Last updated: May 4, 2026
Most natural dye molecules do not bond directly to fibre with adequate permanence on their own. Mordants solve this by forming coordination bonds with the dye molecule on one side and ionic bonds with the fibre on the other. The word derives from the Latin mordere — to bite. The metal ion literally bites into both fibre and dye, holding them together.
Potassium Alum
Potassium aluminium sulphate (KAl(SO₄)₂·12H₂O) is the most widely used mordant in natural dyeing. It produces the truest representation of a dye plant's colour — no significant hue shift occurs. Madder with alum gives terracotta-red; weld with alum gives bright yellow; madder and weld overdyed with alum gives orange. It is safe to handle, water-soluble, and suitable for protein and cellulose fibres.
Typical rate: 15–20% weight of fibre (WOF) for wool. Cotton and linen require 20–25% WOF and benefit from a tannin pre-treatment to improve alum adhesion.
Process: Dissolve alum in hot water. Add pre-wetted fibre. Heat to 80–85°C over 45 minutes. Hold at temperature for 45 minutes. Remove and allow to cool in the mordant bath. Mordanted fibre can be dyed immediately or dried and stored for later use.
Over-mordanting with Alum
Using alum above 25% WOF does not improve colour depth and can make the fibre sticky or cause wool to felt more readily. The amount of alum that bonds to the fibre reaches a practical limit; excess alum simply washes out. This is a common point of confusion for new dyers — more mordant is not always better.
Iron Mordant (Ferrous Sulphate)
Iron mordanting consistently shifts colours toward green, grey and dark tones — a phenomenon called saddening. Dye plants that produce warm yellows with alum produce olive-green or khaki with iron. Madder produces grey-mauve or warm brown rather than red. This colour shift is caused by the formation of iron-tannate or iron-flavonoid complexes with different light absorption properties than the alum equivalents.
Typical rate: 2–4% WOF. Iron at higher concentrations damages wool fibre — the metal ions catalyse oxidative degradation of the keratin protein. The resulting fibre becomes brittle and breaks on handling. For this reason, iron is used at low percentages and often added directly to the dyebath (afterbath iron) rather than as a pre-mordant at elevated temperature.
Copper Mordant (Copper Sulphate)
Copper shifts colours toward green and blue-green. It is frequently used with yellow dye plants to produce khaki and moss greens. Copper is more toxic than alum — it is harmful to aquatic organisms and should not be disposed of in household drains. In provinces with active textile waste guidelines (including British Columbia and Ontario), spent copper mordant baths should be collected and disposed of through appropriate chemical waste channels.
Typical rate: 2–3% WOF. Above this level, copper begins to damage fibre, particularly at high temperatures.
Tannin Pre-Mordanting for Cellulose Fibres
Cotton, linen, and other plant-based fibres do not take alum, iron or copper as readily as wool because they lack the reactive amino groups found in keratin. A tannin pre-treatment addresses this by depositing a layer of polyphenols on the fibre surface that both provides direct colour (tannins themselves dye fibre pale tan to brown) and improves affinity for subsequent metal mordants.
Tannin sources in common use:
- Oak galls: High tannin concentration; produces a pale grey on undyed cotton before mordanting
- Sumac leaves: Good tannin content; produces a warm beige and improves alum mordanting significantly
- Black tea: Readily available; lower tannin concentration than oak gall but useful for small batches of cotton
- Rhubarb leaves: Note — rhubarb leaves contain oxalic acid and are toxic; handle with gloves and avoid disposal in compost
After tannin treatment, cotton can be mordanted with alum at 20–25% WOF using the standard process. Colour depth achieved on tannin-treated cotton typically approaches that of unmordanted wool for most dye plants, though the colour temperature (warmth/coolness) may differ.
Mordant Sequencing and Colour Modification
Mordants can be used in sequence to produce compound effects. A common technique is to pre-mordant with alum, dye to a full colour, then add a small amount of iron to the dyebath in the last 10 minutes — this is called an iron afterbath. It saddens the colour selectively without the full commitment of iron pre-mordanting. The same approach with copper produces greening rather than saddening.
A brightness modifier used with yellow dyes is cream of tartar (potassium bitartrate), added to the mordant bath at 6% WOF. It does not change the fundamental colour but improves levelling — the uniformity with which colour distributes across the fibre — and produces a cleaner, more saturated yellow than alum alone.
Disposal Considerations in Canada
Alum mordant spent baths are relatively low-risk and can generally be disposed of in municipal sewage in small quantities, in line with local limits on aluminium discharge. Iron and copper spent baths are environmental hazards at scale. For small-scale home dyeing, the Environment and Climate Change Canada website provides guidance on hazardous household material disposal by province. Most municipalities have seasonal hazardous waste collection programs.