Alum Mordant

Potassium Aluminium Sulphate Dodecahydrate

Potassium Alum is the most commonly used mordant for wool, silk and many natural dyes. It helps suitable plant dyes bond with the fibre and generally produces clear, bright colours without darkening them.

For protein fibres, use 7.5–15% WOF - weight of dry fibre.

Cotton, linen and other cellulose fibres require a different mordanting method. Alum should not be used alone on cellulose.

At a glance

How much to use

For wool and other protein fibres, begin with:

• 7.5% WOF for paler shades or minimum mordant residue
• 10% WOF as a general starting point
• 12–15% WOF for deeper shades

WOF means weight of dry fibre. For example, 10% WOF is 10g Alum for 100g dry fibre.

More Alum does not automatically produce stronger or more colourfast results. Use the lowest amount that works well for your fibre and dye.

Cold mordanting wool

Cold mordanting is our preferred method for wool.

1. Weigh the dry fibre and calculate 7.5–15% WOF Alum.
2. Soak the fibre thoroughly in water.
3. Dissolve Alum in a small amount of hot water.
4. Add it to a container of cool water and stir.
5. Add the wetted fibre and ensure it is fully immersed.
6. Leave for 24–72+ hours, moving the fibre occasionally.
7. Remove, rinse and dye immediately, or dry for future use.

Cream of tartar is not required for cold mordanting.

Hot mordanting wool

1. Weigh the dry fibre and calculate 7.5–15% WOF Alum.
2. Soak the fibre thoroughly.
3. Dissolve the Alum in hot water and add it to the mordanting pot.
4. Optional: add up to 5% WOF cream of tartar.
5. Add the wetted fibre.
6. Slowly raise the bath to approximately 80°C.
7. Hold for one hour, moving the fibre gently.
8. Allow it to cool before rinsing.

Avoid boiling or excessive movement, as this may felt wool.

Silk

For silk, begin with 7.5–10% WOF Alum.

Cold mordanting is particularly suitable. Handle delicate silk gently and sample first.

Cotton, linen and other cellulose fibres

Alum alone is not a complete mordant for cellulose fibres.

For cotton, linen and hemp, use one of our complete cellulose methods:

• Tannin followed by Alum and soda ash
• Aluminium acetate followed by chalk

Do not use the wool recipe directly on cotton or linen.

Good to know

• Dissolve Alum completely before adding the fibre.
• Wet the fibre thoroughly for even uptake.
• Alum is not required for indigo or woad vats.
• When layering indigo with mordanted colours, apply indigo first.
• Alum supports many natural dyes but cannot make a naturally fugitive colour permanent.
• Used mordant baths may retain Alum and can often be saved for reuse.

Storage and safe handling

• Store sealed in a cool, dry place.
• Avoid creating or breathing dust.
• Wear eye protection when handling powder or concentrated solutions.
• Wear gloves where appropriate.
• For textile dyeing and craft use only.
• Not for consumption or cosmetic use.
• Use dedicated dyeing equipment.
• Keep away from food, children and pets.
• Do not release concentrated solutions into drains or natural waterways.

What you receive

Your chosen net weight of potassium aluminium sulphate dodecahydrate, supplied in a sealed pouch.

Cream of tartar, soda ash, tannins and other dyeing materials are sold separately.

Display jars, dyed fibres and photographic props are not included.

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IRON MORDANT ~ Mordant, Colour Changer, increased light fastness

Iron (Ferrous Sulfate) is used alone as a mordant, influencing Colours and to increase light fastness when used in combination with other natural dyes.

Iron is best known for shifting yellows into greens, keep in mind that this is not given and doesn't work with every yellow plant dye, in which case it will be more of a brown. it is used to increase light fastness for weaker plant dyes, bu it will always sadden and/or darken the colours. 

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Basic recipe:

When adding Iron to your dye bath start of with 3% and use it at the end of your dye bath or as an after bath. Allow to oxidize for 10 min for full colour development.

*If you like your colours darker, increase the Iron amount by 1-2% at a time. 

**Leave for 10min than remove from dye bath - Iron will make wool brittle.

Sodium Acetate ~ Used to turn Ferrous Sulfate into Ferrous Acetate; used in the Mordanting Recipe for Cotton and Linen without Tannin (see below)

Sodium acetate, NaCH₃COO, also abbreviated NaOAc, is the sodium salt of acetic acid. We use Sodium Acetate to turn Iron into an easier digestible version of itself, Iron acetate. Similar to 'Iron Water' but you can control the amounts. Iron acetate is not as harsh on the fibres and can be easier digested by nature or your septic tank when turned into rust. Once made, it has no shelf life and needs to be used immediately. 

For mordanting Cellulose fibres, please choose one of the following recipes on our blog:

How to mordant linen and cotton fabrics successfully – AppleOak FibreWorks

How to mordant Linen and Cotton successfully without Tannins – AppleOak FibreWorks

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Basic recipe: 

1part Ferrous Sulfate

1part Sodium Acetate

Mix and dissolve in Water. I find that it makes the iron stronger and we only use 1% of Iron acetate on most recipes. Warm water will also intensify the reaction.

Aluminium Acetate Mordant

For Linen, Cotton, Hemp & Other Cellulose Fibres

Aluminium acetate is a mordant for cotton, linen, hemp, ramie, nettle and other cellulose fibres.

It is particularly useful when a clear, bright dye colour is required. It can be applied after a tannin treatment for the most reliable results or used without tannin when preserving a white background is more important.

