List of natural mordants

what's safe, what's lightfast, what's available

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This week I am coming to you with a post on natural mordants. This article is being published in my Substack's “Plant Dyeing” section. This is where I share tips, recipes, tutorials, and resources for those interested in dyeing with plants. If that’s not something you wish to receive, you can opt-out of this section. Click “Unsubscribe” at the bottom of this email, unfollow this section, and only stay subscribed to the “Studio Practice” section of this newsletter.

I chose this subject to kick off the year because this is the question I get the most often: How can I make botanical colors last using natural methods?

First of all, we have to define natural. I often hear from people, who want to skip “chemical” substances and only work with natural ingredients to make plant colors last. Let me jump in right here: traditional mordants, like aluminum, iron, or copper, are both chemical substances AND they are natural. Just like botanical dyes have their own chemical structure, so do traditional mordants, and so does soy milk or tannins. Chemical and natural are not mutually exclusive.

Now let’s define mordants. A mordant is a substance that connects with fibers and improves the stability of natural color. Mordants make natural dyes less likely to fade or wash off. There are different mordants available, but they all have two things in common—they are always metal salts and they are able to form an insoluble connection with fibers and dyes. There are different metal salts available, like aluminum salts, iron salts, copper salts, etc. They all require different recipes and work best on different types of fibers.

There are other substances that allow for better uptake of plant color, like, for example, soy milk. But in contrast to mordants, soy milk does not form a permanent bond with the fibers and dyes. Rather than a chemical connection, it works as a sort of mechanical “protein film” that layers on top of the fiber and glues the dye and the fiber. This “glue” is soluble and might degrade over time. Therefore, soy milk is not a mordant, it is a binder.

Another alternative to metal mordants is tannins. They can be found in many leaves, seeds, roots, and stems. Just like soy milk, they are not able to form any insoluble bonds with fibers, but they might improve lightfastness. They will usually stay true or might even darken when exposed to sunlight but bleach if vigorously washed.

Mordants (insoluble fixatives)

Aluminum salts are the most commonly used mordants. They are easy to use and make natural dyes lightfast, washfast, and vibrant.

Potassium Aluminium Sulphate (PAS or alum) can be used on animal fibers. It requires a hot mordanting bath. When combined with tannins, it works on plant fibers, too.
Aluminium Acetate is a cold mordant that works best on cellulose fibers and silk. It has to be either “dung” after mordanting or applied onto a layer of tannins, like PAS.
Aluminium Formate is my mordant of choice, a cold bath that doesn’t require any additional treatment and works on all kinds of fibers. Unfortunately, as far as I know, it’s only available in Germany.
Aluminium Lactate is a mordant I recently found out about but still haven’t tried, so I won’t go into any more detail. If it’s available in your country, it might be worth researching. There might be also other aluminum compounds available to you.

Iron salts can be used either as a mordant or as a modifier, as iron “saddens” and darkens many dye plants.

Iron sulfate comes in form of iron crystals. It is best used on plant fibers. It should be used sparsely on animal fibers, as it might deteriorate them.
Iron acetate is a much gentler choice for dyeing wool and silk, but it can’t be purchased, only homemade. Iron acetate has a short shelf life and takes ca. 2 weeks to develop at home, using steel wool and white vinegar.

Copper, tin, and chrome are some of the other metals that can be used in natural dyeing. They might be potentially unsafe to humans or ecosystems. I never felt inclined to work with them so I will leave it only as a mention.

Binders (soluble fixatives)

Soy milk is a great alternative for dyers, who don’t sell their work and don’t care about the extreme longevity of the color. The connection it forms with fibers is not fully stable and might degrade when exposed to sunlight or washed. It is a good way of improving color uptake, though, and it’s safe to use around kids. It can be used in many creative ways and I like to play with it for my own wardrobe.

Tannins can be used by themselves or to improve the lightfastness of other dyes. Tannins come in many colors—beige, yellows, and pinks. Combined with iron, they make a range of dark colors like grey, moss green, and brown. They make a great base for over-dyeing with less stable colors.

Alternative processes

Indigo dyeing is a process that doesn't require mordants. Indigo works by oxidation, which makes this dye into insoluble pigment when exposed to air.

Acid dyeing involves mixing dyes, tannins, and acids in one bath, without mordants. It works only with selected plant dyes and is lightfast but not washfast in high temperatures. I am planning to cover an acid dyeing experiment in one of the future posts.

What’s NOT a mordant (nor a binder)

Salt, white vinegar, baking soda, citric acid—they DO NOT have any influence on the color stability or intensity. They neither fix nor bind natural dyes. They are sometimes listed as fixatives in articles from inexperienced dyers, mostly because of misconceptions and misunderstandings (e.g. mordant = metal salt but ≠ kitchen salt).

Whichever fixative you decide to use, you should run your own lightfastness tests in order to examine how durable your results are. Color stability depends not only on the fixative used but equally on the dye plant you choose to dye with.

Finally, to illustrate how different mordants influence color, I dyed a few swatches pre-treated with different natural substances. I used onion skins on cotton and on wool. Onions make a wonderful dye, they are easy to obtain and allow for much play when combined with mordants and binders.

Top to bottom I used:

• Iron sulfate as a mordant resulting in olive green

• No treatment, just onion skins on washed cotton, making a nice pastel shade

• Soy milk as a binder, enhancing the color to yellow/orange

• Aluminum formate as a mordant, deepening the color to vibrant orange

To the right I also laid two tests on wool:

• Iron sulfate mordant (olive green)

• Aluminum formate mordant (yellow/orange)

Let me know what topics you’d like me to cover in the future. You can either answer to this email, or leave a comment on Substack.

And if you want to dive even deeper into chemistry of mordants, I wrote an eBook about mordants with over 70 pages of useful tips, recipes, project ideas and photos from my studio. I’m proud to say that since publishing it in 2020 it received exclusively 5-star reviews (thank you!!). You can order it here.

Thank you for reading!
Until next time,
Ania