To soak or not to soak?

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Way back in 2016, when I had just begun selling clothes under the label Gumnut Magic, I had a lot of interest from other natural dyers about my eco-print technique. Back then people were mostly eco-printing on wool so I had a lot of questions about how I was managing to get clear, bright prints on cotton. At that stage I’d been eco-printing for a few years and had developed a technique that worked for me. I decided to write an ebook to teach my method.

My first ebook was Gum Leaf Alchemy, so named because I focused specifically on eucalyptus (or gum) leaves. This ebook was specifically about eco-printing on cotton using an iron mordant. Part of the method I described in that ebook was to pre-soak the leaves in water for a few days or a few weeks before eco-printing. This is a way to get clear, bright prints with vein details and variety of colour:

Unfortunately, I now see some dyers swearing that you need to soak all leaves for any type of eco-printing, while others denounce the idea of ever soaking leaves.

But I never said that we should always soak leaves or that it will always give better results. There is definitely a time and place for it. I’ve done lots of tests over the years which have helped me to understand that sometimes soaking leaves is very helpful and sometimes it is not. I’ve also done research into the science of natural dyeing to try to understand the different chemical reactions that occur when using fresh or soaked leaves. I continue to learn and experiment, and I will continue to share my findings. In this post I hope to clarify when it can be useful to soak leaves, and when it is unnecessary.

On tannins and soaking

When I wrote Gum Leaf Alchemy I was mostly eco-printing with eucalyptus leaves and I had found that soaking is an important step for many species of eucalyptus – because of the high tannin content which causes bleeding and patchy prints. But also because they are sometimes quite a tough leaf and sometimes the other colours besides blue are only accessible after soaking. Here you can see the blue, patchy and less detailed prints that can come from fresh leaves:

That said, some fresh eucalyptus leaves give great results, particularly when sourcing leaves from healthy street trees, so it is always worth experimenting with both fresh and soaked leaves to see if there is a difference. It’s a large genus of plants with many different species growing in different conditions, so of course there will be a lot of variation.

For my third ebook, The Leaf Guide, I researched more about tannins and came to understand the mechanism by which soaking leaves improves results. There are three types of tannins: gallic, ellagic and condensed. Gallic tannins react strongly with iron mordant, causing the blue and grey bleeding through the fabric that you can see below:

Luckily gallic tannins are hydrolysable (water soluble), which means that soaking the leaves reduces or removes them, thus reducing or eliminating the bleeding. In that ebook I also noted that it is always worth doing your own tests with your local leaves, rather than assuming that soaking will make a difference. Sometimes it makes a big difference and so it is worth the effort, while other times the difference may be small and so you could decide that it’s not worth the extra hassle. And of course, you may love the dappled blue effects from the tannins interacting with iron, and prefer that to clearer prints on a plain background. There is no right or wrong in eco-printing, just different possible results!

Below I have tried to summarise the circumstances in which pre-soaking your leaves before eco-printing is a good idea, and when it is unnecessary or may even produce worse results. The basic summary? For most methods of eco-printing, it is better to use fresh leaves!

When to soak leaves

• If you are eco-printing onto a plant based fabric (cotton, linen, viscose etc) AND you have pre-mordanted your fabric with iron, then pre-soaking the leaves in water for a few days or longer may reduce bleeding and create clearer prints.

Put another way, I only recommend soaking leaves if you are using the specific technique that I write about in Gum Leaf Alchemy. Even better, only soak them if you have done tests with your local leaves comparing the results from fresh and soaked leaves, so you can be sure that it is a necessary step. While I find that pre-soaking improves results more often than not, for some leaves it makes no difference and, occasionally, it causes worse results for some leaves.

It is also important to understand that removing the gallic tannins may result in clearer prints, but at the expense of having the positive properties of these tannins – namely that they help natural dyes bind to fabric and can also provide a colourfast dye. Gallic tannins actually have better wash- and light-fastness than the condensed tannins which are not water soluble. So even though removing some of the gallic tannins mean that the different colours of the condensed tannins become more visible, your prints will not be as colourfast.

Also, you do want to be careful not to soak your leaves for too long. After some weeks of soaking, these maple leaves gave very beautiful, detailed prints with different colours from each side (top row). But after soaking for several months, the prints were paler which means they will be less colourfast (bottom row):

The interesting thing about these maple prints is that they show that the tannins leach out slowly over time. So when soaking leaves, we want to be hitting a sweet spot where enough of the tannins have been removed that the prints aren’t bleeding, but enough remain that you can get the benefits of them outlined above. There’s no way to accurately judge this, except to aim to soak the leaves for as short a time as possible. I have found it valuable to do tests comparing results from fresh leaves and leaves soaked at increments of 3 days, 1 week, 2 weeks and 3 weeks, so that I can try to find that sweet spot. I encourage you to do your own tests with your local leaves.

