Mordant Chemistry

Part of Natural Dyes & Inks

Most natural dyes wash out of fabric within a few launderings unless they are chemically bonded to the fiber. Mordants are the metallic salts and tannins that create this bond — they are the invisible technology that makes natural dyeing permanent.

Why Mordants Are Essential

A dye molecule sitting on a fiber is just a stain — water lifts it off easily. A mordant forms a chemical bridge between the dye and the fiber, creating a complex that resists washing, light, and wear. The word comes from the Latin mordere (“to bite”), because the mordant “bites” into both fiber and dye, locking them together.

Without mordanting, most plant dyes fade to nothing within 5-10 washes. With proper mordanting, the same dyes last for years or decades.

How Mordants Work

The mechanism involves metallic ions forming coordination bonds:

1. Mordant binds to fiber — metal ions from the mordant attach to reactive sites on the fiber (hydroxyl groups on cellulose, amino groups on protein fibers)
2. Dye binds to mordant — when the mordanted fiber is placed in the dye bath, dye molecules attach to the metal ions
3. Result — a fiber-metal-dye complex that is chemically stable and insoluble in water

Different metals produce different effects because they bond to different dye molecules in different ways, shifting the color.

Alum: The Universal Mordant

Alum (potassium aluminum sulfate, KAl(SO4)2) is the most commonly used mordant in history. It produces clear, bright colors without significantly shifting the hue, making it the standard mordant for nearly all natural dyes.

Finding Alum

Source	Method	Quality
Alunite mineral deposits	Mine and dissolve in water	High — pure alum
Shale and clay deposits	Burn, leach with water, crystallize	Medium — may contain impurities
Urine-soaked earth	Leach and crystallize	Low — mixed salts
Commercial alum (if available)	Purchase	High

Alum from Alunite

Alunite is a sulfate mineral found in volcanic regions and hydrothermally altered rocks:

1. Mine and crush the alunite
2. Roast at moderate heat (500-600°C) for several hours
3. Dissolve the roasted mineral in hot water
4. Filter to remove insoluble material
5. Evaporate slowly — alum crystallizes as clear, octahedral crystals

Using Alum as a Mordant

For protein fibers (wool, silk):

1. Dissolve alum in hot water — use 15-20% of the dry weight of the fiber (e.g., 15-20 g alum per 100 g wool)
2. Add cream of tartar at 6% of fiber weight (helps even absorption and brightens color)
3. Fill the pot with enough water to cover the fiber freely
4. Add wetted fiber and heat slowly to 80-90°C
5. Hold at temperature for 1 hour, stirring gently
6. Remove fiber, squeeze gently (do not rinse), and wrap in a towel
7. Let sit overnight before dyeing — or up to a week for best results

For cellulose fibers (cotton, linen, hemp):

1. Cellulose fibers absorb alum poorly — you must use tannin as a pre-mordant
2. First: soak fiber in a tannin bath (see tannin section below) for 12-24 hours
3. Then: mordant with alum at 20-25% of fiber weight
4. The tannin-alum combination bonds well to cellulose

Protein fibers (wool, silk) mordant easily with alum alone. Cellulose fibers (cotton, linen) require a tannin pre-treatment before alum will bind effectively. This is the single most common reason cotton comes out pale — the mordant did not take.

Iron as a Color Modifier

Iron (ferrous sulfate, FeSO4) is not typically used as a primary mordant but as a “saddening” agent — it darkens and shifts colors toward muted, earthy tones.

Making Iron Water

The simplest iron mordant is made by soaking rusty iron in water with vinegar:

1. Place rusty nails, horseshoes, or iron filings in a jar
2. Cover with equal parts water and vinegar
3. Leave for 2-4 weeks, shaking occasionally
4. The resulting rusty brown liquid is your iron mordant
5. Strain before use

Using Iron

• As a post-bath modifier — dip dyed fiber briefly (5-15 minutes) in iron water to shift the color. Yellow becomes olive-green, red becomes maroon, orange becomes brown.
• Concentration — use sparingly. Iron is potent: 2-4% of fiber weight is sufficient. Excess iron makes fiber brittle over time.
• Color effects:

Original Dye Color	Iron Modifier Result
Yellow (weld)	Olive green
Red (madder)	Dark maroon/brown
Orange	Brown
Blue (indigo)	Darker blue-black
Tan	Dark brown to black

Iron mordant makes fibers brittle with prolonged exposure. Use the minimum amount needed for the desired color shift, and limit soaking time. Historical textiles mordanted heavily with iron often deteriorated faster than those mordanted with alum.

Tannin Mordants

Tannins are large, complex organic molecules found in bark, leaves, galls, and some fruits. They serve as mordants in their own right (especially for browns and blacks) and as essential pre-mordants for cellulose fibers.

