Mordants in Natural Dyeing: The Chemistry of Color Permanence
Without mordants, most natural dyes wash out. The color looks vibrant initially, then fades to nothing after a few washes.
Mordants are metal salts that create chemical bridges between dye molecules and fiber molecules, making the bond permanent. But they do more than just fix color - they shift it. The same plant produces completely different colors depending on which mordant you use.
Madder root with alum mordant creates brick red. Madder with iron mordant produces purple-brown. Same dye source, different metal salt, entirely different color.
This is why mordants matter in natural dyeing. They're not just fixatives - they're color modifiers. Understanding what each mordant does changes what you can create.
What Mordants Actually Are
The word "mordant" comes from Latin mordere - to bite. Mordants bite into fiber, creating attachment points for dye molecules.
Chemically, mordants are metal salts. When dissolved in water, they break into metal ions and other compounds. Those metal ions bond to fiber, then provide sites where dye molecules can attach.
Think of it as creating hooks. Unmordanted fiber is smooth - dye molecules slide across the surface but don't grab hold. Mordanted fiber has metal ions embedded in it. Dye molecules bond to those metal ions and stay there.
The specific metal determines not just how well the dye bonds, but what color results. Different metals have different atomic structures. Those structures interact with dye molecules in different ways, absorbing and reflecting different wavelengths of light.
This is why the same dye produces different colors with different mordants. The dye molecule is the same. The metal it's bonded to changes how that molecule interacts with light.
Alum: The Standard Mordant
Alum - specifically potassium aluminum sulfate - is the most common mordant in natural dyeing.
It produces clear, bright colors. Most natural dyes achieve their "true" color with alum. The reds read as red, yellows as yellow, blues as blue. Alum doesn't shift hue dramatically - it just makes color permanent.
Alum is also relatively benign. It's used in food processing (pickling, baking powder) and water treatment. The toxicity is low compared to other metal mordants. This made alum the accessible choice historically and keeps it popular now.
On protein fibers (wool, silk), alum works exceptionally well. The fiber structure readily accepts aluminum ions, and dye uptake is strong. On cellulose fibers (cotton, linen), alum alone is often insufficient - tannins or other additives improve results.
Traditional alum mordanting involves dissolving alum in hot water, adding fiber, simmering for an hour, then letting it cool overnight. The aluminum ions migrate into the fiber during that time. The fiber is then dyed while still damp.
The concentration matters. Too little alum, and dye doesn't bond completely. Too much, and fiber becomes sticky or stiff. The standard ratio is roughly 15-20% alum by weight of fiber, though this varies by dye and desired intensity.
Iron: The Color Shifter
Iron mordant - typically ferrous sulfate - darkens and neutralizes color.
Yellow dyes become olive or gray-green with iron. Red dyes shift to purple-brown or burgundy. Blue dyes deepen toward black. Iron acts as what dyers call a "sadden" agent - it removes brightness and shifts everything toward darker, muted tones.
This is incredibly useful. If you want subtle earth tones rather than bright colors, iron gets you there. Historical "black" dyes were often dark browns or blues overdyed with iron to push them as dark as possible.
The tradeoff: iron degrades fiber over time. The metal ions create weak points in the fiber structure. This happens slowly - we're talking years or decades, not weeks - but it's why some historical textiles with heavy iron mordanting are more fragile than others.
Dyers manage this by using iron as a modifier rather than the primary mordant. Mordant with alum first, dye, then dip briefly in an iron bath to shift the color darker. This gives you the color modification without prolonged iron exposure weakening the fiber.
Iron concentration is typically lower than alum - 1-2% by weight of fiber. Higher concentrations increase color shift but also increase fiber damage.
Copper: The Green Maker
Copper mordant - usually copper sulfate - shifts colors toward green and blue-green.
Yellow dyes become chartreuse or moss green with copper. Browns shift to olive. Some reds move toward rust. Copper creates the earthy greens that are difficult to achieve otherwise in natural dyeing.
