Alum Mordant

Part of Natural Dyes & Inks

Using alum as a dye mordant to fix colors permanently in fibers.

Why This Matters

Without a mordant, most natural dyes wash out within a few launderings. The color may look brilliant when first applied, but it sits loosely on the fiber surface, held by nothing stronger than physical adhesion. A mordant creates a chemical bridge between the dye molecule and the fiber, locking the color in place permanently. The word itself comes from the Latin “mordere” — to bite.

Alum (potassium aluminum sulfate) is the single most important mordant in the history of textile dyeing. It produces bright, clear colors without shifting the dye’s hue significantly. It doesn’t damage fibers. It’s non-toxic in normal use. And it can be sourced from natural mineral deposits found on every continent.

In a rebuilding scenario, the ability to produce colorfast textiles is not merely aesthetic — it’s practical. Color-coded clothing identifies roles and teams. Durable dyes prevent the psychological toll of wearing perpetually faded, dingy garments. And the chemistry of mordanting leads directly to tanning, water purification, and other aluminum-salt applications.

What Alum Is and How It Works

Chemistry

Alum is a double sulfate salt with the formula KAl(SO₄)₂·12H₂O (potassium aluminum sulfate dodecahydrate). When dissolved in water, it dissociates into potassium, aluminum, and sulfate ions.

The aluminum ions are the active mordanting agent. They bond to hydroxyl groups (-OH) on the fiber surface (particularly cellulose and protein fibers), and simultaneously coordinate with the dye molecule’s oxygen and nitrogen atoms. This creates a stable three-way complex: fiber-aluminum-dye.

Why Alum Works So Well

Property	Benefit
Colorless in solution	Doesn’t alter the dye color
Strong fiber affinity	Bonds well to wool, silk, cotton, and linen
Dye coordination	Forms stable complexes with most plant dye molecules
Gentle on fibers	Doesn’t degrade cellulose or protein
Water soluble	Easy to apply in a simple bath
Lightfastness	Aluminum-dye complexes resist UV degradation

Sourcing Alum

Natural Mineral Deposits

Alum occurs naturally as the mineral alunite (KAl₃(SO₄)₂(OH)₆) and as efflorescent crusts on certain rocks, particularly in volcanic regions and areas with sulfur-bearing clays.

Where to look:

• Volcanic areas — sulfurous hot springs and fumaroles often deposit alum on surrounding rocks
• Shale and clay exposures — particularly those near coal seams or pyrite-bearing rock
• Desert surfaces — white, powdery crusts on alkaline soils sometimes contain alum
• Cave walls — efflorescent mineral deposits in limestone caves

Identification:

• White or colorless crystalline crust
• Astringent (puckering) taste — the definitive field test
• Dissolves readily in warm water
• Solution has a slightly acidic, metallic taste

Processing Alunite

If you find alunite rock:

1. Roast the rock — Heat to approximately 500-600°C for several hours. This converts the alunite to a soluble form.
2. Crush — Break the roasted material to small pieces.
3. Dissolve — Add to hot water and stir until dissolved. Filter through cloth to remove sediment.
4. Crystallize — Allow the solution to cool slowly. Alum crystals form as the solution cools. Collect crystals and redissolve/recrystallize for higher purity if needed.

Making Alum from Clay and Urine

A traditional method used where mineral alum is unavailable:

1. Collect aluminum-rich clay — White or grey kaolin-type clays work best
2. Mix with urine — Aged urine provides both sulfate (from decomposition) and ammonia
3. Allow to react — Let the mixture stand for several weeks, stirring occasionally
4. Leach — Add water, stir, and filter through cloth
5. Evaporate — Boil down the filtered solution until crystals form on cooling
6. Purify — Redissolve and recrystallize

This produces a crude but functional alum. The process was used extensively in medieval Scotland and other regions without natural alum deposits.

Alternative Aluminum-Based Mordants

If purified alum is unavailable, these alternatives provide aluminum ions:

Source	Method	Quality
Aluminum-rich clay	Soak fiber in clay-water slurry	Crude but functional
Wood ash + clay	Leach clay with wood-ash water	Better — ash dissolves more aluminum
Club moss (Lycopodium)	Boil plant material, use liquid as mordant bath	Traditional Scandinavian method
Symplocos leaves	Boil leaves, use liquid	Traditional Asian method — very effective

Mordanting Procedures

Mordanting Wool

Wool is the easiest fiber to mordant because protein fibers have natural affinity for aluminum ions.

