls are all rich tannin sources. Tannin essentially creates a protein-like surface on plant fibres, allowing dyes to grip.

“Same dye, different mordant, different colour: madder + alum gives brick red; madder + iron gives brownish-purple; madder + tin gives a vivid orange-red. The mordant does not just fix the colour — it creates it.”

Mordants and the Modern Natural Dyer

For contemporary knitters and hand-dyers exploring natural dyeing, understanding mordants is the key to unlocking the full palette of plant-based colour. Alum is the standard starting point — it is the safest, most predictable, and most widely available mordant for home use. Iron is popular for its transformative “saddening” effect on greens, but should be kept in a dedicated pot and never mixed with alum baths.

A word on safety: iron and tin mordant run-off is harmful to waterways and should never be poured down the drain untreated. Copper is particularly toxic in water systems. The craft of natural dyeing carries real environmental responsibilities alongside its pleasures. Use separate, dedicated pots for mordanting (never cookware used for food), wear gloves, work in well-ventilated spaces, and research proper disposal for your mordant wastewater.

The natural dyeing revival has brought these ancient techniques back into widespread practice, and mordants are at its heart. From weld to woad, madder to marigold — the extraordinary range of colour available in the plant kingdom only becomes fully accessible with the right mordant in hand.