Like iron, copper is somewhat harsh on fiber. It's not as damaging as iron, but it's more aggressive than alum. And copper sulfate is more toxic to handle - it requires care during preparation and disposal.
Copper mordanting follows similar processes to alum: dissolve in hot water, add fiber, simmer, cool. Concentrations are typically 2-6% by weight of fiber.
Some dyers use copper pots instead of copper mordant. The pot surface releases copper ions during heating, which mordant the fiber and shift color. This is less precise but requires no additional chemicals - just a copper vessel.
Historical dyeing sometimes used verdigris (copper acetate) as a mordant. The color shifts were similar, though the chemistry differs slightly.
Tin: The Brightener
Tin mordant - stannous chloride - intensifies and brightens color.
Yellows become more golden. Reds become more scarlet. Tin doesn't shift hue the way iron or copper do - it amplifies what's already there. The colors become more saturated, more vibrant.
This comes at a cost. Tin is harsh on fiber - more so than iron or copper. It makes fiber brittle if overused. And stannous chloride is corrosive, requiring careful handling.
Because of these drawbacks, tin is often used sparingly, either at low concentrations (1-4% by weight of fiber) or as a post-dye modifier rather than a primary mordant. Dyers call this a "tin bloom" - a brief final dip in dilute tin solution to brighten the color without extended tin exposure.
Tin also doesn't work equally on all fibers. It's most effective on protein fibers. On cellulose, results are less predictable.
Historically, tin was valuable enough that its use indicated expensive textiles. The intensified colors were a mark of quality - and wealth.
Chrome: The Historical Mordant No Longer Used
Chrome mordant - potassium dichromate - produced excellent color permanence and a wide range of hues.
It was widely used in industrial and craft dyeing through the mid-20th century. Then the toxicity became impossible to ignore. Chromium compounds are carcinogenic and environmentally persistent. Disposal contaminates water. Handling creates exposure risk.
Contemporary natural dyeing avoids chrome entirely. The colors it produced can often be approximated with other mordants, and the health and environmental costs aren't worth whatever advantages it offered.
If you encounter historical dyeing recipes mentioning chrome, understand it as a documentation of past practice rather than a current recommendation.
Tannins: The Pre-Mordant for Cellulose Fibers
Tannins aren't metal mordants, but they function similarly for cellulose fibers.
Cotton and linen resist metal mordants. Their smooth cellulose structure doesn't provide good attachment sites for metal ions the way protein fibers do. Tannins solve this by coating the fiber with compounds that then accept metal mordants more readily.
Common tannin sources include oak galls, sumac, tea, and myrobalan. The fiber is simmered in tannin solution, then mordanted with alum. The tannin layer helps the alum bond to the cellulose, which then helps the dye bond to the alum.
This two-step process - tannin then mordant - is sometimes called a "tannin-mordant-dye" sequence. It's standard for achieving good color on cotton with most natural dyes.
Some dyes are substantive on cellulose and don't require mordants at all (black walnut, for example). But for most plant dyes on cotton, tannins are essential.
Pre-Mordanting vs. Post-Mordanting
The mordant can be applied before dyeing or after. Each produces different results.
Pre-mordanting is the standard approach. Mordant the fiber first, then dye. The metal ions are embedded in the fiber before dye molecules arrive. This typically produces the truest color and strongest bond.
Post-mordanting involves dyeing first, then applying mordant. The dye has already bonded to fiber (weakly), and the mordant strengthens that bond. Post-mordanting with iron or copper also shifts the color, since the metal ions interact with dye already in the fiber.
Simultaneous mordanting means adding mordant directly to the dye bath. This is less common and produces less predictable results, but it's faster and was used historically when time mattered more than perfection.
Some dyers use combinations - pre-mordant with alum, dye, then post-mordant with iron to darken. This layers effects.
The Temperature Factor
Mordants typically require heat to bond effectively with fiber.
Most mordanting happens near simmering temperature (180-200°F / 82-93°C) and is maintained for 30-60 minutes. The heat opens fiber structure slightly, allowing metal ions to penetrate. Cooling locks them in place as the fiber structure contracts again.
