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.
This is part 3 of my natural dyeing series. If you want to check out the previous posts, you can have a look here:
Part 2: Natural Dyeing Books
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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 focusses on.
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.
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.
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.
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.
Aluminium acetate is used for mordanting cellulose fibers. In general, the mordanting process is very similar to alum. Since I don’t have any personal experience with it yet, I will not go into more detail here. Aluminium acetate is classified as a hazardous substance and has to be handled with care.
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.
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.
Since tannins are such a broad and interesting topic, I will cover them in more detail in an upcoming post.
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:
- The fiber you want to dye (protein or cellulose)
- The dyestuff you are using.
Don’t use the same pots and utensils for natural dyeing that you use for food preparation. Always wear gloves, safety glasses and protective clothing. Mordanting the fibers 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.
Generally, there are three possibilities when the mordant can be applied during the dyeing process.
- The mordant is applied first
This means that the yarn is initially being mordanted and then dyed afterwards. This is what I normally do.
- 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.
- The fiber is dyed and treated with a mordant afterwards
The dyed material is treated with a mordant.
How to Mordant with Alum (Tutorial)
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.
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.
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.
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.
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:
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.
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. I will cover the exact dyeing process in an upcoming post.
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If you have already experimented with natural dyeing in the past, what kind of mordant do you normally use? Come share in the comments!
 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.
 Vejar, Kristine the MODERN NATURAL DYER: A Comprehensive Guide to Dyeing Silk, Wool, Linen, and Cotton at Home. Abrams 2015, pp. 58-59.
 Dean, Jenny, et al. Wild Colour: How to Make & Use Natural Dyes. Mitchell Beazley, 2018, p. 10.