On Mordanting


Indigo is apparently the gateway drug of the natural dye world. It led me to try dyeing with cochineal, which should have led me to a better understanding of mordanting, except it didn’t. There are a lot of recipes, but not a fat lot of information out there on the science of natural dyeing. There is a lot of information on synthetic dyes. As it turns out, there is a good reason for this.

The history of synthetic dyes is also the history of organic chemistry, so the process of learning how to make synthetic dyes provided the chemical knowledge. The advent of synthetic dyes squeezed out the natural dyers’ guilds, so the new chemical knowledge wasn’t applied backward. There is a lot of good information out there on the invention of mauveine, the first analine dye. If you love history, how synthetic dyes changed the socio-economic world is fascinating, and it in part explains why there is precious little textile manufacturing still done in the US. Some of the more interesting monographs I’ve come across about the chemistry of natural dyeing are from India, Pakistan and Egypt, where there still are textile industries, and where scientists are taking another look at natural dyestuffs in order to have a more sustainable and less toxic impact on their environment. I’ve ended up learning about synthetic dyes, color chemistry, the quantum physics of color, synthetic fibers, and finally, mordanting and natural dyes.

That said, Maiwa and Turkey Red Journal are both excellent resources for natural dyeing information, including the chemistry, and they are both on the forefront of bringing the chemical knowledge back to the natural dye world.

So what is mordanting? If you are using metallic mordants, basically you are making your own acid dyes. Instead of using an acidic bath to promote ionic bonding as with synthetic acid dyes, the metal ions of the metal mordants have a similar polar effect. The mordants form covalent bonds with the color bearing compounds in the dyestuffs, which are the very strong bonds between atoms (sharing electrons in the outer shell). Since they are acid (polar) dyes, they therefore bond better with the positively charged amino acid chains (wool, silk) and poorly with cellulose (cotton, linen).

Tannic acid is a non-metal mordant, but “tannic acid” itself isn’t really a discreet chemical, but rather a broad heading under which several acids fall (which also happen to be tannins): Gallic acid, ellagic acid, and catechic acid. Most of the so-called “substantive” natural dyestuffs that require no mordanting have some form of tannic acid in them, for instance, sumac, pomegranate, fustic and cutch. Tannic acids bond well with protein fibers (think tanning hides), and also with cellulose plant materials. It also bonds well with the metal mordants, so plant fibers normally get pre-mordanted with tannic acid, and then again with the metal mordant.

I really wish I could tell you what chemical bonds are formed between tannic acids and these different fibers, but I’ve had no luck in finding scientific documentation thus far. I have to assume it’s not covalent bonding with cellulose just because fiber reactive synthetic dyes are so much more wash fast. I’m starting a natural dyeing class in October and I’m hoping to get to the bottom of this.

Nowadays aluminum acetate is available to mordant plant materials and the tannic acid step is debatable—this post and this post from Turkey Red Journal do comparisons of dyeing cotton cloth with different configurations of tannic acid/alum/aluminum acetate. Some of their considerations are cost and availability for dyers in poorer countries. Rachel does most of the cotton dyeing between the two of us, so I’m leaving it to her to take good notes on her findings.



This monograph is great in detailing the chemical structure of wool. There is a lot going on in a strand of wool, aside from the positively charged dye sites. There are other chemical bonds that give rise to it’s strength and elasticity, and these are both things that can be affected by Ph, heat, and specific properties of different metal mordants like iron or tin.

There is a time vs. temperature factor in mordanting. A lot of recipes call for simmering your wool in your mordant for an hour, but that can easily lead to felting. Heating up your mordant and letting your wool steep overnight can often produce a more thorough saturation of the fiber and therefore more even dye uptake. Mordanting can take place before, during, or after the dyeing, but if it’s done prior to adding the fiber to the dye pot, there is more control over the mordant-to-fiber ratio, and the mordant bath can continue to be reused. This becomes more important when using the more toxic of the metal mordants, tin, copper and chrome.

Older mordanting recipes called for an excess of the metal mordants to ensure good dye uptake, in part because the strength of the mordant material was not guaranteed. Now we can source mordants with guaranteed strength and purity, so we can be a lot more precise and use recipes that leave little to no extra mordant in the bath. I did some trials with cochineal earlier this summer (that’s the next blog post). Using a recipe for a weighed amount of fiber, I tested my mordant bath to see if it was actually discharged (my copper did not seem to be), by adding more fiber to the “discharged” mordant bath and then soaking it in the dye bath and seeing if the dye strikes or not. When I was done I bottled up and saved my remaining mordant bath rather than tossing it out anywhere.

On hold for the next round of mordanting

On hold for the next round of mordanting

Once the mordant has bonded to the fiber it’s not going anywhere, so you can use different mordanted materials in the same dye pot, which is fun and interesting because you can see the effect the different mordants have.

Some metal mordants are toxic. Chromate poisoning is particularly unpleasant. Oxalic acid, often used to shift colors as an after-mordant is toxic. Synthetic “true black” acid dye is also toxic, as it contains chromium as it’s coloring component. None of theses things belong in the groundwater, or your septic tank, or near kids, pets or livestock. Entrapment is the state wherein metal particles are trapped in the steam from a water bath, and are then able to be inhaled, so don’t mordant in your kitchen. And for that matter…

Oak galls

Oak galls

A cautionary tale: we have a large tanoak tree growing next to the abandoned well out by our barn, and as a good source of natural tannins, I checked the interwebs for what the tannin concentration should be compared to oak galls, etc, for a possible recipe. What I found was that it wasn’t a tanoak. So I used a tree identification website rather than the book with illustrations I’d used initially, and the final question on the flowchart was “do the leaves smell like almonds when crushed?” Ironically as it turns out, this reminded me of the opening lines of Love in the Time of Cholera. My tree does smells like almonds when the leaves are crushed. It’s a cherry laurel, and when you boil the leaves you get hydrogen cyanide, which Nero used to poison his enemies’ wells. So. Back to collecting gall nuts.

