Mordant Chemistry
Part of Textiles and Weaving
Mordants are metallic salt solutions that bond permanently to fiber and then bond again to dye molecules — creating a three-way chemical lock that makes plant dyes colorfast and washfast rather than fugitive.
The word mordant comes from the French and Latin roots for “to bite.” The image is apt: without a mordant, most plant dyes merely coat the surface of fiber like water on glass, washing off after one or two launderings. The mordant bites into the fiber’s molecular structure, creating bonding sites that the dye molecule can then attach to permanently. The result is a color that survives washing, light, and years of wear — compared to weeks without mordanting.
Understanding mordants requires grasping two chemical concepts: that fiber (especially protein fibers like wool) has molecular sites that attract metallic ions, and that many dye molecules are attracted to the same metallic ions. The mordant bridges fiber and dye. Change the mordant metal and you change the color — the same dye plant produces dramatically different hues on alum-mordanted versus iron-mordanted fiber.
Primary Mordants
Alum (Potassium Aluminum Sulfate)
Alum is the universal mordant — the standard against which all others are compared. It produces the truest, brightest representation of most dye colors, does not dull or shift hues, and does not damage fiber even in moderate excess.
Sources:
- Naturally occurring alum minerals (alunite): found in volcanic regions as a white crust on rocks, mined and purified by dissolving in water and allowing to crystallize
- Alum shale: certain clay-rich shales contain aluminum sulfate that can be leached out with water
- Plant ashes: some plants accumulate aluminum compounds — bracken fern ash and oak bark ash can provide enough alum for light mordanting
- Urine: aged urine contains some aluminum but primarily provides ammonium salts, not a reliable alum source
Purifying naturally occurring alum:
- Crush mineral material, dissolve in hot water, filter through cloth to remove insoluble rock
- Allow solution to cool slowly — alum crystals form first (they are more soluble at high temperature, less at low temperature)
- Collect crystals, dissolve again in fresh water, re-crystallize for higher purity
- Pure alum forms clear octahedral crystals with a sharp, astringent taste (safe to taste-test in small amounts)
Usage rate: 15-20% of dry fiber weight. For 100 g of wool: dissolve 15-20 g of alum crystals in enough hot water to cover the fiber (approximately 3-5 liters).
Process:
- Dissolve alum in hot water. Add pre-wetted fiber (fiber must be thoroughly wetted through before mordanting — dry spots do not mordant).
- Heat slowly to a simmer (70-80°C for wool; lower for plant fiber). Hold at temperature for 45-60 minutes.
- Allow to cool in the bath. Squeeze out (do not wring) and proceed directly to the dye bath, or allow to dry and store mordanted fiber for later dyeing.
Color effects with alum mordant: Truest representation of the dye color. Reds are red, yellows are yellow, oranges are orange. Alum adds a slight warmth (yellow cast) compared to iron’s coolness.
Iron (Ferrous Sulfate)
Iron mordant “saddens” (darkens and shifts toward gray-green) all colors. Used intentionally for darker, cooler shades, or for black production (iron + tannin = dark gray to black).
Sources:
- Old rusty iron objects: leave in a weak acid solution (vinegar, or whey from cheesemaking) for 1-2 weeks. The acid dissolves iron oxides. Strain and use the resulting iron-rich liquid as a mordant.
- Iron sulfate minerals: naturally occurring as green crystite (green vitriol) in certain ore deposits
- Blacksmith scale: the scale that falls from iron during hammering contains iron oxides; dissolve in weak acid
Iron liquor from rust: Fill a jar with rusty iron objects (nails, filings, old tools). Cover with a mixture of 1 part vinegar to 3 parts water. Seal and leave 2-4 weeks, shaking occasionally. Strain. The dark brown liquid is iron liquor. Concentration varies — test on a sample of mordanted and dyed fiber before committing a full batch.
Usage rate: 2-5% of dry fiber weight for moderate sadening. More than 5% causes fiber brittleness and degradation — especially with wool. Iron is the only common mordant that damages fiber significantly when overused.
Process: Same as alum. Iron can be used as:
- Primary mordant: Apply iron before dyeing (gives the strongest, most even color shift)
- Afterbath: Apply iron after dyeing (gives a lighter shift; more controllable)
- Iron sadening: Dip briefly in iron solution after dye bath to darken selectively
Safety: Iron sulfate in mordanting concentrations is not particularly hazardous. Avoid ingestion. Rinse equipment thoroughly — iron stains everything it contacts permanently.
Tannin (Tannic Acid)
Tannin is not a metallic mordant but functions similarly for plant (cellulose) fibers. Protein fibers (wool, silk) mordant well with metallic salts. Cellulose fibers (cotton, linen, hemp) mordant less efficiently with metals but bond strongly to tannins. A tannin pre-mordant followed by an alum or iron mordant gives excellent results on plant fiber.
Tannin sources:
| Source | Tannin content | Form |
|---|---|---|
| Oak galls | Very high (50-70%) | Grind dried galls |
| Sumac leaves | High (25-30%) | Dry and grind, or simmer fresh |
| Oak bark | Medium (10-20%) | Simmer bark chips |
| Black tea | Medium | Strong tea solution |
| Walnut leaves | Medium | Simmer fresh or dried |
| Alder bark | Low-medium | Simmer |
Tannin mordant process for plant fiber:
- Simmer tannin material in water 45-60 minutes. Strain.
