a spoon with alum above several natural dye materials (onion skins, madder, fustic, rose petals), mordants for natural dyes

What is a Mordant? (Mordants for Natural Dyes)

In this post I want to highlight the important topic of mordanting and help you chose which mordant to use for your natural dyeing project. Mordants are essential if you are dyeing either protein or cellulose fibers with natural dyes. Therefore, I will explain in detail what a mordant actually is and how to use it.

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When I first started experimenting with natural dyeing, I distinctly remember that the mordanting part of the process confused me the most. Therefore, I will try my best to make this post as clear and simple as possible. Since I mainly dye protein fibers like wool, this is what this post focuses on.

Table of Content:

What is a Mordant?

Types of Mordants

A. Metal Salt Mordants

  • Alum
  • Aluminium Acetate
  • Aluminium Triformate
  • Iron(II) Sulfate (FeSO4)
  • Copper Sulfate (CuSO4 x 5 H2O), Tin Chloride (SnCl2) and Potassium Dichromate (K2Cr2O7)

B. Plant-based Mordants

  • Tannins
  • Soybeans

Which Mordant should I use?

Mordanting Process

How to Mordant with Alum (Tutorial)

Does a Mordant Affect the Color Results?

Does Indigo need a Mordant?

If you want to learn more about mordants for natural dyes, you can check out this video on my Youtube channel:

a hand holding a spoon with a white substance and several dye materials in the background. a text saying mordants for natural dyes.

What is a Mordant?

A mordant or dye fixative is a substance which is used to set dyes on fibers. It forms a coordination complex with the dye which attaches to the fiber. As a result, mordants enhance the wash- and lightfastness of the dyed yarn or fabric.

Types of Mordants

A. Metal Salt Mordants


(KAl(SO4)2 x 12 H2O) or potassium aluminium sulfate  is an inorganic salt. It is commonly used in water purification, leather tanning, as a food additive and in the cosmetics industry.

Alum is classified as nonhazardous by the GHS. The Globally Harmonized System of Classification and Labelling of Chemicals (GHS) is an internationally agreed-upon standard managed by the United Nations. Furthermore it is considered as a generally recognized as safe (GRAS) substance by the FDA. FDA stands for the U.S. Food and Drug Administration.

Due to the common availability and non-toxicity, alum is my preferred mordant.  

a hand holding a spoon with alum

Alum is often used in combination with cream of tartar. Cream of tartar or potassium bitartrate is an assistant that is added to brighten the colors and prevent the wool from damage. It increases the alum uptake by the wool and brings up the pH of the solution. However, the change in pH also means that adding cream of tartar can change the final color result. Cream of tartar is added to the mordanting bath in concentrations between 4 and 10% of the yarn weight.
Potassium bitartrate is a byproduct of winemaking. It can also be used in baking or as a cleaning solution. Like alum, it is classified as a generally recognized as safe substance by the FDA.

Tip: In my personal experience, it is not mandatory to add cream of tartar. You can still get bright colorways without damaging the fibers, even if you don’t add potassium bitartrate.

Aluminium Acetate

Aluminium acetate is used for mordanting cellulose fibers. In general, the mordanting process is very similar to alum.

Aluminium Triformate

Aluminium triformate (also known as aluminium formate) is the aluminium salt of formic acid. It has numerous benefits compared to other mordants. In short, aluminium triformate can be used at room temperature and works for both protein and cellulose fibers.

If you want to know more about aluminium triformate and how to use it, you can have a look at this blogpost: Cold Water Mordant Aluminium Triformate

Iron(II) Sulfate (FeSO4)

Iron(II) sulfate or ferrous sulfate is a green salt. It can damage the wool and make it brittle when used in high concentrations and over longer periods of time. Therefore, I don’t use iron(II) sulfate as a mordant. It is, however, a great option if you are not completely happy with the final color of your yarn and want to modify it. In general, adding an iron modifier will make the colors darker and duller in tone. It can also completely transform a dye color.