Cold mordanting exists but produces weaker bonds. Some dyers use it for delicate fibers that can't handle prolonged heat, accepting the tradeoff of less dye permanence for fiber protection.
The cooling phase matters as much as the heating. Quick cooling can leave mordants unevenly distributed. Slow cooling - leaving fiber in the mordant bath overnight - allows metal ions to migrate fully into the fiber structure.
Mordant Exhaustion and Reuse
Mordant baths don't exhaust completely in one use. After mordanting fiber, metal ions remain in the solution.
The bath can be reused for additional batches, though it weakens with each use. First batch gets full-strength mordanting. Second batch gets slightly less. Third batch might need supplemental mordant added.
Some dyers keep mordant baths going indefinitely, topping up with fresh mordant as needed. Others start fresh each time for consistency.
The exhaustion rate depends on fiber type and amount. A bath used to mordant one pound of wool retains more mordant than a bath used for five pounds. Protein fibers also take up more mordant than cellulose, exhausting the bath faster.
Color Shift Examples: The Same Dye, Different Mordants
Here's what actually happens when you use different mordants with the same dye source:
- Alum → brick red to coral
- Iron → purple-brown to burgundy
- Copper → rust orange
- Tin → bright scarlet
- Alum → golden yellow
- Iron → olive green to gray-green
- Copper → brass to moss green
- Tin → bright gold
Logwood:
- Alum → purple
- Iron → blue-black to black
- Copper → deep purple-blue
- Tin → bright purple
The dye source is the same in each case. The metal mordant creates entirely different colors by changing how light interacts with the dye-metal complex.
This is why mordant knowledge expands your color palette exponentially. One dye source with four mordants gives you four different colors.
Safety and Disposal
Metal mordants require safety considerations.
Alum is the safest - food grade, low toxicity, standard precautions apply (don't ingest, avoid eye contact).
Iron is relatively safe as ferrous sulfate but can stain and corrode. Keep it away from other dye materials. Disposal in normal wastewater is generally acceptable at small scale.
Copper is more toxic. It's harmful to aquatic life even in small concentrations. Disposal requires care - don't pour copper mordant down the drain. Many areas have hazardous waste collection for small amounts of chemicals like copper sulfate.
Tin is corrosive and requires protective equipment (gloves, eye protection). Disposal follows similar guidelines as copper - don't introduce it to water systems.
Chrome shouldn't be used at all due to carcinogenic properties.
Good practice: mordant outdoors or with ventilation, wear gloves, label containers clearly, research local disposal regulations, and use the minimum concentration needed for your desired result.
Historical Mordanting Practices
Before commercial chemical suppliers, dyers sourced mordants from natural materials.
Alum came from alum mines or was extracted from clay and alunite rock. Centers of alum production - Tuscany, the Middle East - controlled significant trade.
Iron came from rusty nails, scrap metal, or iron-rich mud. Ferrous sulfate formed naturally in some water sources.
Copper came from copper vessels, verdigris scraped from copper surfaces, or copper ores.
Tin was extracted from pewter or tin vessels.
Tannins came from oak galls, tree bark, or plant materials high in tannic acid.
The quality varied significantly. Natural alum deposits contained impurities that sometimes affected color unpredictably. Iron from rusty nails produced different results than iron from bog mud. Dyers developed expertise in assessing and using whatever mordant sources were locally available.
Standardized commercial mordants are a relatively recent development - late 19th century onward. Before that, mordanting required both chemical knowledge and material sourcing skill.
The Exception: Substantive Dyes
Some natural dyes don't require mordants. They bond directly to fiber.
These are called substantive dyes. The dye molecule structure allows it to bind to fiber without metal intermediaries.
Black walnut is substantive - it permanently stains fiber without mordants. The tannins in walnut hulls bond directly to cellulose and protein fibers.
Indigo is substantive through a different mechanism - the reduced indigo (leucoindigo) penetrates fiber, then oxidizes back to insoluble pigment trapped inside.
Turmeric contains curcumin, which bonds to fiber readily - though not particularly permanently. It's substantive but not especially lightfast.
