What is a mordant?
A mordant is a fixative that binds dye to fibres, whether fabric or paper. Using a mordant is crucial when dyeing fabric as it prevents the dye from washing straight out, but on paper, of course, this isn’t typically a concern. I still choose to use a mordant when dyeing paper anyway as it will not only fix the dye, but it will also increase the amount of dye that sets in the paper, thereby enhancing the colour and improving lightfastness. This produces better results that last longer.
There are several different mordants that can be used in natural dyeing and ecoprinting. Some mordants can act as a dye, some will only work on certain types of fibres, some can alter the colour of other dyes, and some things that are often described as mordants aren’t actually mordants. The choice of mordant is, therefore, very important.
When working on paper, the type of fibre you are dyeing (wood pulp or cotton rag, for example) is not such an important consideration when compared to the others previously listed, but still one to bear in mind. Another thing to consider is the strength of mordant you use. The amount of mordant needed will vary depending on weight and absorbency of the paper or fabric, so it is good to experiment until you find a combination that works for you.
Why use a mordant?
The primary reason to use a mordant is to improve colourfastness. By using a mordant to fix the dye to the fibres, it helps to prevent the colour from fading over time or washing away. This is particularly important for textiles, especially those that will be repeatedly washed or used, like clothing, but rather less significant for paper. Direct sunlight is also a risk when dyeing both fabric and paper, and a mordant will help improve the lightfastness of a dye. That said, not all natural dyes are as lightfast as others (these are commonly called fugitive dyes) so even with a mordant, you can expect some fading of colour.
As well as preventing colours from fading, mordants can also help to enhance the vibrancy of a natural dye. Mordants allow more dye to fix into the fibres, which results in deeper and more saturated hues. In natural dyeing this is particularly useful as some dyes are quite delicate, so mordants can help to brighten up paler colours.
Different mordants react with natural dyes in different ways. Some mordants, such as alum, produce brighter shades, whilst others, like iron, will produce darker shades. This is a particularly useful way to create a large spectrum of colours from just one dye. By altering the type of mordant used (and the strength of the mordant), one species of leaf can dye paper bright yellow, or olive green or even dark black. You can also combine mordants, either together in one dyeing process, or in series (pre- vs. post-mordanting), to produce even more variety of colour.
What isn’t a mordant?
Before we get into the best mordants to use for dyeing paper, it’s important to clear up some misinformation. When people start natural dyeing, they often want to use things they already have around the house or things that are very easy to get a hold of. Two things that are often misclassified as mordants are salt and vinegar. Although they both play a role in dyeing, neither are actually mordants.
Salt is used in dyeing to increase the attraction between the dye and the fibres (cellulose fibres in particular), which speeds up dye absorption. It doesn’t fix the dye onto the fibres in the way a mordant would, it just allows the whole process to happen quicker. Vinegar, on the other hand, works as a pH modifier which, depending on the dye you are using, can alter the colour produced. However, vinegar on its own won’t fix the colour.
Mordants to use
Alum vs. aluminum acetate
The rule when dyeing is to use alum (potassium aluminium sulphate) for protein (animal-based) fibres (e.g. wool, silk etc) and aluminium acetate for cellulose (plant-based) fibres (e.g. cotton, linen etc). Therefore, when dyeing on most types of paper, the mordant of choice would be aluminium acetate, but in my experience the results produced by alum on paper are also very good, and since it is more readily available and cheaper to buy than aluminium acetate, it is what I tend to use. As a side note: when dyeing paper, regardless of the mordant you use, the most important factor to consider is the contents of the paper. Dyes will only easily be taken up by natural fibres, so papers that contain traces of plastic or have a coating on will always produce paler, lower quality results, even when using a mordant.
Iron
Iron (ferrous sulphate) as a mordant has a distinct effect on the colour of dyes. It produces darker, duller colours when used with natural dyes, and will make ecoprints grey/purple or even black, depending on the strength of it. I can be used in conjunction with other mordants to produce a wide variety of different colours.
A cheap and easy way to make an iron mordant is by leaving rusty objects to sit in water for a couple of weeks until the rust comes off the objects and is suspended in the water. The only downside to this is it is almost impossible to know the strength of the rust water, which can mean unpredictable results whilst dyeing. If you are looking for more consistent results, I do recommend using ferrous sulphate, which can be bought in powder form and mixed with water to create the strength of iron mordant you need. It is different chemically to rust water, but produces similar results in the dyeing process.
Other Mordants
Another mordant that can be used for dyeing, but that I haven’t personally tried yet, is copper (copper sulphate). Copper, like iron, is a saddening mordant and shifts the colour of dyes towards green/blue instead of grey. There are also a few other chemical mordants, such as tin (stannous chloride), chrome (potassium dichromate) or titanium oxalate. However, these tend to be more toxic and therefore are harder or more expensive to buy. Since there are safer and cheaper options available, I haven’t tried any of these other mordants on paper or fabric.
Tannin, which is derived from plant sources such as oak bark, leaves, galls, pomegranate peels, black tea, sumac leaves, and walnut hulls etc. is another mordant. Most tannins can also act as a dye in their own right to produce yellow/brown colours, or can alter other natural dye colours. They are often used in conjunction with other mordants, such as alum (potassium aluminium sulphate) to help bind dyes to cellulose fibres. If you are ecoprinting with leaves, you may find that the leaves you choose already have tannins in. Therefore, either the paper or fabric you are dyeing won’t require pre-mordanting, or if you do use another mordant as well, you may get interesting effects during the dyeing process.
Dyes that don’t need mordants
A dye that doesn’t require a mordant to bind to fibres is known as a substantive dye. As previously mentioned, tannin rich dyes don’t require a mordant to work, because tannin itself is a mordant. A good example, and one that you may have already tried at home, is black tea, which will easily dye paper and fabric on its own.
Indigo is another dye that does not need a mordant, as it bonds differently to fibres compared to other natural dyes. It is a vat dye and therefore the dyeing process is different to that of other natural dyes, and not something I have tried before.
My dyeing process
When dyeing or ecoprinting paper, I like to use a mix of alum and iron mordants. Typically, I pre-mordant the paper by soaking it in alum, and then add iron mordant into the dye bath for boiling. I find this produces the best results for what I want: the alum brings out the colour of the leaves (typically bright yellow/green) and fixes the dye from the dye bath I’m using and iron darkens the colours to create better contrast between the leaf prints and the dyed paper. Iron also seems to affect the veins and edges of the leaves more and so helps to define the leaf print.
The great joy of natural dyeing and ecoprinting, on both paper and fabric, is the chance to experiment. Now that you have more information about mordants, hopefully this takes away some of the mystery of ecodyeing and gives you confidence to try it out yourself!