The importance of using a mordant in natural dyeing

Hi Creative Mamas! I am so excited to share with you a little bit of information about mordants and the role they play within the whole process in Natural dyeing.

As outlined in the Ultimate Guide to Natural Dyeing, there is a process to follow when we are embarking in natural dyeing.

I have created a 8 steps process chart that clearly shows which steps to follow and the order in which they need to be followed. As you can see in the chart below, mordanting comes after fiber classification, weighing and scouring.

This post will go deep into the different kinds of mordants and will outline the basic mordant recipe that I always follow for best results.

However, bare in mind that mordanting is only step 4 within a 8 steps process.

natural dyeing process

Let’s start by defining and understanding what a mordant is.

Mordant Definition

A mordant is defined as a naturally occurring water soluble metallic salt which creates a bond between the dye and the fiber. 

The word “mordant” means “to bite” in french. So, if  we think of the fiber as a surface, we can picture the mordanting process as a way to allow the fiber  to open up in order for the dye to be able to bite into the fiber. 

In order to create colors that are colorfast (meaning that will last forever) it’s very important that you use a mordant to prepare your fabric for dyeing and receiving the color from the dye.

The function of a mordant is therefore to create a bond between the fiber and the dye. There are many types of mordants.

Can I dye without a mordant?

Drop a skein of wool or a piece of fabric into a dye bath (without previously mordanting) and it’s unlikely that they will retain any color.

However, there are some dyes which don’t require mordanting and these are called Substantive dyes.

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Dyes that don’t require mordant

The substantive dyes are the easiest dyes to apply because they will adhere to the fiber without the assistance of a mordant.

They are usually dyes which are rich in tannins including barks and the leaves and fruits of trees. They are sometimes referred as mordant dyes.

Plant Mordants:

  • Walnut
  • Sumac
  • Oak
  • Black tea

Types of Mordant

There are many types of mordants. Some of them are very toxic to the dyer and the environment and some are not. I only use Alum myself since it’s the safest option. Please refer to the safety guidelines section in this post. 

These are the traditional mordants that are widely used and written about:

  • Alum
  • Copper
  • Tin
  • Iron
  • Chrome

Alum Mordant

This includes Potassium Aluminum Sulphate for wool and silk and Aluminum Acetate for cotton and linens.

Allum is the most commonly used mordant. I only use Allum as a mordant myself. I use it in combination with cream of tartar which helps with the brightness of the color. 

Recipe: 10 % Alum (Aluminum Potassium Sulphate ) WOF (weight of fabric) and 8% cream of tartar WOF (weight of fabric).


Copper mordant is mildly poisonous so it needs to be handled with much care. I don’t use copper as a mordant. Some dyers like it because it can be used to green most yellows.  

Recipe: 2 % Copper Sulphate WOF (weight of fabric) and 4% vinegar WOF (weight of fabric).


Tin is also mildly poisonous so xtra care is required again. Most dyers like to use tin as an additive rather than as a mordant because it can make the fiber brittle. 

Recipe: 3 % Tin (weight of fabric)


Iron has been widely used as a mordant. I make my own iron water solution and I love to use it as an after mordant or as a color modifier. 

Iron can make the fibers coarse and can even damage them. It can also create duller color tones but its a great mordant to experiment with. 

Recipe: 5 % Iron WOF (weight of fabric)


Chrome is a very toxic mordant that contaminates the environment and causes irritation. The results from using chrome are very similar to the ones obtained by using copper as a mordant. 

Using different mordants will create different colors

You can create many different colors and tones by using one single dye and mordanting the fiber with different mordants.

The type of mordant that you use will create totally different results but the amount of mordant will also create a different color, brightness and feel in the resulting fiber.

You can get lost reading about different recipes and methods. I spent a lot of time experimenting with different processes and not always getting satisfying results.

I now stick to what I know works best and delivers consistent results time and time again! My advice is to mordant with Allum for both cellulose and protein fibers and to follow the method outlined in this guide.

Mordant with Alum

I personally mordant with Alum for both protein and cellulose fibers. Below is a detailed description of how I mordant when preparing my wool and my silks.

Note: this mordant recipe is just for wool and silk. For mordanting with cotton and linen check out the recipe in The Ultimate guide to Natural dyeing.

The colors in the photo below were created by dyeing different types of silk and wool using ALUM as a mordant. For more information about the dyes that I used please refer to the Beginners Guide to Natural dyes.

natural dyeing

You can also check out the Ultimate Natural Dyes List with over 40 dyes to choose from to use in combination with Alum.

This post contains affiliate links, which means I receive a small commission, at no extra cost to you, if you make a purchase using this link. Please see my disclosure for more details

Equipment and tools:

Alum Mordant Recipe for wool and silk

10 % Alum (Aluminum Potassium Sulphate ) WOF (weight of fabric) and 8% cream of tartar WOF (weight of fabric).

  • Dissolve Alum in lukewarm water in a glass jar
  • Transfer to a larger stainless steel pot and make sure the Allum is well dissolved.
  • Add more water, then add the wetted fiber in the pot, making sure there is plenty of room
  • Slowly bring the pot to a temperature of 90 degrees C or 190 degrees F over a period of no less than 1 hour
  • Simmer for 40 minutes. Let the bath cool.
pre mordanting

Vinegar as a mordant

Vinegar is not really a mordant, even though its widely used as a basic print enhancer in the case of very basic eco printing or bundle dyeing.

