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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37 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.

      1. Hi I had read that vinegar on cotton can damage the fibre. Do you advise using anything as an after treatment to help alleviate this. Thank you

        1. I know some people soak their fabric in vinegar, mistakenly believing that it is a mordant, and so maybe at that amount it will damage the fabric. But the amount of vinegar from using an iron mordant is going to be tiny. I recommend washing with a pH neutral detergent.

  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.

  3. I’ve been doing a fair bit recently and some with great results, but admittedly mostly I don’t know why so am learning to record my findings more. I have lots of jars of 3 year old rusty solution and will now practice testing strengths but a, confirmed as I thought you need to added a couple of teaspoons of solution to water for example an iron blanket to be soaked in, or is this just for plant dipping. TIA. B

    1. Hi, you can use the mordant either way – dipping leaves in it, or adding a dilute amount to water to soak fabric in. And if it is very strong, I dilute it before I dip the leaves in too.

  4. My first batch of iron mordant is looking very dark, what’s alarming is that it also has developed a thick foam. Any idea what that is, and whether I can safely remove them foam and still use the liquid?

    1. Hi, that’s no problem. If it is very dark, just use less of it. And you can stir in the foam or strain it off.

  5. First of all, I am a beginner at eco printing. I dyed some silk scarves in cochineal, after Mordanting in alum. I produced a bright fuchia and a lavender scarf after adding baking soda. For the fuchia scarf I used an iron blanket. My homemade Iron water looked strong, so I diluted 50/50. The scarf turned dark purple and had some definition of leaves, but not much. Overall, the scarf was very dark. I liked the iron blanket better than the scarf! On a pale avocado dyed scarf, I tried dipping the leaves in the diluted iron water, no iron blanket. Again, turned out Very dark. Is my iron water too strong ? What is the advisable dilution.

    1. Hi Margaret, if the results are too dark then yes, you need to dilute the iron mordant more. Usually when I mordant cotton, I put in just a small splash, perhaps 1 tsp – 1 Tbsp, into a bucket of water that I soak the fabric with. For iron blankets, you will want it a little stronger than this. And if you are dipping leaves in mordant and the results are too dark, try diluting it more next time. It can be a frustrating process testing and trying different things, but you will get there.

  6. García.Alejandra

    Buenas tardes!he hecho impresiones con lienzo de algodon y hojas en hierro,que adquiri en un vivero y no me ha impreso nada .Estoy muy desilusionada!!¡

    1. Hello. That sounds frustrating. There are lots of variables, so it is hard to know exactly why you aren’t getting prints. What sort of leaves did you use? Did you ‘scour’ (wash) your fabric thoroughly?

    1. There probably is an optimum pH level, but I haven’t monitored my mordants so I’m not sure, sorry. It would be a great thing to test.

  7. Pingback: Pale eco-print results: Troubleshooting the cause – Gumnut Magic

    1. Oh dear. It happens to all of us 🙈 It’s good to try to flush out the excess mordant with clean water if possible, but it is still useable and can give good prints. You might just notice that the fabric degrades quicker over time.

  8. Hi there

    I’ve tried using an iron mordant on white silk- and ever single time it stains the silk outside of where the botanical prints are. Is there any way of keeping white silk white when you mordant silk in an iron bath because mine always changed colour and it’s really frustrating me

    1. Hi, it’s a bit hard to tell without seeing your results, but it sounds like the dye is bleeding out from the leaves. I find that soaking the leaves in water for a few days or weeks before use helps a lot to reduce this bleeding. Placing an extra layer of cotton on top of the leaves before rolling up can also help.
      If the staining you are talking about is coming from the iron rather than from bleeding, perhaps you just need to use far less of it, and make sure you dry your mordanted fabric in the shade.
      I hope some of these ideas will help you.

  9. Great to know what the sludge was at the bottom of my iron jars. I tossed it and have started again. I’ll be buying some more vinegar next time at the supermarket.

  10. ALOHA, I am just making my first batch of iron mordant..and I am new to the dyeing process and methods…I am wondering how much of the iron mordant that should be used… Is there a certain amount based on the weight of your fabric. Or you you add a little at a time to the iron bath??…Thank you…

    1. Hi, it isn’t possible to give precise measurements because homemade mordant will vary so much in strength. For very dark mordant I tend to use about half a teaspoon of mordant for each item of clothing, and for pale mordant I use about a tablespoon. It’s a pretty forgiving process and the fabric will only absorb so much anyway.

  11. I want to expand my knowledge of mordants for paper. I use alum to mordant my eco prints and sometimes use my homemade iron solution, but I don’t know how to use purchased copper or iron products…how much to use and what kinds.
    Also, why does my homemade iron solution foam?
    I have so many questions about dyeing on paper with plant materials. Every bit is information helps.

    1. Hi Laura, I don’t actually use store bought mordants so I can’t advise you on that. But what I’d suggest trying is dissolving small amounts of it in warm water and soaking the paper in that. You can do your own experiments about quantity but I’d start small.
      The foam is caused by some kind of chemical reaction, I’m not sure exactly but it is totally normal and doesn’t indicate a problem with the mordant.

    2. Alum is for Protein fibres and Alum Acetate is for Cellulose fibres.
      When using powdered Iron Sulphate or Copper Sulphate bought at the Plant Nursery Stores.. try using only 2.5gms to 4ltrs of water. Dissolve well in a small amount of hot water first.
      You will get different results when you dip your papers ( or cloth) into this mix for different times… longer time darker results… best to play and test. Cheers

  12. Oh dear wish I read your excellent explanation first… I am in middle of my first eco attempt and have already soaked my cotton in a soy milk and water bath for 2 days and currently drying the fabric, am currently steeping black tea in water for my dye and have just put ‘all’ of my (12month old) iron mordant into a pot to soak my leaves in for I’m guessing about an hour?!. None of which sounds correct after reading your posts 🤭 oh well…. I will continue and see what happens! 🤞
    So, the fabric needs to be soaked in water with a Tbs of iron mordant? And the leaves too?

    1. Hi Mel, please don’t worry, there are so many different ways to eco-print and to use mordant! When I prepare my fabric with soy milk binder, I often soak the leaves in mordant just as you are doing. I hope you weren’t frightened off and made an attempt with the fabric and leaves you had. Experimenting is the best way to learn, and you’ll slowly figure out the methods and results that you prefer.

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