The majority of natural dyes, whether chemical dyes or natural dyes, adhere to the fiber through a chemical bond. With straight dye and fiber, this bond is easy to degrade and break. However, some compounds can cause the dye to adhere to the fiber. These compounds are called mordants, and are usually metal salts.
Natural dye mordants include copper, alum, and iron. Chromium is sometimes used, but is so highly toxic that there is more harm than benefit to using it. Alum is the safest of the natural dye mordants, with copper and iron varying in toxicity depending on the form they are in. 
Colour changers:
All the metal mordants for natural dyes influence the color of the finished product. If you have naturally occurring mineral salts in your tap water, these can also have influence on the colors you get when dyeing fiber, no matter what dyes you are using.
Natural dyes are famous for being unpredictable, unrepeatable, and variable. That is where their beauty and artistry comes in. You can get close to being predictable and repeatable when using a natural dye journal.
Alum is both the safest of the natural dye mordants, and the one that preserves the clearest tone. As alum is a clear mineral, it does not impart its own colour to the dye vat. This leaves you with the clearest possible natural dye colour on your dyed fiber or fabric.
Copper is both safe and unsafe depending on the preparation. Dyeing in a copper pot is a safe way to utilize copper as a mild mordant and a strong color shifter. Copper will slightly sadden your colors, adding a green tint to your yellows, and other colours.
Iron is the same as copper with its safety depending on how the mordant is prepared. Using an iron sulfite may make a good mordant, but dying in an iron pan is easier and safer, and will sadden the colors in the same way. Iron saddens colors more than copper, and can turn brown to black depending on the tannin content. Iron can also degrade and degenerate fibres, so using an iron pot as opposed to a pure iron mordant can make your dyed fibers last longer.
Tin is last on this list as it is the most toxic of the listed mordants. Tin is a brightener, and will make your reds and oranges brilliant. Holding vinegar in a tin can for a few weeks can give you enough tin for a mild after-bath mordant, or a quick in-vat side-mordant. You should be extra careful not to ingest tin, and keep it off of any surfaces where food is prepared.
Tannin is used as a sub mordant on cottons, along with alum or another metal salt. Tannin helps baste and cellulous fibers to catch and bond with the dye. You can dye cotton without the tannin mordant, but it will help the dye retain. A tannin mordant will impact the colour of the final dye on the plant fibers as well.
Mordanting Technique Tip:
The safest mordanting method is to do a normal alum mordant. Then dye your fabric, and use the other metal mordants in a mild after-bath solution so that they can change the color. This helps protect your fibers, and also your health when working with mordants, so that you can make sustainable clothing.
For some fiber reactive dyes, vinegar is used as a fixative. Vinegar is not a mordant, but it changes the pH to enable the fiber and dye molecules to bond. Vinegar can be helpful with natural dyes, since some dyes do not reveal their full color unless their pH changes to acid. Specific instances include some yellow natural dyes, and some woad dye baths that I have done.
Always follow safe disposal methods with your metal salt mordants. Also, some metals in your natural tap water can change the brightness and tone of your natural dyes. So experiment, record, and have fun with your mordant adventures.
Back To You:
What are your favorite dye fixatives? What mordants have you used?
Do you have a recipe for using a copper powder as a mordant? I received the mineral form in a kit and am having trouble finding a recipe that doesn’t just list using pennies or tubing. Any WOF % or measurements?
From a quick bit of research, the one piece of information I found recommended using copper as a Weight of Fiber percentage, not exceeding 1.5%. The average was 1%, due to copper’s toxicity. For colorfastness, you can use a 1% copper, and a 4% alum per weight of fiber as a pre-mordant. Then you’ll get the color changing action of copper, and the additional mordant benefit of the non-toxic alum.
I hope that helps!
I´m wondering, since some time I read about soy milk as a mordant. I was just thinking about coloring some wool yarn with vegetable dye. Do you have any experience with that?
Hi Marian,
All natural dyes are technically vegetable dyes since they are largely sourced from plants, excepting insect-based reds of course. Are you meaning you want to dye with actual vegetables, though? Most of those colors are not colorfast…
I wouldn’t recommend soy milk as a mordant, as it is not likely to form the correct chemical bonds with the fabric and dye, as the metal based mordants do. Even the tannin “mordants” are used in connection with a metal salt mordant, and fiber-reactive, weak acid, and other chemical dyes all have the metal salt mordant included in the dye powder.
If you don’t want to use a mordant, you can try the natural dyes that do not need mordants (indigo and walnut do not need a mordant to remain color fast).
I have received a package of dyes from Morocco as a gift. I am wondering how to use them. I have bought white fabric and thought I would prep it by washing it to get rid of the sizing. Then soaking it in an alum bath. Would it them be ready to dye and if you know, how should I mix the powdered dyes?
If the dyes came with instructions, follow them.
If the dyes do not have instructions, if you can determine what type of dyes they are, you should be able to find dyeing instructions either online or from a book. I’m afraid I’d need more information to be able to give solid advice.
What’s the process for safe disposal? (Re: metal based mordants)
For Alum, the remaining bath after mordanting can be poured onto decorative plants that like acidic soil. Alum is completely non-toxic, so the vat itself is simply acidic.
Since I recommend copper and iron as pot based mordants (use a copper lined, or iron pot for the dying), dumping the vat as normal is permissible. The metals will have only impacted the color of the fabric and won’t have seriously leached into the vat liquid.
On the off chance you use an iron sulfate, or a powder based copper mordant – those are toxic and require more careful disposal. This is why I do not recommend using them, or chromium which is even more highly toxic.
I’ve been soaking alum mordanted fabric for about a week because I just haven’t had the time to dye it. Could the mordant have leached out of the fabric in this amount of time? Do you think I need to re-mordant it? Thank you!
Hi…. once I’ve mixed my alum in a bath with water… how long will this mixture last (stay active)
And also the same with copper and iron mineral dips.. I intend to mix with water and keep them in a container to then dip my leaves etc into just before the process of printing?
It should last indefinitely. You’ll just want to replenish the alum content each time you mordant new fabric. I’ve stored alum water for 6 months with no issues. If it starts smelling off, it should be replaced. Copper and Iron dips will have a higher acidity, and should remain good indefinitely. I’ve used copper/vinegar solutions that were stored for several years. Just make sure that copper and iron dips are safely stored – if they are ingested they can be toxic.