"It would be great if you would dye a red and white self-striping yarn. Or red, white, and blue!" I've heard some version of the above many times, and I always smile and thank the person for the suggestion. But in my head, I'm thinking "No way!" But why wouldn't I want to dye a red, white, and blue self-striping yarn? It involves chemistry.
Chemistry? What does chemistry have to do with which colors are dyed together on a skein of yarn? Chemistry is everything.
Think about doing laundry. Has a red sock ever ended up in your white wash? What happens? Now think about each time you've blocked or washed a project. Have you ever ended up with the water being tinted with color? How often was that color in the red or blue spectrum? Red and blue (especially turquoise) dyes have a tendency to not "stick" to the yarn quite as well as other colors and might go back into the water when soaked. Several factors contribute to this phenomenon, but it can have upsetting results: if you've made something using a red and white yarn and the red dye molecules enter the water while soaking, those dye molecules aren't going to automatically go back to the red parts of the project. Some will go into the white sections.
So why does this happen? Note, I'm going to specifically be discussing a type of dye called acid dyes, which is what I use, so some of this would not apply to the types of dyes used for cotton, for example. To set the color for acid dyes, two things are needed: acid + heat. The acid changes the pH level of the water, as these dyes need a lower pH in order to bond with the fiber. Using vinegar or citric acid is most common for getting the right pH level. The heat is also part of this chemical bonding. Without both acid and heat, the dye won't bond to the yarn.
Have you ever wondered why hand-dyed yarn should be washed in cold water? That's because if you add heat back into the equation, you're taking the chance of "reactivating" the dye particles and causing them to unbind and move around. So to combat this possibility, hand-dyed yarn truly must be washed in cold water. And I do mean cold water. If the water coming out of your faucet isn't very cold, add ice cubes to the water to cool it down even more.
So why are red and blue more likely to "bleed"? If you look at the size of the molecules in various dyes, the red and blue dyes usually contain larger molecules. Think of a bunch of balls. If you have a bunch of tiny balls (like in a ball pit for kids), the balls are able to get closer together, without much space in between. So smaller dye molecules are able to "hug" the yarn more tightly. But if you have a bunch of larger balls (now imagine a pit full of large beach balls), there will be a lot more space between the balls. So those larger molecules of dye aren't able to hug the yarn quite as tightly, and it's much easier for them to not be as securely attached, especially when put into an environment that "reactivates" them (such as hot water).
What else can cause yarn to bleed? The water you use to wash your projects may be different from the water used to dye the yarn. And if your water has a different pH level or other different factor (hard water vs soft water, chemicals such as fluoride), it can cause the dye bonds to weaken. Sometimes, the soaps you use to wash your projects can affect the chemical bonds, especially if the soap has a scent to it as the chemicals in the scent can interfere with the dye molecules. So if you're finding yarn bleeding to be a big issue for you, you might try washing using different water and/or a different wool wash.
But what if you have your heart set on a red and white striped sweater? Is there anything you can do so you don't end up with a red and pink sweater?
Honestly, this will save you so much potential heartbreak. Not only should you swatch to check for gauge, but you should also swatch to check what happens after you wash that swatch. So create a fairly generous swatch (at least 4 inches square) using both colors of yarn. Wash this swatch just as you would your sweater: cold water and unscented wool wash (just a drop) is recommended. Let it soak for a good long while. What happens? If the water and white yarn stay completely clear, it just might be possible for you to get the sweater you want (but it's no guarantee).
If the red yarn does tint the water, how much tint is there? If it's just a little, it could just be that the yarn needs to be rinsed a bit more. Do this before you knit the sweater. Put the yarn back into a hank (big loop) and make sure to tie a few figure 8 ties so the yarn doesn't get tangled. Wash the yarn until the water rinses clear. Sometimes using blue Dawn (because it contains a surfactant) can be helpful. Another option is to use Synthrapol, which is a detergent that can help keep excess dye in water from depositing back onto the yarn. (You only need a tiny bit and should ignore the instructions to use hot water as that is for a different type of dye.)
Reset the Color
If the red yarn turns the water very red, so that it is hard to see through the water, you can try the above suggestions. But this also might be a sign that the dye did not bond completely with the yarn. This is the point where you might need to "reset" the dye. You might have seen suggestions to soak the yarn in water and vinegar to set the dye. But that suggestion is only half right. Don't forget that heat is also needed! The best way to do this is to get a stainless steel pot that you won't use for food again. Fill it with enough water to cover the yarn and add a glug of vinegar. Put it on the stove and heat it slowly. Don't let it boil, but it should simmer, for at least 30 minutes, until the water becomes clear. (Some people might suggest microwaving the yarn, but that can lead to burning the yarn, which is an even worse problem, so I don't recommend it.)
So what happens if your swatch turned out okay but you wash your sweater for the first time and—horror! The white areas turned pink!
Do NOT add vinegar or citric acid and/or heat at this point. If you do, you'll successfully have set the red dye into the white sections, making the pink parts permanent. Instead, make sure you are using cold water to wash (add those ice cubes to be sure) and try using a little Synthrapol in the water so any dye that comes out into the water won't resettle back onto the white (in fact, it might be good to add it from the beginning—or one of those color catchers you can use for laundry). Also, do not use bleach because that will damage the wool.
I know it's heartbreaking to have a color bleed onto another area of a project. But if you take the above steps, you'll minimize the risk. (Simply washing in cold water with a non-scented wool wash is the best first line of defense.) But there's still a risk. And that's why I won't dye a red and white or red, white, and blue self-striping colorway.
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 In case you're curious, according to How Acid Dyes Work, the chemistry involved goes like this: "Acid Dyes are so called because they contain acidic molecular groups such as -S03H and work in a low pH environment with a mildly acidic 'fixative' like white vinegar or citric acid. Acid Dyes are used to dye protein fibers (and nylon) which are all made out of proteins with amino groups -NH2 and the bond between the dye and fiber occurs between the basic amino groups and acidic -S03H groups. Acid dyes are thought to fix to fibers by hydrogen bonding, Van der Waals forces and ionic bonding. Fiber━NH2 + HS03━ Dye → Fiber━NH3+-S03━Dye"