Iron Mordant – Ferrous Sulphate
Colour Modifier · Mordant
Iron, or ferrous sulphate, is a powerful natural-dye mordant and colour modifier. It darkens and “saddens” colours, creating olives, greys, browns, charcoals and deeper muted shades.
Yellow dyes may shift towards olive or green, although this depends on the dye plant - some yellows become brown or grey instead. With tannin-rich dyes, iron can produce deep greys and near-blacks.
Iron may improve the lightfastness of some plant dyes, but it cannot make every fugitive colour permanent.
At a glance
| Feature | Details |
|---|---|
| Product | Ferrous sulphate |
| Function | Mordant and colour modifier |
| Suggested quantity | 1–3% WOF |
| Starting quantity | 1% WOF |
| Suitable fibres | Wool, silk, cotton, linen and other textile fibres |
| Best application | Warm and added directly to the dye bath near the end |
| Processing time | Up to 10 minutes, aerating. Repeat up to 3 times. |
| Colour effects | Olive, green, grey, brown, charcoal and muted shades |
How much to use
Begin with:
1g iron per 100g dry fibre
Equivalent to 1% WOF - weight of dry fibre.
For most work, 1–3% WOF is sufficient. Begin at 1% and only add another 1% if repeated immersion and aeration have not produced enough change.
Some colours may not shift noticeably until approximately 5% WOF, but this concentration presents a much greater risk of fibre damage and is not recommended for routine use.
Suggested method
Iron reacts with oxygen, and much of the final colour develops while the fibre is exposed to air.
- Complete the main dyeing process.
- Dissolve 1% WOF ferrous sulphate thoroughly in a little warm water.
- Add the solution directly to the warm dye bath when finished dyeing.
- Immerse the fibre and move it regularly for up to 10 minutes.
- Use a shorter immersion for delicate fibres such as silk.
- Remove the fibre and hang it up or spread it out for approximately 10 minutes, allowing air to reach it evenly.
- Return it to the bath and repeat the immersion and aeration cycle if required.
- Up to three cycles are usually sufficient. Further repetitions generally produce little additional change.
- If the colour remains too light after three cycles, dissolve and add another 1% WOF iron, then repeat the process.
- Rinse thoroughly once the desired shade has developed.
Maintain a little warmth throughout the process. Warm application directly in the dye bath is generally more effective than a cold treatment or separate afterbath.
Watch the colour during every stage. It may continue to darken considerably while hanging in the air.
Making ferrous acetate
We generally prefer to convert ferrous sulphate into a ferrous acetate working solution.
Use:
1 part ferrous sulphate
1 part sodium acetate
- Weigh equal quantities of ferrous sulphate and sodium acetate.
- Dissolve them in warm water and mix thoroughly.
- Add the freshly prepared solution directly to the dye bath.
- Use immediately.
Ferrous acetate can act more strongly than ferrous sulphate alone. Begin with 1% iron, calculated from the weight of the ferrous sulphate - not the combined weight of both ingredients.
For example, for 100g of fibre, use:
- 1g ferrous sulphate
- 1g sodium acetate
Sodium acetate is sold separately.
Possible colour changes
| Original colour | Possible result with iron |
|---|---|
| Yellow | Olive, green, brown or grey |
| Red or pink | Burgundy, plum, brown or muted purple |
| Tannin-rich beige or brown | Grey, charcoal or near-black |
| Bright botanical colours | Darker, softer or more muted shades |
Results depend on the dye, fibre, water chemistry and amount of iron used. Always test a sample first.
Important fibre care
Iron can damage fibres when used excessively or left in contact with them for too long.
For most projects, stay within 1–3% WOF and build the colour through repeated immersion and aeration rather than immediately increasing the iron concentration.
Use shorter immersion periods for silk and other delicate materials. Although some colours may only shift at approximately 5% WOF, this higher concentration presents a significantly greater risk of weakening the textile.
Ferrous acetate is preferable when a more controlled, fibre-friendly iron treatment is required.
Storage and safe handling
- Store tightly sealed in a cool, dry place.
- Harmful if swallowed.
- Causes skin and serious eye irritation.
- Avoid creating or breathing dust.
- Wear gloves and eye protection while measuring and dissolving.
- Work in a well-ventilated area.
- Use dedicated dyeing utensils - never food-preparation equipment.
- Keep away from food, children and pets.
- Do not discharge concentrated solutions into drains or waterways.
- Follow local guidance when disposing of iron-containing solutions.
What you receive
Your chosen net weight of ferrous sulphate, supplied in a sealed pouch.
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