five mini skeins of yarn in greens and yellows, naturally dyed with nettles, rosemary and elder

Iron (Ferrous Sulfate) as a Natural Dye Modifier

This blog post is all about iron (ferrous sulfate) as a natural dye modifier. When it comes to natural dyeing, iron plays an important role and you will come across it frequently. It can be used for different purposes but, personally, I mainly use it as a modifying agent for naturally dyed colorways.

Table of Content:

If you want to learn how to dye yarn with natural dyes, you can check out this step by step tutorial on my Youtube channel:

a woman holding a yellow skein of yarn. a second picture is showing a basket filled with colorful skeins of yarn. a text saying: How to dye yarn at home with natural dyes

Characteristics and Uses of Iron(II) Sulfate

Let’s start off by looking at the characteristics and properties of this substance. Iron(II) sulfate or ferrous sulfate is a green salt. It is a reducing agent and is mainly utilized as a precursor to other iron compounds. The hydrated form is used medically to treat iron deficiency and different industrial applications. Historically, the textile industry valued it as a mordant for hundreds of years. Additionally, iron(II) sulfate can also be applied as a soil amendment for lowering the pH of a high alkaline soil so that plants are able to access the nutrients in the soil [1].

a hand holding a spoon with iron (II) sulfate for natural dyeing

One major drawback is the fact that ferrous sulfate can damage the wool and make it brittle when used in high concentrations and over longer periods of time. Therefore, I don’t use iron(II) sulfate as a mordant. It is, however, a great option if you are not completely happy with the final color of your yarn and want to modify it. In general, adding an iron modifier will make the colors darker and more muted in tone. In some cases it can also completely transform a dye color.

You can learn more about mordants and what I used instead of iron in this blog post.

How to use Iron as a Modifier for Natural Dyeing

  1. If you want to shift the color of the skein as evenly as possible, make sure to scour the yarn in water for about 24 hours before placing it in the iron bath. To do so, fill a bucket or pot with cold water and put the skein in it. Gently press the yarn below the surface and make sure it gets wet all throughout the skein. In the course of the 24 hours, check the yarn from time to time and press it down below the surface again if necessary. When you take the skein out of the water, very gently wring out the excess water. The skein should be evenly wet but not dripping.
  1. Dissolve 1.5 g of ferrous sulfate per 100 g skein of yarn in a pot filled with cold water. Stir until the salt is fully dissolved. Iron sulfate needs to be handled carefully. Don’t forget to wear gloves and safety goggles and work in a well ventilated area. Iron(II) sulfate can be irritating to the eye and skin and is harmful when swallowed. You can have a look at the safety data sheet here. Furthermore, you have to check with your local regulation regarding the waste disposal.
  1. Put the yarn in the iron bath and leave it for 30 minutes without heating. Make sure that the skein is fully immersed in the solution and press it down with a spoon if necessary.
    If you are not satisfied with the color shift after 30 minutes, leave the yarn in the bath a bit longer, but not more than one hour. In my experience, this doesn’t damage the fibers but you can still get great color modifications.

Alternative Method to Making Iron Water for Natural Dyeing

It has become a popular suggestion in the natural dyeing world to create an iron bath by using rusty nails, vinegar and water. I can only assume that this is the case because it feels like a more natural way compared to making an iron bath by using the chemical in its solid form.

Being a chemist by training, I prefer to use iron(II) sulfate as a salt because it allows me to exactly control the concentration of the iron bath. This also leads to more reliable results that are easier to replicate.  

Let’s explore the chemical reaction that is going on in the rust method a bit further. Rust is an iron oxide and consists of hydrous iron(III) oxides (Fe2O3·nH2O) and iron(III) oxide-hydroxide (FeO(OH), Fe(OH)3)) [2]. That is a significant difference to the iron(II) sulfate in salt form.
Adding an acid like vinegar will dissolve the rust particles in the water solution.

You have to be aware that you are still working with chemicals (namely iron salts and an acid), even if placing rusty nails in a jar and adding vinegar and water might feel less intimidating. Whatever method you choose, please make sure to take the necessary precautions.

Which Dye Plants can be modified with Iron?

Here is a list of dye plants that will shift colors when coming in contact with ferrous sulfate. You can use this list as a starting point, but I highly encourage to experiment with different natural dyes and see for yourself.

  • Acorns
  • Alder (Leaves, Cones, Bark)
  • Avocado
  • Blackberry (Leaves, Canes)
  • Birch (Bark, Leaves)
  • Dandelions
  • Elder (Leaves, Bark)
  • Elm (Leaves, Bark)
  • Eucalyptus (Leaves, Bark)
  • Fustic
  • Goldenrod
  • Nettles
  • Oak (Galls, Bark, Leaves)
  • Onion Skins
  • Pine Needles
  • Queen Anne’s Lace
  • Rosemary
  • Rose Petals
  • Turmeric

Examples of Color Modifications through Iron as a Natural Dye Modifier

Not all natural dye colors are susceptible to ferrous sulfate. In these cases, immersing the dyed skein in an iron bath will not result in a color shift. But it is always worth giving it a try because more often than not, colors will change when they come in contact with iron(II) sulfate. Some changes are more significant than others, though. You will have to be willing to experiment and see for yourself.

In general, if the natural dye colors are susceptible to iron(II) sulfate, yellow colorways will often shift towards different shades of green while reds and pinks will shift more towards purple.

five skeins of yarn and a pink rose on a wooden background, three smaller skeins are yellow, one larger one is grey and one larger one beige

All of the skeins you can see in the picture above have been dyed with rose petals (the light yellow skein was created with a different type of rose compared to the bright yellow mini skeins). The grey skein (second to left) originally had a light yellow hue that was shifted towards this vibrant grey through immersing it in an iron bath.

five mini skeins of yarn in greens and yellows, naturally dyed with nettles, rosemary and elder

The two skeins on the left were modified with iron and shifted from a yellow towards green. They were dyed with nettles (far left skein) and rosemary (second to left skein).

six skeins in apricot, peach and dusty pink tones naturally dyed with avocado

All of the mini skeins that you can see in the picture above have been dyed with avocado (either pits or skins). The purple-colored, third to left skein has been modified with iron.
You can find the full tutorial for natural dyeing with avocado in this blog post.

a greyish purple skein of yarn next to a small bouquet of flowers on a white surface

Dyeing with oak galls will result in a light tan/beige colored hue which will shift it’s color quite significantly towards a greyish purple when treated with iron(II) sulfate.

green and yellow socks on a rock in the woods naturally dyed with pine needles

Both of these socks were made out of yarn that has been dyed with pine needles. The upper, light green sock was made out of a yarn that has been modified with iron.

Want to learn more about the fascinating topic of natural yarn dyeing and connect with nature at the same time?

I have created a beginner’s guide to natural dyeing that contains everything you need to know to get started. And the best thing: it is available for free!

four hand dyed skeins of yarn in shades of purple and blue on a wooden surface and a text saying beginner's guide: natural dyeing. everything you need to know to get started dyeing yarn with natural dyes

Pin It For Later: Iron (Ferrous Sulfate) as a Natural Dye Modifier

three pictures of hand dyed yarns that have been treated with iron after the natural dyeing process. a text saying how to use iron as a natural dye modifier

Further Reading:




I am a yarn dyeing artist, writer and educator.
I am also an avid knitter and love to create something with my hands every day.
Read more about me here:

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