Most natural dyes are not substantive. They need mordants to achieve permanence. The substantive dyes are useful precisely because they bypass that requirement.
Why Natural Dyes Need Mordants and Synthetics Don't
Synthetic dyes are engineered to bond to specific fiber types without additional chemicals.
Fiber-reactive dyes contain reactive groups that form covalent bonds with cellulose. Acid dyes bond to protein fibers through ionic attraction and hydrogen bonding. Disperse dyes designed for polyester use different mechanisms entirely.
Natural dye molecules weren't optimized for fiber bonding. They evolved for other purposes in plants - protection from UV light, attraction of pollinators, defense against herbivores. The fact that they produce color on fiber is coincidental.
Mordants compensate for that. They create the chemical bridges that natural dye molecules lack.
This is also why synthetic dyes are more lightfast and washfast - the bonds are stronger and more specific. Natural dyes with mordants approach but rarely match synthetic dye permanence.
The tradeoff is the color quality. Natural dyes produce complex color because the dye molecules are complex mixtures of compounds. Synthetic dyes produce pure color because they're single compounds. Each has aesthetic advantages depending on what you want.
Mordants and Fiber Type
Different fibers require different mordanting approaches.
Wool and silk (protein fibers) mordant easily. Their structure - amino acid chains with reactive sites - accepts metal ions readily. Alum, iron, copper, and tin all work well. Standard mordanting produces strong dye bonds.
Cotton and linen (cellulose fibers) resist mordants. Their smooth cellulose structure provides fewer attachment points. Tannin pre-treatment helps, but results are still less vibrant than on protein fibers. Some dyes work better on cellulose than others, but none match the ease of dyeing wool.
Silk is particularly receptive to mordants and dyes - it produces the most intense colors with the least material. This is partly why silk was historically associated with rich, vibrant color.
The fiber type determines not just whether mordants work, but how much you need and what results to expect.
The Art of Mordant Combinations
Experienced dyers sometimes use multiple mordants in sequence or combination.
Alum first for strong dye bonding, then iron post-mordant to shift color darker. Or copper and tin combined to create unusual color effects that neither produces alone.
These combinations require understanding how mordants interact. Some combinations are complementary. Others cancel each other out or create undesirable effects.
The experimentation is part of the craft. Historical dye recipes sometimes call for complex mordant sequences developed through generations of trial and error.
Contemporary dyers continue this experimentation, documenting what works and sharing results. The chemistry provides guidelines, but the actual color that emerges from any specific dye-mordant-fiber combination requires testing.
What Permanence Means
"Permanent" in natural dyeing is relative.
A well-mordanted natural dye on wool will last decades without significant fading if protected from strong light and washed gently. The color won't wash out in normal laundering.
But it will fade eventually. UV exposure breaks down dye molecules over time. Frequent washing gradually removes dye. Abrasion wears color away.
This is true for synthetic dyes too - nothing is truly permanent - but natural dyes fade faster. The mordant makes the bond strong enough to be practical, not invincible.
The fading pattern is part of natural-dyed fabric's aesthetic. The color shifts subtly over years. High-wear areas lighten first. The fabric develops character through use.
Some dyes are more permanent than others. Indigo, madder, and weld are known for excellent lightfastness. Turmeric, safflower, and some flower dyes fade quickly. The mordant improves permanence but can't overcome inherent dye molecule instability.
Mordants as Color Palette
Once you understand mordants as color modifiers rather than just fixatives, the palette expands dramatically.
Instead of "I have access to madder and onion skins and goldenrod," you have "I have access to madder in four colors, onion skins in three colors, and goldenrod in three colors."
The same dozen common dye plants become thirty or forty distinct colors through mordant variations.
Add overdyeing - blue from indigo, then yellow from onions, creating green - and the combinations multiply further.
Historical dyers worked within these constraints and created extraordinary color ranges. The limitations forced creativity. The mordants provided the tools.
Contemporary natural dyers have the same tools, plus better understanding of the underlying chemistry. The potential is there. The results depend on experimentation and experience.