Mordanting

What It Is

A mordant (from Latin mordere, "to bite") is a substance — typically a metal salt — that forms a coordination complex with both the dye molecule and the fibre, creating a stable three-way bond. Without a mordant, most natural dyes will wash out within a few uses.

The same dye + different mordant = different colour. This makes mordanting not just a technical step but a creative one.

Three Timing Approaches

• Pre-mordanting (선 매염): Mordant the fibre before dyeing. Most common method. Allows full mordant uptake before colour application.
• Simultaneous mordanting (동시 매염): Add mordant and dye in the same bath. Faster but less controlled.
• Post-mordanting (후 매염): Dye first, mordant after. Used to shift or darken colour. Iron post-mordant (saddening) deepens and darkens most dye colours.

Common Mordants

Alum (백반, Potassium aluminium sulfate): The most widely used natural dye mordant. Gives lighter, brighter colours. Concentration: 25% of fibre weight (hot water). Must always be used — "never skip alum." For animal fibres, use cream of tartar (potassium bitartrate) alongside alum for better colour and fibre protection.

Important: When making alum mordant for Korean traditional painting (아교반수), dissolve alum in cold water only — hot water makes it more acidic and can damage silk. Ratio: agyo (아교) 5 : baekban (백반, alum) 1 (grind alum to fine powder in a mortar before dissolving).

Tannin (탄닌): Gives darker, more muted colours. Tannin-rich materials include oak galls, sumac, black tea, walnut, avocado pits, and bark. Used at approximately 6% of fibre weight (for wool/animal fibre, with cream of tartar). Tannin is often used as a first step for cotton — it helps subsequent mordants and dyes bond to the cellulose fibre.

Soy milk (대두유 매염): A protein mordant for cellulose fibres. Scour the cotton first (1% soap + 1% soda ash), then mordant with soy milk or plain water soy treatment. Used for cochineal on cotton, and for cyanotype on fabric. The soy protein acts as a bridge between the cotton fibre and the dye molecule.

Sodium carbonate / Soda ash (소다회): An alkali modifier, not a true mordant. More intensive colour and powdery texture. Shifts pH alkaline — which changes colour: alkaline = darker, more muted; acid = brighter, clearer.

Iron (철): Post-mordant. "Saddening" — deepens, greens, and darkens most colours. Small amounts (1–2%) used after dyeing. Overuse damages fibres.

Ox gall (소담즙): Not a mordant — a wetting agent. Helps dye penetrate fibres evenly, especially silk. Used in small amounts in the dye bath.

pH and Colour

pH significantly affects natural dye colour:

• Alkaline (above pH 7): Darker, more muted colour. Sodium carbonate, lime, wood ash all shift pH alkaline.
• Acid (below pH 7): Brighter, clearer colour. Vinegar, citric acid (more stable than vinegar), cream of tartar shift pH acid.
• pH-sensitive dyes (red cabbage, beetroot, hollyhock) will change colour dramatically with pH shifts — these can act as pH indicators.

Fibre-Specific Notes

• Wool and silk: pH must not exceed 9. Never 11 — this will damage or dissolve the protein structure. Use warm water (40–60°C), gentle agitation. Cream of tartar + alum together for best results.
• Cotton: Scouring first is essential: 1% soap + 1% soda ash. Then mordant with alum (25%), tannin, or soy milk depending on dye. Mercerized cotton takes dye more readily than unmercerized.
• Silk: Mordant with diluted alum solution (10–15%) in warm water. No boiling.

Mordanting Process (Alum, Pre-mordanting)

1. Dissolve alum in hot water at 25% of fibre weight.
2. Add pre-wetted fibre to the mordant bath.
3. Heat slowly to 80–90°C (wool/silk: do not exceed 80°C).
4. Maintain temperature for 45–60 minutes.
5. Remove fibre, gently squeeze. Do not rinse — proceed directly to dye bath, or store damp wrapped in plastic for up to a week.

Jay's Studio Note

Mordanting is where the same plant material produces completely different results in different hands. In Oaxaca, the same dye bath would be divided into several portions, each mordanted differently — alum, iron, lime, tin — producing a full spectrum from one source plant. The mordant is not a preparatory step; it is a creative decision. Alum + cochineal = bright red. Iron + cochineal = grey-purple. Lime + cochineal = orange-pink. Same insect, completely different colour.

Glossary

매염 = Mordanting · 선 매염 = Pre-mordanting · 동시 매염 = Simultaneous mordanting · 후 매염 = Post-mordanting · 백반 = Alum (potassium aluminium sulfate) · 탄닌 = Tannin · 소다회 = Soda ash (sodium carbonate) · 명반 = Alum (alternative Korean term)