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How to make an Iron Mordant and Copper Mordant for Natural Dyeing and Eco Printing

If you have dabbled with natural dyeing, eco dyeing or eco printing on fabric, paper or yarn, then you are probably familiar with the use of Mordants to help affix the dye and make it more permanent.

This mordant process can be as simple as soaking your fabric in Soy Milk or Alum Powder. It can also get much more complicated, sometimes involving multiple steps over several days.

Liquid Iron or rust water is fairly simple and inexpensive to make. All you need is some rusty bits of metal, white vinegar, some water and time for this Iron Mordant Recipe.

Bonus: I will also share with you how to make a Copper Mordant so make sure and read to the bottom for that easy recipe.

Closeup of a large Mason jar with rusty nails and liquid

Basic steps for making an iron mordant

Place rusty nails, screws and other hardware in a quart size canning jar
Add water and white vinegar
Let sit for a week or more

What is a Mordant?

In a nutshell, a mordant or fixative is a substance (often metallic) that helps the natural dye adhere to your material. It ‘fixes’ the dye. Confusing, right?

Well, the problem with fabric dyes made from plants, food or mushrooms is that they are usually not colorfast and so they rinse right out of your fabric.

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Itis so disheartening when you work really hard to make a beautiful color dye stock only to have it immediately wash out. That’s where a ‘mordant’ comes in.

Simply put, it acts as a bond between your fabric and your natural dye stock. So, unless your dye material contains its own tannins (which I’ll explain more later), a mordant is necessary to make the color ‘stick’.

Mordants (and Modifiers) work for dyeing with natural materials such as leaves, flowers and vegetables but they are also great for Eco Printing or Eco Dyeing.

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A pile of rusted nails used for a dyeing mordant

Nails soaking in water and vinegar solution in a Mason jar with clean trees in the background

What is the Difference between a Mordant and a Modifier?

As I explained above, a Mordant is a substance which helps the natural dye adhere to fabric or yarn, making it more colorfast or washfast. A Modifier, on the other hand, is a substance which is used to alter the final color, often done after dyeing.

In some cases, the modifier will deepen the original color. With an Iron Mordant, the final color is darkened or ‘saddened’. Other mordants can brighten the final dye.

In the photos below, I pre-mordanted the cotton fabric and the yarn with Alum Powder and dyed them using Blackberry Leaves, Queen Ann’s Lace Flowers, and Avocado Pits and Skins.

After dyeing, I dipped the fabrics in liquid Iron Modifier which instantly turned the colors a deeper brown, gray and blue.

Cotton fabric and yarn: top half is off white and tan and the bottom half is dark brown and gray from the Queen Ann's Lace dye and an Iron Modifier — **Cotton fabric and wool yarn dyed pre-treated with Alum Mordant and dyed naturally with Queen Ann’s Lace flowers. This was then dipped in an Iron Modifier**

Cotton fabric and yarn: top half is off white and tan and the bottom half is dark blue from the Blackberry leaf dye and an Iron Modifier — **Cotton fabric and wool yarn that was mordanted with Alum and dyed naturally with Blackberry Leaves and then dipped in an Iron Modifier**

Cotton fabric and wool yarn dyed naturally with Avocados and an Iron Modifier
Cotton fabric and yarn: top half is pale pink and the bottom half is dark blue and gray from the Avocado dye and an Iron Modifier — **Cotton fabric and wool yarn that I mordanted with Alum and dyed naturally with Avocados. The bottom was dipped in an Iron Modifier**

The next photo shows a piece of cotton fabric that I mordanted with Alum Powder. Then I Eco Printed it with leaves from a Willow Tree. As you can see, there is a huge difference in the color of the prints.

I did not pre-treat the leaves on the left side. The leaves on the right side were dipped in the iron modifier before printing.

On left: Cotton fabric pre treated with an Alum Mordant with printed Willow leaves.
On right: Cotton fabric pre treated with an Alum Mordant with printed Willow leaves dipped into an iron modifier — **Cotton fabric that I pre-treated with an Alum Mordant and then Eco Printed with Willow leaves. The leaves on the right side were dipped in an Iron Modifier**

Mordant Safety

Iron and Copper Mordants are pretty safe to use for mordanting with a few precautions:

Wear gloves to protect your skin. And, if you are going to be heating these metals, you should probably do that outdoors to minimize exposure to the fumes.

Finally, liquid iron and copper mordants are TOXIC if ingested! Keep away from children and pets!

How Do You Make an Iron Mordant?

You can easily make your own iron mordant (modifier) or rust water with this super simple recipe. Just remember, it does need to sit for a week or two so keep that in mind when you decide to dye your materials.

Here is how you can easily make your own iron mordant or modifier:

Iron Mordant Recipe

Handful of Rusted Nails, Nuts, Bolts, etc.
1 Cup White Vinegar
2 Cups Water
1 Quart Mason Jar

Place the rusty nails, vinegar and water in the Mason Jar and loosely screw on the top. Let this sit for about 2 weeks.

As you use the iron liquid, simply replace the water/vinegar in the jar at the same 2/1 ratio.

Where Can I Buy an Iron Mordant?

If you don’t happen to have rusty nails laying around, you can also make your liquid iron with this Ferrous Sulphate powder. There are some upsides to using ferrous sulphate rather than making your own.

First, by carefully measuring the powder, you can control the strength of the iron in the liquid. Secondly, you know the purity or your mordant. and finally, there is no waiting period.

How Do You Make a Copper Mordant?

Not only can you use iron to make a mordant, but you can do the same thing with copper!

Personally, I have only used copper as a modifier, not a mordant but it’s my understanding that it works well for both.

Copper tends to shift a dye color to green or blue. This is made exactly like the iron mordant recipe … just put it in mason jar and wait.

Copper Mordant Recipe

Piece of Copper pipe or tubing
1 Cup White Vinegar
2 Cups Water
1 Quart Mason Jar

Place all in your mason jar and screw the top on loosely. Let sit for about 2 weeks.

If you don’t have any copper available, you can also order copper sulphate crystals online.

Eco Printed Oak Leaves on a white paper background — **Paper with Eco Printed Oak Leaves dipped in liquid Copper Modifier**

Mason jar containing clear liquid and a coil of copper tubing with trees and a shed in the background

Which Natural Dyes don’t need a Mordant?

Plants that contain tannins are naturally colorfast and therefore don’t require a mordant. These include Black Walnut, Acorns and Sumac.

If you have ever made a dye with black walnuts, you know that stuff stains EVERYTHING! No mordant needed.