- Cool to 60-70°C. Add pre-wetted fabric.
- Hold at 60-70°C for 1 hour, stirring gently.
- Remove fiber. Do not rinse.
- Proceed to alum or iron mordant bath, then dye bath.
Tannin as color: Many tannin sources add color themselves — walnut tans the fiber brown, oak galls add a yellow-tan. This base color can be welcome (providing a deeper, earthier tone) or unwanted (interfering with intended color). Use pale tannin sources (cream of tartar) when the goal is a mordant assistant with minimal color contribution.
Secondary Mordants and Additives
Cream of Tartar (Potassium Bitartrate)
Not a mordant itself, but a mordant assistant that:
- Brightens and clarifies colors (especially reds from madder)
- Protects wool fiber in the dye bath
- Adjusts pH to slightly acid, which benefits most dye-fiber bonds
Source: Crystallizes naturally inside wine barrels and grape-pressing equipment. Collect the white crust that forms on the inside of wine fermentation vessels.
Usage: 6% of dry fiber weight, added to both the mordant bath and the dye bath.
Copper (Copper Sulfate / Blue Vitriol)
Copper mordant shifts colors toward green and teal. Used for producing greens from yellow dye plants without needing a separate blue vat.
Sources: Blue vitriol deposits in copper mining areas; copper dissolved in weak acid (vinegar dissolves copper readily from copper vessels or scrap).
Usage rate: 2-3% of dry fiber weight. Copper damages wool at higher concentrations — use with caution.
Color effects: Yellow (weld, goldenrod) → green-gold; Red (madder) → brownish brick red; Tannin-dyed fiber → olive green.
Ammonia (Urine)
Aged urine (1-2 weeks old) provides ammonia, which acts as an alkali mordant assistant and color modifier. Historical use was universal — urine was collected in every textile workshop for this purpose.
Function: Opens the scales of wool fiber, allowing mordant and dye penetration. Shifts some colors (especially indigo/woad) to brighter blue. Acts as a reducing agent in woad and indigo vats.
Process: Age urine in a sealed container 1-2 weeks. The urea decomposes to ammonia. Use as an additive (1 part aged urine to 5-10 parts water in the mordant or dye bath), or as a pre-soak for protein fiber before mordanting.
Mordanting Different Fibers
Different fibers take mordants differently. This is one of the most important variables in dye work.
| Fiber type | Mordanting behavior | Best mordant approach |
|---|---|---|
| Wool | Excellent metallic mordant uptake | Standard alum or iron process |
| Silk | Very good, similar to wool | Same as wool, but lower temperatures |
| Linen | Poor metallic mordant uptake | Tannin pre-mordant essential |
| Cotton | Poor metallic mordant uptake | Tannin pre-mordant essential |
| Hemp | Moderate, better than cotton | Tannin pre-mordant recommended |
| Nettle fiber | Similar to linen | Tannin pre-mordant recommended |
Temperature sensitivity: Wool mordants and dyes at 70-80°C safely. Higher temperatures cause felting. Silk tolerates even less heat — keep below 60°C. Cotton and linen can be mordanted and dyed near boiling (95°C) without damage.
Safety and Waste Management
Toxicity:
- Alum: Low toxicity. Safe to handle without special precautions. Mordant baths safe to dispose in soil (diluted).
- Iron: Low toxicity. Some environmental concern if concentrated iron solutions enter waterways — dilute substantially before disposal.
- Copper: Moderate toxicity to aquatic organisms. Never dispose copper mordant baths directly into streams or ponds. Allow to evaporate or dilute heavily before soil disposal.
- Chromium and tin: These historical mordants produce excellent colors but are toxic — avoid entirely if alternative mordants achieve acceptable results.
Vessel management: Keep dedicated dye/mordant vessels clearly marked. Never use mordanting vessels for food. Wooden or clay vessels absorb mordant chemicals permanently. Copper vessels leach copper into solution continuously — helpful for copper mordanting, problematic for everything else.
Mordant Sequencing and Testing
Experienced dyers use mordant sequencing to produce colors that simple single-mordant processes cannot achieve.
Alum then iron (shift after dyeing):
- Mordant with alum → Dye → Move fiber to brief iron afterbath → Rinse Result: The iron shifts the color after the dye has bonded, giving a slightly darkened, cooler version that has the lightfastness of alum mordanting.
Sample skeins: Always dye a small sample skein (5-10 g) before committing the full fiber batch. Mordanting and dye baths vary batch to batch — the sample reveals the actual color achieved and allows adjustment.
Standardizing mordant baths: Weigh mordant chemicals carefully on a simple balance scale. Inconsistent mordant quantity is the primary cause of inconsistent color across batches. Keep a dye notebook recording weight of mordant, water volume, temperature, and duration for every batch you produce.
Mordant chemistry is reproducible science. With accurate measurement and consistent process, the same mordant and dye plant will produce the same color batch after batch — the foundation of a reliable textile dyeing operation.