Bleeding, it plagues every dyer, whether indie or commercial. As dyers we have a responsibility to minimise as much bleeding as possible.
Before starting a project:
When creating something that uses different colourways to create stripes, fair isle, mosaic, etc, I always recommend that you swatch. This isn't just to test whether the colours look together but to test for any bleeding. The last thing you want is to knit something using contrasting yarns only to find that the darker yarn bleeds like mad and causes backstaining on the lighter yarn.
How I set the colours:
It happens to the best of dyers. Personally I steam set every single yarn for three hours (yes, you read that right - three hours! Much longer than most dyers) and then I leave it to cool down naturally in the steamer overnight. Many dyeing books and resources advocate this need for dyeing quickly with a quick blast in the microwave to set colours or steam for half an hour. In my experience, I couldn't get saturated colours to set with such short time frames but other dyers' mileage might vary.
Then I rinse the yarns the next day using hand warm water. I don't use wool washes because of any sensitivities that people may have. I might like my yarn to smell of eau d'unicorn but it doesn't mean I should inflict that upon you, besides it might give you hives! On the whole, my yarns rarely bleed but occasionally one might slip through the net and you might experience minimal bleeding. This isn't a cause of alarm unless it bleeds, and bleeds, and bleeds and the bleeding just never stops. Minimal bleeding isn't a fault, but never ending bleeding is. Please, please, please contact me if you experience the latter with my yarns.
Just as a heads up, yarns that contain silk and non-superwash wool are more likely to be bleeders/crockers than others such as Lord of Silk DK & Aran .
Okay, so you know your yarn is a bleeder:
Still in hank form, add one big figure of eight tie in the middle, preferably in undyed cotton, or string. Give the yarn a wash in the same way you would wash your FO. Bear in mind that I don't use wool wash so whilst the yarn will rinse clear for me with just water, the wool wash you use may leech out some colour. I personally am a fan of Navia wool wash or using just a standard washing up liquid like Fairy (Dawn, if you're across the pond). In my experience, I find scented wool washes to be the ones that leech out colour and I don't recommend using them. Blue dye molecules are slightly bigger than any other colour and are more likely to contribute to bleeding.
It's bleeding, I've been told to wash it with vinegar to set the colour!
I swear that this is one big urban myth that circulates Ravelry and I have no idea where it originates. Vinegar, or citric acid, is used to set acid dyes on protein fibres in conjunction with heat. On it's own, it will NOT set colour. If it did, I wouldn't need to steam the yarn, right? If you're familiar with dyeing your hair, especially with some of the colours on the crazier end of the scale, then vinegar is used to soften down the cuticles on the hair shaft which get opened up during the dye process. Wool is very similar in this regard so vinegar will soften the fibre, flatten down those cuticles and that's it. You need heat too.
It's bleeding but the colour of the yarn isn't changing:
This is because the dye has been set correctly on the yarn but has either not been rinsed properly or your water is too hot or you're using a wool wash that is leeching out the colour. Just keep washing and rinsing to remove the excess dye. If the colour of the yarn is changing, or fading, that means the dye has not been set properly at all. If the colour is staying the same, you don't need to reset it, it's just excess dye that is coming away. Fibre can only hold so much before it starts spitting forth.
One thing to bear in mind is that the composition of the water that I use in the studio may be different from the water that you use, i.e. slightly different pH level, hard water v soft water, presence of added chemicals such as fluoride. Whilst I can try to ensure that yarn and fibre will not bleed in my water conditions, I am unable to give that guarantee to your own water conditions. It's all chemistry.
How do I set the dye?
Heat. You need heat. Using a stainless steel pan, one you'll never use again for food, fill with cold tap water, add a decent sized glug of vinegar or a tablespoon or so of citric acid if you happen to have it about for brewing alcohol or making bath bombs, and dunk in your yarn or FO. Slowly heat the pan for about 30 minutes and occasionally stir gently. Very gently, especially if the yarn is not superwash. You don't want to boil the water but you want to get it simmering. Take the pan off the heat, put the lid on and walk away. Seriously, just leave it, preferably overnight. You should find that the water is completely clear the next morning, meaning the dye molecules should have bonded to the yarn. Set the dye slowly, there's no rush. I'm of the opinion that dyeing should be a slow process, just like knitting. Step away from nuking it in the microwave. If you've used different colourways in a project and did not check for bleeding prior to knitting, try using a colour catcher when washing but it is always best to check for bleeding before you commence your project.
This is a specialist detergent, not available at the supermarket, that prevents backstaining which is useful if you've discovered your darker yarn has bled onto your lighter contrast yarn. It also strips oil from coned yarns and used for washing fleece. I'll be straight up honest with you, it stinks. It has this sort of chemically smell that hits you at the back of the throat but it's useful to have some to hand. Hopefully it'll languish in the cupboard under the kitchen sink and you'll never need to use it but if you do, you only need a couple of drops in your water to remove excess dye. It prevents that excess dye from depositing itself back onto the yarn so hopefully it should clean up that lighter contrast colour. I say hopefully, nothing is ever a guarantee. Have a look on eBay for a supplier. Dharma Trading also does it's own version that is just as good.
Crocking, what's that all about?
Have you ever knitted or crocheted something and found that your hands were getting stained? That's crocking. It's the same thing as buying dark colour jeans and it has a tag that warns about dye transfer. Crocking is generally caused by the pH value of your skin pulling out the dye molecules. It doesn't mean that the yarn will bleed though once it hits water, I mean it might, but it doesn't mean that it definitely will. I find that this is more likely to happen with colours that use blue dyes and/or yarns made from non superwash wool and silk. I'm looking at you dark saturated purple and vibrant green! If you want to check that your FO will bleed, rinse the project in free flowing water before leaving to soak which will prevent back staining.
Knots in the yarn
I try very hard not to sell yarns that contain knots, especially in superwash yarns as the yarn is trickier to join together neatly. Generally these are sold as seconds and the listing will state that there is a knotted join. Yarns that have not been superwash treated are allowed to pass my "no knots in the skein" rule as long as it is only the one knotted join. Non superwash wool can easy be spit-spliced and felted. More often than not, yarn is joined together in the mills in a splicing method which is perfectly strong enough to knit through. Please bear in mind that my skein winding equipment is motorised so it is more difficult for me to always catch knots.
More often than not at the mill, yarn ends are thermally joined together, so fused not knotted. The fuses do not show once knitted though.