Homemade iron mordant: what factors affect mordant quality?

Making an iron mordant is simple – get some pieces of iron, preferably rusty, put them in a jar and cover with vinegar. Let this sit for a couple of weeks, or until the liquid changes colour, and then use. (I go deeper into this method in my eco-printing ebook, Gum Leaf Alchemy).

But there are different factors affecting the quality of iron mordants. Here’s three pieces of cotton cut from the same piece of fabric and eco-printed with the same eucalyptus leaves, but with quite different results:

The difference between each sample was the quality of the iron mordant used. Let’s take a closer look at each.

Sample 1: A poor quality eco-print 

The mordant on this piece had been diluted with water a few days before eco-printing. It was a strong mordant that I was trying to dilute. The results are very pale and fuzzy:

The problem is that the vinegar is a vital component of the mordant because of its acidity. When water is added to iron mordant, it raises the pH and changes the iron from a dissolved form to a solid form. This is sometimes quite visible, it looks almost like the mordant has curdled. In this state, during the mordanting process it stays suspended in the liquid rather than bonding to the fabric.

This process is not immediate (otherwise it would not be possible to put iron mordant into water to mordant fabric). But it will be visible if you save any leftover mordanting water for a few days or longer, especially if it was quite strong. You will be able to observe the iron separating into small clumps and settling at the bottom of the container. 

So, the moral is don’t add water to your mordant. If you have an overly strong mordant, you can either use less of it or add vinegar to dilute it.

Sample 2: A mediocre eco-print 

The mordant on this piece was very old, and had been soaking for over 2 years. Although the mordant itself looked great, very dark and strong, the prints are fairly washed out, without the crisp detail I sometimes get on cotton:

The reason for this is that vinegar will lost its acidity over time, creating a similar problem to adding water to a mordant. Ideally you will be using your mordant regularly, and topping it up with more vinegar each time. If you have a very old mordant (unused for 6 months or longer), try adding new vinegar, letting it rest at least a few days before you use it again. If you still get disappointing results, you may need to make a new mordant instead.

So here the lesson is that the colour and darkness of the mordant won’t necessarily predict results. And that it is important to keep topping up your mordant with vinegar.

Sample 3: A high quality eco-print 

This last piece was made from a mordant that was only 2 months old but contained a lot of rusty metal so was a good strong colour. There is crisp vein detail on each leaf and lovely blue dots dappled across the brown leaf prints:

These are optimal results for eco-printing on cotton – crisp, detailed and dark. The pale prints still look nice but will fade much faster.

This sample shows that iron, vinegar and the right amount of time (not too little, not too much) are all you need to create a great iron mordant.

Other iron mordant considerations 

Often people think that the brightness or paleness of a print is caused by using more or less mordant. But actually, other factors are far more important. How well the fabric was scoured, how much pigment is in the leaves, and the quality of the mordant can all drastically impact the results you get. The fabric will only absorb a certain amount of mordant, and a small amount is surprisingly potent. So even when using homemade mordants that aren’t measurable, it is not easy to drastically over- or under-mordant your fabric.

One final mordant variable is when you have used up the liquid of a mordant, but have iron sludge at the bottom of the jar. Once you top it up with fresh vinegar, you need to let it sit for a few days or longer otherwise it will also give pale results.

So if you have been getting poor or mediocre results when eco-printing on cotton, I encourage you to consider whether your mordant might be the reason why. If you need more help with getting great results on cotton, check out my eco-printing ebooks for in-depth information about every step of the process.

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6 thoughts on “Homemade iron mordant: what factors affect mordant quality?”

  1. Thanks Louise, it’s sweet of you to always update your students with new tips. I have been adding vinegar into iron mordant. It works great, thanks again – Nat

    1. I get so excited when I learn something new that I want to share it with everyone! So glad this has been working for you.

  2. Thankyou for your generous information/confirmation re iron mordant on cotton. I was slowly coming to this conclusion after documenting a series of failures! You have explained it so succinctly. Thanks again.

    1. I’m sure there are more intricacies I haven’t worked out yet, but I’m glad my current understandings are useful to you.

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