Materials:

• 10-15% alum by weight of fiber (e.g., 10-15g alum per 100g of dry wool)
• 5-6% cream of tartar (optional but improves evenness and brightens colors)
• Enough water to submerge the fiber freely

Procedure:

1. Weigh the dry fiber — All mordant quantities are calculated as a percentage of the dry fiber weight (called “weight of goods” or WOG).

2. Dissolve alum — Add alum (and cream of tartar if using) to warm water and stir until completely dissolved. Use a non-reactive vessel — ceramic, glass, enamel, or stainless steel. Never use aluminum pots (uncontrolled mordant pickup) or iron pots (color shift).

3. Wet the fiber — Soak wool in warm water for at least 30 minutes. This opens the fiber structure and ensures even mordant absorption. Squeeze gently — do not wring.

4. Add fiber to mordant bath — Place wet wool in the alum solution. Ensure it’s fully submerged and can move freely.

5. Heat slowly — Raise temperature to a gentle simmer (80-85°C) over 30-45 minutes. Never boil wool — it felts and shrinks irreversibly.

6. Hold at temperature — Maintain the simmer for 45-60 minutes, gently turning the fiber occasionally.

7. Cool in the bath — Turn off heat and allow the fiber to cool in the mordant bath, ideally overnight. This maximizes absorption.

8. Remove and rinse — Gently squeeze out excess liquid. Rinse briefly in water of similar temperature. The fiber is now mordanted and ready for dyeing.

Mordant in Advance

Mordanted wool can be dried and stored for weeks or months before dyeing. This lets you batch your mordanting when alum is available and dye whenever you have fresh dye materials.

Mordanting Cotton and Linen (Cellulose Fibers)

Cellulose fibers are much harder to mordant than protein fibers. They have fewer binding sites for aluminum ions and require a multi-step process.

Tannin pre-treatment (essential for cellulose):

1. Prepare tannin bath — Boil oak galls, sumac leaves, pomegranate rinds, or acacia bark in water. Use approximately 10-15% tannin source by weight of fiber.

2. Soak fiber in tannin — Submerge cotton or linen in the tannin bath for 12-24 hours. The tannins bond to cellulose and provide attachment sites for aluminum.

3. Remove and squeeze — Do not rinse.

4. Mordant with alum — Dissolve 15-20% alum in warm water. Soak the tannin-treated fiber for 12-24 hours. Heat is optional but helps (warm, not hot — 50-60°C).

5. Repeat if desired — For deepest colors, repeat the tannin-alum cycle 2-3 times, alternating between tannin bath and alum bath.

6. Rinse gently before dyeing.

Mordanting Silk

Silk is a protein fiber and mordants similarly to wool, but it’s more delicate:

• Use 8-10% alum (slightly less than wool)
• Do not exceed 70°C — silk is damaged by high heat
• Handle very gently — wet silk is fragile
• Soak for 1-2 hours rather than simmering

Troubleshooting Alum Mordanting

Problem	Cause	Solution
Uneven color after dyeing	Fiber not wetted thoroughly; mordant bath too concentrated	Pre-soak fiber longer; use more water
Sticky or stiff fiber	Too much alum used	Reduce alum to 10% WOG; rinse more thoroughly
Color washes out despite mordanting	Insufficient contact time; bath too cool	Extend mordanting time; ensure proper temperature
White residue on dried fiber	Excess alum crystallizing	Reduce alum percentage; rinse after mordanting
Fiber feels harsh	Over-mordanting; cream of tartar too high	Reduce quantities; handle gently

Alum in Combination with Other Mordants

Alum can be used alongside other mordants for specific effects:

Combination	Effect	Application
Alum + cream of tartar	Brightens colors, improves evenness	Standard for wool
Alum + tannin	Enables cellulose mordanting	Essential for cotton/linen
Alum then iron (after-bath)	Darkens and saddens colors	Greens from yellow dyes; grey tones
Alum then copper	Shifts yellows toward green	Enhancing plant greens

Iron After-Baths

Iron sulfate (copperas) used after alum mordanting dramatically shifts colors toward dark greens, greys, and blacks. Use sparingly — 1-2% WOG maximum. Excess iron makes fibers brittle over time.

Storage and Shelf Life

• Crystal alum — Stores indefinitely if kept dry. Exposure to humid air causes the crystals to develop a white powdery surface (dehydration) but they remain functional.
• Alum solution — Prepared solutions last weeks but may develop mold growth from organic contaminants. Add fresh water as needed.
• Mordanted fiber — Properly mordanted and dried fiber retains its mordant for months. Store in a dry, dark place. Label clearly — mordanted fiber looks identical to untreated fiber.