A mordant is a metal salt, usually alum (potassium aluminium sulfate), that forms a bridge between a plant dye and the fibre so the colour survives washing. The carrier’s standard is alum at 15-20% of the dry weight of the fabric, dissolved in hot water and simmered with the cloth for 1-2 hours, then squeezed out without rinsing. Iron, used briefly after dyeing, shifts colours toward brown and black, and too long a dip makes cloth brittle.
Part of Organic Chemistry
Pretreating fibre with a metal salt so that plant dyes bond permanently to it.
The Metal Is The Bridge
Most plant dyes are not held to fibre on their own; they wash out. The carrier explains that a mordant, a metallic salt, binds the dye to the fibre. Metal ions attach to the fibre on one side and to the dye on the other, so the colour sits in a stable complex. The chemistry is covered in Mordant Chemistry and Mordant Chemistry (clothing).
What You Need
- Alum, potassium aluminium sulfate (Alum Mordant).
- A large non-reactive pot and enough water for the cloth to move freely.
- Tannin (oak galls, bark, sumac) for cotton and linen, which do not take alum alone (Tannin Fixation).
- Iron liquor for darkening (Iron Modifier).
- A scale to weigh dry fabric.
Method: Alum Mordanting Per The Carrier
Step 1: Weigh the dry fabric.
Step 2: Dissolve alum in hot water at 15-20% of the fabric weight, the carrier’s figure.
Step 3: Soak and simmer gently for 1-2 hours.
Step 4: Remove and squeeze out the excess. The carrier says not to rinse.
Step 5: Dye at once, or dry for later use.
Iron modifier. The carrier’s recipe soaks rusty nails in vinegar for two weeks, strains it, and dips the dyed cloth briefly to move colours toward brown or black. The carrier warns that too long makes the fabric brittle.
The Corpus Gives Different Alum Percentages; None Is Changed Here
The carrier gives one range, 15-20% of fabric weight. Mordant Chemistry gives 15-20% for wool and silk and 20-25% for cotton and linen. Alum Mordant gives 10-15% for wool at 80-85 °C and says cotton and linen need a tannin pretreatment. Dyeing Process gives 12% for wool. So the percentage depends on fibre and source. Where they disagree, the lower figure is the cheaper and the less likely to leave the wool harsh; the carrier does not say what too much alum does, so none of the pages is altered here. Test a swatch.
Worked Example: Alum For A Batch
For 800 g of dry wool at the carrier’s range, 0.15 × 800 = 120 g to 0.20 × 800 = 160 g of alum. At the 10-15% wool range on the Alum Mordant page the same cloth takes 0.10 × 800 = 80 g to 0.15 × 800 = 120 g. The two ranges overlap only at 120 g, a workable starting point; the colour result is checked by a fastness test.
Common Mistakes
- Alum on cotton without tannin, which then washes out.
- Rinsing after mordanting, which removes the unfixed alum the carrier says to leave.
- Leaving the iron dip too long. Fabric becomes brittle.
- Mordanting in a pot used for food.
Safety
Metal mordants other than alum and iron are poisonous
Alum and rusty-iron liquor are low in toxicity, but copper, tin and chrome mordants are not, and spent mordant baths carry dissolved metals. Do not pour them into streams, wells or drains that feed water supplies; follow Waste Treatment. Hot baths scald.
What’s Next
- Learn next: Ink Production — iron and tannin used for writing
- Combine with: Dyes and Pigments — the colours the mordant fixes