Mordant Exploration

(originally posted 11/13/2020)

If you've followed many modern natural dyers you have probably noticed that mordants, substances used to fix dyes, are somewhat controversial. Some may wish to avoid working with metallic salts for health and environmental concerns and others, myself included, may be interested in keeping their sources as local as possible. Certainly, working with alum and iron mordants is the closest to “laboratory chemistry” that I get in my work with natural dyes and using these factory-made products raises a lot of questions for me. Are they “safe” for the dyer? Where do they come from? How are they made? Are there any alternatives? I’ve spent some time exploring these questions and here are my thoughts.

** I referenced the book below, "Natural Dyes" by Dominique Cardon, for much of the historical information stated here. I reference this book constantly as it contains an incredible depth of information on both plant chemistry and the historical and cultural context of natural dyes. I could not recommend this book enough, well worth the investment! **

(this book is amazing)

So, what is a mordant?

While some natural dyes have an affinity for fibers and can be used without a mordant, the most prominent example being indigo, the great majority of natural colors require the use of a mordant to fix the dye. The most commonly used mordants are mineral salts that combine with dye molecules to create a chemical complex within the fiber. Other substances used to enhance dyeing results (historically these included urine, blood, and animal fats as well as plant extractions) can be considered mordants as well. The word "mordant" comes from the latin mordere, meaning “to bite”, and you can see in the examples below that mordants allow dyes to really hold onto fibers, especially cellulose fibers like cotton and linen. The contrast between mordanted and unmordanted cotton is key in printing and painting with natural dyes.

In this pattern the birds are printed with an iron mordant paste, while the red-capped wings are printed with an alum mordant paste. Once the mordants are set, the entire piece is put into a madder dye bath, and the background/unmordanted section remains nearly devoid of color.

Mordants have been used in dyeing for thousands of years. Aluminum and iron, the most abundant metals on the earth’s surface, form the two most widely used mordants and the two I use in my work today. Other metallic salts used as mordants include tin, titanium, copper, and chromium (which is quite toxic). “Alum” refers to several forms of an aluminum salt, often potassium alum sulphate, and has had many industrial and medical uses both currently and throughout human history. I focus on alum here because it is central to my dyeing and printing practice and because, unlike iron, it is not a material I can salvage from rusty nails in my backyard.

This sample demonstrates the range of colors available using different mordant pastes in a single dye vat. Alum and iron mordant pastes were used.

Potash alum occurs naturally in desert regions and in some volcanic areas, including the western Egyptian desert where evidence of ancient alum harvests can still be seen today. Much of the alum used in antiquity, however, was manufactured from alunite or alumstone in a hugely laborious process that yielded relatively small amounts of usable alum. Still, alum was such an important commodity from the Middle Ages onward, that alumstone mining altered landscapes across Europe, the Middle East, China and Japan, and to a lesser or lesser-known extent, the Americas.

Currently I purchase alum (usually potassium alum sulphate) and iron sulphate that is industrially produced. I use these both in their sulphate forms with animal fibers, and I make an acetate by adding acetic acid (vinegar) for cellulose fibers. These come in powder form in plastic bags from who-knows-where and I’ve always had some hesitation around these purchases. Alum is considered relatively non-toxic, iron perhaps even moreso as it is an essential mineral for human bodies, though I do still always wear gloves when I work with the mordants. I also try to be conscious about wastewater in my work. While city wastewater treatment will isolate out contaminates (actually using alum sulphate as a flocculant!), I’m usually thinking more about containing my water cycle on my own property. Ideally, I’d be collecting all my water from rain or streams and giving it back to my plants (though this is certainly not the case yet!), and I’d rather keep industrially made mordants out of the picture. So, are there any alternatives to using metallic salt mordants?

Mordant alternatives

Along with mineral earth sources of alum, plants have also been used as a source of alum, especially in parts of the world where alum wasn’t naturally available in antiquity. A very small number of plants take up aluminum from the soil and store it, and some of these plants have been used by dyers in various parts of the world to aid in dyeing. Lycopods from northern Europe, shrubs from the genera Symplocos and Memecylon in India, southeast Asia, and Japan, and shrubs from the Rubiaceae family in Africa have been used by traditional dyers. I have also read that lycopods, also known as clubmosses, were used by native American peoples.

A sampling of the diversity of clubmoss species in my local forest.