I’ve also detailed some of the results of my experiments in The Leaf Guide. In this ebook, I show examples of 25 different leaves eco-printed on cotton, with information about how long I soaked the leaves for each sample, and whether soaking made much of a difference.

Here’s an example of a cotton tshirt pre-mordanted with iron, where the leaves where soaked for a perfect amount of time. The prints are still strong and not washed out, there is no bleeding, and there is a good range of colours and some very nice vein detail:

When not to soak leaves

• If you are eco-printing on wool, it is unnecessary to soak your leaves.

This is true even when using iron on wool (which you should always do sparingly). I think this is because the proteins in wool are able to bind more plant dyes directly where the leaves are touching it, rather than the excess dye bleeding through layers. It’s still okay to use soaked leaves on wool, if that’s all you have to work with, as long as you haven’t soaked them for too long. If you soak them for weeks or longer, you may notice the results become more pale.

• If you are using any other mordant, it is unnecessary to soak your leaves.

As far as I am aware, the strong reaction and bleeding only occurs with the combination of gallic tannins and iron mordant. So if you are using alum or any other mordant, you can use fresh leaves.

• If you are using iron dipped leaves on plant based fabric (either plain fabric or prepared with soy milk binder), it is unnecessary to soak your leaves.

This is a method I have been using more over the past few years, because it removes the need to keep track of how long leaves have been soaking for. And the combination of iron dipped leaves and fabric prepared with soy milk provides strong, colourfast results. I explain this process in detail in my new ebook, Soy Milk Binder for eco-printing and natural dyeing. Here’s some of the results I show in that ebook for eco-printing with fresh deciduous leaves, both plain and dipped in iron:

So why can you use fresh leaves if you are dipping them in iron, rather than coating the whole piece of fabric in them? My hypothesis is that because the iron is only on the leaf, and is only making contact with the same part of the fabric as the leaf is, there is no way for it to bleed through to the rest of the fabric. Whereas when you mordant the whole piece of fabric with iron, any tannin that hasn’t bound to the piece of cotton it is pressed against is going to bleed through and bind to any available iron mordant.

When comparing these results with the maple prints above, you can see that the results using iron-dipped fresh leaves are slightly different to the soaked leaves. There is a bit less crispness and vein definition when using the iron-dipped fresh leaves on soy milk binder. But for me, the convenience of using fresh leaves sometimes outweighs this.

So if you can use fresh leaves, why would you soak them?

Hopefully by now you understand that soaking leaves is useful just when using an iron mordant on plant based fabric, and that it is also possible to eco-print with an iron mordant with fresh leaves. It seems to me that there are three options for getting clear prints on plant based fabric, if you want to use an iron mordant. They have differing levels of colourfastness, and of ease of preparation:

• Fresh leaves dipped in iron, on plain fabric

This is the simplest option, which requires no advanced preparation besides making the iron mordant. But results will be the least colourfast. A good option for testing new leaves, or if you want a simple method and don’t mind re-printing your clothing regularly.

• Soaked leaves on iron mordanted fabric

This option has better colourfastness, and the fabric is simpler to prepare with iron mordant than with soy milk binder. But you need to be organised about soaking the leaves in advance. This is the method I teach in my ebook Gum Leaf Alchemy and my Living Colour ecourses. I like the results from this method the best, because when leave have been soaked an optimal length of time (not too short or too long), the results are bright and clear with lots of detail.

• Fresh leaves dipped in iron, on fabric prepared with soy milk binder

This option probably has the best colourfastness, because you are getting the benefits of both soy milk binder and iron mordant. This is the method I teach in my new ebook, Soy Milk Binder, and in my ecourse of the same name. It’s a longer process preparing the fabric with the soy milk binder, but if you do an occasional big batch you’ll always have fabric ready to use, and you don’t need to worry about soaking leaves in advance. Using soy milk binder also means that you can dye the background of the fabric, either before or after eco-printing.

Final thoughts

There are many different ways to prepare fabric and to eco-print. I love working with iron mordant because it can bring out great colours and interesting details such as crisp vein lines. Because the gallic tannins in many leaves react with iron mordant, sometimes a little and sometimes a lot, it’s worth considering whether you mind this bleeding, and whether you want to take steps to avoid it.

I’d love to hear your thoughts. Was this post useful? Do you soak your leaves before eco-printing? Sometimes, always, never? Have you learnt anything from your own research and experiments?