Mordant

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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Combining iron and alum mordants for eco-printing

Although iron is my trusty favourite mordant- easy to make, relatively low toxicity, and easy to dispose of- I have been experimenting with alum lately. My first results of eucalyptus on alum were not very promising. Fluro yellow is not my favourite colour. But then I tried double-mordanting, combining both iron and alum. I hoped that the saddening effect of iron would help tone down the brightness of the alum and bring in more subtleties. And it worked beautifully!

Double mordanted with alum and iron

The alum has brought out these beautiful oranges and yellows from the eucalyptus leaves, while the iron gave more depth and detail.

How to mordant cotton

This onesie was bit of a failure originally. It had pale, faded grey prints, probably from not being scoured properly. The alum and iron mordanting prepared it well for a second round of eco-printing, and it was transformed into one of my favourite pieces so far. Aren’t the gumnuts exquisite?!

Mordanting cotton with alum and iron

To do this double mordanting, I scoured the cotton, gave it a short dunk in one mordant, let it dry and cure, then dunked it in the other. Then I eco-printed the fabric in my usual way, using the instructions I share in my ebook Gum Leaf Alchemy.

I haven’t yet tested to see if it makes a difference to do either the iron or the alum first. It is definitely worth continuing to play with this combination!

Combining alum and iron mordants to eco-print on cotton

Mordanting with blood

I’ve been curious about mordanting with blood ever since I read India Flint’s passing reference to it in Second Skin. I’m not a squeamish person and I have ready access to a cruelty-free form of blood each month with my menstrual cycle, so I thought I would give it a go. I wondered if it would give similar results to other protein mordants, or whether the particular properties of blood would effect the results.

Mordanting fabric with blood

To prepare the fabric, I simply soaked it for about an hour in the water I had soaked my cloth pads in. Then I lay it flat to dry and let it set for about a week before eco-printing.

Eco-print with eucalyptus leaves on fabric mordanted with blood

The mordant definitely worked, although the prints were fairly light. Next time I would try to use more blood and soak the fabric for longer to create a stronger mordant. I might also try a series of ‘dipping and drying’ as is done with soy mordants, and see if this gives more vibrant results.

Eucalyptus eco-print on blood mordanted fabric