Though several species of lycopods are abundantly common in my local woods, some species have been overexploited in Europe and, partially due to their long germination periods, are near extinction. Besides their use in dyeing, clubmoss spores were harvested for medical purposes and for use as a flammable flash powder for early photography and theatrical effects. Since, as I mentioned, my woods are blanketed in a beautiful diversity of lycopods, I have sustainably harvested just a handful or so for experimenting with natural dyes. There are a couple of species of lycopods in New England that are quite rare so if you want to experiment with clubmoss please know which species you’re harvesting and only take a few from where they’re most abundant. I selected only those growing in conifer-dominant areas as lower pH/higher acidity increases the biological availability of metals in the soil and therefore in the plant.

For these experiments I used 40 g of raw lycopods (all the species combined), minced them up as finely as I could, covered them with water in a stainless steel pot, and left them to simmer lightly for a couple of hours. Over the course of three or four days, I simmered the material whenever I was around the house, and let it soak and cool the rest of the time. I finally strained the solution, which was a light yellowy brown and smelled delicious, and added 10 g of fiber. This included a mixture of scoured but untreated cotton, cotton pre-soaked in a sumac leaf tannin bath, and wool. Sadly I had no silk handy to experiment with! The fibers were kept just below a simmer for at least 2 hours, and then left to soak overnight.

The fiber samples were then dyed with madder and weld, with some completely unmordanted pieces and some alum mordanted pieces for comparison. As you can see on the madder-dyed samples below, the lycopod-mordanted pieces are muted as compared to the mineral salt-mordanted pieces, but are really lovely colors. On the wool samples, the lycopod mordant gave a much more magenta hue, as opposed to the crimsony red of the alum sulphate.

Madder dyed cotton samples, from left to right: untreated, lycopodium treated, lycopodium and sumac tannin treated (2 pieces), alum acetate mordant.

Madder dyed wool, lycopodium mordant left, alum sulphate mordant right.

These preliminary results got me thinking about the possibilities for other plant mordant qualities. Traditional natural dyeing recipes often involve plants besides those that give color, some of which may be high in tannins and others that may assist the dye in other ways, perhaps because they are metal accumulators. I’ve been enjoying the very slow, intentional process of color extraction and layering several plants together, which seems to me a way of imbueing the fiber with the complexity of a landscape. I am certain there are other plants in my landscape that contain keys to locking in unique and lasting colors and I plan to explore other potential plant mordants. From what I've learned about metal availability in soils with low pH, I think acidic soils are the first place I'll be looking.

Symplocos

I also recently acquired a Symplocos paniculata plant, which a local natural dyer friend picked up for me. I’ve read that the ashes of this plant are traditionally used as a mordant in Japan, though I imagine that once this shrub gets established I will probably just take the leaves to make a mordant bath. I am waiting for the leaves to fall before winter to do some small experiments. Apparently, many other Symplocos species grow abundantly well in warmer areas in the U.S., where they have been introduced as ornamentals and may even be invasive in some areas.

My newly planted and rather scrawny, but hopefully hardy Symplocos paniculata, mulched with pine needles.

Mature Symplocos paniculata, also called sapphire berry.

(Kristine Paulus (kpaulus) at flickr, CC BY 2.0 <https://creativecommons.org/licenses/by/2.0>, via Wikimedia Commons)

Soy milk

I use soy milk all the time as an after treatment in my work with indigo. First of all, I love the process of blending up frothy, earthy soy milk from soaked soy beans and straining it through my fabric. Though I haven’t done lightfastness tests myself, from what I’ve seen in John Marshall’s book “Singing the Blues”, soy milk can make a great difference in the longevity of indigo blue hues. I’ve used it when painting with pigments on fabric with really fantastic results. Though I think of soy milk as more of a binder than a mordant, I know that other dyers use it in the same way one would a mordant (as a pretreatment before immersion dyeing). The soy milk does not fix the color in the same brilliant way that metallic salt mordants do, but it certainly seems to increase color uptake and longevity. Soy milk can also be a good option, especially if you're going for earth tones and don't intend to wash the fabric.

While I’m not sure how these plant mordants would translate to my printing practice, I’m very excited to realize that I may just be surrounded by potential mordant sources. Certainly making use of the materials at hand in my surrounding landscape is a key part of my practice, and an aspect that I will always be exploring and expanding.