Rich Tannin Sources

Source	Tannin Content	Availability
Oak galls	Very high (50-70%)	Oak trees with gall wasps
Sumac leaves	High (25-30%)	Temperate woodlands
Oak bark	High (10-20%)	Any oak species
Tea leaves	Moderate (10-15%)	If cultivated
Pomegranate rind	High (20-30%)	Mediterranean, subtropical
Chestnut wood	High (8-12%)	Temperate forests
Black walnut hulls	Very high	Eastern N. America
Acacia bark	High	Tropical/subtropical

Tannin Pre-Mordant for Cotton

1. Prepare a tannin bath — simmer bark, galls, or leaves in water for 1-2 hours
2. Strain out plant material
3. Soak pre-wetted cotton or linen in the tannin bath for 12-24 hours (overnight)
4. Remove and wring out gently — do not rinse
5. Proceed immediately to alum mordanting

This tannin-alum sequence is called the “tannin-alum-tannin” (TAT) method when a second tannin bath follows. TAT produces the most permanent results on cellulose fibers.

Other Mordants

Copper (Cupric Sulfate)

• Shifts colors toward green and blue-green
• Toxic — handle with gloves, do not dump waste water on food gardens
• Use at 3-5% of fiber weight
• Can be made by dissolving copper in vinegar over several weeks

Tin (Stannous Chloride)

• Brightens colors dramatically — makes reds more scarlet, yellows more vivid
• Very small quantities needed (1-2% of fiber weight)
• Excess tin makes fiber extremely brittle
• Difficult to obtain without industrial chemistry

Chrome (Potassium Dichromate)

• Produces deep, rich, lightfast colors
• Highly toxic and carcinogenic — avoid unless no alternative exists
• Historically important but should be the mordant of last resort

For a rebuilding civilization, alum + iron + tannin cover 95% of mordanting needs. Copper adds some range. Tin and chrome are nice-to-have but not essential and carry significant toxicity concerns.

Mordanting Methods

The timing of mordant application relative to dyeing affects the final result:

Pre-Mordanting (Most Common)

Apply the mordant before dyeing. This is the standard method for most applications.

Advantages: Most even color, most predictable results, mordant can be applied days or weeks before dyeing.

Simultaneous Mordanting

Add mordant directly to the dye bath. Simpler but less predictable.

Advantages: Saves time and fuel (one heating instead of two). Disadvantages: Can produce uneven coloring, some mordant-dye combinations react poorly.

Post-Mordanting (Saddening)

Apply mordant after dyeing to modify the color. The iron darkening technique described above is a post-mordant.

Advantages: Lets you adjust color after seeing the initial dye result. Disadvantages: Less permanent bond, more prone to uneven results.

Mordant Disposal

Mordant baths — especially iron, copper, and tin — should not be poured on food gardens or into waterways. Alum is relatively benign but should still be disposed of thoughtfully. Options: pour on non-food areas, let evaporate, or neutralize with lime before disposal.

Mordanting Troubleshooting

Problem	Cause	Solution
Uneven color	Fiber not wetted evenly before mordanting	Always pre-soak fiber in warm water 30 min before mordanting
Color washes out immediately	Mordant did not take (common on cotton)	Use tannin pre-mordant, increase alum percentage
Colors duller than expected	Iron contamination from pot or water	Use non-iron vessels (glass, stainless, enamel); test water
Brittle fiber after dyeing	Too much iron or tin mordant	Reduce mordant percentage; shorten soak time
Streaky or spotted color	Fiber bunched up in mordant bath	Use plenty of water; move fiber gently during mordanting

Common Mistakes

1. Mordanting cotton like wool — cellulose fibers need tannin pre-treatment. Alum alone washes off cotton within a few dye baths.
2. Too much iron — a little darkens elegantly; too much turns everything gray-black and weakens the fiber. Start with 2% and increase only if needed.
3. Not pre-wetting fiber — dry fiber repels mordant solution, causing uneven uptake. Always soak in warm water for 30 minutes first.
4. Boiling protein fibers — wool felts and silk damages above 85°C. Keep temperatures below 90°C and handle gently.
5. Mixing mordants carelessly — alum then iron gives one result; iron then alum gives another. Understand the sequence before combining methods.

Summary

Mordant Chemistry — At a Glance

• Mordants create chemical bridges between dye and fiber, making color permanent
• Alum is the universal mordant: 15-20% of fiber weight for wool/silk, 20-25% for cotton (with tannin pre-treatment)
• Iron darkens and mutes colors (“saddening”) — use sparingly at 2-4% to avoid fiber damage
• Tannin is essential for mordanting cellulose fibers (cotton, linen) — use oak galls, bark, or sumac
• Pre-mordanting gives the most even, predictable results
• Always pre-wet fiber before mordanting for even absorption
• For 95% of needs, alum + iron + tannin is sufficient — copper adds some range, tin and chrome are optional and toxic
• Dispose of mordant baths responsibly — do not pour iron or copper waste on food gardens