There are three types of fibres - protein, cellulose, and synthetic. Synthetic fibres, such as polyester, are made from petroleum, and will not dye using natural dyes. Protein fibres come from animals, the most common types being wool and silk, but can also include things such as fur and feathers, as well as many of the luxury fibres such as angora, cashmere, alpaca, etc. Cellulose fibres come from plants, the most common being cotton and linen, but also include hemp, bamboo, etc.
I tend to use the terms protein/animal and cellulose/plant interchangeably, as I am not a stickler for precision, but there are a couple of fibres that can trip up those categories, such as soy fibre, which of course comes from a plant, but is high enough in protein to be treated as a protein fibre. But soy fibre is also relatively uncommon. The other fibre that throws people off is rayon, which many folks think of as a synthetic. But it is actually what is known as a regenerative fibre, and is created through extensive processing, but it's base is (usually) wood pulp. So it also a cellulose fibre, and can be dyed as such.
Now the question is - why should I care? Well, it's because the different types of fibres react VERY differently to the mordants and dyes, and need to be treated differently. While to us they can seem like the same thing, and we might think that fabric is fabric, or yarn is yarn, the two types of fibres are chemically very different. Essentially, fibres are long chains of linked units, and the units that make up protein fibres are made of amino acids, whereas the cellulose units are made of beta-glucose. In short, the chemical reactions that are taking place when we dye, take place differently on the different types of units.
Secondly, as you may have discovered, animal fibres have a much greater affinity for natural dyes, and often our results will be much brighter, deeper, and longer lasting. This is also to do with the structure of the fibres - anyone with a chemistry background will have learned the difference between amorphous and crystalline structures - and just so we're clear, that person is not me! But essentially (and feel free to write and correct me on this!) crystalline structures are concentrated, highly organized chains of these above mentioned units. And amorphous means things are much looser, much more loosey goosey and not so organized. And these are the places that allow the mordants and dyes to penetrate and hold on. Those unstructured little spots that let all the good stuff in! While those crystalline units, all assembled and arranged so perfectly, don't allow for as much penetration. And... you guessed it, wool, for example, is made of both amorphous and crystalline, while cotton, for example, is mostly crystalline. Whew. Are you with me?
Now it is of course possible to dye without understanding all of this chemistry. What you basically need to know is that protein fibres easily form bonds with mordants (which we'll talk about next time!), and the dyes bond with the mordants. And plant fibres, well... they don't. So we have to add another layer, another step, which is to first treat our plant fibres with something that is rich in tannin. Tannins are soluble in water, so we can attach them to our fibre. Once we have, the mordants then form a bond with the tannin, which has become insoluble on our fibre, and we can then use our dyes to bond with the mordants. So much bonding! And that's that! Easy peasy, right?
Beyond all this chemistry, there are many other considerations when choosing your fibres for dyeing. First of all, what if a fibre has both plant and animal in it? Well - the different components may dye somewhat differently, and you may end up with a shade that is different than if the fibre was only plant or only animal. But go for it! You do have to complete the tannin step though. The yarn pictured at top is a merino and linen blend, and it dyes beautifully.
What if a fibre has some synthetic content to it? I still say go for it - though your results will vary depending how much synthetic. For example, a sock yarn, with 95% wool and 5% elastine, you likely won't notice much difference from a 100% yarn. But fabric that is 50/50 poly/cotton, you can still dye, but only the cotton content will take the dye, so it be quite pale, and quite difficult to achieve a dark shade.
Whether your item is old or new is also important - if you are dyeing an old t-shirt that has been washed many many times, those fibres will have already begun to break down quite a bit, and will be more open and receptive to the mordants and dyes. Compare this to piece goods such as cotton tote bags or napkins, which will still have a layer of the natural waxes and pectins that cotton contains AND will have often been coated with chemicals to make them stain resistant (and/or fire retardant and wrinkle resistant).
In a similar way, you may notice that a superwash merino yarn dyes quite brilliantly compared to a regular merino yarn. This is because the scales of the wool which cause felting have been partially removed (which is also what makes it so soft), again, allowing the mordants and dyes to penetrate more easily.
Finally, the initial colour of your fibre will also affect your results. You don't have to dye pure white items - you can dye things that are unbleached, or even things that are fully coloured. Basic colour mixing rules will apply. You can read more about overdyeing in this post.
Phew, that was a long one! And I know it wasn't the most exciting topic, but I promise you we will move on to more colourful things soon! In the meantime, if you made it this far, thanks for reading, and I will be back with more!
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