Tip: I have great success with dissolving 1.5 g of ferrous sulfate per 100 g skein of yarn in cold water. I put the yarn in the iron bath and leave it for 30 minutes without heating. If I am not satisfied with the result, I will leave the yarn in the bath a bit longer, but never more than one hour. In my experience, this doesn’t damage the fibers but you can still get great color modifications.

a hand holding a spoon with iron (II) sulfate for natural dyeing

Iron(II) sulfate needs to be handled carefully. You have to wear safety glasses, gloves and protective clothing. It is an irritant to the skin and eye and is harmful if swallowed. You can have a look at the safety data sheet here. Furthermore, you have to check with your local regulation regarding the waste disposal.
Iron (II) sulfate is a reducing agent. It is used industrially mainly as a precursor to other iron compounds and various applications. On the other hand, it is also used to treat and prevent iron deficiency anemia.
As you can see, benefit and hazard are often present at the same time in chemical substances.

Copper Sulfate (CuSO4 x 5 H2O), Tin Chloride (SnCl2) and Potassium Dichromate (K2Cr2O7)

These three chemicals can function as mordants or modifiers as well. However, they are toxic and have to be disposed as hazardous waste. Therefore, I don’t have any personal experience with these compounds and recommend using nontoxic alternatives.

B. Plant-based Mordants


Tannins or tannoids are a class of astringent, bitter compounds naturally occurring in plants. Those polyphenolic biomolecules are present in many different plants. They can occur in various parts of the plants, like the stems, buds, seeds or leaves. Tannins play an important role in the process of tanning leather. They also have antibacterial properties[1].

In high concentrations, tannins can act as natural mordants which makes it possible to skip the mordanting part of the natural dyeing process. Some dye materials with high tannin concentrations are: tree barks, avocado pits, oak leaves and galls.

You can learn about tannins in this blog post: Tannins as Natural Mordants


When dyeing cellulose fibers, it is possible to use soy milk as a natural mordant. I will not go into more detail here since I mainly dye protein fibers and have little experience with it. But in case you want to know more about this method, I recommend the book “Botanical Colour at your Fingertips” by Rebecca Desnos. She covers the topic comprehensively.

Which Mordant should I use?

You have to keep in mind that the type of mordant you choose depends on two things:

  1. The fiber you want to dye (protein or cellulose)
  2. The dyestuff you are using.

Mordanting Process

Generally, there are three possibilities when the mordant can be applied during the dyeing process.

  1. The mordant is applied first

This means that the yarn is initially being mordanted and then dyed afterwards. This is what I normally do.

  1. The mordant is added to the dye bath

It is also possible to add the mordant directly into the dye bath itself. However, the results of this method are not always as good compared to the first option.

  1. The fiber is dyed and treated with a mordant afterwards

The dyed material is treated with a mordant.

How to Mordant with Alum (Tutorial)

List of Materials

Undyed skein of yarn100 g
You can simply use an undyed, natural skein of yarn. Maybe you even have some suitable yarn already in your stash. If you want to use a yarn that is in the form of a ball or cake, you have to create a skein first. This can be done by using a swift (this is the one I use and can recommend).
Alum14 g
Alum or potassium alum (KAl(SO4)2·12H2O) is a mordant which means that it is used to set dyes on fibers by forming a coordination complex with the dye. It increases the fastness of the dye.
PotYou can use an old pot or acquire one just for natural dyeing purposes. This is the size I use if I dye only one skein at a time, it has a holding capacity of 6 qt (5.7 L).
Bucket or Washing PanTo scour and wash the yarn.
Kitchen ScaleTo measure the alum. This is the one I use.
TieTo secure the yarn and avoid tangling. You have to use something that is stable in boiling water and doesn’t give off any color. I usually use some kind of package cord.
SpoonWooden or stainless steel
Dish ShoapWhatever you have on hand.
This is used to scour and wash the yarn.
Wool Laundry DetergentThis is the one I use and can recommend. It is gentle and doesn’t have any smell.