These types of processes are more experimental and will yield different and unexpected results every time. For best results check in eco printing you should follow these eco printing guidelines.

The vinegar can act as a mild mordant but it’s not a reliable one when you are after solid good results. I wouldn’t use vinegar to dye a length or fabric or a skein of wool. 

fabric print vinegar solution

Soy milk as a mordant

Soy milk is sometimes used as a mordant  in cellulose fibers such as cotton and linen. It’s actually not a mordant but a binder. It makes cellulose fibres act more like protein fibres (wool and silk) which naturally tend to absorb dye color much better. 

Soaking your fiber in soy milk it’s a great totally natural alternative to mordanting. 

Urine as a mordant

Urine was a very important mordant used in the olden days. Sometimes mixed with Alum or simply used by itself, urine can be a powerful mordant because it decays into ammonia. 

Urine works really well with wool and to soften leather. 

Extending the color palette using iron as a post mordant

You can expand your color palette by using Alum as a mordant and using iron as a post mordant or color modifier. You can also use iron as a mordant instead of Allum but I only do this sparingly because I am aware that the iron may damage the fibers.

However, when I am in an experimental mood I like to play around a bit wit iron water solution as a pre and post mordant. the results are fascinating.

The illustration below shows the color achieved by dyeing with Eucalyptus and modifying with iron water solution as as post dyeing step.

natural dyeing

If you want to experiment with different colors and tones that can be achieved by playing around with modifiers and mordants such as iron water and lemon (acid modifier) check out the results obtained when dyeing with avocados and onion skins. Super fun to explore!

Also, if you are interested in making black from acorns check out the full step by step tutorial on how to transform gold color from acorns into a deep black.

Safety guidelines when using mordants

  • Get into the habit of wearing a face mask when mordanting
  • Use rubber gloves
  • Never handle mordants near food or drinks
  • Keep all mordants and modifiers out of direct sunlight
  • Never use a cooking pot or utensil for cooking after using it for dyeing
  • If you are unsure if a substance is toxic assume it is

Frequently asked questions:

Do I always need to use a mordant in natural dyeing?

Not always but unless you are using substantive dyes you will require a mordant to make sure that the dye bonds with the fiber. the other exception is indigo. There is no need to mordant a fiber if you are using an indigo vat for dyeing.

Are mordants toxic?

Many mordants are. I recommend using Allum because even though you must handle with care it’s the safest and most enviromentally friendly option.

Can I use the mordant water in the dye bath?

Yes, you can totally use the same water. However, I recommend that you use only half of the water. If there is any potential damage to the fibers from the mordanting liquid you will minimize that by adding some fresh water.

Can I mordant fabric, dry it and have it ready for dyeing at a later date?

Yes, you totally can. I have done this many times but I only use this method for swatches and samples. So, if you have some fabric or yarn leftover which has been mordanted but not dyed you can dry it and use it as a test for testing dyes etc before dyeing big quantities.

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natural dyeing

My final thoughts are that alum sulphate mordant is the best option for dyeing protein fibers and alum acetate mordant is the best way to go when dealing with cellulose fibers.

Please email me any questions that you may have and I will be happy to answer it 🙂

6 thoughts on “The importance of using a mordant in natural dyeing”

  1. Hello,
    I am not sure if you are still active on this blog. I have a group of ladies that get together once a month and try a crat. Mostly using reduce reuse and recycle policies. we are going to be trying out dying in a couple of weeks. I dont like the idea of using alum so am going to try soy milk. Would like to use mostly substantive dyes too. I live in Tropical australia so cannot get all the plants you talk about. It is all about experimenting. Thank you so mcuh for all the information you have put out.

    • Hi Gillian, thanks for your message. Yes I am very active on this blog and thanks for reminding me that I need to publish a post about soy mil as a mordant. If you live in Australia Eucalyptus will be available to you and the color is just amazing! You can also try tannin as a mordant. Cheers and hope the session goes great!

  2. Hi,
    I’m wanting to dye 1×1 ribbing knit fabric cotton/ poly blend with Rooibos tea to make a buttery cream colour, I did try this without any mordants on a sample and the colour was perfect, but I need to do a bigger quantity for commercial garments will the colour wash out over time? I’m just unsure of the colourfastness.


    • Hey Ashlee, the quick answer is yes, it will. However a cream color is very light and if you are after a shade above white you might be ok but whatever you dye with natural dyes without mordant {except using dyes that have tanning (tea)} it will fade away. Also be careful of the poly content in the fiber, the cotton will be taking the color but not the color, hope this helps!

  3. Hello, I am about to dye a pair of baggy, heavy cotton Indian pants that were a moss green new, but most of the colour has washed out. I am using a purply-red plant that is used for tea here in Ecuador. I plan to use urine as a mordant, but I can´t find the proportions anywhere. I don´t have a scale, so don´t know how much the pants weigh. Should the urine be used neat or watered down? And how much?
    Thanks so much.

    • Hi Annie, thanks for reaching out. Unfortunately I can’t answer your question with certainty since I have never actually used urine as mordant. I used to teach my students how some cultures used this method in the past but never actually experimented with it myself. There are no proportions in the old written texts that i have either so I am not sure what formula you would use for successful results. My advice is that you try using urine in its pure form and then in a diluted form with water and test in a piece of cloth before getting to your pants…maybe use the 10% WOF mordant as a guide to start with… sorry I can’t be more help. Would love to know how you get on! Cheers


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