Use at 5-8% WOF - weight of dry fibre.

At a glance

How much to use

Use:

5-8g aluminium acetate per 100g dry fibre

Equivalent to 5-8% WOF.

For example:

Suggested method

1. Scour: Scour the cellulose fibre thoroughly.
2. Optional tannin treatment: For the most reliable colour uptake and durability, treat the fibre with a suitable tannin first.
3. Weigh: Calculate 5–8% WOF aluminium acetate.
4. Dissolve: Dissolve the powder in warm water.
5. Mordant: Add the wetted fibre and enough warm water for it to move freely. Keep at approximately 48–60°C for 1–2 hours, moving occasionally. It may also be left overnight.
6. Remove: Remove the fibre and gently squeeze out the excess mordant. Do not rinse.
7. Chalk: Place the fibre in a warm bath containing approximately 10g calcium carbonate per litre of water. Leave for 30–60 minutes.
8. Rinse: Rinse thoroughly to remove excess chalk.
9. Dye: Proceed directly to dyeing or dry the prepared fibre for later use.

Very hard water can cause the aluminium to precipitate prematurely. Use soft water where possible.

With or without tannin?

Tannin followed by aluminium acetate

This is our preferred approach when maximum dye performance is the priority.

Low-colour tannins include:

• Tara
• Oak gall
• Sumac gallnut

Myrobalan, acacia and chestnut may also be used where a warmer background colour is suitable.

Aluminium acetate without tannin

Tannin can be omitted where maintaining the natural whiteness of cotton or linen is particularly important.

Apply the aluminium acetate directly to thoroughly scoured fibre and complete the process with a chalk bath.

This is useful for pale pinks, delicate yellows and other colours where even a light tannin background may influence the shade. However, the best fastness results are achieved with tannin as a first step.

Homemade aluminium acetate

Aluminium acetate can also be prepared from potassium alum and sodium acetate.

This produces a fresh, concentrated solution and uses a slightly different application method from the ready-made powder sold here.

Find our homemade aluminium acetate method here.

Good to know

• Aluminium acetate must be followed by a chalk bath.
• Alum and aluminium acetate are not interchangeable in recipes.
• Use Alum for most wool and silk mordanting.
• Aluminium acetate is primarily intended for cellulose fibres.
• Indigo and woad do not require mordanting.
• When combining indigo with mordanted colours, apply indigo first to avoid contaminating the vat.
• Always sample before beginning an important project.

Storage and safe handling

• Store sealed in a cool, dry place.
• Avoid creating or breathing dust.
• Work in a well-ventilated area.
• Wear gloves and eye protection.
• Avoid contact with the eyes, mouth and open wounds.
• For textile dyeing and craft use only.
• Not for consumption or cosmetic use.
• Use dedicated dyeing equipment.
• Keep away from food, children and pets.
• Do not release concentrated mordant solutions into drains or natural waterways.

What you receive

Your chosen net weight of aluminium acetate powder, supplied in a sealed pouch.

Tannins, calcium carbonate, dyes and other auxiliaries are sold separately.

Display jars, dyed textiles and photographic props are not included.

Learn with us

Join The Natural Dyers & Growers Academy to share your results and explore natural colour alongside fellow dyers and growers.

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A Modern Aluminium Mordant for Linen, Hemp & Cotton

Aluminium lactate is a highly effective mordant for natural dyeing on cellulose fibres such as linen, hemp, cotton, ramie and nettle.

Produced from lactic acid obtained through the fermentation of renewable raw materials, it offers a more sustainable alternative to aluminium acetate while producing bright, clear and colourfast results.

Fully soluble at room temperature and compliant with GOTS 6.0 standards, aluminium lactate is easy to use and suitable for both beginners and experienced natural dyers.

Product Details

Our Approach to Mordanting Cellulose Fibres

At AppleOak FibreWorks, we recommend a tannin-first approach when mordanting cellulose fibres.

Unlike wool and silk, cellulose fibres have relatively few natural binding sites for mordants. While aluminium mordants can improve dye uptake on their own, we have consistently found that a preliminary tannin treatment produces stronger and more durable results.

The tannin acts as a bridge between the fibre, the aluminium mordant and the dye, helping improve colour depth, wash fastness and long-term durability.

Step 1 – Tannin Bath

Low-colour tannins:

• Oak Gallnut
• Sumach Gallnut
• Tara Powder

For a background colour:

• Acacia Extract
• Myrobalan
• Chestnut Extract

Step 2 – Aluminium Lactate Bath

Use aluminium lactate at approximately 5–8% WOF (weight of fibre).

A Note on Dunging & Chalk Baths

Some modern recipes recommend aluminium mordants followed by bran, chalk or dunging baths.

While these methods may improve dye uptake, our own experience and understanding of the chemistry suggest they do not create the same durable tannin–aluminium complex traditionally used on cellulose fibres.

For projects where wash fastness matters, we continue to recommend tannin followed by aluminium mordanting.

Suggested Usage Rates

Traditional Recipes, AppleOak Methods & Learning Resources

Natural dyeing is full of conflicting recipes and advice. Over the years we have tested a wide range of mordanting methods on linen, hemp and cotton and shared our findings through both our blog and the AppleOak Academy.

Traditional recipes, AppleOak cellulose mordant guides and in-depth educational resources can be found here:

AppleOak Blog

https://appleoakfibreworks.com/blogs/news/tagged/how-to-mordant-linen

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Please Note

For textile and craft use only.

Not intended for food, cosmetic or medicinal applications.