NB: This post assumes you have some dyeing experience and understand the process of mordanting.
You know that dry pucker-y mouth feel you get from drinking teas or red wines? That's tannin, a group of bitter astringent phenol compounds that occur naturally in plants. Tannin’s biological function is as a natural antimicrobial, protecting against pathogens that might harm the plant. Many natural dyes contain tannins, e.g. Pomegranate, Walnut, Cutch and Eucalyptus. When I first started dyeing, I didn't understand the role of tannins at all. I found it all a bit confusing and admit I still have a lot to learn, so I'm open to any corrections. I hoping this article will give you a head start into understanding this important compound. If you are using plants for dyeing, then you are using tannins, so it's helpful to have a basic understanding of how they fit into the process and how to use them to your advantage.
Tannins have many commercial, nutritional and medicinal applications but for natural dyers they have 2 main uses. Tannins can either be a) a mordant or b) a source of colour, although I find the two are not mutually exclusive.
Tannins as mordant
As a mordant, tannins are best used on cellulose fibres on their own or in conjunction with alum. Tannins bond well with cellulose fibres, but alum does not. Mordanting first with a tannin followed subsequently with alum creates a sort of sandwich, allowing the natural dye to bond with the fabric. This two step tannin-alum method is described on many natural dyeing websites (Google it my friends), and I may do another blog on that in the future. But remember, as tannin is also a dye, it will add colour to your fabric so if you want the original colour of your fabric to remain the same (e.g. white), aluminium acetate will be a cleaner option for your cottons, linen and other plant based fibres.
Tannins as dyes.
Tannins will give shades of tan, gold, yellows, or brown-reds depending on the source e.g. pomegranate yields a soft gold and cutch a browny-red. On their own, used as an immersion dye bath, you can achieve a range of organic and neutral warm colours.
Things get more interesting when iron salts are added. Iron modifies the colours of tannins, changing them to deeper browns, greens, greys and sometimes bluey-purple. This is due to a reaction between tannic acid and iron ions which form iron tannate. It's often referred to as 'saddening' colours, but as this reaction usually makes me happy I prefer to call it enhancing or shifting. Iron can be added in many forms; ferrous sulfate crystals, home made iron water or the use of rusty objects.
Lay down a base of tannin colour under other dyes, madder for example, and you can increase the depth of colour.
And then, when the tannin comes directly from a leaf eco print on top of, or under, another base dye and iron is added, it creates a whole other layer of complexity and well - all sorts of magic happens. The potential of tannins is mind-blowing. Often, I find myself examining all the details of a finished piece, dissecting out the variables (like peeling the Shrek onion) in order to understand – what just happened?
All tannins come from botanical sources but some are stronger than others. Gall nut, Quebracho, Myrobalan and Fustic are amongst the strongest and therefore are sold commercially as extracts. In New Zealand, we rely on imported sources of these particular tannins. But of course, you have a local source of tannin at your fingertips every time you pick up a leaf, flower, seed or piece of bark. Exploring the potential of tannins found in your own natural environment is a box of wonders for the natural dyer to open and sift through. Understandably, you could spend years experimenting with found tannins, I know I have. I’m not saying, don’t buy commercially prepared tannins, they are extremely useful to have in your stash. I’m just suggesting that there are other tannins to be explored that, if sourced locally, reduce the carbon footprint of importing and widen your repertoire of colours.
Tannins are within our local plants including New Zealand natives. The clues are there if we look to indigenous Māori practices. For example, Hinau bark with iron rich mud gives the black that you can see in this early kahu raranga (woven cape).
So how do you know if the plant you have contains tannin? One quick indicator for tannins is to do an test as follows:
Gather plant material from a single source. Place your leaves in a stainless pan and cover with clean rain water. Simmer for at least an hour. You may see the water get darker, or notice an astringent smell, both are important clues. Strain the liquid and pour some into a small glass jar with a decent pinch of ferrous sulfate, or add a rusty object and leave to sit. If the plant matter contains sufficient tannin, it will bond with the iron and turn the water darker, maybe grey to black or a purplish hue. This bonding may not be instant, be patient.
Another way is to make up a natural dye bath from your plant matter as usual and dye some swatches of pre-mordanted fabric. Then soak a swatch of that fabric in a solution of iron water, leave for a few hours (overnight if you have the patience). If it darkens or turns green/grey/black, that is the tannin-iron reaction happening. I suggest you keep a record of the results. I always think I’m going to remember, then I always don’t. I have devised a form that I use to record my tannin experiments. You are welcome to download a free PDF copy from here.
Other things of interest to note.
If there is tannin in the plant matter, then there’s also a solid chance the leaf will give you an eco print with our without iron - test to see.
Iron will break down fibres over time. Use it sparingly and cautiously if you want your dyed fabrics to stay intact. It’s also toxic to children, please keep your iron well away from little hands with enquiring minds.
Always, always collect your plant material sustainably. Use windfall, deadwood, prunings or florist waste. I can’t stress that enough. (see my blog post on Foraging & Tikanga).
Take advantage of incidental tannins that leach out when plants sit in water over time – like in a vase of wilted flowers. You’ll also be conserving water.
If this captures your attention, start building up a library of available local tannins you can use as immersive dye baths. A good starting point are eucalyptus leaves, which will smell divine as you ‘cook’ them to extract colour. Don’t limit yourself to leaves either. Seeds, seedpods, stalks and bark are very much worth trying. For example, try simmering bark fallen from a eucalyptus tree.
I don’t usually get all science-y about dyeing, preferring to leave a lot up to serendipity, but exploring the potential of tannins has given me the joy of discovery and added layers of complexity to my work. Knowledge is empowering right? I’d love to hear what you discover, or see images of your own experiments if you decide to jump down this rabbit hole with me. Drop me a comment below or tag me in your Instagram pictures @nukutextiles with the hashtag #tanninexplorationnz