Precautions: Don’t use the same pots and utensils for natural dyeing that you use for food preparation. Always wear gloves. Creating the dye solution and the dyeing process itself should be done in a well ventilated area. I often use a cooking plate that I put on my patio.

A. Standard Method

Fill the pot with some lukewarm tap water. You need 14 g of alum for a 100 g skein of yarn. I usually dissolve the alum in a small jar with some warm water by stirring it in with a spoon first. Then I pour it into the pot and add the wet yarn. Make sure that the alum solution covers the yarn completely.

a scale with a jar containing alum salt on it along a bucket of alum with a spoon in it

Heat slowly and let it simmer for an hour. Let the pot cool down completely before taking out the yarn. Gently wring out the excess liquid and place the yarn in a bucket or washing pan with clear water. Then you can either hang the skein to dry for a later dyeing project or immediately use it for dyeing.

undyed cream skein of yarn in a pot with water

It is possible to reuse the mordant bath a second time. You simply have to add an additional 2 g of alum for a 100 g skein of yarn. After using the mordant bath twice, discard it by pouring it down the drain[2].

B. Cool Mordanting Method

Mordanting with alum without heating is possible as well. After preparing the mordant bath, add the wet yarn and let it sit for at least 12 hours[3].

Does a Mordant affect the Color Results?

There are several cases where the use of a mordant will have an effect on the final yarn color. If you use iron, it will generally dull the color and sometimes completely transform it. Let me show you two examples:

five skeins of yarn and a pink rose on a wooden background, three smaller skeins are yellow, one larger one is grey and one larger one beige
Adding the dyed skeins of yarn to an iron bath completely changed the color from yellow and beige to grey

Another interesting example is avocado dye. Avocado pits contain high concentrations of tannins. Therefore it is possible to dye fibers with avocados without using an additional mordant. Interestingly, the presence or absence of a mordant has a significant effect on the final color as you can see in the picture. The peach and rust skeins were mordanted with alum before dyeing. The dusty pink skein was dyed without the use of a mordant.

three skeins of yarn in pink/peach colours on a wooden background
from left to right: no mordant, mordanted with alum (2nd exhaust), mordanted with alum (1st exhaust)

Does Indigo need a Mordant?

No. Indigo and woad don’t need the addition of a mordant. Both of these dye materials are vat dyes and require a different dyeing method in general, because they are not soluble in water.

You can learn more about dyeing yarn with indigo in this blog post: How to Dye Yarn with Indigo

Want to learn how to dye yarn using natural dyes?

I have created a beginner’s guide to natural dyeing that contains everything you need to know to get started. And the best thing: it is available for free!

four hand dyed skeins of yarn in shades of purple and blue on a wooden surface and a text saying beginner's guide: www.rosemaryandpinesfiberarts.de. natural dyeing. everything you need to know to get started dyeing yarn with natural dyes

Pin It For Later: What is a Mordant?

four colourful skeins of yarn and a text "mordants for natural dyes. www.rosemaryandpinesfiberarts.de"


[1] Prabhu, K. H.; Teli, M. D. (1 December 2014). “Eco-dyeing using Tamarindus indica L. seed coat tannin as a natural mordant for textiles with antibacterial activity”. Journal of Saudi Chemical Society. 18 (6): 864–872.

[2] Vejar, Kristine the MODERN NATURAL DYER: A Comprehensive Guide to Dyeing Silk, Wool, Linen, and Cotton at Home. Abrams 2015, pp. 58-59.

[3] Dean, Jenny, et al. Wild Colour: How to Make & Use Natural Dyes. Mitchell Beazley, 2018, p. 10.


I am a yarn dyeing artist, writer and educator.
I am also an avid knitter and love to create something with my hands every day.
Read more about me here:

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