The Science of Dyeing


What is color? When I studied philosophy as an undergrad, it was always treated as a “secondary quality”, that is, something that’s not intrinsic to the nature of the thing itself. And while it’s true that how we see color is a subjective function of our eyes and processing in our brains, the colors of things is entirely dependent on the physical makeup of those things. When we see color, we are seeing into the atomic and subatomic nature of things. In other words, a tree is green in a forest even if no one’s around.


To understand how color works, you need a little quantum physics. This monograph on color chemistry is concise, well-written, and with a little patience, accessible even for people like me who have only high school level chemistry and physics. If you are at all interested in how dyeing works, it explains everything.


I’m also slogging through this one. It’s highly technical and I can only digest a few pages at a time, but it details all the general information in the first book. If you want more after reading The Chemical History of Color, then this is for you.


To very generally sum up, the visible spectrum that our eyes can detect takes place in a really small range of wavelengths, from red to violet. Everything of shorter wavelength then the red range is the infrared, and everything longer than the violet is the ultraviolet. How these wavelength are generated or influenced happens at the quantum level, with the interactions of the electrons within an atom or a molecule. The electrons need to be understood as waves, not particles as I learned in high school chemistry. There are four or five different models that explain wavelength production, depending on the arrangement of electrons in their shells around the nucleus, and how they combine, or don’t combine with other atoms. What’s neat about all of this is that our eyes are seeing what’s going on at the quantum level! (That’s my take on it. I can’t think of any good reason why humans spend so much time and effort changing the color of things, if not to influence the building blocks of the world itself.)

Natural dyeing shows us that there are some plants and insects that impart good, lasting color, and some that are fugitive. The beginning attempts at synthesizing these color compounds were all trial and error, but now computer modeling can predict what wavelengths a particular molecular configuration should yield, and how to bind it to a particular fiber. It should be noted that two things dyers care about, light-fastness and wash-fastness are two separate issues. Light-fastness depends on the ultra-violet spectrums’s influence, whereas wash-fastness depends on the type of bond with the fiber (for the most part). Ultraviolet wavelengths can greatly influence the visible spectrum. We see this when colors fade in the sunlight. This often comes into play in natural dyeing (with black beans and berries for example)…one of the advantages of synthetic dyes is that they’ve been designed to be less susceptible to this effect. Another advantage of synthetic dyes is their leveling ability, that is, to dye evenly. They’ve been designed to bond weakly with the fiber so that they can actually un-bond and re-bond, rather than strike all at once in a concentrated area. Some of the molecules used to produce color are quite large, especially in the blue range. This is why even when using an acid dye, there is still blue left in the dye bath even though it is fully exhausted. The color producing part of the molecule is so large that it will actually break off from the part that bonds to the fiber during the leveling process. One of the mysteries of indigo is how it’s able to produce a blue color out of a relatively small molecule (there are several theories).

Synthetic dyes are often described as brighter than their natural counterparts. This is because the synthetic dye molecule is emitting a vary narrow, specific wavelength, where a natural dyestuff, as a complex plant material, is emitting a broader range of wavelengths within that color band. Different mordants also affect the color in natural dyeing. The metals used in mordanting not only have the necessary number of electrons in their outer shells to form covalent bonds with the dyestuffs, but of themselves have different wavelength properties…precisely because of how the electrons are composed around the nucleus of the atom. (This website/app of the periodic table is great. It shows everything you might want to know about each element, down to the electron spins in each orbit.)

Color aside, to understand how dyeing works, you need chemistry: the chemistry of the fiber being dyed, and the chemistry of the dye. Here are two excellent blogs that explain the chemistry of synthetic dyeing in simple terms:

Gnomespun Yarns

Paula Burch’s All About Hand Dyeing

Again to sum up, there are different types of bonds that can be formed, and they depend entirely on what you are dyeing: the amino acid chains of proteins, or hydroxide chains of cellulose plant material, maybe a mixture of both in the case of synthetic fibers, (or none of these in the case of polyesters). Animal fibers have positively charged receptor sites, so ionic bonding occurs with acid dyes (and also some hydrogen bonding, which is like ionic bonding but smaller). Plant material’s OH hydroxide chains don’t have the positve charge sites that animal fibers do, so fiber reactive dyes are designed to form covalent bonds, which are very strong, in a basic, rather than acidic bath. Disperse dyes dye plastics at high temperatures and pressures, although there are disperse dyes available for the home dyer that work in the dryer. Direct dyes work through a force called substantivity, and they need to be rather large molecules in order for this force to work. Since they are so large they are not particularly wash fast, and the colors are often duller. They are generally used on plant fibers, and are a component of all-purpose dyes like Rit.

This post by Gnomespun Yarns does a good job explaining the difference between animal fibers and plant fibers, and how it affects dyeing. This one by Paula Burch does a good job explaining the different types of chemical bonds that are made with the various types of synthetic dyes. They are both well written, with nice diagrams, and really explain why it’s important to know the chemistry of what you’re trying to accomplish.

All of this is by way of the next blog post, which is about mordanting. The chemistry of natural dyeing is only very recently becoming well documented, and I’ve found that understanding the technology that succeeds it is the most straightforward way of getting to it’s precursor.