Choosing and using mordants is a really broad topic, so I will be talking about it in chunks. In this segment, I’m talking specifically about the mordanting I did in preparation for dyeing silk and cotton with madder dye. For the sake of simplicity, let me start at the beginning.
What is a mordant and why do we need it?
The word mordant is related to the French word morder, “to bite.” A mordant is a substance that helps the pigment in the dye “bite” into the fiber and hold on. Dyeing fiber is a process of creating a molecular bond between fiber and pigment, and many natural dyes can’t make that bond without assistance. Dyes that require use of a mordant to make this bond are called adjective dyes. Both species of madder I am using during my residency, Rubia cordifolia and Rubia tinctoria (also called tinctorum), are adjective dyes, and they both give significantly different results when they react with different mordants. For some dyes, the choice of mordant doesn’t make such a visible difference in the final color, but madder is beautifully responsive.
Choosing the right mordant for your fiber and desired result and applying that mordant correctly makes all the difference, not only in the appearance of your final product, but also in giving that dyed product the best possible longevity. The mordant helps prevent the dye from washing out or fading from washing, and in certain cases, it also helps the color resist fading from light. Correct mordanting also helps the fiber to take up the pigment more evenly and thoroughly.
Mordant can be applied before dyeing, which is most common, it can be included with the dye, or it can be done after the dye in certain processes, though it’s less common.
Types of mordant
The most commonly used mordants are metal and mineral salts and tannins. Learning to choose the best mordant or combination of mordants for your application takes study and experimentation, but two very basic choices are applicable in most circumstances: alum and tannin. Potassium Aluminum Sulfate, which most dyers refer to as PAS or simply alum, is readily available, inexpensive, it can be used on both cellulose and protein fibers, and it is colorless. Chemically, it is the same stuff as the alum used in baking, but the alum we use in dyeing is much more concentrated. You will also find PAS in the garden center at your favorite box store, and I’m going to take a moment to caution against using that. The garden center PAS is a good deal cheaper than what you buy from a dye supplier, which makes it tempting, but it is less refined and often has iron in it. Iron can cause your dye color to become more gray and dull – we call this saddening the color – and it can react with tannins to make unintended black spots. It really is worth the extra cost to buy the most pure PAS you can get.
Another aluminum salt mordant that many dyers insist is a must-have is aluminum acetate, also known as alum acetate or AA. Alum acetate is used on cellulose fibers (like cotton, linen, and bamboo) and bast fibers (like rayon or tencel), and it gives better color uptake and lightfastness than PAS. Alum Acetate is significantly more expensive than PAS and it can be much harder to get, depending on where you are. I will do more scientific comparisons between the two in the future, but after my first several experiments with AA mordant, I am not convinced that the improvement is worth the additional cost.
Tannin mordants are used because they improve resistance to fading, they can assist the bond between other mordants and fiber, and in the case of madder dyeing, they also deepen the red tones. They can be used on all types of fiber, but they’re most commonly used on cellulose and bast fibers. One reason for that is that protein fibers tend to soak up more color from the tannin mordants, which affects the final dye color, sometimes in undesirable ways. Another reason is that protein fibers don’t need the mordant bonding assistance from tannin, so it’s a largely unnecessary step. Tannins are often used in combination with alum mordant on cellulose or bast fiber. All tannins contribute some color to the fiber they are used on. There are three major types of tannins: Gallic, Ellagic, and Condensed, each with different benefits and drawbacks.
Gallic tannins are easy to find and inexpensive, and the most commonly used are oak gall or Aleppo oak gall, and sumac. These tannins contribute a light brown or reddish-brown tint. When they are combined with iron, they create a dark bluish-gray or black color. These highly concentrated tannins are top choices because they’re typically inexpensive and easy to get, and they have a long shelf-life, as long as they’re kept dry and out of direct sunlight. Gallic tannins require extra attention when you’re applying them because they can deposit unevenly on the fiber, leaving a mottled appearance that may not even out with dyeing.
Ellagic tannins come from sources like myrobalan and pomegranate rinds. They contribute a light yellow tint to the fiber, which can vary from pale green to pale brown with yellow tones. Typically, the color from ellagic tannins affects the final color less than gallic tannins do. Elagic tannins are also less likely than gallic tannins to create uneven coloration on the fiber. One drawback to using ellagic tannin is that you need to use more plant matter to get results, so there is more rinsing and cleaning to do afterward. That said, I have found that for most applications, I like using myrobalan for a tannin mordant much more than I like using oak or Aleppo gall.
The third major class of tannins are called condensed tannins, and they include sources like cutch, quebracho, black tea, and chestnut bark. Condensed tannins, weirdly, are the weakest sources of tannin among the major types. Condensed tannins oxidize quickly, which gives them more red and brown tones, especially in a higher pH (more alkaline) solution, but the resulting colors tend to be less lightfast than colors dyed with gallic or ellagic tannin mordants.
The amount of mordant material used is based on the weight of the fiber to be mordanted. Check a trusted resource for the correct percentage range to use for for your fiber type and amount. Experience tells me that alum doesn’t tend to clump, so I simply sprinkle it over the surface of a pot of warm (around 100F) water, stir it in, and continue to stir occasionally as the water heats. Very easy!
The tannin mordants I use are all plant matter and, with the exception of pomegranate, they’re powdered. These powders will clump tenaciously in the pot if they’re not mixed in correctly, and that means potential spots on your fabric. To prevent this, I paste the powders so they will mix in more thoroughly when I add them to the pot of warm water. I use a small stainless steel bowl with a flat bottom and a glass tool called a muller. The mullers I use were handmade for me by a very talented friend who works in glass. They have a flat bottom to mash the powder clumps and mix it evenly into a paste using a little warm water from the pot I am going to mix the paste into. Once the paste is well mixed and there are no clumps or dry powder left, I add warm water a second time to make the paste into a looser mixture that will mix more easily into the pot of water.
These are examples of the cottons I mordanted to dye in madder. Each one was mordanted in alum then rinsed thoroughly, then in various types of tannin, and then in alum again. Many dyers refer to this process as ATA, for short.
After a thorough rinsing, all of my alum-mordanted cellulose fibers get a chalk bath. I prepare a solution of chalk (calcium carbonate) powder and water by heating the water to about 100F and whisking the chalk powder in until the lumps were gone. I place the still-damp fabric in the water and worked it in (squish the fabric under water to make sure it soaks it up) and simmer it for about half an hour (a little longer for very heavy fabrics). The chalk bath helps weakly bonded alum make a solid bond, and it washes out unbonded alum. This matters because if unbonded alum gets into the dye pot, it will bond with the pigment and prevent it from bonding to the fiber. I have done side-by-side comparisons and in my opinion, the chalk bath consistently works to make a brighter final color, and it’s well worth the additional effort and time. Chalk powder is very cheap and easy to get. My best results have come from doing the chalk bath and rinse-out immediately before dyeing. If you need to stop during the process, my suggestion is to rinse out after the last alum bath and allow the fabric to dry, then soak it in water and do the chalk bath when you’re ready to start dyeing.
Fun with chemistry
Sometimes, chemistry is a bunch of critical-but-boring knowledge that you need to have in order to make dyeing work. Once in a while, chemistry is really fun and produces spectacular results. I did a batch of fun chemistry in the middle of all this scouring and mordanting, to brighten my day (literally) and make it easier to wait to be able to start dyeing. I used a metal salt called titanium oxalate (TO for short) to mordant some silk and some cotton. TO contributes a fairly bright yellow when used in an appropriate concentration, and as a mordant, it improves lightfastness and wash-fastness. It boosts the orange and red tones in the dye you use over it, and it produces spectacular oranges (particularly on silk) when it touches tannin. That’s why TO is popular among botanical printers; it makes gorgeous, bright prints of tannin-rich leaves. This reaction is very strong, and surprisingly sensitive. I dried my hand after sticking them in the tannin pot, but did not rinse them first. When I picked up the TO-mordanted silk, I left bright orange fingerprints behind. Luckily, they didn’t show up after I submerged and soaked the fabric. After dyeing that piece in madder, it turned out a bright orange California poppy color!